LENTICEL DAMAGE ON ‘B74’ MANGO FRUIT372944/s4247519...i LENTICEL DAMAGE ON ‘B74’ MANGO FRUIT Tuan Minh Nguyen B.Sc Food Technology M.Sc Agriculture Food Production A thesis

i

LENTICEL DAMAGE ON ‘B74’ MANGO FRUIT

Tuan Minh Nguyen

B.Sc Food Technology

M.Sc Agriculture Food Production

A thesis submitted for the degree of Doctor of Philosophy at

The University of Queensland in 2015

School of Agriculture and Food Sciences

ii

Abstract

The mango (Mangifera indica L.) is a fruit crop of considerable economic and social importance.

Widely renowned as the king of fruits, mangoes are produced in tropical and sub-tropical regions.

In Australia, the cultivar B74 (marketed as CalypsoTM) is a recently developed hybrid cross between

Kensington Pride (KP) and Sensation. Damage to lenticels, macroscopic cavities on the surface of

fruit that facilitate gas exchange, limits the marketability of B74 fruit. Lenticel damage (LD)

exhibits as a dark area surrounding the lenticel cavity, which reduces the visual appeal of the fruit.

In the current research, the working hypothesis that high cell turgor within the lenticel cavity

predisposes mango fruit to LD that is exacerbated by other stresses was tested.

The characteristics and morphology of lenticels during fruit development of B74 and four other

mango cultivars were comparatively examined. Lenticels on B74 fruit formed from stomata on

young fruit and, more so, from cracking of the cuticle during later stages of fruit growth. B74 fruit

had 2- to 7-times higher lenticel density at commercial harvest maturity than KP, Honey Gold and

R2E2 fruit. Sensation fruit had a 2-fold higher lenticel density than B74. The high lenticel density

on B74 fruit, as inherited from Sensation, may explain its greater susceptibility to developing

commercially significant LD. Accumulation of condensed phenolics around damaged B74 lenticels

was also observed macroscopically and microscopically. A larger proportion of lenticels on B74

fruit had smaller chambers at commercial harvest as compared to KP, perhaps because more

lenticels formed later from cracks. There was no spatial link in terms of close proximity between

sub-cuticlar epidermal resin ducts and LD. This lack of association suggests that external stressors

are key LD-causing agents.

The impact of tree irrigation was investigated towards discerning pre-disposing factors to LD.

Withholding irrigation from B74 trees for 3 - 8 weeks prior to harvest reduced soil water content

and leaf stomatal conductance. However, it had no appreciable effects on either fruit water status or

lenticel morphology. The severity of LD on fruit at eating ripe was not reduced by irrigation

treatment, although withholding irrigation for 4 weeks prior to harvest increased LD severity on

fruit at eating ripe and 7 days after eating ripe. Dipping B74 fruit after harvest into water increased

LD severity by ~ 1.7 - 2.0-fold after standard commercial pre-harvest irrigation. Exposure of fruit

to γ-irradiation also increased LD irrespective of the irrigation treatment. Where fruit do not require

irradiation, ceasing irrigation from 3 - 8 weeks before harvest might represent a cost saving.

iii

The susceptibility of B74 fruit to developing LD in response to commercial packing house

operations, specifically solution dipping and brushing, were investigated. Postharvest operations

concomitantly increased LD severity and resulted in pigment accumulation in epidermal cells

surrounding the lenticel cavity. LD severity increased on fruit that were agitated in water, possibly

because it contributed to the breaking of air films over fruit surfaces, thereby extending wetting and

facilitating water entry into lenticels. Treatment of fruit with a non-ionic surfactant solution

increased LD severity, presumably by enhancing cuticle wettability and / or if the organic active

components were phytotoxic to cell membranes. Treating fruit with NaCl solutions (2 - 3%) that

aimed to lower solution osmotic potential to approximate cell water potential (~ -0.8 MPa) resulted

in an undesired increase in LD severity. Similarly, treatment with polyethylene glycol 6000 also

resulted in higher LD severity. Brushing B74 fruit with different types of brushes for varying

durations did not consistently increase LD severity in the presence of water.

The efficacy of pre- and postharvest bagging and coating treatments were tested to potentially

reduce LD on B74 fruit. Two treatments, on-tree bagging with paper bags at 2 months and on-tree

coating with 2.5% TFC (a carnauba-based wax) at 1 day before harvest, significantly reduced LD

severity. Adding a 0.1% surfactant (Maxx Organosilicone Surfactant™) to 20% TFC reduced LD

severity by up to 43% until 9 days after the full yellow skin colour stage as compared to the

untreated control. Postharvest coating with 30% TFC plus surfactant also reduced LD severity by

up to 50% on fruit at the eating ripe stage as compared to the control. No bagging or coating

treatment effects were evident on lenticel morphology. However, TFC at 15% or higher covered

the fruit surface and the lenticel cavity, which appeared to minimise external stress impacts on

lenticels.

Overall, the research revealed that the relatively high lenticel density on B74 fruit may explain its

susceptibility to commercially significant LD during typical postharvest handling. The research

also highlighted that external stressors, namely exposure of fruit to water, were key LD-causing

agents. Covering fruit with a paper bag and coating fruit with TFC plus 0.1% surfactant before or

after harvest effectively reduced LD. These proposed control methods may contribute to further

enhancing the commercial prospects of mango cultivars such as B74 that are relatively susceptible

to developing LD.

iv

Declaration by author

This thesis is composed of my original work, and contains no material previously published or

written by another person except where due reference has been made in the text. I have clearly

stated the contribution by others to jointly-authored works that I have included in my thesis.

I have clearly stated the contribution of others to my thesis as a whole, including statistical

assistance, survey design, data analysis, significant technical procedures, professional editorial

advice, and any other original research work used or reported in my thesis. The content of my thesis

is the result of work I have carried out since the commencement of my research higher degree

candidature and does not include a substantial part of work that has been submitted to qualify for

the award of any other degree or diploma in any university or other tertiary institution. I have

clearly stated which parts of my thesis, if any, have been submitted to qualify for another award.

I acknowledge that an electronic copy of my thesis must be lodged with the University Library and,

subject to the policy and procedures of The University of Queensland, the thesis be made available

for research and study in accordance with the Copyright Act 1968 unless a period of embargo has

been approved by the Dean of the Graduate School.

I acknowledge that copyright of all material contained in my thesis resides with the copyright

holder(s) of that material. Where appropriate I have obtained copyright permission from the

copyright holder to reproduce material in this thesis.

v

Publications during candidature

A. Proceeding paper

Nguyen TM, Joyce DCA, Gupta MB, Hofman PJC, Macnish AJD, Marques JRE (2014) Pre-harvest

bagging and coating treatment effects on lenticel discolouration in ‘B74’ mango fruit. Acta

Horticulturae (submitted).

B. Magazine article

Roberto MarquesA, Minh NguyenB, Peter HofmanC and Daryl JoyceD (2013) Do late-harvested

mangoes develop more lenticel damage after harvest. Mango Matters - Autumn, 29-31. A and B

were responsible for carrying out the experiment, recoding and analysing data. C and D were

responsible for advising on the experiment design and critically revising the manuscript.

Publications included in this thesis

Nguyen TMA, Joyce DCB, Gupta MC, Hofman PJD, Macnish AJE, Marques JRF (2014) Pre-harvest

bagging and coating treatment effects on lenticel discolouration in ‘B74’ mango fruit. Acta

Horticulturae, In press. (incorporated as Chapter 7).

A was responsible for conducting experiment and drafting the manuscript. B, C, D and E were

responsible for concept initiation and critically revising the manuscript. F contributed to manuscript

conception.

vi

Contributions by others to the thesis

1. Professor Daryl Joyce (Thesis Principal Advisor; UQ Professor): conception and design of the

project; advising on individual experiments; critical revising drafts of the work so as to contribute to

interpretation.

2. Dr Peter Hofman (Thesis Advisor; Senior Principal Horticulturist - Postharvest; Queensland

Department of Agriculture and Fisheries): advising on individual experiments; critical revising drafts

of the work so as to contribute to interpretation.

3. Dr Andrew Macnish (Thesis Advisor; Horticultural Scientist - Postharvest; Queensland Department

of Agriculture and Fisheries): advising on individual experiments; critical revising drafts of the work

so as to contribute to interpretation.

4. Dr Madan Gupta (Thesis Advisor; UQ Senior Lecturer): advising on individual experiments in

chapters 1, 2, 5, 6, 7 and 8; critical revising drafts of the work so as to contribute to interpretation.

5. Bob Mayer (Biometrician; Queensland Department of Agriculture and Fisheries): advising on

experimental design and data analysis.

6. Dr. Chisty Macnish (Editor; University of Sunshine Coast): review of thesis chapters for professional

editing.

Statement of parts of the thesis submitted to qualify for the award of another degree

“None.”

vii

Acknowledgements

I would like to express my gratitude and appreciation to my principal advisor Professor Daryl Joyce

for his critical guidance, constructive criticism and inspiration. I am also sincerely grateful to my

co-advisors, Dr Peter Hofman, Dr Andrew Macnish, and Dr Madan Gupta for their expert advice,

constructive comments, and kind encouragement in my research. I gratefully acknowledge their

contributions in the preparation of papers, manuscripts, and this thesis. Apart from academic

contributions, I would like to acknowledge their understanding, which enabled me to successfully

complete this PhD study.

Financial support for my study was provided by a John Allwright Fellowship (JAF) funded by the

Australian Centre for International Agricultural Research (ACIAR). I would also like to thank the

UG Postgraduate School for the UQ International Quarter I-2015 Tuition Fee Scholarship. The

research budget and conference expenses were provided by the Horticulture Innovation Australia

(HIA) Project No. MG10008. This mango project was funded by HIA through voluntary

contributions from Harvest Fresh Fruits Company Ltd. and matched funds from the Australian

Government. The project was also financially supported by The Queensland Government through

the Department of Agriculture and Fisheries (DAF) and the University of Queensland (UQ). I

gratefully acknowledge industry support from Simpson Farms and Oolloo farms in supplying fruit

necessary to complete this research.

I am deeply indebted to Robert Nissen, a former ACIAR project leader, who supported me in

applying for the JA fellowship. Assistance and cooperation from current and former staff members

of the DAF Supply Chain Innovation Team, including Dr Roberto Marques, Dr Bhavisha Mehta,

Ms Leanne Taylor, Mr Jonathan Smith and Ms Barbara Stubbings, are highly appreciated. I would

like to express my sincere gratitude to other staff members at the DAF Maroochy Research Facility,

especially Mr. Bob Mayer for his advice regarding statistics. I would like too to thank Ms Sharon

Harvey at ACIAR, Ms Catherine Fitzgerald at UQ International Scholarship Unit, and Ms Kaye

Hunt at UQ School of Agriculture and Food Sciences for their kind administrative support, plus Dr

Christy Macnish for English editing of my thesis. Fellow postgraduate students Muhammad Sohail

Mazhar, Xi Yu, Tram Anh San, and Li Guoqin were always friendly and supportive and I hereby

express my thanks to them. I reserve special thanks for my beloved wife, Nho, and my son, Hieu,

for their immense sacrifices throughout the period of my PhD work.

viii

Keywords

Lenticel discolouration, bagging, brushing, coatings, irrigation, lenticel morphology, wetting angle.

Australian and New Zealand Standard Research Classifications (ANZSRC)

ANZSRC code: 070605, Post Harvest Horticultural Technology, 100%

Fields of Research (FoR) Classification

FoR code: 0706, Horticultural Production, 100%

ix

TABLE OF CONTENTS

CHAPTER 1: GENERAL INTRODUCTION .................................................................................. 1

1.1 Research background ............................................................................................................. 1

1.2 Research approach ................................................................................................................. 3

1.3 Thesis composition ................................................................................................................ 5

CHAPTER 2: LITERATURE REVIEW .......................................................................................... 6

2.1 Physiological disorders of fruits ............................................................................................ 6

2.2 Lenticel damage in mango fruit ............................................................................................. 8

2.2.1 Lenticel origin in mango fruit .................................................................................... 8

2.2.2 Function and morphology of lenticels ....................................................................... 9

2.2.3 Epicuticular wax and cutin layer in lenticel damage ............................................... 10

2.2.4 Differences in lenticel sensitivity among cultivars .................................................. 11

2.2.5 Lenticel damage mechanisms .................................................................................. 12

2.2.6 Pre-harvest and postharvest factors causing lenticel damage .................................. 14

2.2.7 Conditions for free water entering into lenticel cavities .......................................... 16

2.2.8 Conditions for free water infiltration into cells ........................................................ 18

2.2.9 Stresses on lenticels ................................................................................................. 19

2.2.10 Substrates, enzymes and factors involved in lenticel damage process .................... 20

2.2.11 Discolouration process in lenticels .......................................................................... 23

2.2.12 Role of lenticel damage in fruit shelf life ................................................................ 25

2.3 Study directions ................................................................................................................... 25

CHAPTER 3: GENETIC INHERITANCE AND SENSITIVITY TO DEVELOPING

LENTICEL DAMAGE ON B74 MANGO FRUIT ................................................. 28

3.1 Introduction .......................................................................................................................... 28

3.2 Materials and methods ......................................................................................................... 29

3.2.1 Sample collection sites ............................................................................................. 29

3.2.2 Experiments ............................................................................................................. 29

3.2.3 Assessments ............................................................................................................. 30

3.2.4 Statistical analyses ................................................................................................... 34

3.3 Results .................................................................................................................................. 34

3.3.1 Lenticel density and sensitivity to lenticel damage ................................................. 34

3.3.2 Lenticel origin .......................................................................................................... 37

3.3.3 Mechanism of lenticel damage on B74 mango fruit ................................................ 39

3.4 Discussion ............................................................................................................................ 45

3.4.1 Sensitivity of B74 mango fruit to lenticel damage .................................................. 45

3.4.2 Lenticel origin .......................................................................................................... 46

3.4.3 Mechanism of lenticel damage on B74 mango fruit ................................................ 48

x

CHAPTER 4: PREDISPOSING FACTORS TO LENTICEL DAMAGE ON B74 MANGO

FRUIT - A RETROSPECTIVE ANALYSIS OF DATA ........................................ 50

4.1 Introduction .......................................................................................................................... 50


4.2.1 Fruit .......................................................................................................................... 51

4.2.2 Assessments ............................................................................................................. 53


4.3 Results .................................................................................................................................. 53

4.3.1 Farms effects ............................................................................................................ 53

4.3.2 Interaction between farm and season ....................................................................... 54

4.3.3 Location in the block................................................................................................ 55

4.3.4 Effect of harvesting and handling methods on lenticel damage severity ................. 55

4.3.5 Effect of brushing associated with water on lenticel damage severity .................... 56

4.3.6 Effect of coatings and irradiation on lenticel damage severity ................................ 57

4.4 Discussion ............................................................................................................................ 57

CHAPTER 5: EFFECT OF WITHHOLDING IRRIGATION ON LENTICEL DAMAGE ON

B74 MANGO FRUIT .............................................................................................. 60

5.1 Introduction .......................................................................................................................... 60


5.2.1 Experiment sites ....................................................................................................... 61

5.2.2 Treatments and handling procedure ......................................................................... 61

5.2.3 Assessments ............................................................................................................. 66


5.3 Results .................................................................................................................................. 69

5.3.1 Experiment 1: Withholding irrigation for 3 weeks in 2012 ..................................... 69

5.3.2 Experiment 2: Withholding irrigations for 4 and 8 weeks in 2013 .......................... 74

5.4 Discussion ............................................................................................................................ 81

CHAPTER 6: EFFECTS OF POSTHARVEST PRACTICES ON LENTICEL DAMAGE ON

B74 MANGO FRUIT .............................................................................................. 84

6.1 Introduction .......................................................................................................................... 84


6.2.1 Fruit .......................................................................................................................... 85

6.2.2 Experiments ............................................................................................................. 85

6.2.3 Assessments ............................................................................................................. 88


6.3 Results .................................................................................................................................. 91

6.3.1 Effects of commercial operations ............................................................................ 91

6.3.2 Effects of solution treatments .................................................................................. 93

6.3.3 Effects of brushing treatments ................................................................................. 97

6.4 Discussion .......................................................................................................................... 100

xi

CHAPTER 7: EFFICACY OF BAGGING AND COATING TREATMENT TO REDUCE

LENTICEL DAMAGE ON B74 AND CAT CHU MANGO FRUIT ................... 104

7.1 Introduction ........................................................................................................................ 104

7.2 Materials and methods ....................................................................................................... 106

7.2.1 Fruit ........................................................................................................................ 106

7.2.2 Fruit bags and coating compounds......................................................................... 106

7.2.3 Treatments .............................................................................................................. 107

7.2.4 Fruit harvest and processing .................................................................................. 110

7.2.5 Assessments ........................................................................................................... 111

7.2.6 Experiment design and statistical analyses ............................................................ 114

7.3 Results ................................................................................................................................ 114

7.3.1 Experiment 1: pre-harvest bagging and coatings of B74 fruit in 2011/12.. ........... 114

7.3.2 Experiment 2: pre-harvest bagging and coatings of B74 fruit in 2012/13.. ........... 117

7.3.3 Experiment 3: pre-harvest bagging and coatings of B74 fruit in 2013/14 ............. 120

7.3.4 Experiment 4: postharvest coatings of Cat Chu fruit in 2014/15 ........................... 125

7.3.5 Experiment 5: postharvest coatings of B74 fruit in 2014/15 ................................. 126

7.4 Discussion .......................................................................................................................... 130

CHAPTER 8: GENERAL DISCUSSION .................................................................................... 135

8.1 Lenticel damage sensitivity, genetic inheritance and lenticel damage mechanism of B74

fruit ..................................................................................................................................... 136

8.2 Improvement of pre- and postharvest management through irrigation, solution treatments

and brushing to minimise lenticel damage on B74 fruit .................................................... 137

8.3 Potential for bagging and coatings to reduce lenticel damage on B74 fruit ...................... 139

8.4 Directions for future research ............................................................................................ 139

REFERENCES……………............................................................................................................. 141

APPENDICES.. ............................................................................................................................... 153

Appendix 1. Field site of B74 mango production for the irrigation experiments ...................... 153

Appendix 2. Soil compositions of the two sites for irrigation experiments ............................... 153

Appendix 3. Irrigation schedule of the standard irrigation ........................................................ 154

Appendix 4. Effect of withholding irrigation for 3 weeks on leaf water status in 2012 ............ 154

Appendix 5. Effect of withholding irrigation for 3 weeks on fruit pericarp relative water

content in 2012 ....................................................................................................... 155

Appendix 6. Effect of withholding irrigations for 4 and 8 weeks on dry matter and fruit

firmness in 2013 .................................................................................................... 155

Appendix 7. Effect of pre-harvest bagging and coatings of B74 mango fruit on

fruit firmness in 2011/12 ....................................................................................... 156


fruit firmness in 2012/13………. .......................................................................... 156

xii


dry matter, fruit firmness and sensorial evaluation in 2013/14 ............................. 157

Appendix 10. Effect of postharvest coatings of Cat Chu mango fruit on fruit firmness in

2014/15................................................................................................................... 157

Appendix 11. Effect of postharvest coatings of B74 mango fruit on fruit attributes in

2014/15. ................................................................................................................. 158

xiii

LIST OF TABLES

Table 2.1. Listing of some important physiological disorders of fruits ....................................... 7

Table 3.1. Rating scales for lenticel damage severity on the skin of B74 mango ...................... 32

Table 3.2. The density of lenticels (number per cm2) on hard green-mature fruit. If

followed by different letters, means for lenticel density within each column

are significantly different (P < 0.05) by LSD (P = 0.05). ........................................ 35

Table 3.3. Lenticel damage severity (0 = no damage to 5 = severe damage) and

proportion (%) of damaged lenticels on the fruit skin. If followed by different

letters, means for lenticel damage within each column are significantly

different (P < 0.05) by LSD (P = 0.05)... ................................................................. 36

Table 3.4. Dyed lenticel density (per cm²) and proportion of dyed lenticels (%) on green-

mature mango fruit in South-East Queensland. If followed by different

letters, means for dyed lenticel density and proportion of dyed lenticels within

each column are significantly different (P < 0.05) by LSD (P = 0.05). .................. 37

Table 4.1. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 fruit

harvested from different farms and assessed at full yellow skin colour. If

followed by different letters, means for lenticel damage severity within each

column in the same season and region are significantly different (P < 0.05) by

LSD (P = 0.05) ......................................................................................................... 54

Table 4.2. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 mango

fruit harvested from different farms and seasons, and assessed at full yellow

skin colour. If followed by different letters, means for lenticel damage

severity within each column are significantly different (P < 0.05) by LSD (P

= 0.05) ...................................................................................................................... 55


harvested from different locations in a farm in Childers, South-East

Queensland in the 2009/10 season. If followed by different letters, means for

lenticel damage severity are significantly different (P < 0.05) by LSD (P =

0.05) ......................................................................................................................... 55

Table 4.4. Lenticel damage severity (0 = no damage to 5 = severe damage) of B74 fruit

harvested from the tree directly into trays (off tree) or collected at the end of a

packing line (commercial practice). If followed by different letters, means for

lenticel damage severity in the same farm are significantly different (P <

0.05) by LSD (P = 0.05). ......................................................................................... 56

Table 4.5. Lenticel damage severity (0 = no damage to 5 = severe damage) of B74 fruit

harvested directly from tree without any postharvest treatment. Fruit were

then brushed with or without water spray. A set of fruit served as the control.

If followed by different letters, means for lenticel damage severity are

significantly different (P < 0.05) by LSD (P = 0.05)...………….. .......................... 56

xiv


coated and / or irradiated and assessed at full yellow skin colour. The fruit

were then coated with Natural Shine™ TFC210 (three part wax one part

water) and / or irradiated (398 Gy). If followed by different letters, means for

lenticel damage severity are significantly different (P < 0.05) by LSD (P =

0.05) ......................................................................................................................... 57

Table 5.1. Average day and night temperatures (°C), relative humidity (RH, %), and total

rainfall (mm) per week during the 4 weeks from 16th October to 13th

November 2012 before harvest at the experiment site.………. ............................... 69

Table 5.2. Effects of irrigation treatments of B74 mango tree on soil moisture content (%)

at three soil depths from 3 weeks before harvest until harvest in 2012. If

followed by different letters, means for soil moisture content within each

week before harvest are significantly different (P < 0.05) by LSD (P = 0.05) ....... 70

Table 5.3. Effects of irrigation treatments on the gape (mm) of a cut and water potential

(MPa) of B74 fruit. The harvested fruit were subjected to either water

dipping or not and then to either irradiation or not. The tests were done on

the day of harvest for the no-irradiation treatments, or 3 days after harvest for

the irradiation treatments. If followed by different letters, means for gape or

water potential are significantly different (P < 0.05) by LSD (P = 0.05) ............... 72

Table 5.4. Effects of irrigation treatments on lenticel damage severity (0 = no damage to 5

= severe damage) on the B74 fruit. The harvested fruit were subjected to

either water dipping or not and then to either irradiation or not. If followed

by different letters, means for lenticel damage severity at full yellow or 7

days later are significantly different (P < 0.05) by LSD (P = 0.05).... .................... 73

Table 5.5. Average day and night temperatures (ºC) and relative humidity (%, RH) and

the total rainfall (mm) during 8 weeks before harvest (17th September to 12th

November 2013) at the experiment site ………… .................................................. 75

Table 5.6. Effects of irrigation treatments on water potential (MPa) and gape (mm) of

B74 fruit in 2013. If followed by different letters, means for water potential

or gape within each column at the same site are significantly different (P <

0.05) by LSD (P = 0.05). ......................................................................................... 78

Table 5.7. Effects of irrigation treatments on lenticel damage severity (0 = no damage to 5

= severe damage) of B74 fruit in 2013. If followed by different letters, means

for lenticel damage severity within each column at the same site are

significantly different (P < 0.05) by LSD (P = 0.05). .............................................. 80

Table 6.1. Solution treatments to investigate the effect of solutions on lenticel damage and

fruit quality attributes of B74 mango fruit. Fruit were treated with DI water,

DI water +agitation, 0.1% surfactant, NaCl at 1, 2, and 3%, and PEG 6000 at

150, 300, and 400 g.kg-1 H2O.………….................................................................. 86

Table 6.2. Brushing treatments to investigate the effects of brushing with or without water

spray for 1 or 2 minutes on lenticel damage and fruit quality attributes of B74

mango fruit ............................................................................................................... 88

xv

Table 6.3. Effects of various serial commercial packing line steps on wetting angle (°) of

5 µl DI water droplets on the fruit surface at harvest and lenticel damage

severity (0 = no damage to 5 = severe damage) of B74 fruit at full yellow. If

followed by different letters, treatment means for wetting angle or lenticel

damage severity within each column are significantly different (P < 0.05) by

LSD (P = 0.05)... ...................................................................................................... 91

Table 6.4. Effect of solution treatments to B74 mango fruit on lenticel damage severity (0

= no damage to 5 = severe damage), subjective firmness (0 = hard to 4 = soft)

and objective firmness (Aweta reading). If followed by different letters,

means for lenticel damage severity, subjective firmness, or objective firmness

within each column are significantly different (P < 0.05) by LSD (P = 0.05).. ...... 94

Table 6.5. Effects of brushing treatments to B74 mango fruit on lenticel damage severity

(0 = no damage to 5 = severe damage), skin glossiness (1 = less glossiness to

6 = almost glossiness), dyed lenticel index (1 = almost no dyed lenticels to 6

= almost dyed lenticels), and blotchy dye (1 = almost no blotchy area to 6 =

almost blotchy area). If followed by different letters, means for lenticel

damage severity, glossiness, dyed lenticel index, or blotchy dye index within

each column are significantly different (P < 0.05) by LSD (P = 0.05) ................... 98

Table 6.6. Effects of brushing treatments to B74 mango fruit on wetting angle (°) of 5 µl

DI water on the fruit surface and skin browning (0 = no skin browning to 5 =

severe skin browning) at full yellow (eating ripe) and at 7 days after full

colour. If followed by different letters, means of wetting angle or skin

browning within each column are significantly different (P < 0.05) by LSD

(P = 0.05). ................................................................................................................ 99

Table 7.1. Treatments applied in the 2011/12, 2012/13 and 2013/14 seasons to test effects

of bagging and coatings applied before harvest on LD and fruit attributes of

mango fruit B74. Postharvest coatings were also applied in the 2014/15

season to test effects of coatings for harvested B74 and Cat Chu fruit on

quality, including lenticel damage. ........................................................................ 109

Table 7.2. Effects of pre-harvest bagging and coatings of B74 mango fruit on lenticel

damage severity (0 = no damage to 5 = severe damage), dyed lenticel index

(1 = almost no dyed lenticels to 6 = almost dyed lenticels) and proportion (%)

of dyed lenticels in the 2011/12 season. If followed by different letters,

means for lenticel damage severity, dyed lenticel index, or proportion of dyed

lenticels within each column are significantly different (P < 0.05) by LSD (P

= 0.05). ................................................................................................................... 115


damage severity (0 = no damage to 5 = severe damage), dyed lenticel index

(1 = almost no dyed lenticels to 6 = almost dyed lenticels), and weight loss

(% of initial fresh weight). If followed by different letters, means for lenticel

damage severity, dyed lenticel index or weight loss within each column are

significantly different (P < 0.05) by LSD (P = 0.05). ............................................ 117

xvi

Table 7.4. Wetting angle (º) on glass slides and on the skin surfaces of B74 fruit and

proportion (%) of lenticels taking up solutions of DI water plus surfactant,

15% TFC plus surfactant, or 30% TFC plus surfactant. If followed by

different letters, means for wetting angle on the glass slide, wetting angle on

the fruit surface, or proportion of dyed lenticels within each column are

significantly different (P < 0.05) by LSD (P = 0.05). ............................................ 120


damage severity (0 = no damage to 5 = severe damage) and dyed lenticel

index (1 = almost no dyed lenticels to 6 = almost dyed lenticels) in the

2013/14 season. If followed by different letters, means for lenticel damage

severity or dyed lenticel index within each column are significantly different

(P < 0.05) by LSD (P = 0.05) ................................................................................ 121

Table 7.6. Effects of postharvest coatings of B74 mango fruit on lenticel damage severity

(0 = no damage to 5 = severe damage) in the 2014/15 season. If followed by

different letters, means for lenticel damage severity within each column are

significantly different (P < 0.05) by LSD (P = 0.05)... .......................................... 127

xvii

LIST OF FIGURES

Figure 1.1. Photographs of the tree, fruit and lenticels of B74 mango including a mango

tree bearing green-mature fruit (A), a fruit at eating ripe with no lenticel

damage (B), a fruit with severe lenticel damage (C), and the macroscopic

appearance of a discoloured lenticel on a fruit (D).... ................................................ 2

Figure 1.2. A proposed model for the research hypothesis in which free water enters fruit

lenticels and increases strain such that the perceived stress invokes defensive

browning reactions in cells lining the lenticel cavity. ................................................ 4

Figure 2.1. A light micrograph of a transverse tissue section through the skin of a green-

mature B74 mango fruit showing a lenticel with sub-cuticular cavity (Lc), the

fruit cuticle layer (Cl) and a resin duct (Rd). The section was stained with

Toluidine Blue. Scale bar represents 50 µm (Nguyen, unpublished 2012). ............. 9

Figure 2.2. Equilibrium stage of 5 µl solute droplet on the surface of a mango fruit ................. 17

Figure 2.3. Reduction of quinones by oxidant chemicals (from Robards et al. 1999) ................ 22

Figure 2.4. Model of lenticel damage process in which an undamaged lenticel become

damaged. Factors causing the damage were proposed and preventing

solutions were suggested .......................................................................................... 26

Figure 3.1. The five locations on the fruit surface of mango fruit used to determine

lenticel number, density and aperture. ..................................................................... 31

Figure 3.2. Changes in lenticel numbers (per fruit) on B74, KP, HG and R2E2 fruit in the

2011/12 season (A), on B74 and KP fruit in the 2012/13 season (B) in South-

East Queensland, on B74, KP and Sensation fruit in the 2013/14 season (C) in

North Queensland, and in lenticel aperture diameter (µm) on KP, B74 and

Sensation fruit in the 2013/14 season (D) over sequential development stages.

The vertical bar represents the LSD at P = 0.05 in A, B and C. The vertical

bar in lenticel aperture diameter graph (D) represents the standard errors of

means..…………….. ............................................................................................... 38

Figure 3.3. External feature (Ef) of a lenticel on B74 fruit at 1cm diameter (A) revealed

its similarity to a stomata shape (St) on mango leaf (B) using the surface

replica method and observed by light microscopy. A light micrograph of a

transverse 5 µm-thick section of a lenticel at this stage as stained with

Toluidine Blue (C) showed a distinct surface cell layer (Sc) and an open pore

generally similar to stomata. A light micrograph of an unstained transverse

section of a lenticel (D) showed browning of cell / tissues (Bt) in the lenticel

cavity (Lc). The transverse sections (C, D) were examined using the paraffin

embedding method. Scale bars represent 10 µm.………….. ................................. 39

Figure 3.4. Light micrographs showing lenticels and resin ducts in B74 (A), KP (B), HG

(C) and Sensation (D) fruit harvested at the green-mature stage. The

transverse 5 µm-thick sections with lenticel cavity (Lc) were examined using

the paraffin embedding method and stained with Toluidine Blue. Scale bars

represent 50 µm ........................................................................................................ 40

xviii

Figure 3.5. Light micrographs of an undamaged lenticel (A, intact lenticel cavity, Lc) and

a damaged lenticel (B, darkened cavity) of B74 fruit harvested at 7 cm length.

Sections of lenticel cavity (Lc) and resin ducts (Rd) were examined using the

paraffin embedding method and stained with Toluidine Blue. The blue tissue

staining inside the oval is indicative of phenolic accumulation (Pa) around the

damaged lenticel cavity with brown tissues (Bt). Scale bars represent 50 µm ....... 41

Figure 3.6. Series transverse sections through a lenticel on B74 fruit showed browning

tissues (Bt) in the lenticel cavity (Lc) and an intact resin duct (Rd) adjacent to

the lenticel. Scale bars represent 50 µm.. ................................................................ 42

Figure 3.7. External features and transverse sections of four common types of lenticels on

B74 fruit at full yellow of undamaged lenticel (A and B), slightly damaged

lenticel (C and D), small dark spot (E and F), and severe damaged lenticels

(G and H). Brown tissues (Bt) surrounded the discoloured lenticel (C, D, E,

F, G and H). The sections were examined using the paraffin embedding

method. Scale bars represent 50 µm ....................................................................... 43

Figure 3.8. External feature of a damaged lenticel with red halo and an undamaged

lenticel (A) and transverse section of a red lenticel (B) on B74 fruit at full

yellow. The light micrograph of a damaged lenticel (Lc) with brown tissues

(Bt) showed the red pigment accumulation in the surface epidermal cell

layers). The transverse section (B) was examined using the hand-sectioning

method. Scale bars represent 50 µm ....................................................................... 44

Figure 3.9. Light micrographs lenticel structures for dyed and un-dyed lenticels of B74

fruit. Un-dyed lenticel (A) and dyed lenticel (B) were examined using the

paraffin embedding method. Dyed lenticel (C) was examined using hand-

sectioning. Blue dye was observed in the lenticel cavity (Lc) in (C). Scale

bars represent 50 µm ................................................................................................ 44

Figure 3.10. Dissecting light micrograph (A) and fluorescence micrograph (B) revealing

opening pore of lenticels and natural wax deposition near and inside the

lenticel cavity of B74 fruit. The explant containing a lenticel (Lc) was

stained with 0.1% Arcidine Orange, a fluorescence dye and observed under a

fluorescence microscopy (B). Scale bars represent 50 µm. .................................... 45

Figure 5.1. Flow chart of the irrigation experiment in 2012 in Katherine, Northern

Territory ................................................................................................................... 63

Figure 5.2. Flow chart of the irrigation experiment in 2013 in Katherine, Northern

Territory ................................................................................................................... 65

Figure 5.3. Effects of irrigation treatments of B74 mango tree on leaf stomatal

conductance (mmol.m-2.s-1) during the day before harvest. If followed by

different letters, means for stomatal conductance within each time of the day

are significantly different (P < 0.05) by LSD (P = 0.05) ........................................ 71

Figure 5.4. Unstained transverse 5 µm-thick tissues sections of B74 fruit at full yellow of

withholding irrigation + no irradiation (A), withholding irrigation +

irradiation (B), standard irrigation + no irradiation (C), and standard irrigation

+ irradiation (D). Brown tissues (Bt) surrounded the discoloured lenticel (B,

D). The lenticel cavity (Lc) and resin canal (Rc) are also evident. Scale bars

represent 50 μm.. ...................................................................................................... 74

xix

Figure 5.5. Effects of irrigation treatments of B74 mango tree on soil moisture content

(%) at three soil depths from 8 weeks before harvest until harvest in 2013. If

followed by different letters, means for soil moisture content at each week of

assessment in the same graph are significantly different (P < 0.05) by LSD (P

= 0.05) ...................................................................................................................... 76

Figure 5.6. Effects of irrigation treatments of B74 mango tree on stomatal conductance

(mmol.m-2.s-1) at 2 days (site 1) and 1 day (site 2) before harvest in 2013. If

followed by different letters, means for leaf stomatal conductance at each

time of the day in the same graph are significantly different (P < 0.05) by

LSD (P = 0.05). ........................................................................................................ 77

Figure 5.7. Effects of irrigation treatments on fruit weight loss (% of initial fresh weight)

of B74 fruit after harvest in 2013. If followed by different letters, means for

fruit weight loss at each assessment time at each graph are significantly

different (P < 0.05) by LSD (P = 0.05)..... ............................................................... 79

Figure 5.8. Unstained transverse 5 µm-thick tissues sections of B74 mango fruit at full

yellow skin (2013) of standard irrigation at site 1 (A); withholding irrigation

for 4 weeks at site 1 (B); withholding irrigation for 8 weeks at site 1 (C);

standard irrigation at site 2 (D); and withholding irrigation for 4 weeks at site

2 (E). Brown tissues (Bt) surrounded the discoloured lenticel (B, D). The

lenticel cavity (Lc) and resin canal (Rc) are also evident. Scale bars represent

50 μm. ...................................................................................................................... 81

Figure 6.1. A wetting angle of a 5 μL DI water droplet on the surface of a mango fruit ............ 89

Figure 6.2. Unstained transverse tissues sections of B74 mango fruit at full yellow skin.

Fruit were sampled along a commercial packing line at directly off the tree

(A), after detergent treatment (B), after brushing (C), and after packing (D).

Lenticel morphology was observed following the hand sectioning method.

Brown tissues (Bt) surrounded the discoloured lenticel. Scale bars represent

50 μm…………………. .......................................................................................... 92

Figure 6.3. Changes in appearance of the same lenticel from 2 days to 15 days after

harvest on a typical B74 fruit picked and packed under commercial

conditions. Lenticels at 2 days (A); 7 days (B); 9 days (full yellow, C); 12

days (D); and 15 days after harvest (E). Brown tissues (Bt) surrounding

discolouring lenticels. Scale bars represent 50 μm. ................................................ 93

Figure 6.4. Changes in lenticel morphology of typical lenticels from 2 days to 15 days

after harvest on a B74 fruit. The fruit were picked and packed under

commercial conditions. Unstained transverse 5 µm-thick tissue sections were

sampled at: 2 days (A); 7 days (B); 9 days (full yellow, C); 12 days (D); and,

15 days after harvest (E). Brown tissues (Bt) form around the discolouring

lenticels (B, C, D, E). Scale bar present 50 μm ...................................................... 93

Figure 6.5. Unstained transverse 5 µm-thick tissue sections of lenticels on B74 mango

fruit at full yellow for the treatments: off-tree control (A), end of packing line

control (B), DI water (C), surfactant 0.1% (D), NaCl 1% (E), NaCl 3% (F),

PEG 150 g.kg-1 H2O (G), and PEG 400 g.kg-1 H2O (H). Brown tissues (Bt)

surrounded discoloured lenticels (B, D, E, G, H). Scale bars represent 50 μm. ..... 96

xx

Figure 6.6. Unstained transverse sections of B74 mango fruit at 7 days after full yellow

skin (eating soft) for treatments of: hard dry brushing for 1 minute (A), hard

wet brushing for 1 minute (B), soft dry brushing for 1 minute (C), and soft

wet brushing for 1 minute (D). Tissues were fixed in fixative solution (FAA)

and embedded in paraffin wax. Brown tissues (Bt) around discoloured

lenticels (A, B, C, D). Scale bars represent 50 μm. .............................................. 100

Figure 7.1. The locations on the surface of mango fruit used to measure wetting angles. ....... 112

Figure 7.2. Transverse 5 µm thick tissue sections through non-discoloured lenticels (A and

C) and discoloured lenticels (B and D) of B74 mango fruit at full yellow

colour in the 2011/12 season. Sectioned tissues were either left unstained (A

and B) or stained with Toluidine Blue (C and D). Blue stained tissue (circle

in D) highlights accumulation of phenolic compounds. Brown tissues (Bt)

surrounded the discoloured lenticel (B and D). The lenticel cavity (Lc) and

resin canal (Rc) are also evident. Scale bars represent 50 μm.. ............................ 116

Figure 7.3. Lenticel damage severity (0 = no damage to 5 = severe damage) in B74

mango fruit at harvest and at 8 days after harvest (eating ripe) versus dyed

lenticel index (1 = almost no dyed lenticels to 6 = almost dyed lenticels) at

harvest. Each data point in the graph represents the average means of each

treatment................................................................................................................. 118

Figure 7.4. Unstained transverse 5 µm-thick tissue sections of B74 fruit at full colour in

2012/13 for the control (A), bagging (B), 5% RainGard coating (C), 5%

RainGard coating + water (D), 5% Raynox (E), and 2.5% TFC coating (F).

Brown tissues (Bt) surrounded the discoloured lenticel (B, D). The lenticel

cavity (Lc) and resin canal (Rc) are also evident. Scale bars represent 50 μm ..... 119

Figure 7.5. Effects of pre-harvest bagging and coatings of B74 mango fruit on fruit

weight loss (% of initial fresh weight) and skin colour (1 = green to 6 =

yellow) during ripening in the 2013/14 season. The vertical bars with the

symbols represent the standard error for the mean values where they exceed

the symbol size. ...................................................................................................... 122

Figure 7.6. Transverse 5 µm thick tissue sections through a non-discoloured lenticel (A)

and a discoloured lenticel (B) of B74 mango fruit at full yellow colour in the

2013/14 season. Sectioned tissues were stained with Sudan IV. Red stained

tissue highlights the location of lipid-rich cuticle layers (CI). Brown tissues

(Bt) surround the discoloured lenticel (B). The lenticel cavity (Lc) and resin

canal (Rc) are also evident. Scale bars represent 50 μm. ...................................... 123

Figure 7.7. External features of a typical lenticel on B74 mango for non-bagged or non-

coated control fruit (A), for a 20% TFC plus surfactant coated fruit (B), and

for a 20% TFC plus surfactant in admixture with 0.1% Brilliant Blue dye in

the 2013/14 season. The arrows show TFC wax filling lenticel cavity. Scale

bars represent 50 μm .............................................................................................. 124

xxi

Figure 7.8. Transverse sections of B74 fruit at full colour stage in the 2013/14 season.

The typical sections of the non-bagged or non-coated control lenticels (A) and

of the pre-harvest coatings with 20% TFC plus surfactant lenticels (B) were

obtained by hand sectioning. Sudan IV stained lipids of the cuticle layer,

turning a red colour. Coating with 20% TFC plus surfactant (Wl) covered the

fruit surface and entered the lenticel chamber. Red stained tissue highlights

the location of lipid rich cuticle layers (CI). Brown tissues (Bt) surround the

lenticel cavities (Lc). Scale bars represent 50 μm................................................. 124

Figure 7.9. Effect of postharvest coatings of Cat Chu mango fruit on lenticel damage

severity (0 = no damage to 5 = severe damage) and weight loss (% of initial

fresh weight) during ripening at 20 °C. The vertical bars with the symbols

represent the standard error for the mean values where they exceed the

symbol size ............................................................................................................. 125

Figure 7.10. Effects of postharvest coatings of Cat Chu mango fruit on skin colour (1 =

green to 6 = yellow) during ripening at 20 °C. Non-coated fruit served as the

control. The vertical bars with the symbols represent the standard error for

the mean values where they exceed the symbol size ............................................. 126

Figure 7.11. Effects of postharvest coatings of B74 mango fruit on fruit weight loss (% of

initial fresh weight) during ripening at 20 °C, 85% RH in the 2014/15 season.

The vertical bars with the symbols represent the standard error for the mean

values where they exceed the symbol size. ............................................................ 127

Figure 7.12. Effects of postharvest coatings of the B74 mango fruit on skin colour, as

measured subjectively (1 = green to 6 = yellow) or objectively (L*, a* and b*

value) during ripening at 20 °C in the 2014/15 season. The vertical bars with

the symbols represent the standard error for the mean values where they

exceed the symbol size……. .................................................................................. 128

Figure 7.13. Lenticel morphology of B74 mango fruit during ripening at 20 °C in the

2014/15 season. Harvested fruit were coated with 15% TFC (B), 15% TFC

plus surfactant (C), or 30% TFC plus surfactant (D). Non-coated fruit served

as the control (A). Transverse sections were obtained by cutting fresh

samples (hand sectioning) and then stained with Sudan IV. The dye stained

the lipids of the cuticle layer (Cl) a red colour, but did not stain the TFC wax

layer. Arrows show a TFC wax (Wl) cover on the fruit surface and into the

lenticel cavity (Lc). Scale bars represent 50 μm.. ................................................. 130

xxii

LIST OF ABBREVIATIONS USED IN THE THESIS

AO Acridine orange

DI water Deionised water

DM Dry matter

DW Dry weight

e.g. For example

et al. And others

FAA solution Solution of 95% ethyl alcohol, 50 mL: glacial acetic acid, 5 mL: 37%

formaldehyde, 10 mL: distilled water, 35 mL

FW Fresh weight

h Hour

HG Honey Gold

i.e For example

KP Kensington Pride

LD Lenticel damage

LSD Least significant difference

min Minute

Min.-max. Minimum to maximum

mo Month

MRF Maroochy Research Facility

NT Northern Territory

n.s. Not significant

OP Osmotic potential

PAL Phenylalanine ammonialyases

PEG Polyethylene glycol

POD Peroxidases

PPO Polyphenoloxidases

RH Relative humidity

ROS Reactive oxygen species

RWC Relative water content

SM Soil moisture

sec Second

TFC Natural Shine™ TFC210, a carnauba wax for coating fruit

TW Turgid weight

Viz. Namely

xxiii

WP Water potential

w Week

WW Wet weight

1

Chapter 1

GENERAL INTRODUCTION

1.1 Research background

Mango (Mangifera indica L.) is one of the most important fruit crops in the world (Palafox-Carlos

et al. 2012). Global production of mango fruit increased from 15.7 to 34.3 million tonnes between

1990 and 2008 (IMS 2010). In Australia, mango is grown in tropical and subtropical regions, and

its production is estimated to be 38,500 tonnes for the fresh market (AMIA 2011). Mango is grown

in many production regions across the Northern Territory, Western Australia, Queensland and New

South Wales. Production is predominately of the Kensington Pride (KP) mango cultivar (Hofman

et al. 2010a, Jacobi and Giles 1997, Johnson 2000). Yearly production of KP is unpredictable, with

pronounced annual yield fluctuations (Whiley et al. 2001). The second largest production is of the

B74 and this cultivar has reliable and high yields (Hofman et al. 2010a, Whiley et al. 2006). Honey

Gold (HG) cultivar is the third one, but it suffers from certain physiological disorders, such as

‘under skin browning’ (Winston et al. 2010). Other cultivars available later in the season include

Palmer, Keitt, Kent, Pearl and Brooks (AMIA 2011).

The B74 mango cultivar (Fig. 1.1 A) was bred in Queensland in 1999. It originated from a

controlled cross of KP, an Australian cultivar, and Sensation, an American cultivar (Hofman et al.

2010b). B74 has been commercialised under the trading name Calypso™. This cultivar’s

production contributes to consistent supply for the duration of the Australian mango harvesting

season from October to February (Hofman et al. 2010b, Whiley et al. 2006). Its production

currently accounts for 20% of total Australian mango production and continues to increase. Its fruit

have a mild specific flavour and an attractive appearance. The fruit skin has a bright yellow colour

overlaid with red blush on the shoulder when fruit reach the eating-ripe stage (Fig. 1.1 B). Among

other favourable attributes, the fruit have a small seed, good texture and fibre-free flesh. However,

B74 mango fruit can develop dark spots on the skin surface that reduces its cosmetic value. This

discolouration is localised to the cells around lenticels (Hofman et al. 2010b). When the lenticels

become damaged, dark spots manifest on the fruit surface (Fig. 1.1 C). These spot become

prominent as dark pigments form around the lenticel cavity (Fig. 1.1 D).

2

Figure 1.1. Photographs of the tree, fruit and lenticels of B74 mango including a mango tree

bearing green-mature fruit (A), a fruit at eating ripe with no lenticel damage (B), a fruit with severe

lenticel damage (C), and the macroscopic appearance of a discoloured lenticel on a fruit (D).

Lenticels are macro-pores on plants that facilitate gas exchange (Dietz et al. 1988b). Lenticels are

present on the surfaces of many fruits, including mango (Hofman et al. 2010a). Lenticel damage

(LD) is a biological process that manifests as darkened tissues around lenticels. The tissue

discolouration is a symptomatic defence mechanism against stress (Du Plooy et al. 2006). Because

LD reduces the visual appearance quality of the fruit skin, understanding and minimising this

process has been a priority research area for a number of fruits, including apple (Curry et al. 2008),

avocado (Everett et al. 2008) and mango (Hofman et al. 2010b). The LD symptom on apple is

typically visualised as round, discoloured pits around the lenticel (Curry et al. 2008). On avocado,

LD appears as 1-5 mm diameter dark dots on the fruit skin (Everett et al. 2008). Lenticel

D

A

B C

3

discolouration on mango fruit is usually evident as dark spots, sometimes surrounded by a green or

red halo of pigmented tissue (Du Plooy et al. 2006, O'Hare et al. 1999).

Lenticel discolouration is usually not visible on green-mature mango fruit. Rather, this

discolouration generally expresses on fruit during postharvest handling and marketing (Cronje

2009a, Hofman et al. 2010b). The severity of LD is typically less for fruit that are not exposed to

any postharvest treatments (Curry et al. 2008, Hofman et al. 2010b). In contrast, LD expression

can increase after fruit are processed on a commercial packing line. Many current commercial

handling practices used by the Australian mango industry can exacerbate LD on the fruit surface.

These harvest and postharvest practices include washing fruit in solutions in harvest aid machines

and dipping fruit in hot fungicide solution, respectively. Moreover, LD is especially problematic

after fruit are exposed to insect disinfestation treatments with ionising radiation or hot water and

vapour (Self et al. 2006, Whiley et al. 2006).

For some fruits such as European pear, LD that is limited to pinprick spots over the fruit surface is

generally accepted by consumers. For many other fruits, such as apple and mango, those afflicted

with severe LD are often rejected by buyers due to their unattractive appearance and can necessitate

a high cost for traders and retailers in repacking fruit (Bezuidenhout et al. 2005, Curry et al. 2008,

Tamjinda et al. 1992). LD is a common problem for many mango cultivars, including Keitt,

Tommy Atkins and B74 (Bezuidenhout et al. 2005, Hofman et al. 2010a). Mango fruit with high

LD severity may be downgraded in quality or become unmarketable. LD is possibly the main

factor reducing consumer preference and posing a barrier for the export of B74 fruit (Hofman et al.

2010b). Reducing losses due to LD is of significant economic importance for B74 mango.

Accordingly, understanding the mechanism regulating LD and devising appropriate management

strategies are important to achieve better marketing outcomes.

1.2 Research approach

Mango fruit are exposed to various aqueous solutions during pre-harvest, harvest and postharvest

phases. These include exposure to rain, water in mechanical harvest aid machines, detergent

washes for sap burn prevention, postharvest fungicide application, and condensation during

postharvest handling. The main hypothesis of this study is that free water is the trigger for LD. It is

postulated that free water entering into the lenticel cavity will increase the turgor pressure in

adjacent cells. This stress, applied as a pressure increase, will lead to tissue strain-based induction

of plant defence mechanisms at the cellular and tissue level. Stress is an external force that causes

4

the cell response. The response can be elastic when the cell reforms its original structure after the

stress is removed (Pereira and Calbo 2000). Increasing stress results in an inelastic response such

that cell walls and membranes are damaged (strain), and an enzymatic browning reaction is invoked

(Fig. 1.2).

Figure 1.2. A proposed model for the research hypothesis in which free water enters fruit lenticels

and increases strain such that the perceived stress invokes defensive browning reactions in cells

lining the lenticel cavity.

Lenticel damage is a topical physiological disorder of the commercially important B74 mango fruit

that are the focus of this study. The core aim of the project was to better understand the complex

physicochemical processes that lead to the discolouration of lenticels and why and how LD

manifests in mango fruit. Through understanding the physicochemical mechanisms involved,

effective novel measures to minimise LD in B74 mango fruit could potentially be devised. Thus,

this study was also intended to enhance the commercial prospects of B74 mango.

The specific aims of the study were:

To understand lenticel damage sensitivity, generic inheritance and lenticel damage mechanism

of B74 mango fruit.

To carry out a series of experiments that involved modifying plant water status and the entry of

water into fruit lenticels in order to test the research hypothesis.

To minimise fruit / water contact through pre- and postharvest bagging or surface coatings,

which may reduce LD on the skin of B74 mango fruit.

Free water

Entry into lenticels

Increased cell turgidity

Dark pigment formation

due to stress response

Cell perception of strain

Other stresses; (e.g. physical

irradiation injury)

5

1.3 Thesis composition

This chapter (Chapter 1) provides an overview of the thesis subject and research program. Chapter

2 reviews the literature on physiological disorders for fruits and lenticel disorder, and suggests

research approaches to understand this disorder. Chapter 3 involves a chronological and

comparative study of B74 fruit with four other commercial mango cultivars (HG, KP, R2E2 and

Sensation) to better understand the high LD sensitivity of B74 and the mechanism of the disorder.

Chapter 4 assesses potentially predisposing factors to LD by reanalysing unpublished data set from

a previous mango research project ‘Development of Best Practice Protocols for production of B74

Mango’. Chapter 5 examines the effect of withholding irrigation on LD and the relationship

between the fruit water status and LD. Chapter 6 investigates the roles that postharvest solutions

dipping and brushing may play as potential contributors to LD. Chapter 7 examines the effect of

pre- and postharvest bagging and surface coatings on LD. Chapter 8 discusses and concludes the

findings for all of the chapters outlined above, and suggests the future directions to reduce LD in

commercial practice.

6

Chapter 2

LITERATURE REVIEW

2.1 Physiological disorders of fruits

Many fruits are highly perishable once harvested from the mother plant and often exhibit a short

shelf life at ambient temperature. Physiological disorders that occur externally and internally can

contribute to this limited postharvest life. Economically significant volumes of fruits may not reach

the end consumers due to such disorders. Abnormal or physiological disorders are typically caused

by failures in metabolism (Sauco 2009). Curry et al. (2008) considered that fruit disorders result

from dysfunctions of biological processes influenced by pre-harvest factors, such as nutrition,

weather, irrigation, and orchard management and by postharvest practices including harvesting,

handling, and storage. Physiological disorders frequently present in many fruits and may vary with

cultivar. Common examples of fruit physiological disorders are listed in Table 2.1.

Factors which contribute to the occurrence of physiological disorders are complex (Sauco 2009).

Accordingly, many causes and mechanisms of such disorders are still unknown. One of the main

external disorders of mango fruit is lenticel damage (LD) which causes high economic losses.

Understanding the underlying mechanism/s potentially contributes to developing treatments to

minimise or even prevent physiological disorders.

7

Table 2.1. Listing of some important physiological disorders of fruits.

Disorders Species Characteristics Possible causes References

External disorders

Fruit

cracking

Cherry, apple,

persimmon

Splitting or cracking on the fruit skin Long rainy periods during fruit

development

Byers et al. 1990, Iwanami et al. 2002,

Peschel et al. 2007, Yamada et al. 1987,

Rind

disorder

Mandarin,

orange

Sunken colourless areas of the peel that

develop into reddish-brown, dry areas

Relative humidity at fruit colour

break, treatment for de-greening

Agusti et al. 2001, Cronje et al. 2011

Under skin

browning

Mango Browning underneath epidermal layers Mechanical injuries during

transportation

Hofman et al. 2010a,

Winston et al. 2010

Lenticel

damage

Apple, avocado,

mango

Darkening of tissue surrounding lenticels

from red, brown to black

Production conditions, weather,

postharvest handlings

Curry et al. 2008, Duvenhage 1993,

Everett et al. 2008, Hofman et al. 2010b

Fruit skin

pitting

Kiwifruit,

mango

Discrete sunken pits on the fruit surface Maturity, calcium, boron

deficiency, chilling injuries

Ferguson et al. 2003, Sharma and Singh

2009

Internal disorders

Soft nose Mango Flesh softening at the fruit apex Low calcium and other minerals Burdon et al. 1991

Water-core Pear, pineapple Translucent water soaked appearance Low calcium concentration Kajiura et al. 1976, Pesis 2005

Core

browning

Apple, pear Softening and browning of flesh tissues,

no external symptoms

Imbalance of biological

processes

Feys et al. 1980, Franck et al. 2007,

Hernandez-Sanchez et al. 2007,

Spongy

tissue

Mango Spongy pulp with discolouration, air sacks

and off-flavour, no external symptoms

Deficiency of nutrients, high

humidity at harvest

Janave and Sharma 2008,

Shivashankara and Mathai 1999,

Fruit

softening

Persimmon Sudden softening of whole fruit Soil moisture stress, storage

condition, exogenous ethylene

Ortiz et al. 2005, Park and Lee 2006,

Payasi et al. 2009

8

2.2 Lenticel damage in mango fruit

Lenticel damage (LD) has been reported to occur on several fruit crops including apple (Curry et al.

2008), avocado (Everett et al. 2008), cherry (Peschel et al. 2003) and mango (Du Plooy et al.

2009a, Gazzola et al. 2004). Relatively little work has been completed and that no practical

solutions have been developed so far. In order to devise strategies to reduce LD, a better

understanding of lenticel features is required. The following review will focus on mango lenticels.

It will provide an overview of current knowledge of lenticel origin and function and the potential

factors that cause LD on mango fruit.

2.2.1 Lenticel origin in mango fruit

Lenticels are macro-pores on the surfaces of plant organs (e.g. roots, stems, fruits) that function to

facilitate gas exchange (Dietz et al. 1988a). Lenticels on mango fruit have been reported to

originate from stomata (Bezuidenhout et al. 2005). Stomata are present on the surface of mango

fruit from fruit set and are relatively prominent on young fruit. Stomata generally become

dysfunctional due to rapid fruit surface expansion during growth (Scora et al. 2002). Once

dysfunctional, stomata may develop into lenticels when they are mechanically disrupted and

lenticels may cover the fruit skin at random (Bezuidenhout et al. 2005, Tamjinda et al. 1992).

A number of authors have studied the transition of stomata into lenticels for several mango

cultivars. Bezuidenhout et al. (2005) found that the guard cells of Tommy Atkins mango formed on

very young fruit of just 3 mm diameter and were flush with the fruit surface. Bally (1999)

suggested that stomata on 5 cm-long Kensington Pride (KP) mango fruit were still functional.

Bezuidenhout et al. (2005) found that stomata on Tommy Atkins mango of 50 mm length ruptured

and became lenticels because of rapid enlargement and division of sub-epidermal cells compared to

epidermal cells. The guard cells on Tommy Atkins ruptured and then the sub-stomatal cavity

enlarged and the cuticle continued to thicken and enter the cavity (Bezuidenhout et al. 2005). The

lenticels were shown to be surrounded by epidermal cells which were flattened, elongated and

covered by a waxy cuticle. In fruit of 100 mm length, the lenticel continued to enlarge and

pigments were stored in sub-lenticellular cell vacuoles (Bezuidenhout et al. 2005). In Namdokmai

mango, Tamjinda et al. (1992) reported that stomata on fruit 1 month after full bloom were visible

as white spots of a diameter of 1 μm. After 3 months from fruit set, lenticels on Namdokmai mango

had a small hole with degenerated cells near the hole. The cell walls of lenticels became thin

9

compared to the cell walls between lenticels (Tamjinda et al. 1992). Similarly, Bally (1999) stated

that stomata on KP mango fruit became dysfunctional after the fruit reached ~ 35 mm diameter.

Overall, most authors indicate that lenticels originate from stomata which lose their function

(Bezuidenhout et al. 2005, Bally 1999, Tamjinda et al. 1992). However, Dietz et al. (1988a) stated

that lenticels may develop from either stomata and / or cracks of the fruit epidermis. The origin of

lenticels is still questionable. Moreover, there is not a clear understanding of why LD only becomes

evident when mango fruit reach harvest maturity even though the lenticels formed at early stages of

fruit development. More study is needed to fully appreciate how lenticels originate and develop

during fruit growth and ripening, including for the main mango cultivars in Australia.

2.2.2 Function and morphology of lenticels

Lenticels are present on the mango fruit surface and have been shown to play a role in gas exchange

(e.g. respiration and transpiration) (Dietz et al. 1988b, Rymbai et al. 2012). Mango fruit lenticels

are superficial structures with a sub-cuticular cavity (Bezuidenhout et al. 2005, Du Plooy et al.

2006; Fig. 2.1). Adjacent to lenticels are the fruit cuticle and epidermal cell layers plus sub-

cuticular resin ducts.

Figure 2.1. A light micrograph of a transverse tissue section through the skin of a green-mature

B74 mango fruit showing a lenticel with sub-cuticular cavity (Lc), the fruit cuticle layer (Cl) and a

resin duct (Rd). The section was stained with Toluidine Blue. Scale bar represents 50 µm

(Nguyen, unpublished 2012).

Cl Lc

Rd

10

During mango fruit growth and development, the lenticel structure and dimension changes

significantly (Bally 1999, Tamjinda et al. 1992). LD usually becomes obvious when fruit reaches

the green-mature stage and may increase after harvest and during ripening. The discolouration of

lenticels depends upon cultivar, environment, management and postharvest handling practices

(Oosthuyse 2002, Du Plooy et al. 2004, Self et al. 2006). Damage of lenticels is visualised as the

formation of small dark pigmented spots up to prominent areas around lenticels (Bezuidenhout et

al. 2005, Self et al. 2006). LD typically does not penetrate beneath the epidermal cell layers (Dietz

et al. 1988b).

2.2.3 Epicuticular wax and cutin layer in lenticel damage

The mango fruit is covered by a cuticle which consists of epicuticular wax and cutin layers (Yeats

et al. 2012). The cuticle is a hydrophobic layer that has an important role in fruit protection,

including water loss prevention and water shedding, and in maintaining gas exchange during fruit

development and ripening (Eigenbrode and Espelie 1995, Hess and Foy 2000). Cutin is an

insoluble polyester of hydroxyl fatty acid. The cuticle layer is covered and infiltrated with wax as a

mixture of highly lipophylic aliphatics (Yeats et al. 2012). Polymeric cutin compounds mostly

consist of ω- and mid-chain hydroxyl and epoxy C16 and C18 fatty acid, and glycerol. Wax is

comprised of monomeric fatty acids of straight chain C20 to C60 aliphatics and can also contain

triterpenoids, phenylpropanoids, and flavonoids (Samuels et al. 2008). Enzymes play a governing

role in wax and cutin biosynthesis (Samuels et al. 2008, Yeats et al. 2012). Wax is synthesised in

the reticulum of epidermal cells and then transported to the surface (Samuels et al. 2008). The main

process of wax biosynthesis is elongation of C16 and C18 acids into long-chain C20 to C60 fatty

acids and secondary compounds such as alcohols, alkanes, and ketones.

The waxy cuticle of Tommy Atkins, Kent and Keitt mango fruit becomes relatively smooth from

anthesis to post-anthesis (Du Plooy et al. 2004). At this early stage, the wax layer consists of a

partly soluble fraction (Du Plooy et al. 2004). Because the fruit surface expands rapidly during fruit

growth and development, the waxy cuticle layers have been found to divide up into many platelets

upon sub-divisional shearing and cracking on KP (Bally 1999) and Tommy Atkins, Kent and Keitt

(Du Plooy et al. 2004). At the green-mature stage, the outermost layer of the cuticle on Keitt

mango fruit consists of microscopic wax crystals that form an irregular surface and protrude from

an amorphous wax film underneath (Prinsloo et al. 2004). Crystalline wax and amorphous wax

layers have chemical differences (Du Plooy et al. 2004). The uppermost wax layer has long-chain

11

aliphatic constituents, while the amorphous wax layer is rich in compounds with aromatic rings and

unsaturated bonds (Prinsloo et al. 2004).

The wax covering around lenticels is also chemically and physically different from the surrounding

areas (Prinsloo et al. 2004). Wax in the lenticel perimeter of Keitt fruit lacked crystalline aliphatic

compounds, while the surroundings outermost wax layer was rich in long chain aliphatic and

unsaturated compounds (Prinsloo et al. 2004). Cuticle thicknesses between the lenticels of Tommy

Atkins, Kent and Keitt mango fruit are similar from anthesis to maturity, but cuticle deposition

inside lenticels may differ among the three cultivars (Du Plooy et al. 2004). Tommy Atkins and

Keitt cultivars have similar chemical compositions in the wax layer (Prinsloo et al. 2004). The wax

layer around lenticels is high in aromatic components (Du Plooy et al. 2004). The crystalline waxy

layer may play a role in the reflection of UV-B radiation in order to reduce heating on the fruit

surface. This layer potentially minimises photo-oxidative injuries due to excess radiation. LD

sensitivity was not related to cuticle permeability (Gazzola et al. 2004).

Water permeability of the fruit surface is affected by the physical and chemical properties of the

cuticle layer, especially the chemical composition and epicuticular wax structure (Petit-Jimenez et

al. 2009). During fruit development, wax layers are born and change in structure to meet their

protective role. A glossy wax layer may increase the wetting angle of water on the fruit surface.

Therefore, water may more easily run off the fruit surface. When mangoes travel through a

commercial packing line, severe damage can occur to the surface structure of fruit. Du Plooy et al.

(2002) reported that mechanical damage during packing flattened the wax crystals and debris was

found inside lenticel cavities. Also, postharvest treatments may damage this area differently to the

rest of the skin (Du Plooy et al. 2004). In addition, postharvest handling activities that cause

damage to the wax layer may facilitate water entry (Bally 1999).

2.2.4 Differences in lenticel sensitivity among cultivars

The morphology of lenticels was reported by Du Plooy et al. (2009b) to vary for different mango

cultivars. Keitt mango exhibited the most lenticel discolouration and Kent mango the least among

three selected cultivars that included Tommy Atkins (Du Plooy et al. 2004). Lenticels of Tommy

Atkins mango fruit had a small aperture with a relative large lenticel chamber and thin, loose and

diminishing wax. In contrast, lenticels of Keitt had a markedly thinner cutin layer over the cells at

the bottom of the lenticel chamber. Kent fruit had lenticels with a small and shallow cavity and a

continuous coverage of cutin and wax on the surface (Du Plooy et al. 2004, Du Plooy et al. 2006).

12

Lenticels of Tommy Atkins mango fruit are easily discoloured due to lack of a cork cambium

(Bezuidenhout et al. 2005). Tamjinda et al. (1992) examined the Falan mango cultivar, which does

not exhibit lenticel discolouration. It has a cork cambium which prevented the shearing of cells,

and a disorganised cellular structure within the lenticel chamber, and subsequent discolouration.

The authors compared the lenticel structure of Falan mango, a tolerant cultivar, and Namdokmai

mango, a sensitive variety. For Falan mango, the lenticel protrudes from the fruit surface and it has

thicker walls and shallower lenticel cavities than Namdokmai.

According to Dietz et al. (1988b), the number of lenticels on mango fruit also varied with cultivar.

Moreover, the total number of lenticels (preformed stomata and cracks in cuticles) had a positive

and linear correlation to fruit weight loss during storage of five mango cultivars including Mallika,

Alphonso, Dashehari, Pairi and Totapuri (Dietz et al. 1988b). Overall, there is not a consistent

explanation in the literature concerning the differences in LD among mango cultivars. Lenticel

shape, dimension, density and distribution may play a significant role in LD. Differences in the

shape and dimension of lenticels at various positions on the fruit surface may contribute to the

lenticel severity. Cultivars with a high density of lenticels may also have high potential for LD.

2.2.5 Lenticel damage mechanisms

First theory on lenticel damage mechanism

Several authors have suggested that latex originating from the resin ducts which ramify throughout

the mango exocarp may contribute to LD. Sap can potentially leak out of resin ducts and enter

lenticels due to high pressure. Laticifiers progressively become closer to the mango fruit surface

and adjacent lenticels as the fruit develop (Loveys et al. 1992).

In mature mango fruit on the tree, droplets of resin have been observed to pool above lenticels on

the surface of some fruit (Bezuidenhout et al. 2005). Most damaged cell walls inside a lenticel

cavity may be located near resin ducts (Du Plooy et al. 2009a). During times of high fruit water

potential, for instance under wet cool conditions, resin ducts may rupture. At this point, sap leaking

from a resin duct adjacent to a lenticel cavity may possibly move into cell walls around the cavity

wherein oxygen concentrations could be higher, allowing oxidation to take place (Bezuidenhout et

al. 2005).

Sap of mango fruit consists of two phases, these being an oil compound and a protein-

polysaccharide fraction that account for about 10% and 90% of total sap volume, respectively

13

(Loveys et al. 1992). Mango fruit sap content, pH and composition depend on cultivar, year,

growing location and fruit maturity (Hassan et al. 2009). Oil components of the sap have been

shown to elicit a stronger effect on skin and LD than the protein-polysaccharide components

(O’Hare and Prasad 1992). Major components of the oil phase are terpene-like compounds.

Terpinolene was found to be the most abundant compound in the oil fraction at over 80% in KP

mango, but it accounted for only 3.33% of the oil fraction in Irwin mango sap (Loveys et al. 1992).

This study also showed that terpinolene was the main constituent causing the sap-burn symptom on

mango skin. Terpinolene is an oil component which has a very low wetting angle (Prabhu et al.

2009). This likely facilitates terpinolene to enter lenticel cavities easily. Associated with the

terpinolene reaction, enzymes in the fruit skin played a significant role in the sap burn damage

(Loveys et al. 1992). Robinson et al. (1993) maintained that LD was caused by terpenoids from sap

latex which caused vacuoles to lyse and allow phenolic compounds to come in contact with

polyphenol oxidase enzymes. As a result of either of these discolouration scenarios, mango

lenticels change from purple to dark brown, resulting in the typical discolouration.

Second theory of lenticel damage

Based on experimental observations, a number of authors have suggested that LD may also be

regulated by external factors. The lenticel has been described as a weak point on the fruit skin and

is associated with several physiological disorders (Brown and Considine 1982). Aqueous solutions

that penetrate into lenticels have been associated with tissue discolouration (Cronje 2009a, O’Hare

et al. 1999, Self et al. 2006, Shorter and Joyce 1998). Water penetration can lead to overly turgid

cells inside the lenticel cavity (Everett et al. 2008). The influence of aqueous solutions on LD

depends on the ease of water entry into the lenticels (Peschel et al. 2003), which is likely regulated

by the waxy cuticle and anatomical characteristics of lenticels. Additional stresses along the

packing line, such as mechanical injuries, and other postharvest processes can lead to damage of

waxy layers on the fruit surface and the cell membranes inside lenticel cavities (Du Plooy et al.

2002, Self et al. 2006). Such perturbations can facilitate browning enzymes to come into contact

with phenolic substrates, potentially causing discolouration of lenticels.

Overall, the first theory for explaining the possible causes of LD in mango may not answer why

some lenticels show darkening while other lenticels on the same fruit skin surface area do not

darken when mutually proximal to underlying resin ducts. This hypothesis also may not explain

why some lenticels became discoloured when fruit were hanging on trees. The second theory is

based on observations that external stresses appear to be associated with LD. It is established that

openings of lenticels are weak points on the fruit surface for external stress influences and also that

14

free water is a key trigger for LD. Accordingly, the present study proposes that water infiltration

into the lenticel cavity increases turgor in adjacent cells that leads to tissue strain, cellular stress and

defence processes, including enzymatic browning.

2.2.6 Pre-harvest and postharvest factors causing lenticel damage

Lenticel discolouration reduces consumer satisfaction leading to economic loss. The degree of the

disorder depends on the mango cultivar and is affected by weather conditions, agronomic practices

and postharvest activities during treatment, distribution and marketing (Cronje 2009a, Hofman et al.

2010b). Many studies on various aspects of mango lenticel discolouration have been undertaken;

e.g. Cronje 2009a, Cronje 2009b, Hofman et al. 2010b, Oosthuyse 2002, Oosthuyse 2007a.

Pre-harvest factors affecting lenticel damage

The harvest maturity of mango fruit intended for cold storage can have a profound influence on LD

(Oosthuyse 2002). Late harvested fruit had more LD at 1 week after developing full ripe skin

colour as compared to fruit harvested earlier (Oosthuyse 2002, Whiley et al. 2006). Shaded fruit

from within the tree canopy were found to develop more lenticel spotting than sun-exposed fruit,

especially when harvested at a more advanced maturity (Whiley et al. 2006). Although, when the

fruit from both canopy positions ripened further, there was no difference in their potential to express

the disorder (Whiley et al. 2006). Calcium treatment that was applied to improve fruit firmness in

the pre-harvest period was reported to have an adverse impact on LD in Sensation and KP mango

(Joyce et al. 2001). The severity of LD expression depended on the fruit harvest maturity of both

cultivars (Joyce et al. 2001).

Applying water deficit stress to KP trees for either 7.5 or 1.5 weeks before harvest was reported by

Simmons et al. (1995) to slightly reduce lenticel spotting and increase dry matter of fruit at the ripe

stage. Similarly, reducing soil water potential from -30 to -70 kPa over the last 3 weeks before

harvest reduced the proportion of LD that developed on Keitt mango fruit (Cronje 2009a). Thus,

regulated irrigation stress may potentially reduce LD, possibly due to reduced fruit turgor.

Postharvest factors affecting lenticel damage

Harvest practices can also contribute to LD. Tommy Atkins mango fruit picked with long stems

that were immediately de-sapped prior to processing on a commercial packing line showed less LD

compared to fruit that were de-sapped at 15, 30 or 45 minutes later after harvest (Cronje 2009a).

Delaying de-sapping may reduce the volume of sap in the protein fraction of liquid excluded from

15

stem. Similarly, Hofman et al. (2010b) reported that harvesting B74 mango fruit directly from trees

into cardboard trays and with no further treatments or exposure to water resulted in much less LD

than fruit collected at the end of a commercial packing line. The effect of the commercial picking

and packing processes on LD was also evident for Tommy Atkins (Self et al. 2006). Black lenticel

spotting markedly increased for fruit exposed to the commercial practices of calcium hydroxide

dipping, waxing and pre-cooling. Commercial practices involving solutions may increase cell

turgor by fruit contact with aqueous solutions. Harvest practices may also damage the waxy layer

on the fruit skin (Du Plooy et al. 2002). Bally (1999) suggested that this layer may play a

significant role in lenticel protection by minimising water entry into lenticels and preventing water

loss from lenticels.

Detergents such as Mango Wash® are used to minimise sap-burn on the fruit skin surface of

mangoes during harvesting (O’Hare and Prasad 1992). However, these detergent solutions can

cause LD (Willis and Duvenhage 2002, Whiley et al. 2006). Willis and Duvenhage (2002) showed

that Heidi mango was exacerbated when the fruit were treated with lime-based solutions like

Sporekill®. Nonetheless, LD on other cultivars such as Tommy Atkins, Sensation and Kent was not

affected by the Sporekill® treatment. Red and black lenticel spots developed on Tommy Atkins

mango fruit following treatment with a calcium hydroxide solution used to prevent sap-burn (Self et

al. 2006). Whiley et al. (2006) showed that B74 mango fruit treated with detergents developed

more lenticel blackening as fruit began to ripen. High pH solutions possibly cause cell membrane

damage inside lenticel cavities as they uptake and absorb water.

Post-harvest treatments for improving mango fruit firmness or controlling pest and disease can also

increase LD. For example, dipping KP in a 6% CaCl2 solution resulted in skin damage around

lenticels (Shorter and Joyce 1998). Exposing mango fruit to hot water treatments is widely used for

phyto-sanitary quarantine requirements (Self et al. 2006). However, LD was worsened by dipping

KP mango fruit into hot water at 47 °C for 10 - 15 minutes (Jacobi et al. 2001). A hot fungicide dip

or spray with SpinFlo also increased LD on B74 mango (Hofman et al. 2010b). While water may

be the key trigger for LD, high solution temperature may also cause cell membrane damage

associated with LD.

Films and coatings can be applied to form an artificial barrier around fruit and so may affect LD.

Coatings are used commercially for apple (carnauba wax) and for mango and lychee (e.g. chitosan)

(Curry et al. 2008, Jiang et al. 2005, Zhu et al. 2008b). Hofman et al. (2010b) reported that

polyethylene wax, bees wax and a mixture of olive oil with beeswax applied postharvest had no

16

effect on LD on B74 fruit. In contrast, carnauba wax application significantly reduced LD on B74

fruit. However, coating with high concentrations of carnauba wax increased fruit off-odours

(Hofman et al. 2010b). Appropriate coatings possibly reduce water entry into lenticels and thereby

reduced lenticel sensitivity to stress. Packing Tommy Atkins and Keitt mango fruit in polyethylene

film after harvest dramatically reduced the development of red or green spots on the fruit surface,

but also reduced peel colour development after cold storage (Pesis et al. 2000). It is possible that

modified atmosphere packaging may have reduced water loss from lenticels and reduced oxygen

concentrations around fruit tissues, a key component of oxidative browning reactions.

Mechanical injuries incurred on commercial packing lines have been reported to exacerbate LD

(Cronje 2009b, Self et al. 2006). Cronje (2009b) found that mechanical injury due to brushing

increased LD evident at the ripe stage. Postharvest handling may contribute to lenticel severity in a

cumulative way (Self et al. 2006). Mechanical injuries as physical stresses to lenticels may cause

cell membrane damage and increased water loss.

Exposure of mango fruit to -irradiation as an insect disinfestation treatment is required by some

export markets, like to New Zealand from Australia (Johnson and Hofman 2009). Irradiation with

doses of 300 or 600 Gy increased LD severity of KP fruit (McLauchlan et al. 1990). Irradiation can

cause many negative effects on mango fruit, including softening, uneven ripening and surface

damage (Sivakumar et al. 2011). Irradiation delayed the ripening process of Tommy Atkins mango

(Durigan et al. 2004). Irradiation of B74 fruit after commercial picking and packing increased LD

within 3 days of the irradiation treatment compared to non-irradiated fruit (Hofman et al. 2010b).

The authors also showed that treating B74 fruit with de-ionised water or detergent solution

increased LD upon irradiation. Also, brushing further increased the sensitivity of B74 mango fruit

to LD caused by irradiation (Hofman et al. 2010b). Thus, exposure to water increases the

sensitivity of lenticels to irradiation possibly in association with more turgid cells being more

vulnerable to irradiation stress. Cold treatments before irradiation or nitrogen atmospheres did not

reduce LD of KP fruit (McLauchlan et al. 1990). Based on the literature, it appears that commercial

picking and packing practices increase lenticel sensitivity to irradiation damage.

2.2.7 Conditions for free water entering into lenticel cavities

Water and solutions are hypothesised to be key factors contributing to LD (Everett et al. 2008, Self

et al. 2006). High cell turgor within the lenticel cavity caused by entry of water and solutes

potentially predisposes mango fruit to LD exacerbated by other stresses such as irradiation

17

treatment. Liquid entry into a lenticel cavity may be influenced by wettability of the specific liquid

on the fruit surface. Wettability of the leaf surface is a pre-condition for liquid penetration of

stomata (Schonherr and Bukovac 1971) and similar cavities. Spontaneous infiltration of stomata

takes place when the liquid contact angle is smaller than the wall angle of the aperture wall. This

may also be true for lenticels on the fruit skin. Wettability of a liquid on a surface is characterised

by the wetting angle or contact angle (Ruckenstei and Lee 1975). A droplet of liquid would spread

on a fruit surface until an equilibrium state is established with a static contact angle. The

equilibrium is determined by the Young’s equation (Fig. 2.2). In it, γSL is the liquid-solid surface

tension, γLG is the liquid-gas surface tension, and γSG is the solid-gas surface tension, and θc is

wetting angle (Lamour and Hamraoui 2010, Prabhu et al. 2009).

Figure 2.2. Equilibrium stage of 5 µl solute droplet on the surface of a mango fruit.

The wetting angle of liquid on a fruit surface depends on the surface tension of the liquid-fruit

surface, the surface tension of liquid-gas and the surface tension of fruit surface-gas (Peschel et al.

2003, Prabhu et al. 2009). The roughness of the solid surface also influences the wetting angle of a

liquid drop in equilibrium (Alberti and De Simone 2005). With a given liquid, such as deionised

water, the wetting angle is characteristic for a fruit surface. For instance, the contact angle of a

water droplet on mature cherry fruit was 94.2° on average (Peschel et al. 2003). This study also

showed that wetting angle on sweet cherry fruit was not different between fruit surface regions or

cherry cultivars. During the postharvest period, the waxy cuticle layer of mango fruit can be

affected (Du Plooy et al. 2002) and thereby may modify the wetting angle of the fruit surface.

Contact angles have been assessed as a condition for stomatal uptake (Eichert and Burkhardt 2001).

Thus, contact angles may help to understand the first condition for liquid entry into lenticels and

help to explain the relationship between a specific liquid and LD.

The contact angle of a liquid on a fruit surface can be modified by adding a surfactant (Peschel et

al. 2003). The authors found that a liquid with high surface tension has a high wetting angle, while

a liquid with low surface tension has a low wetting angle. By reducing the surface tension of a

γSG = γSL + γLG cos θc

Rd

γLG

γSL γSG θc

18

liquid with surfactants, a non-wetting fruit or leaf surface can become partially or completely wet.

Surfactants may be absorbed and so the surface tension of fruit surface-gas increases and the edges

of liquid drop become more hydrophilic. As a result, the liquid spreads on fruit surface quickly and

the wetting angle becomes very small (Peschel et al. 2003). A liquid with surfactant can easily

enter the lenticel cavity and may thereby increase the potential for LD. This principle can

potentially also apply conversely in that a fruit coating with low wettability may reduce relative to

the entry of water into the lenticel.

2.2.8 Conditions for free water infiltration into cells

An aqueous solution entering the lenticel cavity possibly infiltrates into adjacent cells depending on

the balance of osmotic potential of the solution and water potential of the epidermal cells. A

solution with low osmotic potential may cause dehydration of cells, while a solution with high

osmotic potential may facilitate free water to penetrate the cell membrane, thus leading to increased

cell turgor. For example, immersion of apple tissue in 50% sucrose can lead to cell death due to

dehydration (Mavroudis et al. 2004). A very low osmotic potential solution stops seed germination

because of reduced water uptake (El-Siddig et al. 2004).

In a plant cell or tissue, the potential for water uptake (negative water potential) is equal to the sum

of osmotic potential of the cell (negative solute potential) and pressure potential (positive turgor

pressure): -ψw = -ψo + -ψp (where, ψw = water potential, ψo = osmotic potential of solute, and ψp =

cell or tissue turgor (from Shackel 1987 and Burdon and Clark 2001).

Fruit osmotic potential in planta is affected by the osmotic potential of the culture medium (Bolarin

et al. 2001). Tomato fruit grown in saline conditions had a lower osmotic potential compared to the

control (Bolarin et al. 2001). Principal cell solutes which contribute to osmotic potential are K+,

Na+, Cl-, NO3- and reducing sugars (Cochrane and Cochrane 2005). Water potential of fruit can be

estimated with a pressure chamber, thermocouple, or a series of solutions with known osmotic

potentials. The osmotic potential of cells can be measured by obtaining their sap and testing for

equilibrium with a series of solutions of known osmotic potential (Jobling et al. 1997). The

osmotic potential of a medium can also be determined by measuring the freezing point depression

of filtrate using an osmometer (Do and Cormier 1990). Common solutes with characterised

osmotic potentials in aqueous solution include NaCl, KCl, sucrose, NH4Cl, CaCl2, NaNO3, MgSO4,

D-mannitol, D-fructose, polyethylene glycol 6000 and polyethylene glycol 12000 (Cochrane and

Cochrane 2005). Potentially, an equilibrium balance between the osmotic potential of the solution

19

used in postharvest treatments and fruit water potential may minimise water uptake into fruit and so

possibly reduce LD.

2.2.9 Stresses on lenticels

Stress factors stimulate the alteration of cell metabolic processes. Adverse effects of stress on fruit

may lead to postharvest injuries or physiological disorders (Kays 2004). Lenticels are relatively

weak points on the fruit skin and external stress may affect fruit through lenticels and the lenticel

cavity. The discolouration of lenticels can be the result of abiotic stress (e.g. temperature, water

loss, mechanical injuries) and biotic stress (e.g. fungal infection, insect infestation).

Temperature

High temperature possibly leads to partial dehydration of fruit and degradation of cellular

components. Dehydration of Kwai May Pink lychee fruit causes skin browning (Underhill and

Simons 1993). Dehydration during storage of Gala apple can lead to the lenticel breakdown

disorder (Curry et al. 2008).

Chilling injury is the result of exposure of plant tissue to low non-freezing temperatures (Purvis

2004). The primary injury results from an initial rapid disruption of metabolic process that is

reversible if the temperature is raised to non-chilling conditions (Raison and Lyons 1986).

Secondary injuries that follow as a consequence of primary injuries are irreversible. Chilling

temperatures change the physical characteristics of cellular membranes (Salvador et al. 2006).

Chilling also invokes oxidative stress that produces active oxygen species in the tissues (Scandalios

1993). These processes lead to other secondary effects including sub-cellular decompartmentation,

electrolyte leakage and enzyme release (Chidtragoola et al. 2011). Storage of mango fruit below 12

ºC can cause chilling injuries (Mitra and Baldwin 1997). Oosthuyse (2002) found that LD

increased markedly with reductions in cold storage temperatures below 12, 11, 9.5 and 8 ºC for Zill,

Tommy Atkins, Heidi, Sensation and Keitt mango fruit, respectively. Common visual symptoms of

chilling injury in mango fruit are scald-like discolouration, dark lenticels, pitting of the skin, flesh

discolouration and development of off-flavours (Nair et al. 2003).

Mechanical stress

Physical injuries occurring during harvesting and handling operations in the packing house or

transporting can stress fruit (Kays 2004). Physical injuries may lead to cell membrane damage and

water loss. For example, abrasion of Honey Gold mango fruit skin resulted in an under skin

20

browning disorder that spread from the abrasion site (Hofman et al. 2010b). Brushing the skin of

Keitt mango fruit also increased LD (Cronje 2009b).

Irradiation

Exposure to -irradiation is required as a quarantine treatment for Australian mangoes imported into

New Zealand (Hofman et al. 2010a). Irradiation induces reactive oxygen species (ROS) in plant

tissue such as superoxide radicals, hydroxyl radicals and hydrogen peroxide (Ahuja et al. 2014).

Irradiation treatment can also increase the activity of phenylalanine ammonialyase (PAL) during

subsequent mango storage (Reyes and Cisneros-Zevallos 2007). Irradiation at 543 Gy was reported

by Hofman et al. (2010b) increased LD in B74 mango. Similarly, irradiation from 300 to 900 Gy

increased LD on Tommy Atkins and Keitt mango fruit (Grove et al. 2004) and irradiation at 300 or

600 Gy increased LD of KP mango fruit (Johnson et al. 1990).

Biotic stress

Microbial (e.g. fungal, bacterial) infection and also pest (e.g. insect) infestation can lead to

increased polyphenol oxidase (PPO) activity in fruit tissues and produce brown phenolic polymers

in defence against the stressors. The peel of mango fruit is naturally rich in phenolic compounds

(Ribeiro et al. 2008). However, Tamjinda et al. (1992) reported that no pathological factors were

involved in LD on the skin of Namdokmai mango fruit.

2.2.10 Substrates, enzymes and factors involved in lenticel damage process

Lenticel discolouration occurs within the epidermal cells around lenticel cavities. Discolouration is

the result of enzymatic reactions of polymerisation. The degree of discolouration may be affected

the type and concentration of phenolic substrates, activities of PPO, peroxidase (POD) and PAL

enzymes, exposure to oxygen, and reactive oxygen species (ROS).

Phenolics

Phenolic compounds are the key substrates for browning processes (Du Plooy et al. 2009a). They

are phytochemical compounds which have at least one benzene ring with one or more -OH groups

that can bond to methyl, methoxyl, amino or glycosyl groups (Beckman 2000). Phenolics are

naturally stored mainly within the cell vacuole which is surrounded by a lipoprotein membrane and

a small amount of phenolics are stored within cell membrane (Beckman 2000). The free form of

phenolics is not common in plants, and they mostly exist in glycosylated compounds such as

xanthone glycosides and flavonoid glycosides, in which form they become non-toxic to cells (Ajila

21

et al. 2010, Beckman 2000). Glycosidases hydrolyse the glycoside bond to release the phenolics,

the substrates of the browning process (Beckman 2000). Phenolics are mostly present in fruit as

three forms of i. flavonoids (viz., flavonols, flavanones, isoflavones, flavan-3-ols, flavones and

anthocyanins), ii. phenolic acids (iiz., hydroxycinnamic acid such as ferulic acid, p-coumaric acid,

caffeic acid, chlorogenic acid, and hydroxybenzoic acids such as gallic, vanillic, ellagic, syringic

acids), and iii. tannins (viz., proanthocyanidins, gallotannin or tannic acid) (Haminiuk et al. 2012,

Masibo and He 2008, Palafox-Carlos et al. 2012). Many phenolics have antioxidant characteristics

such as gallic acid, protocatechuic acid, chlorogenic acid and vanillic acid (Palafox-Carlos et al.

2012, Robards et al. 1999). The main phenolics in mango peel are mangiferin, syringic acid,

quercitin, ellagic acid, and glycosylated gallic acid (Ajila et al. 2010, Berardini et al. 2005). The

main phenolics in mango peel are mangiferin, syringic acid, quercitin, ellagic acid, and glycosylated

gallic acid (Ajila et al. 2010, Berardini et al. 2005). In Irwin mango fruit, more polyphenols and

flavonoids were found in the peel than the flesh (Kim et al. 2010). High concentrations of

phenolics in mango peel may have a role in cell protection when the epidermal cell layer is

damaged; thereby they may contribute to LD.

Polyphenol oxidase

Polyphenol oxidase (PPO, diphenol oxidase, phenolase, or o-diphenolase) is a copper-containing

enzyme present in most plants (Ndiaye et al. 2009). PPO is generally considered the main enzyme

involved in the browning processes of plant tissues (Robard et al. 1999). PPO is located in plastids

and mitochondria of cells. When plant tissue is injured, PPO is de-compartmented and comes into

contact with substrates released by damaged vacuoles (Loveys et al. 1992). PPO catalyses the

conversion of monophenols into quinones, which has different spectral characteristics depending on

the monophonols and pH (Robard et al. 1999). In KP mango fruit, PPO activity was higher in the

skin than in flesh and in ripe than green fruit (Robinson et al. 1993). The role of PPO activity

presumably becomes relatively more important in cell protection when mango fruit were ripe. The

activity of PPO varied from cultivar to cultivar, being high in KP and low in Irwin mango fruit

(Robinson et al. 1993). Mango cultivar with high PPO activity on the fruit skin may be highly

sensitive to LD.

The oxidation of phenols may be prevented by antioxidant chemicals, such as ascorbic acid,

cysteine and sodium metabisulphite (Queiroz et al. 2008). Antioxidant chemicals may prevent the

first enzymatic browning reaction steps because they converse quinones into phenolic compounds

(Fig. 2.3). Antioxidants may prevent LD by stopping the first step of the browning process.

22

PPO Condensation

Phenol Quinone Polymer phenolics (dark pigments)

Antioxidant chemicals

Figure 2.3. Reduction of quinones by oxidant chemicals (from Robards et al. 1999).

The activity of PPO may be inhibited by temperatures of 70 - 90 °C (Queiroz et al. 2008).

However, treatment of fresh mango fruit at these high temperatures would damage the fruit tissues.

Peroxidases

Peroxidases (PODs) are common in plant tissues. PODs are iron-containing enzymes which

catalyse phenolics using active oxygen from the peroxide group (Dogan et al. 2007, Ndiaye et al.

2009). PODs are oxide-reductive enzymes and often located in the outermost cell layer of fruits

(Toivonen and Brummeli 2008). PODs oxidise phenolics by generating a hydrogen peroxide group

(Jiang et al. 2004). They may have a significant role in cell defence (Zhu et al. 2008a). PODs are

thermal resistant enzymes (Ndiaye et al. 2009). The authors found that PODs in mango slice can be

completely inactivated when treated with water vapour at 94 ºC for 5 - 7 minutes. PODs activity in

the yellow peel of ripe Namdokmai is higher than in the green peel of ripe Tongdum mango fruit

(Ketsa et al. 1999).

Phenylalanine ammonialyase

Phenylalanine ammonialyase (PAL) plays a regulatory role in the synthesis of phenolic compounds

in plant tissues (Reyes and Cisneros-Zevallos 2007). PAL catalyses conversion of phenylalanine

into cinamic acid, constituting the first step in phenolic synthesis in plants. This enzyme increases

the amounts of phenolic substrates for browning reactions. High activities of PAL accelerate

phenolics oxidation due in the presence of high concentrations of substrates. Chidtragoola et al.

(2011) found that PAL activity in the Namdokmai mango peel increased during cold storage and

there was a positive correlation between skin browning and PAL activity.

Role of oxygen

Oxygen is an essential component of the browning reaction. Oxygen is directly involved in the

initial step of the enzymatic oxidation to form quinones (Robard et al. 1999). Dipping minimally

processed fruit products products under water or using treatments that reduce oxygen availability

can reduce browning. Contact with oxygen accelerated the oxidation and enzymatic browning

23

reaction that leads to discolouration in minimally processed fruits and vegetables (Lin and Zhao

2007). Reducing oxygen in and around lenticels may potentially reduce LD.

Reactive oxygen species

Reactive oxygen species (ROS) are formed during physiological stress and physical stresses; e.g.

irradiation and chilling. Major ROS species include superoxide radical (O2-), hydrogen peroxide

(H2O2), hydroxyl radical (OH-) and single oxygen (O.) (Ahuja et al. 2014). Highly reactive ROS

react with lipoprotein membranes causing their membrane dysfunction and loss of cell

compartmentation (Ahuja et al. 2014). Upon damage to membranes and membrane bound and

containing cell organelles, phenolic compounds and browning enzymes mix, allowing browning

reactions to take place. Phenolics naturally exist in plant tissues and some play important roles as

antioxidants able to inactivate free radicals (Haminiuk et al. 2012). Their oxidation reactions lead

to dark pigment formation.

2.2.11 Discolouration process in lenticels

Discolouration processes in lenticels may be similar to browning processes occurring generally in

fresh horticultural produce. Damaged lenticels in mango fruit are observed as dark coloured spots

wherein their cells discolour. Discolouration of lenticels is mostly the result of enzymatic

oxidation, then following non enzymatic auto-condensation (Bezuidenhout et al. 2005, Robards et

al. 1999, Robinson et al. 1993).

First step in enzymatic oxidation

The enzymatic oxidation reaction starts with hydroxylation of mono-phenol to o-diphenols and then

the oxidation of o-diphenols to o-quinones with the participation of oxygen (Queiroz et al. 2008,

Robard et al. 1999).

Polymerisation reactions of quinones

The quinones formed from the first oxidative step are relatively unstable and undergo further

reaction (Robard et al. 1999). Quinones non-enzymatically condense to dark pigments

(Chidtragoola et al. 2011). They can react with other phenolic compounds to form condensation

products (Robard et al. 1999).

24

Cell decompartmentation process under stress and pigments formation in lenticels

In healthy tissue, browning enzymes are separated from their phenolic substrates in the intact cells.

Under physical damage (e.g. brushing) or physiological damage (e.g. chilling), cell membranes are

disrupted and compartmentation is reduced or lost and direct contact takes place between browning

enzymes and their phenolic substrates (Robinson et al. 1993). As reported above, the phenolics are

converted to o-quinones by PPO enzymes (Chidtragoola et al. 2011). Thereafter, non-enzymatic

condensation of the quinones takes place to form polymeric compounds. Melanin is a final product

with a black colour (Queiroz et al. 2008). Exposure to oxygen is important for enzymatic

oxidation. PAL enzyme facilitates browning by creating more phenolic substrates. Glycosidase

also facilitates browning by releasing free phenolic compounds (Beckman 2000).

Pigments formation in lenticels

Some pigment accumulation occurs in the vacuoles of sub-lenticellular cells during fruit maturation

of Tommy Atkins and becomes more visible at maturity (Bezuidenhout et al. 2005). The colour of

lenticels varies with mango fruit position on the tree, otherwise from fruit to fruit, and among

cultivars. Non-discoloured lenticels contain simple phenolics (Du Plooy et al. 2004). With non-

damaged lenticels of Keitt mango fruit, tissues surrounding the cavity contain high concentrations

of free acid phenylpropanoids such as ferulic acid, caffeic acid and flavonoids (Du Plooy et al.

2009a). In this condition, no anthocyanin was detected by fluorescence and there was no phenolic

deposition inside the tissues around the undamaged lenticels (Du Plooy et al. 2009a). The wax

lining the lenticel cavity contains chlorogenic or gallic acid components (Du Plooy et al. 2009a). In

reddened lenticels, accumulation of phenolics was observed in the cell walls and vacuoles of the

tissue inside the cavity. The cells lining the cavity surface contained condensed soluble ester-bound

phenolic compounds (Du Plooy et al. 2009a), resulting in discolouration (Du Plooy et al. 2006). In

a red lenticel cavity, there can be living cells and intact vacuoles (Du Plooy et al. 2004). In dark

lenticels of Keitt fruit, the black colour is associated with the condensation of polymeric phenolics

(Du Plooy et al. 2009a). Bezuidenhout et al. (2005) determined that blackened lenticels had no

vacuolar pigmentation, but the cell walls were discoloured. Tamjinda et al. (1992) suggest that

cells with discoloured cytoplasm are dead or degrading. As determined by Du Plooy et al. (2004),

black lenticels had no living cells in the black area as evidenced by the absence of fluorescent

cytoplasm.

25

2.2.12 Role of lenticel damage in fruit shelf life

Discolouration of lenticels leads to a significant postharvest problem in poor visual fruit quality.

However, the discolouration may have a biological role in fruit shelf life. It may be that

accumulation of polymeric phenolic compounds in cell walls around lenticels creates a defensive

barrier between the relatively exposed lenticel cavity and the surrounding soft cells of the fruit (Du

Plooy et al. 2006). Permeation of phenolics to neighbouring cells is potentially restricted by

reinforced cell walls, thereby isolating the stressor and the stress reaction associated with

discolouration of affected cells (Du Plooy et al. 2009a). Polyphenolic compounds in discoloured

lenticels formed with involvement of cell wall bound quinones may act as a toxic barrier to

pathogen entry (Du Plooy et al. 2004). The blackened lenticel cells may have no living cellular

contents (Du Plooy et al. 2004) and so stressors like microorganisms cannot penetrate deeper into

fruit (Bezuidenhout et al. 2005). Tamjinda et al. (1992) considered, however, that no pathological

factors are typically involved in LD. In this regard, redressing the metabolic balance to meet the

expectations of consumers is important if this is practically achievable in a commercial context.

2.3 Study directions

LD may have a role in the protection of mango fruit, but this metabolic process results in economic

losses due to reductions in consumer acceptability in the marketplace, including export markets.

Research into the causal factors and mechanisms of LD has been undertaken for several different

mango cultivars. However, little or no comprehensive research has been conducted to date on

commercially important Australian cultivars, including the relatively new B74 mango cultivar. B74

is highly sensitive to LD and no strategies have been found thus far for preventing this disorder.

Moreover, there is still uncertainty as to the origin of the lenticels on the surface of mature fruit and

specific mechanisms of LD. It has been proposed that resin leaks from sub-epidermal latex vessels

toward the lenticel to initiate and / or worsen discolouration. However, this proposition does not

account for non-discolouration of other lenticels on the same area of fruit skin. Based on this

literature review, a model has been proposed to describe the process of how a non-discoloured

lenticel becomes discoloured. Factors that likely cause the discolouration process are also outlined

in the model and several potential strategies, to be investigated in this study, are proposed (Fig. 2.4).

26

Figure 2.4. Model of lenticel damage process in which an undamaged lenticel become damaged.

Factors causing the damage were proposed and preventing solutions were suggested.

Factors Lenticel damage process Proposed control methods

Occurrence of cracking

and shearing of wax

and cuticle covering

lenticels

Increased cell turgidity

(water infiltration into

the lenticel

Pigment formation due

to stress response (phenolic compound

condensation)

Cell perception of

strain (damage of cell

walls and membranes

inside the lenticel cavity)

Damaged lenticel

(destructed structure and

necrosis cells in the

lenticel cavity)

Cultivars, climate,

harvesting and

postharvest activities

Physical and

mechanical injuries

(e.g. brushing,

temperature,

irradiation)

Prevent oxygen contact

(modified atmosphere

packaging: films, bags;

controlled atmosphere

storage: O2, CO2, N2);

inactivate browning

enzymes (anti-oxidants).

Apply wax coatings (new protection barrier)

Minimise postharvest

activities (do not use hot-

vapour treatment,

irradiation…)

Undamaged lenticels

Water-based

solution contact

(rain water,

detergent,

fungicide)

Oxygen contact,

PPO, POD, PAL

enzymes, soluble

phenolic substrates

liberation

Reduce cell turgor (water

logging prevention,

withholding irrigation);

balance osmotic potential

(solution osmotic pressure =

cell osmotic pressure);

apply bagging, coatings

(prevent fruit-water contact)

27

As outlined in subsequent chapters of this thesis, the current study focuses on the potentially more

likely role of free water in causing LD due to suddenly increased cell turgor in the lenticel chamber.

Lenticel characteristics and structure are, therefore, investigated for B74 with a view to

understanding the pronounced sensitivity of lenticel of this cultivar and the underpinning damage

mechanism. In terms of commercial practices in Australia, contributing factors from farm,

harvesting season, packing line practices are reviewed from previous research projects on B74.

Optimising irrigation regimes to reduce LD are also tested. Management measures to minimise

handling effects including fruit exposure to water and solutes, and brushing are examined. Potential

control methods through bagging and coatings in the pre- and postharvest periods to reduce LD are

investigated.

28

Chapter 3

GENETIC INHERITANCE AND SENSITIVITY TO DEVELOPING LENTICEL DAMAGE

ON B74 MANGO FRUIT

3.1 Introduction

Lenticels are small pores on the fruit surfaces and they facilitate gas exchange (Dietz et al. 1988b).

The origin of lenticels on mango fruit is potentially from stomata (Bezuidenhout et al. 2005). Bally

(1999) also reported that stomata were evident from anthesis until KP mango fruit reached ~ 50 mm

long. The stomata then started to degenerate, forming lenticels. However, Dietz et al. (1988a)

suggested that lenticels can develop from either stomata or cracks in the cuticle.

Different mango cultivars can exhibit varying degrees of LD (Bally et al. 1997). Lenticels on Keitt

fruit have been considered more susceptible to LD than lenticels on Kent mango fruit (Cronje

2009b, Du Plooy et al. 2009b). Genotypic differences in susceptibility might be associated with

lenticel structure and / or wax deposition inside the cavity (Du Plooy et al. 2004, Du Plooy et al.

2009b). Differing lenticel sensitivity could also be associated with thickness of walls of cells inside

the cavity. Tamjinda et al. (1992) proposed this to explain the difference between Namdokmai, a

sensitive cultivar, and Falan, a tolerant cultivar. Bezuidenhout et al. (2005) suggested that LD

occurred when there was a lack of cork cambium cells in the lenticel cavity.

The mechanism of LD in mango is not fully understood (Rymbai et al. 2012). Of two general

explanations, the first theory involves latex arising from resin ducts which ramify throughout the

mango exocarp (Bezuidenhout et al. 2005, Du Plooy et al. 2009a). During times of high fruit water

potential (e.g. under wet conditions), resin ducts may rupture. Thereupon, latex could move

apoplastically towards the fruit surface and lenticels (Du Plooy et al. 2006, Loveys et al. 1992).

However, this theory does not explain why some lenticels show darkening and others do not in the

same area of the fruit skin. The second theory of LD involves factors outside the fruit (Tamjinda et

al. 1992). Water penetration can cause highly turgid cells in the lenticel cavity (Everett et al. 2008,

Self et al. (2006). Additional stresses, such as mechanical injuries, can cause endomembrane

dysfunction and associated release and mixing of browning enzymes and their phenolic substrates

(Beckman 2000). Such browning reactions lead to coloured polyphenols accumulating in response

to stressors (Du Plooy et al. 2004).

29

Little comprehensive research on LD has been conducted to date on B74 and other commercially

important Australian cultivars. Moreover, there is uncertainty concerning lenticel origin,

mechanisms of LD, and relative sensitivity to LD among mango cultivars. The present study

incorporates a comparative morphological study of the highly susceptible B74 cultivar, its parents

(KP, Sensation) and other commercially important Australian mango cultivars (HG, R2E2) towards

better understanding LD.

3.2 Materials and methods

3.2.1 Sample collection sites

In the 2011/12 mango season, fruit samples were collected from B74 mango trees at a commercial

farm near Childers, South-East Queensland (25°14’S, 152°37’E). Three other cultivars (KP, HG,

R2E2), were collected from a commercial farm near Bundaberg (24°98’S, 152°09’E). The towns

are nearby each other in South-East Queensland. In the 2012/13 season, B74 and KP fruit were

collected from the Childers farm and HG and R2E2 were collected from the Bundaberg farm. In

the 2013/14 season, B74, KP and Sensation fruit were collected from a commercial orchard near

Mareeba, North Queensland (17°10’S, 145°46’E). In the 2014/15 season, B74, KP and Sensation

fruit were collected the Mareeba farm and treated with key postharvest operations in line with

commercial practices.

3.2.2 Experiments

Experiment 1: Understanding the lenticel sensitivity of B74 vs. KP, HG and R2E2

This study aimed to understand why B74 fruit have a greater postharvest problem with LD, as

compared to KP (one parent) and the other commercial cultivars (HG, R2E2). Lenticel

characteristics, such as lenticel density on the fruit surface and dye uptake capacity, may play

determining roles in LD severity. Fruit of the B74, HG, KP and R2E2 cultivars were sampled at 1

month (fruit of ~ 1 cm in length), at 2 months (fruit of ~ 3 cm in length), at 3 months (fruit of ~ 5

cm in length), at 4 months (fruit of ~ 7 cm in length) and at commercial harvest (green-mature

fruit). They were harvested directly from trees into cardboard trays without any postharvest

treatments. The fruit were then transported by car on the same day as harvest to the Maroochy

Research Facility (MRF) near Nambour, South-East Queensland for assessment. At each collection

time, 30 fruit per cultivar were collected to assess the lenticel characteristics of lenticel number,

lenticel density, LD, and dye infiltration into lenticels as elaborated below. This study was repeated

30

in South-East Queensland in the 2012/13 season with the same interval of collection time.

Experiment 2: Comparison of B74 vs. its parent cultivars (Kensington Pride and Sensation)

In the 2013/14 season, the lenticel origin and LD mechanisms in B74 fruit were investigated and

compared to that for the parent cultivars KP and Sensation in order to better understand the genetic

inheritance of LD. Fruit were collected randomly at intervals of ~ 2 months, from a very young

development stage (fruit of 1 cm in length) to the green-mature stage. Fruit were harvested direct

from the trees to trays without any postharvest treatments. The fruit were then transported by car to

Cairns, North Queensland on the day of harvest within 1 hour and air-freighted to Brisbane, South-

East Queensland. On the following day, the fruit were then transported by car to the MRF near

Nambour, within 1.5 hours for assessment. At each collection time, 30 fruit per cultivar were

collected to assess lenticel characteristics of lenticel number, lenticel density, LD, lenticel aperture

diameter, lenticel external features, and lenticel morphology as per below.

This study was repeated in the 2014/15 season in Mareeba, North Queensland. Key commercial

postharvest operations (including detergent dipping, fungicide treatment and brushing) were applied

to fruit of B74 and its parent cultivars to understand the effect of postharvest treatments on fruit

sensitivity to developing LD. Green-mature fruit of B74, KP, Sensation (15 fruit per cultivar) were

harvested direct from the trees to trays. The fruit were then transported to the (MRF as described

above. The fruit were de-sapped in a detergent (0.25% Mango Wash®, Septone Ltd., Queensland,

Australia) for 1 minute and placed in the shade to dry. The fruit were treated with a fungicide (0.55

mL.L-1 Sportak®, a.i. prochloraz, Bayer Crop Science Ltd., Victoria, Australia) for 30 seconds and

then air-dried for 30 minutes. Treated fruit were placed onto a brushing unit (Adds Up Engineering

Ltd., Bundaberg, Australia) that consisted of dry soft brushes rotating at 84 revolutions.minute-1 for

1 minute in line with commercial practice. Fruit were exposed to 10 µL.L-1 ethylene at 20 °C and

80 - 85% relative humidity (RH) for 2 days. They were then allowed to ripen at 20 °C and 80 -

85% RH. Proportion of damaged lenticels and LD severity were assessed at harvest and at full

yellow colour.

3.2.3 Assessments

Lenticel number and density

The number of lenticels within 3 cm2 circles at five different skin surface sites on each fruit (Fig.

3.1) was enumerated. For fruit at the early stage (~ 1 cm in length) and at the young stage (~ 3 cm

in length), lenticels were counted using an epidermal nail polish paint-and-peel cast method (Kakani

31

et al. 2003). Briefly, sites on the fruit (10 fruit per cultivar at each collection time) were coated

with clear nail polish gel (Express Finish Advanced WearTM, Maybelline Llc., New York, USA)

and allowed to air dry for 30 minutes. The film was then peeled using a surgical blade (Swann-

Morton Co. Ltd., Shelfield, England) and then placed on its air contact surface side onto a glass

microscope slide. Lenticel counts were completed in two random fields of view per nail polish strip

under a light microscope (Nikon Eclipse TS100, Nikon Inc., Tokyo, Japan) at x 200-400

magnification. For fruit from 3 months after flowering (~ 5 cm in length) to green-mature fruit, the

number of lenticels in each skin surface site (Fig. 3.1) was counted. The lenticel density (per cm²)

on the fruit was determined as the total number of lenticels per unit surface area (Schotsmans et al.

2004). Lenticel density each fruit was calculated as the average lenticel density of five selected

parts of each fruit. Total number of lenticel per fruit was estimated as lenticel density each fruit

(/cm²) x fruit surface area (cm²). Fruit surface area (cm²) was calculated with the assumption that

the fruit was elliptical in shape. Thus, fruit surface area was calculated from the fruit polar (a cm)

and equatorial (b cm) radii using the formula:

fruit surface area (cm²) = 2Pi x b[(b + a²/(a² − b²)1/2) x arcsin ((a² - b²)1/2/a)]; here, Pi = 3.1415, a =

fruit polar radius (cm), and b = equatorial radius (cm) (Khanal et al. 2011).

Figure 3.1. The five locations on the surface of mango fruit used to determine lenticel number,

density and aperture.

Dye infiltration

To estimate the potential of water to enter into lenticels, 10 fruit per cultivar at each collection time

were submerged in a 1 g.L-1 Brilliant Blue dye solution (All Colour Supplies Pty Ltd., New South

Wales, Australia) that had been equilibrated to 25 °C. The dye bath with the fruit was then moved

into a 12 °C cold room for 16 hours. Thereupon, the fruit were taken to the laboratory, removed

Stem end face

Beak

face

Convex

face Cheek

face

Bottom face

32

from the dye solution, rinsed three times in running tap water, and blotted dry with paper towels.

Dyed and non-dyed lenticels within four 2 cm-diameter circles around on the fruit equator (each of

the two cheeks and two opposite sides to the cheeks) were then counted with the aid of a

magnifying glass (Maggylamp model M.L. 2141, Newbound Pty. Ltd, NSW, Australia). The

proportion (%) of dyed lenticels was calculated as: = 100 x count of dyed lenticels / count of total

lenticels.

Lenticel damage severity

Lenticel damage severity on each fruit was rated at full colour on a 0-5 scale based on both the

degree of damage of the lenticels and the overall area of skin affected (Table 3.1; Hofman et al.

2010b).

Table 3.1. Rating scales for lenticel damage severity on the skin of B74 mango

Rating Lenticel damage severity*

0 Nil

1

Light spots on not more than 25% of the surface or dense pronounced spots on

not more than 5% of the surface, not cracked

2

Light spots on not more than 50% of the surface or dense pronounced spots on

not more than 10% of the surface, not cracked

3

Scattered pronounced spots on not more than 50% of the surface, or dense

pronounced spots on not more than 25% of the surface, not cracked

4 Dense pronounced spots on not more than 50% of the surface

5 Dense pronounced spots on more than 50% of the surface

* Dense = spots no more than 5 mm apart. Light = ‘pinprick’ size. Pronounced = more

than half pinhead size, dark coloured.

Lenticel aperture diameter measurement and lenticel external features

The diameter of lenticel openings was quantified in 1 mm planar sections from skin explants

comprised of the epidermis and exocarp tissue. The sections containing lenticels were excised

using a surgical blade (Swann-Morton Co. Ltd., Shelfield, England) from five positions (Fig. 3.1)

on each 10 fruit. The thin sections were incubated for 2 minutes in 0.1% acridine orange (AO;

Sigma-Aldrich Corporation Co., Missouri, USA) and placed onto a glass slide with skin surface up

to the objective lens. The sections were viewed under an Olympus BH-2 epifluorescence

microscope fitted with an Olympus HBO 103 W/2 burner (Olympus Corporation Co., Tokyo,

33

Japan). They were viewed at x100 and x200 magnification. According to the manufacturer, AO is

a cell-permeable metachromatic fluorescent cationic dye that can indicate DNA and RNA of cells.

Using AO, external features of lenticels could be distinctively observed under the fluorescent

microscope as a green halo, while other cuticlar areas did not stain. The plan view features of

lenticels stained with the fluorescence dye were photographed under white light illumination using

a digital camera (Canon DOS40D, Canon Inc., Tokyo, Japan). The dimensions of lenticel openings

were measured using an eyepiece micrometer calibrated using a haemocytometer (Khanal et al.

2011). The eye piece micrometer was fitted with an Olympus BH-2 microscope (Olympus

Corporation Co., Tokyo, Japan).

Lenticel morphology

The morphology of lenticels was studied using fresh or fixed tissue sections through the fruit skin.

For examination of fresh tissues, five lenticels and surrounding tissues were each excised from five

randomly selected but representative fruit per treatment. The tissue explants were 20 mm-long x 5

mm-wide x 3 mm-deep and comprised of the epidermis and exocarp tissue. Relatively thin tissue

sections were cut through each explant by hand using a surgical blade (Swann-Morton Co. Ltd.,

Shelfield, England). The sections containing lenticels were stained with Sudan IV, which stains

lipids in the cuticle red (O’Brien and McCully 1981). The sections were examined under a light

microscope (Nikon Eclipse TS100, Nikon Inc., Tokyo, Japan) and photographed using a digital

camera (Canon DOS40D, Canon Inc., Tokyo, Japan).

For examination of fixed tissues, lenticel tissues were excised from five randomly selected but

representative fruit for each cultivar at each collection time. Explants that were 10 mm-long x 5

mm wide x 3 mm-deep were fixed in a FAA solution (95% ethyl alcohol, 50 mL: glacial acetic acid,

5 mL: 37% formaldehyde, 10 mL: distilled water, 35 mL). Fixed tissues were dehydrated in a

graded ethanol series of 50, 70, 85, 95, and 100%. They were then gradually passed in a xylene /

ethanol series of 1/2; 1/1; 2/1; and 100% (5 minutes at each step). They were next infiltrated with

paraffin wax in 100% xylene for 48 hours until saturated and then embedded in paraffin wax.

Transverse tissue sections (5 µm) were cut using a rotary microtome (InterlapsTM, International

Biology Laboratories, Haryana, India) with a Feather microtome blade (Feather Safety Razor Co.

Ltd., Osaka, Japan). The 5 µm sections were mounted on glass slides using a Para-Tissuer pen (Ted

Pella, Inc., California, USA) and allowed to dry at room temperature (ca. 22 ºC) for 12 hours. The

sections were de-waxed by firstly transferring slides into 100% xylene and then 50/50 ethanol:

xylene for 5 minutes at each step and then into absolute alcohol. The slides were then hydrated by

passing through a graded series of decreasing ethanol concentrations of 95%, 70%, 50%, 30% (5

34

minutes at each step) and finally into distilled water. The sections were either left unstained or were

stained with 0.05% Toluidine Blue (Sigma-Aldrich Corporation Co., Missouri, USA), which stains

polyphenols blue-green. Toluidine Blue was applied to the sections on glass slides for 5 seconds,

then rinsed off with water for 10 seconds. The sections were examined under a light microscope

(Nikon Eclipse TS100, Nikon Inc., Tokyo, Japan) and photographed using a digital camera (Canon

DOS40D, Canon Inc., Tokyo, Japan). The dimensions of lenticels were measured using eye piece

micrometer fitted with an Olympus BH-2 epifluorescence microscope (Olympus Corporation Co.,

Tokyo, Japan).

3.2.4 Statistical analyses

Statistical analyses of data were performed using Genstat® 16 for Windows® (VSN International

Ltd., Hertfordshire, UK). Analysis of variance (ANOVA) was by the General Analysis of Variance

model, with cultivar as the ‘treatment’. The protected least significant difference (LSD) procedure

was used to test for significant differences at P = 0.05 between treatment means for the parameters

of lenticel density, lenticels damage severity, proportion of damaged lenticels, dyed lenticel density

and proportion of dyed lenticels.

3.3 Results

3.3.1 Lenticel density and sensitivity to lenticel damage

Green-mature B74 fruit exhibited a 2 - 7-fold higher lenticel density than mature KP (the pollen

parent), HG and R2E2 fruit as measured over two seasons for mangoes produced in South-East

Queensland (Table 3.2). The high density of lenticels on B74 fruit relative to KP fruit suggests that

this trait evidently arose from its other parent cultivar, Sensation. In the 2013/14 season, Sensation

fruit were assessed to investigate the presumed genetic inheritance by B74 of the high lenticel

density characteristic. B74 fruit produced in North Queensland in 2013/14 displayed less than half

the number of lenticels as Sensation fruit, but still had three times as many lenticels as KP. Thus,

based on these observations, the Sensation genotype appears to contribute to the high number of

lenticels on B74 fruit.

35

Table 3.2. The density of lenticels (number per cm2) on hard green-mature fruit. If followed by

different letters, means for lenticel density within each column are significantly different (P < 0.05)

by LSD (P = 0.05).

Cultivar

Lenticel density (number per cm2)

2011/12 season

(South-East Queensland)

2012/13 season

(South-East Queensland)

2013/14 season

(North Queensland)

B74 60.7 c 40.6 c 27.9 b

KP 8.4 a 9.0 a 8.5 a

HG 13.1 b 15.4 b -

R2E2 9.9 ab 6.6 a -

Sensation - - 60.4 c

LSD 0.05 3.4 4.5 6.6

‘-’ no data.

The severity of lenticel discolouration on ripe (full yellow skin) B74 fruit produced in South-East

Queensland and not exposed to postharvest treatments was less than that on KP and HG fruit in the

2011/12 season (Table 3.3). R2E2 had the highest LD severity at harvest (green-mature) and ripe

(full yellow) stages of all the tested cultivars. When fruit were harvested from a North Queensland

orchard in the 2013/14 season, there was no difference in the severity of LD on B74 and KP fruit at

harvest and full ripe.

When Sensation fruit were also assessed in the 2013/14 season, the severity of LD on this cultivar

was lower than that on B74 and KP fruit (Table 3.3). Without any postharvest treatments,

Sensation fruit appear to be less sensitive to developing LD as compared to its progeny B74 and the

commercial KP cultivar. However, when fruit were harvested from the North Queensland orchard

in the 2014/15 season and exposed to key postharvest treatments (detergent dipping, fungicide

treatment and brushing), the severity of LD on the skin of B74 fruit was lower than KP fruit at both

harvest and full colour. The severity of LD on B74 fruit was also lower than on Sensation fruit at

full colour. The proportion of damaged lenticels on KP fruit at full colour was 2.2-fold higher than

on Sensation fruit and 6.6-fold higher than on B74 fruit. Taken overall, these datasets provide

circumstantial evidence that the KP genotype appears to contribute to the sensitivity of B74

lenticels to LD.

36

Table 3.3. Lenticel damage severity (0 = no damage to 5 = severe damage) and proportion (%) of

damaged lenticels on the fruit skin. If followed by different letters, means for lenticel damage

within each column are significantly different (P < 0.05) by LSD (P = 0.05).

Without postharvest treatments With postharvest treatments

Cultivar


(0 - 5)

Lenticel damage

severity (0 - 5)

Proportion (%) of

damaged lenticels

2011/12 season 2013/14 season 2014/15 season

At

harvest

At full

yellow

At

harvest

At full

yellow

At

harvest

At full

yellow

At full

yellow

B74 0.4 a 0.3 a 0.8 b 1.2 b 0.1 a 1.5 a 10.0 a

KP 0.6 a 1.0 b 0.7 b 1.1 b 1.2 b 2.4 c 65.5 c

HG 1.0 b 1.1 b - - - - -

R2E2 2.0 c 2.4 c - - - -

Sensation - - 0.1 a 0.3 a 0.1 a 1.9 b 29.5 b

LSD 0.05 0.3 0.4 0.5 0.5 0.3 0.4 8.4

‘-’ no data.

The uptake of food dye by lenticels via immersion of freshly-harvested fruit in dye solutions may

indicate potential for LD. In two seasons, B74 fruit displayed ~ 3 times higher dyed lenticel density

than did KP, HG and R2E2 fruit (Table 3.4). However, only half of the lenticels on B74 fruit

showed dye uptake. The proportion (%) of dyed lenticels on B74 fruit was lower than all the other

tested cultivars in both years. This indicated B74 fruit harvested directly from tree had less

potential for LD, as compared to KP, HG and R2E2 fruit.

37

Table 3.4. Dyed lenticel density (per cm²) and proportion of dyed lenticels (%) on green-mature

mango fruit in South-East Queensland. If followed by different letters, means for dyed lenticel

density and proportion of dyed lenticels within each column are significantly different (P < 0.05) by

LSD (P = 0.05).

Cultivar

2011/12 season 2012/13 season

Dyed lenticels

density (per cm²)

Proportion of dyed

lenticels (%)

Dyed lenticel

density (per cm²)

Proportion of

dyed lenticels (%)

B74 19.6 b 49.5 a 18.3 b 53.0 a

KP 5.4 a 90.2 c 6.2 a 56.9 ab

HG 6.1 a 66.5 b 6.7 a 63.8 b

R2E2 5.6 a 77.0 b 7.8 a 94.8 c

LSD 0.05 1.9 10.9 1.8 8.2

3.3.2 Lenticel origin

In the 2011/12 fruiting season, the number of lenticels on B74 fruit steadily increased during

development from young 1 cm-long fruit (1 month after flowering) to the green-mature commercial

stage (at harvest) (Fig. 3.2A, B and C). In contrast, the number of lenticels on KP, HG and R2E2

fruit increased only from early young fruit until 2 months after flowering, and thereafter changed

little. At harvest, B74 fruit had twice as many lenticels as KP (one of the parent of B74), HG and

R2E2. B74 and KP fruit harvested in the 2012/13 fruiting season in the same region (South-East

Queensland) also showed a similar trend in lenticel development.

In the 2013/14 fruiting season, Sensation, the other parent of B74, was included to assess this

character. The total number of lenticels that developed on B74 fruit was 50% fewer than the

number that developed on Sensation fruit, but 3 times greater than KP fruit (Fig. 3.2C). This

finding is consistent with abovementioned findings for lenticel density on B74 fruit and its parents.

It highlights that new lenticels are continuously forming during fruit development on B74 and

Sensation in North Queensland. During fruit development, lenticel aperture diameter increased

gradually from first collection time until harvest time for B74, KP and Sensation fruit (Fig. 3.2D).

At harvest, B74 fruit had an intermediate lenticel opening diameter as compared to its parent

cultivars, KP and Sensation. This finding reconfirms new-born lenticels on the fruit surface in line

with increasing lenticel numbers on B74 and Sensation fruit.

38

(A ) 2011/12

30d 67d 88d 124dat harvest

0

5000

10000

15000B74

KP HG R2E2

(B) 2012/13

30d 63d 93d 122dat harvest

0

5000

10000

15000

B74

KP

(C) 2013/14

30 d 90 d at harvest

0

5000

10000

15000B74

KP Sensation

Le

ntice

l num

be

r (p

er

fruit)

Le

ntice

l num

be

r (p

er

fruit)

Le

ntice

l num

be

r (p

er

fruit)

Collection time (days after full flowering and at harvest)

30 d 90 d at harvest

Le

ntice

l ap

ert

ure

dia

me

ter

(µm

)

0

50

100

150

200

B74

KP

Sensation

(D) 2013/14

Figure 3.2. Changes in lenticel numbers (per fruit) on B74, KP, HG and R2E2 fruit in the 2011/12

season (A), on B74 and KP fruit in the 2012/13 season (B) in South-East Queensland, on B74, KP

and Sensation fruit in the 2013/14 season (C) in North Queensland, and in lenticel aperture diameter

(µm) on KP, B74 and Sensation fruit in the 2013/14 season (D) over sequential development stages.

The vertical bar represents the LSD at P = 0.05 in A, B and C. The vertical bar in lenticel aperture

diameter graph (D) represents the standard errors of means.

Lenticels were evident on the B74 fruit at 1 cm in length (at ~ 1 month after flowering) (Fig. 3.3A).

The external features of lenticels on B74 fruit were similar to the form of stomata on the mature

mango leaf (Fig. 3.3B). Light micrographs of lenticel morphology showed that the first epidermal

cell layer of fruit at ~ 1 cm length is distinctive from other sub-cuticular epidermal cells as a narrow

cell layer immediately adjacent to the cuticle (Fig. 3.3C). Lenticels even at a young fruit

development age of ~ 1 cm length manifest damage as browning of cells and tissue in the lenticel

cavity (Fig. 3.3D).

39

Figure 3.3. External feature (Ef) of a lenticel on B74 fruit at 1cm diameter (A) revealed its

similarity to a stomata shape (St) on mango leaf (B) using the surface replica method and observed

by light microscopy. A light micrograph of a transverse 5 µm-thick section of a lenticel at this

stage as stained with Toluidine Blue (C) showed a distinct surface cell layer (Sc) and an open pore

generally similar to stomata. A light micrograph of an unstained transverse section of a lenticel (D)

showed browning of cell / tissues (Bt) in the lenticel cavity (Lc). The transverse sections (C, D)

were examined using the paraffin embedding method. Scale bars represent 10 µm.

3.3.3 Mechanism of lenticel damage on B74 mango fruit

No clear differences in lenticel morphology were observed among B74, KP, HG and Sensation

mango fruit collected at the green-mature stage (Fig. 3.4). Damaged lenticels often showed blue

coloured walls of the cells lining the lenticel cavity when stained with Toluidine Blue, which

suggests high phenolics concentrations. Therefore, no link between lenticel damage and lenticel

structure was observed in the studied mango cultivars.

C D

A B

Bt

Lc

St Ef

Lc

Cl Sc

40

Figure 3.4. Light micrographs showing lenticels and resin ducts in B74 (A), KP (B), HG (C) and

Sensation (D) fruit harvested at the green-mature stage. The transverse 5 µm-thick sections with

lenticel cavity (Lc) were examined using the paraffin embedding method and stained with Toluidine

Blue. Scale bars represent 50 µm.

Unlike on fruit at an early development stage, lenticels on later stage mango fruit had no distinctive

first sub-cuticular epidermal cell layer (Fig. 3.5). When stained with Toluidine Blue, non-

discoloured lenticels showed purple staining of the walls of cells around the lenticel cavity (Fig.

3.5A). In contrast, the walls of cells in discoloured lenticel developed a blue colour in damaged

(browned) tissue (Fig. 3.5B). Again, resin ducts near discoloured lenticel cavities and cells and

tissues between lenticel cavities and resin ducts were evidently intact and not discoloured (Fig. 3.5A

and B).

B

D

A

C

Lc

Lc

Lc

Lc

41

Figure 3.5. Light micrographs of an undamaged lenticel (A, intact lenticel cavity, Lc) and a

damaged lenticel (B, darkened cavity) of B74 fruit harvested at 7 cm length. Sections of lenticel

cavity (Lc) and resin ducts (Rd) were examined using the paraffin embedding method and stained

with Toluidine Blue. The blue tissue staining inside the oval is indicative of phenolic accumulation

(Pa) around the damaged lenticel cavity with brown tissues (Bt). Scale bars represent 50 µm.

The light micrographs from serial cuts though a damaged lenticel also revealed brown tissues in the

lenticel chamber and intact resin channels underneath the lenticel (Fig. 3.6). There were also no

discoloured cells between resin ducts and the damaged lenticel cavity. This suggests that there was

no relation between LD and resin leak from resin ducts.

B

A

Lc Lc Pa

Bt

Rd

Rd

42

Figure 3.6. Series transverse sections through a lenticel on B74 fruit showed browning tissues (Bt)

in the lenticel cavity (Lc) and an intact resin duct (Rd) adjacent to the lenticel. Scale bars represent

50 µm.

In general, B74 fruit exhibits four typical lenticel types, viz. undamaged, slightly damaged, lenticels

with a small dark spot, and severely damaged lenticels (Fig. 3.7). Damaged and undamaged

lenticels were present on the same fruit surface area. Non-discoloured lenticels of B74 fruit showed

no dark pigments in cells around the lenticel chamber (Fig. 3.7A), while an increasing intensity and

extent of pigmentation is obvious in more discoloured lenticels (Fig. 3.7B, C and D).

Lc

Rd

Rd

Rd

Rd

Rd

Lc

Lc

Bt

A B

C D

F G

Rd

Lc

Lc Bt

43

Figure 3.7. External features and transverse sections of four common types of lenticels on B74

fruit at full yellow of undamaged lenticel (A and B), slightly damaged lenticel (C and D), small

dark spot (E and F), and severe damaged lenticels (G and H). Brown tissues (Bt) surrounded the

discoloured lenticel (C, D, E, F, G and H). The sections were examined using the paraffin

embedding method. Scale bars represent 50 µm.

A

C

E

G

F

H

Bt

Bt

Bt

Bt

Bt

D

Bt

B

44

Damaged lenticels are present on the B74 fruit surface at random. In some cases, damaged lenticels

surrounded by a red halo (red lenticels) were observed only on the blushed area of B74 fruit (Fig.

3.8A), while undamaged lenticels had no red halo. Longitudinal sections through lenticels

surrounded by a red halo revealed that the red pigmentation was also observed in the surface

epidermal cell layer (Fig. 3.8B).

Figure 3.8. External feature of a damaged lenticel with red halo and an undamaged lenticel (A) and

transverse section of a red lenticel (B) on B74 fruit at full yellow. The light micrograph of a

damaged lenticel (Lc) with brown tissues (Bt) showed the red pigment accumulation in the surface

epidermal cell layers). The transverse section (B) was examined using the hand-sectioning method.

Scale bars represent 50 µm.

In a given fruit, some lenticels were discoloured while others were not. The morphology of selected

dyed and non-dyed lenticels was compared.

Figure 3.9. Light micrographs lenticel structures for dyed and un-dyed lenticels of B74 fruit. Un-

dyed lenticel (A) and dyed lenticel (B) were examined using the paraffin embedding method. Dyed

lenticel (C) was examined using hand-sectioning. Blue dye was observed in the lenticel cavity (Lc)

in (C). Scale bars represent 50 µm.

A B

Undamaged

lenticel

Damaged

lenticel

Lc Red pigment

Red halo

Bt

Bt

C B A

Lc Lc Lc

Dye

45

Light micrographs revealed that all lenticels were open to the fruit surface (Fig. 3.9A and B).

Moreover, non-dyed and dyed lenticels had generally similar morphology and anatomy. The dye is

not observed in Fig. 3.9A and B because it possibly faded during fixation. However, using hand-

sectioning, the blue dye was evident in dyed lenticels (Fig. 3.9C).

Wax deposition on the fruit surface and near lenticel cavities was observed (Fig. 3.10A). Moreover,

when stained with 0.1% acridine orange, wax also observed in the lenticel cavity (Fig. 3.10B).

There were no splits or cracks of cuticle on the fruit surface. Only round or oval pores were

observed under the fluorescence microscopy.

Figure 3.10. Dissecting light micrograph (A) and fluorescence micrograph (B) revealing opening

pore of lenticels and natural wax deposition near and inside the lenticel cavity of B74 fruit. The

explant containing a lenticel (Lc) was stained with 0.1% Arcidine Orange, a fluorescence dye and

observed under a fluorescence microscopy (B). Scale bars represent 50 µm.

3.4 Discussion

3.4.1 Sensitivity of B74 mango fruit to lenticel damage

Lenticel damage is the physiological disorder occurring on the fruit epidermis and its discolouration

reduced the cosmetic of fruit. B74 fruit are distinguished by their relatively high lenticel density.

Other Australian mango cultivars such as KP, HG and R2E2 have significantly (P < 0.05) fewer

lenticels than B74 fruit (Table 3.2 and Fig. 3.2). The high number of lenticels on B74 fruit

evidently accounts for why LD can have such an impact on the appearance of this cultivar. Dietz et

al. (1988b) also reported large differences in lenticel density among Indian mango cultivars,

B A

Lc Lc

Wax

Wax

46

including Mallika, Totapuri, Pairi, Alphonso and Dashehari. The skin of Mallika and Totapuri fruit

had a high lenticel density of 33 - 35 lenticels per cm², which is slightly lower than B74 fruit.

Lenticel density of Pairi, Alphonso and Dashehari mango fruit varied from 19 - 23 lenticels per cm²,

which is slightly higher than KP, HG and R2E2 fruit. The latter are anecdotally believed relatively

tolerant to LD.

Mangoes with a higher number of lenticels on the fruit surface may have more potential to be

damaged. However, without any postharvest treatments, B74 fruit had similar or less LD severity

than KP, HG and R2E2, but higher LD severity than that observed on Sensation fruit (Table 3.3).

The sensitivity of B74 lenticels to LD may originate from KP, the pollen parent, possibly due to

browning enzyme activities. Robinson et al. (1993) found that the activity of polyphenol oxydase

(PPO) in KP mango fruit skin varied from cultivar to cultivar, being high in KP and low in the

Irwin fruit. B74 fruit are highly sensitive to LD. It is possible that PPO activity is also inherently

high in B74 fruit. Dye uptake patterns were possibly associated with lenticels that developed

typical LD. Again, B74 fruit possessed the highest lenticel density at the green-mature stage as

compared to KP, HG and R2E2, so this cultivar displayed about 3 times the dyed lenticel density

relative to the three other cultivars (Table 3.4). However, only half of the lenticels on B74 fruit

demonstrated dye uptake, which was less than the three comparators (KP, HG and Sensation).

Marques et al. (2013) also established a link between food dye uptake at harvest and LD at full

colour for B74 fruit of different maturity. Lenticels which took up dye in green-mature fruit may

have potential to express damage symptoms at ripe. Fruit of the B74 cultivar may therefore be less

susceptible to LD than the other tested cultivars in term of lenticel percentage.

3.4.2 Lenticel origin

Lenticels in mango fruit have previously been reported to originate from dysfunctional stomata and

/ or cracks in the fruit cuticle (Bezuidenhout et al. 2005, Dietz et al. 1988a, Du Plooy et al. 2009a).

In the current study, the origin of lenticels in the commercial cultivars KP, HG, R2E2, B74 and

Sensation was studied with a view to better understanding LD processes. Unlike KP, HG and R2E2

fruit, significantly (P < 0.05) more lenticels developed on B74 and Sensation fruit during fruit

development (Fig. 3.2A, B and C). Sensation is the seed parent of B74, and it evidently contributed

to the relatively high lenticel number and density of B74 fruit. Lenticels on B74 and Sensation fruit

also continued to form throughout fruit development while lenticels on KP, HG and R2E2 ceased

forming after 2 months from flowering. It is likely that stomata did not continue to form at or after

1 month on B74 fruit, because no functional stomata were observed on the fruit at / after this stage.

47

Therefore, the lenticels that formed after this period were likely from cracks in the cuticle plus the

epidermal cells in association with fruit expansion from about 1 month after fruit set. This finding

is consistent with the suggestions of Dietz et al. (1988a) who indicated that lenticels were from pre-

formed stomata and from cracks in the cuticle in Mallika and Totapari mango fruit. In all the

cultivars examined in this study, lenticels only formed from round or oval cracking of the fruit

cuticle. This may suggest that cuticle and epidermal cell cracks progressed very quickly into

lenticels during rapid fruit expansion, possibly because of a thinner cuticle and epidermal cells. In

other fruits such as persimmon, cracks (splitting) with long shapes on the fruit cuticle are generally

associated with the fast process of rain absorption through the fruit skin with high sugar content

(Yamada et al. 1987).

The size of the lenticel chamber significantly (P < 0.05) increased during the development of B74,

KP and Sensation fruit Fig. 3.2D). All these cultivars had a similar lenticel aperture diameter

(about 15 µm) at an early fruit age, but the aperture of B74 lenticels was 2.5 - 3 times smaller than

those on KP fruit at harvest, presumably because lenticels formed later in B74 fruit. These findings

suggested that a significant proportion of the lenticels on B74 fruit formed from cracks that

appeared during the rapid fruit expansion phase, with less fruit expansion from then to maturity to

allow large lenticel apertures. This finding is in agreement with Tamjinda et al. (1992), who

reported that during development of Namdokmai mango, the fruit surface enlarged rapidly, such

that the lenticel opening became wide.

Lenticels in young B74 fruit of 1 cm length had external features similar to the stomata on mango

leaves, viz. top protruding characteristics and a special first epidermal cell layer, which may explain

their origin from stomata (Fig. 3.3A, B and C). However, in these young fruit there were no

obvious guard cells which suggested that the stomata were dysfunctional and were transitioning into

lenticels (Fig. 3.3C and D). Dysfunction of stomata on B74 fruit occurred much earlier than in the

previous findings of Bally (1999) and Bezuidenhout et al. (2005). Bally (1999) stated stomata on

KP mango became ruptured when the fruit reached 5 cm length. Stomata on Tommy Atkins mango

remained intact until the fruit reached 2 - 3 cm in diameter (Bezuidenhout et al. 2005). At an early

stage of B74 fruit, some lenticels exhibited discoloured brown tissues. Similarly, discoloured

lenticels on KP and Sensation fruit of 1 cm length were also recorded (pictures not shown).

48

3.4.3 Mechanism of lenticel damage on B74 mango fruit

There was no consistent spatial link between resin canals and LD in B74 fruit that might be

explained between by local leakage of the phytotoxic resin (Du Plooy et al. 2006, Loveys et al.

1992). Thus, stress from pre-harvest and postharvest operations was probably the main factor

leading to LD in B74 fruit. Where high humidity or free water contribute, wax coverage of lenticels

externally and / or internally may play a role in lenticel protection. The presence of cutin in lenticel

chambers and the degree of cellular organisation within lenticels might mediate their sensitivity to

LD (Du Plooy et al. 2004, Du Plooy et al. 2009b). These differences in lenticel structure were not

observed in this study (Fig. 3.4). Sectioned lenticel tissues stained with Toluidine Blue showed

relatively pronounced blue colouration around damaged lenticels (Fig. 3.5B). The blue staining

indicates an accumulation of phenolics in and around damaged cells surrounding the lenticel cavity.

Condensation of phenolics in the tissues may contribute to defence against stressors (Du Plooy et

al. 2006). In some case, B74 developed red halo around the damaged lenticels (Fig. 3.8) possibly

because of high anthocyanin accumulation around the damaged lenticels, which may also contribute

to form a protective barrier for damaged lenticels.

Wax deposition was observed on the fruit surface and lenticel chambers of B74 fruit (Fig. 3.10).

Wax deposition in the lenticel cavity was not evident in lenticels in transverse light micrographs for

tissue that was fixed and embedded. This was because of the paraffin wax solvent, xylene. Wax

layers on mango fruit form and modify during fruit growth and development (Bally 1999). The

protective capacity of wax may, however, decrease over time due to its shearing and cracking as

lenticels enlarge during fruit development. Also, wax may be eroded during commercial

postharvest practices (Du Plooy et al. 2002), thereby reducing any protective effects.

Consequently, lenticels may be rendered more open to water infiltration and other. Additionally,

they could be more exposed to high atmospheric oxygen that is essential for oxidative tissue

browning reactions. Overall, regarding the distinct lenticel characteristics of B74 as inherited from

its parents, a generic LD control strategy could be by preventing certain external stressors (e.g. free

water) from getting into the lenticel cavity.

In summary, lenticel number on B74 at harvest of the green-mature fruit was intermediate between

its parents. B74 had distinctive characteristics with high lenticel number and density at harvest, due

to the genetic heritance from Sensation, one of its parent cultivar. This may explain its potential to

develop commercially significant LD. However, the discolouration sensitivity of B74 possibly

came from KP, one of its parent cultivar. Lenticels on B74 mango were formed partly from stomata

49

at an early fruit age, but mainly from cracks of cuticles during fruit growth. A larger proportion of

the lenticels in B74 fruit had smaller chambers as compared to KP because of their later formation

during fruit development. No link between resin duct damage and LD suggested external stressors

may be the key factors causing LD. Epicuticular wax possibly played an important role in

protection of lenticels from external stress.

50

Chapter 4

PREDISPOSING FACTORS TO LENTICEL DAMAGE ON B74 MANGO FRUIT –

A RETROSPECTIVE ANALYSIS OF DATA

4.1 Introduction

B74 mango is a relatively new cultivar bred in Queensland and it is now grown in other Australian

states under varying growing conditions (Whiley et al. 2001, Whiley et al. 2006). The early-season

of B74 fruit supply is from October to November in the Northern Territory, followed by middle-

season in December in North Queensland and late season ending in February in South-East

Queensland and northern New South Wales (Hofman et al. 2010b). B74 mango fruit have an

attractive appearance with a pronounced attractive red blush colour on the shoulder. However, the

cultivar is highly susceptible to lenticel damage (LD) (Hofman et al. 2010b). LD on fruit is

typically not prominent at harvest time, but it often exacerbated in fruit coming off the commercial

packing line. It is especially evident when fruit reach the ripe stage (Hofman et al. 2010a). LD

results in significant monetary value loss due to lower selling price.

To support this new cultivar, the production and postharvest handling of B74 had been researched

in two Horticulture Australia Ltd (HAL)-funded projects: HAL FR02049 and HAL MG06005

(Whiley et al. 2006, Hofman et al. 2010b). The first project (FR02049) entitled ‘Development of

best practice pre- and postharvest protocols for production of Calypso™ mango’ focused on

production and postharvest handlings that included tree pruning systems, nutritional requirements,

flowering manipulation, harvest maturity methods, fruit ripening processes, storage and

disinfestation procedures. The second project, MG06005, entitled ‘Development of best practice

pre- and postharvest protocols for production of Calypso™ mango: Phase II’ researched extending

the harvest season, maximising eating quality, and improving appearance quality, transport systems,

storage methods and market access. Within the data sets arising from these projects an opportunity

was discerned to potentially elucidate effects of growing locations and seasons.

The purpose of this desk top study was to analyse data from the aforementioned projects in a

different way. In doing so, it was thought to identify key predisposing factors among current

commercial practices that contribute to LD. The results of this study were anticipated to help to

better understand LD with a view to reducing the negative impacts of any predisposing factors.

51


4.2.1 Fruit

This study is a re-evaluation of selected data sets for B74 mango fruit from HAL MG06005 project

in particular (Hofman et al. 2010b). All selected experiments involved ripening with 10 µL.L-1

ethylene at 20 °C for 2 days and then holding fruit at 20 °C and ~ 85% relative humidity (RH) until

full colour (eating ripe). The experiments were conducted in the 2007/08, 2008/09, and 2009/10

fruiting seasons. Approval from the project leader and team members was obtained before

proceeding with the retrospective study. The B74 mango fruit were harvested from the main

growing regions of Darwin and Katherine (Northern Territory), Mareeba (North Queensland), and

Bundaberg and Childers (South-East Queensland). Fruit were harvested at ~ 14% dry matter and

transported by airplane and / or car to the Maroochy Research Facility near Nambour, Queensland.

They were then ripened with ethylene (as above) and assessed for LD severity at full yellow colour;

over 90% of yellow or red colour on the fruit surface.

Differences among farms in the same region and in the same season

The effects of farms (sites) on the severity of LD that developed on B74 mango were analysed in

three successive harvest seasons. The data were grouped into harvest season within each of

2007/08, 2008/09 and 2009/10, and within the regions of South-East Queensland, North

Queensland and Northern Territory. Fruit were harvested and packed using commercial practices.

Thus, they were dipped into detergent to prevent sab-burn, treated with fungicide, brushed, sorted

and packed into cardboard trays. They were then exposed to 10 µL.L-1 ethylene at 20 °C for 2 days,

and subsequently stored at 20 °C and ~ 85% RH until they attained full yellow skin colour with no

green skin colour remaining. This condition was taken to represent the eating ripe stage. The

differences in LD severity among farms at the same region and in the same season were analysed at

this stage.

Interaction between season and farm in the same region

B74 fruit were collected from the end of two separate packing lines at each of two commercial

farms, Acacia and Oolloo, in the Northern Territory during the 2008/09 and 2009/10 harvest

seasons. The effect on LD severity across farms and seasons was analysed.

52

Effect of location in the farm on lenticel damage severity

B74 fruit were collected at the end of a commercial packing line at a farm in Childers, South-East

Queensland in the 2009/10 season. They were divided into three groups based on the slope of

growing block area encompassing the top, middle and bottom of a hill. Effect of slope position on

LD severity was assessed.

Effect of harvesting and handling methods across farms on lenticel damage severity

Two approaches (i.e., off-tree method vs. standard commercial practice) to harvesting and handling

fruit were compared across three farms in South-East Queensland in the 2007/08 season. In the off-

tree approach, fruit were carefully harvested by hand, de-sapped on racks in the field, and packed

into trays without further postharvest handling and treatment. In the commercial practice, fruit were

harvested and de-sapped with Mango Wash solution and then passed over a commercial packing

line which included brushing, fungicide (0.55 mL.L-1 Sportak®, a.i. prochloraz, Bayer Crop

Science Ltd., Victoria, Australia) application, drying, and packing.

Effect of brushing treatments on lenticel damage severity

Fruit were collected from a commercial farm in Childers, South-East Queensland in the 2009/10

season. They were harvested by the off-tree approach such that the fruit were picked and de-sapped

in the field without exposure to water or water-based chemicals. Thereafter, brushing with or

without water were undertaken using the brushing units of the packing line. The control fruit were

not exposed to brushing. The fruit were also irradiated (~ 398 Gy) and then ripened at the

postharvest laboratory of Maroochy Research Facility at Nambour, Queensland to assess LD

severity at full colour.

Effect of coatings on lenticel damage severity

Fruit were obtained off the end of a packing line on a commercial farm at Childers, South-East

Queensland in the 2009/10 season. The coating solution was a commercial carnauba-based

formulation (Natural Shine™ TFC210) diluted with water at the ratio of three part wax to one part

water. The coating was applied to individual fruit using a soft cloth. For irradiation, fruit were

irradiated with an average dose of 398 Gy. The coating and irradiation treatments were evaluated

either alone or in combination. The control fruit were not coated or irradiated. LD severity was

assessed and treatments compared for fruit when they reached full yellow colour.

53

4.2.2 Assessments

Lenticel damage severity on each fruit was rated at full colour on a 0 - 5 scale based on both the

degree of damage of the lenticels and the overall area of skin affected. Rating scales for LD

severity on the skin of B74 fruit was described in Table 3.1 (Hofman et al. 2010b).


Data were analysed using one-way ANOVAs using the ‘General Analysis of Variance’ model of

Genstat 16 for Windows (VSN International Ltd., Hertfordshire, UK) for farms, seasons, and

brushing or coatings as treatments. Two factorial interaction between season and farm in the same

region was analysed at P = 0.05. The protected least significant difference (LSD) procedure at P =

0.05 was used to test for differences between treatment means.

4.3 Results

4.3.1 Farms effects

In 2007/08, there were no significant (P > 0.05) differences in LD severity among three farms in

South-East Queensland (Table 4.1). Similarly, there was no significant (P > 0.05) difference in LD

severity among farms in Northern Territory in 2008/09 fruiting season. However, in this season

(2008/09), the result for LD severity in South-East Queensland was dissimilar to that in the

Northern Territory. In the 2009/10 season, LD severity among the different farms was significantly

(P < 0.05) different in both Northern Territory and in North Queensland. In 2009/10 season, fruit

from Petani farm in North Queensland had the most severe LD severity (3.8).

54

Table 4.1. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 fruit harvested

from different farms and assessed at full yellow skin colour. If followed by different letters, means

for lenticel damage severity within each column in the same season and region are significantly

different (P < 0.05) by LSD (P = 0.05).

Season Region Farm Lenticel damage severity (0 - 5)

2007/08 South-East Queensland Oolloo 1.1

Simpson B 2 1.0

Simpson B 6 1.1

LSD 0.05 n.s.

2008/09 Northern Territory Acacia Hills 2.4

Oolloo Katherine 1.7

Oolloo Darwin 1.8

LSD 0.05 n.s.

South-East Queensland Oolloo B 21 1.3b

Oolloo B 30 0.9a

Stewart Brothers 1.5b

LSD 0.05 0.37

2009/10

Northern Territory Acacia Hills 1.0b

Oolloo Katherine K2 0.4a

Mataranka 1.3b

LSD 0.05 0.43

Northern Queensland Oolloo Dimbulah 1.2a

Petani 3.8b

Stewart Brothers 1.5a

LSD 0.05 0.57

n.s. = not significant.

4.3.2 Interaction between farm and season

There was no significant interaction between farm and season in LD severity on B74 mango fruit at

full yellow. The value probability at P = 0.05 was 0.841. This indicated that farm and season

independently affected LD severity on B74 mango fruit. However, there was a significant (P <

0.05) difference in LD severity between farms across seasons (Table 4.2). There was also a

significant (P < 0.05) difference in LD severity between seasons across farms.

55

Table 4.2. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 mango fruit

harvested from different farms and seasons, and assessed at full yellow skin colour. If followed by

different letters, means for lenticel damage severity within each column are significantly different

(P < 0.05) by LSD (P = 0.05).

Farm Lenticel damage severity (0 - 5) Season Lenticel damage severity (0 - 5)

Acacia 1.6 a 2008/09 season 2.1 a

Katherine 1.0 b 2009/10 season 0.7 b

LSD 0.05 0.47 LSD 0.05 0.40

4.3.3 Location in the block

There was a significant (P < 0.05) difference in LD severity across the three different growing

positions within one block of trees on the one farm (Table 4.3). Fruit harvested from trees on the

blocks at the top and in the middle of the hill slope had similar LD severity. In contrast, LD

severity on fruit harvested from trees at the bottom of the slope was the high at 2.19.

Table 4.3. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 fruit harvested

from different locations in a farm in Childers, South-East Queensland in the 2009/10 season. If

followed by different letters, means for lenticel damage severity are significantly different (P <

0.05) by LSD (P = 0.05).

Section in the farm Lenticel damage severity (0 - 5)

Bottom of the hill slope 2.2b

Middle of the hill slope 1.6a

Top of the hill slope 1.2a

LSD 0.05 0.56

4.3.4 Effect of harvesting and handling methods on lenticel damage severity

LD severity of fruit harvested and handled by the off-tree approach and by the commercial practice

approach was significantly (P < 0.05) different at all three farms (Table 4.4). The off-tree

harvesting method resulted in significantly (P < 0.05) lower LD severity than the commercial

practice method. At the Oolloo farm in particular, commercial practice contributed to a high LD

severity (3.6).

56

Table 4.4. Lenticel damage severity (0 = no damage to 5 = severe damage) of B74 fruit harvested

from the tree directly into trays (off tree) or collected at the end of a packing line (commercial

practice). If followed by different letters, means for lenticel damage severity in the same farm are

significantly different (P < 0.05) by LSD (P = 0.05).

Farm Harvesting approach Lenticel damage severity (0 - 5)

Oolloo

Off-tree 0.2a

Commercial practice 3.6b

LSD 0.05 0.48

S2

Off-tree 0.1a


LSD 0.05 0.65

S6

Off-tree 0.1a


LSD 0.05 0.64

4.3.5 Effect of brushing associated with water on lenticel damage severity

There was a significant (P < 0.05) effect of brushing on LD severity of B74 mango fruit (Table 4.5).

LD severity was highest on fruit exposed to the brushing plus water spraying treatment at 2.9. It

was lowest in the unbrushed control fruit at 0.2. Brushing without water increased LD severity

slightly. Brushing with water resulted in a high LD severity at full colour.

Table 4.5. Lenticel damage severity (0 = no damage to 5 = severe damage) of B74 fruit harvested

directly from tree without any postharvest treatment. Fruit were then brushed with or without water

spray. A set of fruit served as the control. If followed by different letters, means for lenticel

damage severity are significantly different (P < 0.05) by LSD (P = 0.05).

Treatment Lenticel damage severity (0 - 5)

Control 0.2a

Brushing without water 0.7b

Brushing with water 2.9c

LSD 0.05 0.18

57

4.3.6 Effect of coatings and irradiation on lenticel damage severity

There were significant (P < 0.05) treatment effects of coatings and irradiation on LD severity

(Table 4.6). Coating treatments significantly (P < 0.05) reduced LD severity. Fruit that were

coated but not exposed to irradiation developed the least LD severity. Irradiated fruit without an

artificial surface coating displayed the most LD severity. LD severity of coated and irradiated fruit

was lower than fruit treated with irradiation alone.

Table 4.6. Lenticel damage severity (0 = no damage to 5 = severe damage) on B74 fruit coated and

/ or irradiated and assessed at full yellow skin colour. The fruit were then coated with Natural

Shine™ TFC210 (three part wax one part water) and / or irradiated (398 Gy). If followed by

different letters, means for lenticel damage severity are significantly different (P < 0.05) by LSD (P

= 0.05).


Control 1.7b

Coating 0.7a

Irradiation 3.7c

Coating with irradiation 1.8b

LSD 0.05 0.30

4.4 Discussion

This retrospective analysis highlighted the significant variation in LD severity observed across

region to region and season to season. Previous studies have indicated that weather conditions near

or at harvest may impact on LD severity. Oosthuyse (1998) showed that wet conditions near

harvest increased LD on Tommy Atkins mango fruit. Similarly, LD on avocado fruit was more

severe in a wet season (Duvenhage 1993). Pesis et al. (2000) considered that wind and low

temperatures in the pre-harvest period may cause LD on mango fruit. Oosthuyse (2002) stated that

mango fruit harvested during a wet period (high fruit turgidity) often had more LD than fruit

harvested during dry conditions (low fruit turgidity). Interestingly, Cronje (2009a) also found that

immediate packing of Keitt mango fruit after harvest lead to higher LD than did delaying packing.

Delaying packing may reduce fruit water status, which possibly contribute to reduce LD.

58

The aspect or growing position on a farm had an impact locally of low LD severity on B74 fruit

from trees at the top and middle of the sloping block and high LD severity on those from the bottom

of the block. The lower section of the block may, for example, have had relatively greater

propensity to water logging. Alternatively, higher soil water content possibly resulted in increased

LD severity on B74 mango fruit from the low area due to higher fruit cell turgidity. Cronje (2009a)

found that soil drying over 3 weeks before harvest reduced LD on Tommy Atkins and Keitt

mangoes. Production factors such as irrigation and spray (e.g. pesticide application) as well as

harvest and postharvest handling treatment methods applied variably across farms may we have

contributed to differences in LD between farms in the 2008/09 (South-East Queensland) and in

2009/10 (North Queensland and the Northern Territory) seasons.

Harvesting and handling activities had a significant (P < 0.05) effect on LD severity on B74 mango

fruit. This finding is consistent with that of Self et al. (2006). These authors found that postharvest

operations increased LD on Tommy Atkins fruit. It was considered that there are various stresses to

which the fruit are exposed in commercial picking and packing processes. For example, KP mango

fruit treated with detergents expressed more LD than fruit treated with water alone when they

reached the ripe stage (O'Hare et al. 1999, Whiley et al. 2006). In the present retrospective analysis

study, brushing had a negative impact on LD of irradiated fruit, especially when combined with

water. Similarly, Cronje (2009b) found that brushing in a packing house contributed to LD on Keitt

mango fruit. Moreover, brushing associated with water spraying increased the LD. Brushing may

reduce the wax layer on Kent mango (Du Plooy et al. 2002) and thereby facilitate water entry into

lenticel cavities. Carnauba wax treatments may help to reduce LD, including for irradiated fruit. A

coating may protect the lenticel from physical (e.g. increased turgor) stresses by creating an

artificial hydrophobic barrier over them. However, coating with a 75% carnauba wax treatment

induced negative effects in B74 fruit, such as retarded colour development and evolution of off-

odours at the ripe fruit stage.

Overall, this retrospective analysis has shown that LD on B74 fruit varied with growing location

(site) and fruiting season. It also demonstrated that harvest and postharvest handling and

treatments, including washing, brushing and irradiation, were key contributing factors to LD

developing on B74 mango fruit. The mechanisms of action associated with these factors in eliciting

LD are, however, largely unknown. Similarly, pre-harvest practices that might reduce LD, such as

reducing water availability in the pre-harvest period, are not well characterised. Postharvest

treatments that reduce free water effects and / or involve gentler or no brushing merit further

investigation. Coating of B74 mango fruit with 75% TFC after harvest reduced LD, presumably by

59

minimising fruit wetting and / or other stresses on lenticels. However, in order to maintain normal

fruit physiological metabolism, a more suitable coating would need to be identified. In this regard,

treatment variables might include alternative coating materials, varying coating thickness and

lenticel chamber cover and / or filling.

60

Chapter 5

EFFECT OF WITHHOLDING IRRIGATION ON LENTICEL DAMAGE ON

B74 MANGO FRUIT

5.1 Introduction

Lenticel damage (LD) often appears as brown or black spots on the surface of mango fruit (Hofman

et al. 2010a). This disorder is sometimes associated with free water contact on fruit before and / or

after harvest, such as from rainfall and aqueous solutions used in commercial harvest and / or

postharvest practices (O’Hare et al. 1999, Oosthuyse 1998, Self et al. 2006, Whiley et al. 2006). As

well as fruit to water contact, the fruit water status per se may also play an important role in LD

because it affects cell turgor. For instance, fruit with turgid lenticel cells can be relatively more

sensitive to physical damage (Cronje 2009a, Everett et al. 2008). The water status of fruit can be

affected by many factors, such as irrigation regimes, and handing techniques.

Dry soil conditions may affect the water status of leaves, stems and fruits. Soil and tree water status

can be estimated using soil water content, leaf stomatal conductance and leaf relative water contents

(Jongdee et al. 2002, Swati et al. 2000). Leaf and stem water potential can also be used as

indicators for irrigation scheduling and to assess tree response to water stress (Al-Yahyai 2012,

Swati et al. 2000). Dry soil conditions can be controlled by irrigation regimes. Previous studies

have suggested that deficit or no irrigation before harvest can reduce LD in some mango cultivars

(Cronje 2009a, Simmons et al. 1998). However, deficit (reduced water amount) irrigation over a

long period had generally minor effects of yield and fruit size reductions for Chok Anan mango

fruit (Spreer et al. 2007, Spreer et al. 2009). Exposure to a slight drought stress did not affect fruit

quality and the ripening process (Spreer et al. 2009). Hence, regulated irrigation reduction may be

a feasible strategy to reduce LD.

Treatment with ionising irradiation is a phytosanitary requirement for mangoes exported from

Australia to New Zealand and the USA (Johnson and Hofman 2009). Exposure to -irradiation can,

however, negatively affect mango fruit by causing accelerated softening, uneven ripening, skin

browning and LD (Durigan et al. 2004, Grove et al. 2004, Sivakumar et al. 2011). Discolouration

of lenticels is the result of enzymatic oxidation including polyphenol oxidase (PPO) and peroxidase

(POD) (Bezuidenhout et al. 2005, Robinson et al. 1993). Different cultivars may vary in their

61

response to irradiation due to physicochemical differences, such as browning enzyme activities like

PPO, POD. The activity of PPO is high in KP versus low in Irwin fruit (Robinson et al. 1993).

POD activity in the yellow peel of ripe Namdokmai is higher than in the green peel of ripe

Tongdum mango fruit (Ketsa et al. 1999). In the same fruit, PPO activity was higher in the skin

than in flesh and in ripe than green KP fruit (Robinson et al. 1993). Withholding irrigation may

reduce fruit and tree water status and potentially contribute to reduction of LD when fruit is

irradiated.

The present study investigated the potential of withholding irrigation to reduce LD on B74 fruit

grown in a hot and dry tropical area. The study tested the hypothesis that reducing fruit water

availability during late stages of fruit growth by withholding irrigation would minimise LD on

mango fruit, especially when fruit are subsequently stressed by exposure to water contact and

irradiation.


5.2.1 Experiment sites

The experiments were conducted on a commercial B74 mango farm near Katherine in the Northern

Territory (14°60”S; 132°00”E) during 2012 and 2013. The two experiment sites on the farm

(Appendix 1) were different in soil composition (data are presented in Appendix 2). For each

irrigation treatment, ten B74 trees of similar age, growth status and fruit bearing were selected from

a single row. Black plastic sheets measuring 4 x 4 m were placed on the soil surface around the

trunks of trees in the withholding irrigation treatments to minimise the effects of any rainfall during

the experiments.

5.2.2 Treatments and handling procedure

In 2012, B74 trees at site 1 were either exposed to standard irrigation or withholding irrigation for

the last 3 weeks of fruit production. For trees that received irrigation, water was delivered via one

micro-sprinkler near the tree trunk once per day (6 days per week) with 70 - 105 L water/tree/day

(data are presented in Appendix 3). The two different treatments were applied in parallel and

immediately adjacent rows (each treatment had a separate row of 20 trees). Fruit at harvest were

transported to a research laboratory in Katherine, Northern Territory (14°47”S; 132°30”E). Half of

the fruit from each irrigation treatment were dipped into deionised water for 2 minutes within 2

62

hours of harvest. The other half of the fruit was not exposed to water after harvest. After

assessment for fruit water status, the remaining fruit were transported to Darwin, Northern Territory

within 4 hours and then air-freighted to Brisbane, Queensland within 24 hours. There, the fruit

were transported by car to and then irradiated at Steritech Pty Ltd in Narangba. The fruit were

subjected to either irradiation with a dose of 0 (for four non-irradiated treatments) or 494 Gy (min.-

max. 408 - 629 Gy) -irradiation from a Cobalt 60 source at 20 °C (for four irradiated treatments).

The fruit were then transported by car to the Maroochy Research Facility (MRF) laboratories at

Nambour, Queensland within 1.5 hours for serial assessments during ripening after irradiation. The

fruit were treated with 10 μL.L-1 ethylene at 20 °C for 2 days and then allowed to ripen at 20 °C.

The experiment in 2012 is outlined in Fig. 5.1.

63

Figure 5.1. Flow chart of the irrigation experiment in 2012 in Katherine, Northern Territory

No

irrigation,

no DI

water, no

irradiation

40 fruit

Two parallel and adjacent rows were selected at 4 weeks before commercial harvest.

Withhholding and standard irrigations were applied in each row. Ten trees per row were

selected with similar canopy and fruit yield

.

No

irrigation,

no DI

water,

irradiation

40 fruit

At harvest in NT, 80

fruit from four non-

irradiated treatments

(20 fruit / treatment)

were used to assess

fruit relative water

content, gape and

fruit water potential.

160 fruit of four irradiated treatments and 80 fruit of four non-

irradiated treatments were transported by car and plane to Brisbane,

Queensland.

Fruit were gassed at 10 μL.L-1 ethylene at

20 °C for 2 days, and then stored at 20 °C.

During ripening, 20 fruit / treatment (eight

treatments) were used to assess lenticel damage

and lenticel morphology.

No

irrigation,

DI water,

no

irradiation

40 fruit

Withholding irrigation for 3 weeks before

harvest

Standard irrigation (commercial

irrigation)

No

irrigation,

DI water,

irradiation

40 fruit

Fruit was harvested at 15% dry matter by off-tree method (no contact with chemical or pass

into postharvest handlings), 160 fruit from each irrigation (16 fruit / tree). Each irrigation,

fruit were divided into four groups for water dipping for 2 minutes and irradiation.

Sample soil and measure the soil

moisture content

Before fruit harvest 1 day, 30 leaves (three

leaves / tree, 10 trees / irrigation) were

selected for stomatal conductance.

At harvesting, 100 leaves (10 leaves / tree,

10 trees / irrigation) were harvested for

relative water content and water content.

Standard

irrigation,

no DI

water, no

irradiation

40 fruit

Standard

irrigation,

no DI

water,

irradiation

40 fruit

Standard

irrigation,

DI water,

no

irradiation

40 fruit

Standard

irrigation,

DI water,

irradiation

40 fruit

Right after irradiation, at the MRF,

Nambour: 80 fruit from four irradiated

treatments (20 fruit / treatment) were used

to assess fruit relative water content, gape,

water potential.

160 fruit were irradiated (494

Gy) in Brisbane, and then

transported by car to

Nambour, Queensland.

80 fruit of four non-irradiated

treatments were transported by

car to Nambour, Queensland.

64

In 2013, two experiments were conducted at two separate growing sites with differing soil types on

the above farm. The first experiment was applied in three parallel and adjacent rows at the same

site as used the previous year (site 1). The trees were exposed to withholding irrigation for 8 and 4

weeks. Additional trees that received standard irrigation served as the controls. The second

experiment was applied in two parallel and adjacent rows at another site (site 2) on the farm. The

trees were exposed to withholding irrigation for 4 weeks and additional trees that received standard

irrigation served as the controls. At harvest, fruit and leaves from each treatment were assessed at a

research laboratory in Katherine for water status. The remaining fruit from the total of five

irrigation treatments were transported by car to Darwin and then air-freighted to Brisbane as

described above. There, the fruit were transported by car to and then irradiated at Steritech in

Narangba with an average dose of 409 Gy (min.-max. of 358 - 489 Gy). After irradiation, the fruit

were transported by car to MRF for ripening initiation as described above and assessments during

ripening. The experiment in 2013 is outlined in Fig. 5.2.

65

Figure 5.2. Flow chart of the irrigation experiment in 2013 in Katherine, Northern Territory

Site 1:

withholding

irrigation for

8 weeks,

50 fruit

Fruit were harvested at 15% DM by off-tree method (no further packing line practices),

150 fruit from site 1 and 100 fruit from site 2 (five fruit / tree).

Site 1:

withholding

irrigation for

4 weeks,

50 fruit

Site 2:

withholding

irrigation for

4 weeks,

50 fruit

Site 2:

Standard

irrigation,

50 fruit

Harvested when 15% DM,

off-tree harvest

150 fruit was air freighted to

MRS, irradiated (409 Gy) in

Brisbane

Ripened at 20 °C, 2 days,

then stored at 20 °C until

over ripe: 30 fruit / treatment

Site 1:

standard

irrigation,

50 fruit

Site 1 (at the same position of the previous year):

-Three parallel and adjacent rows were selected

at 8 weeks before commercial harvest. Ten trees

per row (one row / treatment) were selected with

similar canopy and fruit yield. Irrigations were

withheld for 8 and 4 weeks before harvest.

Standard irrigation served as the control.

Site 2 (from other site of the orchard): Two

parallel and adjacent rows were selected at 8

weeks before commercial harvest. Ten trees

per row (one row / treatment) were selected

with similar canopy and fruit yield.

Irrigation was withheld for 4 weeks and

standard irrigation served as the control.

During ripening,

20 fruit / treatment from

five treatments were used

to assess water loss,

firmness and lenticel

damage and lenticel

morphology in Nambour,

Right at harvest,

20 fruit /

treatment from

five treatments

were used to

assess fruit dry

matter, gape and

water potential in

Katherine, NT

Before harvest, soil moisture content

during 8 weeks before harvest was

tested. Stomatal conductance at 1 day

before harvest was measured.

66

5.2.3 Assessments

Soil moisture content

In 2012, soil samples were collected from four replicate sampling points along each treatment row

at 3, 2, 1 weeks before harvest and at harvest at 0 - 15 cm, 15 - 30 cm and 30 - 60 cm depth. In

2013, soil samples were collected from three sampling points along the treatment row at 8, 4, 2, 1

weeks before harvest and 0 week (at harvest) at 0 - 10 cm, 10 - 20 cm and 20 - 30 cm depth. The

samples were collected using a soil corer. Three to four core samples of about 50 g per sample

were collected at each sampling point. Samples were weighed and recorded for wet weight (WW)

and dried at 105 - 110 C° until the sample weight was constant. The samples were then weighed

immediately for dry weight (DW). Soil moisture (SM) content was calculated by the formula: SM

(%) = 100 x (WW – DM)/WW.

Leaf stomatal conductance

Stomatal conductance was measured on the days before harvest using a portable steady state

porometer (SC-1 Leaf Porometer, Decagon Devices Inc., Washington, USA). For each tree, three

fully expanded upper canopy leaves adjacent to fruit were selected. The measurements were

recorded from the lower side of the leaves. For each treatment, 30 leaves (three leaves per tree and

10 trees per treatment) were measured every 2 hours from 600 h to 1800 h during that day.

Leaf relative water content and water content

At harvest, 10 healthy, fully developed topmost leaves on the shoots bearing fruit were selected

from each tree and placed into closed plastic bags. A total of 100 leaves per treatment were placed

in a covered tray and transported to a laboratory in Katherine. Leaves were gently cleaned with soft

paper towels to remove dust. A 1 cm-diameter cork borer was used to excise a disc of tissue from

the middle of each leaf. The fresh leaf discs were weighed (FW) using analytical scales with a

precision of 0.001g. The leaf discs were then placed abaxial sides down onto a filter paper

(Qualitative No. 1, Whatman PLC., Buckinghamshire, UK) pre-moistened with distilled water.

They were then equilibrated in a covered Petri dish for 3 hours at room temperature (25 ºC) under a

well-lit area. After quickly blotting dry the leaf discs with tissue paper, they were reweighed and

their turgid weight (TW) was recorded. The samples were then dried at 65 °C for about 72 hours

and weighed again to enable calculation of dry weight (DW) (Boyer et al. 2008, Elsheery and Cao

2008). Leaf relative water content (RWC %) and water content (WC %) were calculated by the

formulae: RWC (%) = 100 x (FW – DW)/(TW – DW); and, WC (%) = 100 x (FW – DM)/FW.

67

Fruit relative water content

A cork borer and razor blade were used to excise five 1 cm-diameter x 5 mm-thick discs of flesh

tissues from the centre of a cheek from 20 individual fruit. The discs were individually weighed for

fresh weight (FW). The discs were floated skin side down on a paper filter paper on distilled water

for 4 hours in a covered Petri dish at 20 °C in the dark to rehydrate (Landrigan et al. 1996). After 4

hours, the samples were blotted dry, reweighed and their turgid weight (TW) was recorded. They

were placed in an oven at 65 °C for 3 days and weighed again to enable calculation of their dry

weight (DW). Relative water content index was calculated after Burdon and Clark (2001): RWC

(%) = 100 x (FW – DW)/(TW – DW).

Water potential

Fruit tissue water potential was determined by recording the weight change of outer pericarp tissue

samples immersed in five polyethylene glycol (PEG) 6000 solutions of 0, 100 200, 300, 400 g/kg

H2O at 25 °C for 3 hours. Five disks of 1 cm-diameter and 2 mm-thickness were excised from the

centre of a cheek on each of 10 - 20 fruit per treatment. Disks were marked with their number and

weighed before and after dipping using analytical scales with a precision of 0.001 or 0.0001g. The

point at which there was no weight change was determined by plotting the weight changes against

solution osmotic potential (modified from Burdon and Clark 2001).

Fruit gape

Gape is the width and length of the slit that develops after cutting through the epidermis of a fruit

(after Hatfield and Knee 1988, Saftner and Conway 1998). Using a sharp, thin and narrow blade, a

1 cm-deep x 2 cm-long longitudinal cut was made into the cheek of green-mature B74 fruit. After

30 minutes, the length and the width of the slit at the widest point was measured with a digital

caliper (Digimatic, Mitutoyo Corporation, Kanagawa, Japan) with a precision of 0.01 mm. Each

cut was made at a vertical angle to the fruit surface. For each treatment, five replicate fruit were

used with two positions in a cheek areas of each fruit was selected.

Fruit dry matter

A single green-mature fruit from each of the 10 replicate trees per treatment were selected and a

flesh sample was cut from the cheek to a depth of 3 cm. Fruit samples were peeled and flesh

samples (about 20 g) were diced and placed in plastic trays. Each sample was weighed for FW.

The proportion (%) of dry matter (DM) of the fruit pulp was determined after drying at 65 °C to

constant weight (Lechaudel et al. 2002, Hassan et al. 2009) using the formula: DM (%) = 100 x

DM/FW (%).

68

Objective firmness

The firmness of individual fruit was also measured objectively using an acoustic firmness tester

(AFS Aweta, Nootdrop, The Netherlands) based on analysis of the resonance frequencies when the

fruit surface is tapped (Ketelaere et al. 2006). The Aweta is a non-destructive system and its units

decrease as fruit soften.

Fruit weight loss

Fruit were individually weighed at harvest and every 2 - 3 days thereafter during ripening to enable

calculation of weight loss. Weight loss was calculated based on the proportion of fresh weight and

the initial fresh weight of fruit.


Lenticel damage severity on each fruit was rated at harvest and during ripening using a 0-5 scale

based on both the degree of damage to the lenticels and the overall area of skin affected. Rating

scales for LD severity on the skin of B74 fruit was described in Table 3.1 (Hofman et al. 2010b).

Lenticel morphology

The morphology examination of lenticels using fixed tissue sections was described in chapter 3 (the

section on Lenticels morphology in 3.2.3 Assessment).


Data were statistically analysed as one-way ANOVAs using the ‘General Analysis of Variance’

model of Genstat16 for Windows (VSN International Ltd., Hertfordshire, UK) with irrigation as

the ‘treatment’ structure. The protected least significant difference (LSD) procedure at P = 0.05

was used to test for differences between treatment means for soil moisture content, soil water

content, leaf stomatal conductance, leaf water content and relative water content, gape, fruit relative

water content, water potential, skin colour, fruit firmness and LD severity.

69

5.3 Results

5.3.1 Experiment 1: Withholding irrigation for 3 weeks in 2012

Temperature, humidity and rainfall for 4 weeks before harvest in 2012

The average weekly day and night temperatures at the experiment site near Katherine, Northern

Territory, varied from 31.7 - 34.1 ºC and 28.2 - 29.8 ºC, respectively, during the last 4 weeks

leading up to harvest (Table 5.1). The difference between day and night temperatures was 3 - 5 °C.

Both day and night temperatures were slightly cooler during the last week before harvest. Relative

humidity (RH, %) did not vary greatly between day and night but increased towards harvest time.

There was ≤ 1 mm of rain during the third and fourth weeks before harvest. However, higher

rainfall was recorded during the second week and also during the week immediately before harvest.

Table 5.1. Average day and night temperatures (°C), relative humidity (RH, %), and total rainfall

(mm) per week during the 4 weeks from 16th October to 13th November 2012 before harvest at the

experiment site.

Weather data Time before harvest (weeks)

4th 3rd 2nd 1st

Average day temperature (°C) 33.0 34.0 34.1 31.7

Average night temperature (°C) 28.7 29.8 29.2 28.2

Average day RH (%) 49.3 45.8 53.5 64.1

Average night RH (%) 54.3 53.4 66.9 75.7

Rainfall (mm) 0 1 41 18

Effects of withholding irrigation for 3 weeks on soil moisture content

There was a significant (P < 0.05) effect of irrigation treatment on soil moisture content from 3, 2, 1

and 0 weeks before harvest (Table 5.2). Withholding irrigation from 3 weeks before harvest

resulted in lower soil moisture contents than soil exposed to standard irrigation during 3 weeks

before harvest except at the soil depth of 30 - 60 cm at 3 and 2 weeks before harvest. Where

irrigation was withheld, the soil moisture content was similar for the three soil depths. However,

with standard irrigation, the 0 - 15 cm layer had higher water content than the 15 - 60 cm layers at 3

and 1 weeks before harvest and at harvest.

70

Table 5.2. Effects of irrigation treatments of B74 mango tree on soil moisture content (%) at three

soil depths from 3 weeks before harvest until harvest in 2012. If followed by different letters,

means for soil moisture content within each week before harvest are significantly different (P <

0.05) by LSD (P = 0.05).

Time before

harvest

Soil depth

Soil moisture content (%)

Standard irrigation Withholding irrigation for 3 weeks

3 weeks 0 - 15 cm 8.8 c 7.2 b

15 - 30 cm 7.2 b 6.1 a

30 - 60 cm 7.1 ab 6.5 ab

LSD 0.05 1.10

2 weeks 0 - 15 cm 8.1 c 4.5 a

15 - 30 cm 6.7 bc 4.2 a

30 - 60 cm 6.6 bc 5.4 ab

LSD 0.05 1.84

1 week 0 - 15 cm 9.1 c 5.7 a

15 - 30 cm 7.4 b 5.2 a

30 - 60 cm 7.0 b 5.6 a

LSD 0.05 1.24

0 week

(at harvest)

0 - 15 cm 8.0 c 4.5 a

15 - 30 cm 6.9 b 4.5 a

30 - 60 cm 6.8 b 4.8 a

LSD 0.05 0.90

71

Effects of withholding irrigation for 3 weeks on leaf stomatal conductance

The stomatal conductance by leaves on B74 trees exposed to standard irrigation or withholding

irrigation for the last 3 weeks before fruit harvest increased during the morning to a maximum

conductance at between 1200 h - 1400 h and thereafter decreased during the afternoon (Fig. 5.3).

There were no significant (P > 0.05) differences in the stomatal conductance by leaves on trees

exposed to either irrigation treatment from 630 h to 1230 h. However, from 1200 h to 1830 h, the

stomatal conductance by leaves on trees in which irrigation was withheld was significantly (P <

0.05) lower than those exposed to standard irrigation.

Time of the day

630

h - 8

30 h

830

h - 1

030

h

1030

h -

1230

h

1230

h -

1430

h

1430

h -

1630

h

1630

h -

1830

h

Sto

ma

tal c

ond

ucta

nce

(m

mo

l.m-2

.s-1

)

40

60

80

100

120

a

a

a

a

a

a

a

aa

b

b

b

Withholding irrigation for 3 weeks

Standard irrigation

Figure 5.3. Effects of irrigation treatments of B74 mango tree on leaf stomatal conductance

(mmol.m-2.s-1) during the day before harvest. If followed by different letters, means for stomatal

conductance within each time of the day are significantly different (P < 0.05) by LSD (P = 0.05).

Effects of withholding irrigation for 3 weeks on leaf relative water content and water content

Leaf relative water content and water content were not affected by the irrigation treatments (data are

presented in Appendix 4). Across the two treatments, the trees had an average relative leaf water

content of 66.8% and an average leaf water content of 39.0%.

Effects of withholding irrigation for 3 weeks on fruit gape and water potential

Fruit harvested off trees to which irrigation had been withheld generally developed a lower gape

than fruit from trees exposed to standard irrigation (Table 5.3). However, this difference was not

observed for fruit dipped in water and non-irradiated. There were no clear differences in gape

between water dipped and non-water dipped fruit. The irradiated fruit had significantly (P < 0.05) a

72

smaller gape than the irradiated fruit. The reason was that the irradiation test was applied 3 days

later, following transport from Katherine, Northern Territory to Brisbane, Queensland for

irradiation and to Nambour, Queensland for ripening and assessments.

Table 5.3. Effects of irrigation treatments on the gape (mm) of a cut and water potential (MPa) of

B74 fruit. The harvested fruit were subjected to either water dipping or not and then to either

irradiation or not. The tests were done on the day of harvest for the no-irradiation treatments, or 3

days after harvest for the irradiation treatments. If followed by different letters, means for gape or

water potential are significantly different (P < 0.05) by LSD (P = 0.05).

Parameter Treatment No water Water

No irradiation Irradiation No irradiation Irradiation

Gape (mm) Standard irrigation 1.1 e 0.4 b 1.0 d 0.4 b

Withholding irrigation 0.9 c 0.3 a 1.0 cd 0.3 a

LSD 0.05 0.08

Water potential

(MPa)

Standard irrigation -0.6 cd -1.1 ab -0.5 d -1.0 b

Withholding irrigation -0.8 c -1.2 a -0.6 d -1.2 a

LSD 0.05 0.14

There were minor significant effects of irrigation or postharvest water treatment on water potential.

Water potentials varied from -0.5 to -1.2 MPa on average across treatments (Table 5.3). The water

potential of irradiated fruit was significantly (P < 0.05) more negative than that of the non-

irradiated fruit. Similar to the gape response, the delay in testing possibly led to the differences in

water potential.

Effects of withholding irrigation for 3 weeks on relative water content of the outer pericarp

tissue

The irrigation treatments had no significant (P > 0.05) effect on the relative water content of the

outer pericarp tissue (data are presented in Appendix 5). Their values varied from 94.1 to 95.1%.

Effects of withholding irrigation for 3 weeks on lenticel damage severity

There were significant (P < 0.05) effects of irrigation treatments on LD severity on fruit at full

yellow skin and 7 days after full yellow following water dipping and irradiation (Table 5.4).

Irradiated fruit had higher LD severity than non-irradiated fruit at full colour and at 7 days after full

73

colour. Water had no effect on LD severity at full colour stage, but it increased LD severity on fruit

from the standard irrigation at 7 days later.

Table 5.4. Effects of irrigation treatments on lenticel damage severity (0 = no damage to 5 = severe

damage) on the B74 fruit. The harvested fruit were subjected to either water dipping or not and

then to either irradiation or not. If followed by different letters, means for lenticel damage severity

at full yellow or 7 days later are significantly different (P < 0.05) by LSD (P = 0.05).

Time of

assessment Treatment

Lenticel damage severity (0 - 5)

No water Water

No irradiation Irradiation No Irradiation Irradiation

At full yellow Standard irrigation 0.4 a 0.9 bc 0.5 ab 1.0 c

Withholding irrigation 0.3 a 1.1 c 0.3 a 1.1 c

LSD 0.05 0.42

At 7 days after

full yellow

Standard irrigation 0.9 a 1.5 abc 1.8 c 2.5 d

Withholding irrigation 1.1 ab 3.2 e 1.6 bc 2.9 de

LSD 0.05 0.60

Effects of withholding irrigation for 3 weeks on lenticel morphology

Only difference in lenticel morphology among irradiation treatments and no irradiation treatments

was observed. Light micrographs in Fig. 5.4 revealed more extensive browning in cells around the

lenticel cavity in irradiated fruit as compared to non-irradiated B74 fruit at full yellow. In general,

neither irrigation nor water treatment had any obvious effect on lenticel morphology.

74

Figure 5.4. Unstained transverse 5 µm-thick tissues sections of B74 fruit at full yellow of

withholding irrigation + no irradiation (A), withholding irrigation + irradiation (B), standard

irrigation + no irradiation (C), and standard irrigation + irradiation (D). Brown tissues (Bt)

surrounded the discoloured lenticel (B, D). The lenticel cavity (Lc) and resin canal (Rc) are also

evident. Scale bars represent 50 μm.

5.3.2 Experiment 2: Withholding irrigations for 4 and 8 weeks in 2013

The temperature, relative humidity and rainfall for 8 weeks before harvest in 2013

The temperature varied from 27.4 - 34.5 ºC and the average temperature difference between day and

night was from 4 - 5 ºC (Table 5.5). Average day and night temperature slightly increased from

eighth week to first week before harvest. Relative humidity was low in Katherine and it varied

from 44.0 - 63.6 %. There was little rain during the 8 weeks before harvest in 2013. Rainfall

during 3 weeks before harvest in 2013 was much lower than that in the previous year (Table 5.1).

A

C

B

D

Lc Lc

Rc

Lc Lc

Bt

Bt

75

Table 5.5. Average day and night temperatures (ºC) and relative humidity (%, RH) and the total

rainfall (mm) during 8 weeks before harvest (17th September to 12th November 2013) at the

experiment site.

Time before harvest (weeks)

8th 7th 6th 5th 4th 3rd 2nd 1st

Average day temperature (ºC) 32.0 32.7 33.2 34.2 33.5 34.3 34.5 34.4

Average night temperature (ºC) 28.4 27.4 27.9 29.7 29.0 29.4 29.4 29.2

Average day RH (%) 55.1 44.8 44.0 46.9 49.0 47.3 48.2 49.5

Average night RH (%) 63.6 49.5 52.0 52.2 58.3 57.5 58.9 61.9

Rainfall (mm) 5 0 4 2 0 3 0 3

Effects of withholding irrigations for 4 and 8 weeks on soil moisture content

There were significant (P < 0.05) differences in soil moisture content among irrigation treatments at

both sites on the commercial farm (Fig. 5.5). At site 1, withholding irrigation for 8 weeks resulted

in significant (P < 0.05) lower soil moisture content at all soil depths by 4 weeks before harvest.

Withholding irrigation for 4 weeks significantly (P < 0.05) reduced soil moisture content by 2

weeks before harvest as compared to the standard irrigation. Therefore, the 8-week irrigation

treatment resulted in at least 4 weeks of significant (P < 0.05) difference in soil moisture content as

compared to only 2 weeks for the 4-week irrigation treatment. At site 2, there was no significant (P

> 0.05) difference in soil moisture content by 2 weeks before harvest.

76

Soil m

ois

ture

con

ten

t (%

)

2

4

6

8

10

8 4 2 1 0

2

4

6

8

10

2

4

6

8

10

Standard irrigation

Withholding irrigation for 4 weeks Withholding irrigation for 8 weeks

b

b

b

b

b

a a

a

a

aaa

bb

bb

ba

a a

aaaa

bb

b b

ab

aa

a

a

aaa

Depth: 10-20 cm

Depth: 0-10 cm

Depth: 20-30 cm


Site 1

a

a

a

a

a

a

a

a

a

Site 1

Site 1


2

4

6

8

10

Soil m

ois

ture

con

ten

t (%

)

2

4

6

8

10

8 4 2 1 0

2

4

6

8

10

Standard irrigation


Depth: 10-20 cm

Depth: 0-10 cm

Depth: 20-30 cm

a

b

a

b

a

b

a

b

bb

aa

aa a

bb

b

Site 2

Site 2

Site 2

a

a

a

a

aa

a

a

a

a

a

a

Figure 5.5. Effects of irrigation treatments of B74 mango tree on soil moisture content (%) at three

soil depths from 8 weeks before harvest until harvest in 2013. If followed by different letters,

means for soil moisture content at each week of assessment in the same graph are significantly


77

Effects of withholding irrigations for 4 and 8 weeks on leaf stomatal conductance

At site 1, leaf stomatal conductance from 0900 h to 1500 h was significantly (P < 0.05) lower on

trees exposed to withholding irrigation than those treated with standard irrigation (Fig. 5.6). The

differences in leaf stomatal conductance between irrigated trees and non-irrigated trees suggested

tree leaves adapted to water deficit stress by controlling the opening of the stomata. There was no

difference in stomatal conductance between leaves on trees exposed to 4 or 8 weeks of withholding

irrigation. The similar pattern in stomatal conductance among both withholding irrigation

treatments suggests both withholding irrigations for 4 and 8 weeks had similar water stress. At site

2, trees that received withholding irrigation also had lower leaf stomatal conductance than trees

exposed to standard irrigation. These trends suggest an irrigation treatment response on tree water

status. The pattern for the standard irrigation treatment suggests relatively high tree water status.

Time of the day

700 h - 900 h

900 h - 1100 h

1100 h - 1300 h

1300 h - 1500 h

1500 h - 1700 h

1700 h - 1800 h

20

40

60

80

100

120

Standard irrigation


700 h - 900 h

900 h - 1100 h

1100 h - 1300 h

1300 h - 1500 h

1500 h -1700 h

Sto

mata

l co

nd

ucta

nce

(m

mo

l.m

-2.s

-1)

20

40

60

80

100

120

Standard irrigation

Withholding irrigation for 4 weeksWithholding irrigation for 8 weeks

b bb

a a

aa a

a

aa

aa

b

b b

b b

Site 1 Site 2

Time of the day

a

a

a

a

a

a

a

a a

Figure 5.6. Effects of irrigation treatments of B74 mango tree on stomatal conductance (mmol.m-

2.s-1) at 2 days (site 1) and 1 day (site 2) before harvest in 2013. If followed by different letters,

means for leaf stomatal conductance at each time of the day in the same graph are significantly


Effects of withholding irrigations for 4 and 8 weeks on water potential and fruit gape

Fruit from trees in which irrigation was withheld had similar water potential to fruit from standard

irrigations at both sites (Table 5.6). In contrast, there were irrigation treatment effects on gape for

both sites. A smaller gape was recorded on fruit from trees exposed to 4 weeks of withholding

irrigation at the both site 1 and site 2 suggesting a lower water status existed in these fruit.

78

Table 5.6. Effects of irrigation treatments on water potential (MPa) and gape (mm) of B74 fruit in

2013. If followed by different letters, means for water potential or gape within each column at the

same site are significantly different (P < 0.05) by LSD (P = 0.05).

Site Treatment Water potential (MPa) Gape (mm)

Site 1 Standard irrigation -1.5 ab 0.73 b

Withholding irrigation for 4 weeks -1.6 a 0.57 a

Withholding irrigation for 8 weeks -1.3 b 0.63 ab

LSD 0.05 0.22 0.11

Site 2 Standard irrigation -1.2 1.11 b

Withholding irrigation for 4 weeks -1.2 0.89 a

LSD 0.05 n.s. 0.16

Effects of withholding irrigations for 4 and 8 weeks on flesh dry matter and firmness

There were no significant (P > 0.05) irrigation treatment effects on fruit dry matter or objective

firmness at full colour and 7 days after full colour at either site 1 or site 2 (data are presented in

Appendix 6). The average dry matter at harvest was 18.4 % at site 1 and 17.6 % at site 2. The

average firmness of fruit at full colour was 21.4 and 26.3 as measured by the Aweta at site 1 and 2,

respectively. The average firmness at 7 days after full colour was 18.3 and 19.7 (Aweta) at site 1

and 2, respectively.

Effects of withholding irrigations for 4 and 8 weeks on fruit weight loss

There were significant (P < 0.05) irrigation treatment effects on fruit weight loss during ripening at

both sites (Fig. 5.7). Fruit from withholding irrigation for 8 weeks lost significantly (P < 0.05) less

weight during the middle and later stages of ripening than fruit from standard irrigation. However,

no difference between withholding irrigation for 4 weeks and standard irrigation was noted. A

similar response was observed with the irrigation treatment at site 2. However, fruit from

withholding irrigation for 4 weeks at this site had lower weight loss during the middle and later

stages of ripening than fruit from standard irrigation at the same site.

79

Time after harvest (days)

5 8 11 13 15 17

97

98

99

100

Standard irrigation


Fru

it w

eig

ht

loss (

% o

f in

itia

l fre

sh w

eig

ht)

97

98

99

100

Standard irrigation



a

b

b

b

b

b

a

aa

a

a

a

a

aa

a

a

a

a

a

b

b

b

b

b

Site 1

Site 2

aa

aa

a

Figure 5.7. Effects of irrigation treatments on fruit weight loss (% of initial fresh weight) of B74

fruit after harvest in 2013. If followed by different letters, means for fruit weight loss at each

assessment time at each graph are significantly different (P < 0.05) by LSD (P = 0.05).

Effects of withholding irrigations for 4 and 8 weeks on lenticel damage severity

Lenticel damage severity on fruit that received 4 weeks of withholding irrigation was significantly

(P < 0.05) higher than that of fruit that received standard irrigation at the both sites at full yellow

and 7 days later (Table 5.7). LD severity on fruit from trees exposed to 8 weeks of withholding

irrigation at site 1 was similar to LD severity on fruit from the standard irrigation treatment.

80

Table 5.7. Effects of irrigation treatments on lenticel damage severity (0 = no damage to 5 = severe

damage) of B74 fruit in 2013. If followed by different letters, means for lenticel damage severity

within each column at the same site are significantly different (P < 0.05) by LSD (P = 0.05).

Effects of withholding irrigations for 4 and 8 weeks on lenticel morphology

Light micrographs showed that there was irrigation effect on lenticel structure (Fig. 5.8).

Pronounced browning in lenticel tissues was observed when fruit from all treatments were

irradiated with the average dose of 409 Gy. In most cases, the browning of cells occurred in a

narrow tissue layer in lenticel cavities. In many cases, browning was observed in cells which were

not adjacent to lenticel cavities as a consequence of irradiation.

Site Treatment

Lenticel damage severity (0 - 5)

At full yellow At 7 days after full yellow

Site 1 Standard irrigation 3.2 a 3.1 a

Withholding irrigation for 4 weeks 4.1 b 4.0 b

Withholding irrigation for 8 weeks 3.4 a 3.4 a

LSD 0.05 0.51 0.42

Site 2 Standard irrigation 2.2 a 2.0 a

Withholding irrigation for 4 weeks 3.0 b 2.9 b

LSD 0.05 0.42 0.30

81

Figure 5.8. Unstained transverse 5 µm-thick tissues sections of B74 mango fruit at full yellow skin

(2013) of standard irrigation at site 1 (A); withholding irrigation for 4 weeks at site 1 (B);

withholding irrigation for 8 weeks at site 1 (C); standard irrigation at site 2 (D); and withholding

irrigation for 4 weeks at site 2 (E). Brown tissues (Bt) surrounded the discoloured lenticel (B, D).

The lenticel cavity (Lc) and resin canal (Rc) are also evident. Scale bars represent 50 μm.

5.4 Discussion

The results of this research suggest that withholding irrigation from 3 to 8 weeks before harvest did

not reduce LD severity on B74 mango fruit. Rather, withholding irrigation for 4 weeks before

harvest actually increased LD severity in some cases, as compared to standard irrigation. This

possible effect could be counteracted by stress with withholding irrigation. This response was

contrary to the findings of Cronje (2009a) and Simmons et al. (1998). They found that withholding

irrigation reduced LD severity in ripe fruit. This discrepancy of effect could be because of different

cultivars, soils and climatic conditions.

Withholding irrigation for 3, 4 or 8 weeks before harvest significantly (P < 0.05) reduced soil

moisture content during the 2 weeks immediately before harvest (Table 5.2; Fig. 5.5). This impact

may suggest that trees in which irrigation was withheld accessed less water than trees in the

standard irrigation. There was a similar pattern of leaf stomatal conductance with the peak during

the noon of the day for leaves from trees exposed to withholding and standard irrigations (Fig. 5.3;

Fig.5.6). A similar diurnal trend of stomatal conductance for KP and Irwin mango cultivar was

observed by Lu et al. (2012). Withholding irrigation during 3 to 8 weeks before harvest had an

A B C

E D

Bt

Rc

Lc

Bt

Rc

Lc

Bt Rc

Lc

Bt Rc

Lc

Bt

Lc

82

impact on stomatal conductance. Under reduced irrigation, mango trees reduced stomatal

conductance in the afternoon in Katherine. Spreer et al. (2007) also stated that mango trees react

with the limited water supply by reducing stomatal opening.

Water stress during 3, 4 or 8 weeks before harvest had no effect on leaf water status in terms of

relative water content and water content (Appendix 4). These irrigation treatments also had almost

no differences in fruit water status in terms of water potential (Table 5.3; Table 5.7), relative water

content (Appendix 5), firmness (Appendix 6), or lenticel morphology (Fig. 5.4; Fig. 5.8).

Withholding irrigation for 3 - 8 weeks before harvest may not have markedly stressed the B74 trees

and their fruit. Rainfall events before harvest possibly reduced stress more than expected.

Postharvest water dipping for 2 minutes had almost no impacts on flesh through fruit water status in

terms of fruit gape, water potential (Table 5.3), or relative water content (Appendix 5). Water

dipping only increased LD severity of fruit from standard irrigation at 7 days after full yellow

(Table 5.4). Water contact for 2 minutes may have only impact on surface fruit layers, perhaps

through lenticels. This proposition is consistent with previous studies that indicated water was a

trigger for LD (Everett et al. 2008, Oosthuyse 1998). The results confirmed previous findings of

Hofman et al. (2010b) that exposure of B74 fruit to water and irradiation after harvest increases LD.

Withholding irrigation may somehow impact fruit physiology through minimising fruit weight loss

after harvest (Fig. 5.7). It is possible that withholding irrigation decreased fruit transpiration and /

or respiration after harvest, as compared to those exposed to standard irrigation. Interestingly,

withholding irrigation before harvest had almost no effect on fruit dry matter (Appendix 6). The

unexpected lack of effect suggests a potential cost savings strategy for reducing irrigation before

harvest. However, tree performance, yield, fruit quality and LD in the season following would need

to be investigated.

Irradiation increased LD severity and brown tissues in the lenticel cavities in both withholding

irrigation and standard irrigation treatments (Table 5.4; Fig. 5.4). Different cultivars have different

browning enzyme activities (Ketsa et al. 1999, Robinson et al. 1993). PPO activity in KP is higher

than in Irwin fruit (Robinson et al. 1993). B74 is an Australian mango cultivar whose fruit are

highly sensitive to LD (Hofman et al. 2010a). It is possible that browning enzyme activities are

also high in B74 fruit, which is a hybrid of KP and Sensation.

83

Overall, fruit from trees in which irrigation had been withheld had either similar or higher LD than

fruit from the control irrigation treatment. Postharvest water dipping had little or no effects on fruit

water status. However, this postharvest treatment almost doubled LD on fruit from commercially

irrigated trees at the late ripening stage. Irradiation significantly increased LD on fruit for almost

all irrigation treatments with or without postharvest water dipping. Withholding irrigations for 3, 4

or 8 weeks before harvest had virtually no effect on leaf and fruit water status in terms of water

potential and relative water content, or on lenticel morphology. Little or no effects may suggest that

savings in energy costs could be realised by growers by withholding irrigation before harvest.

However, such a practice may lead to increased LD in fruit that are to be irradiated.

.

84

Chapter 6

EFFECTS OF POSTHARVEST HANDLING PRACTICES ON LENTICEL DAMAGE ON

B74 MANGO FRUIT

6.1 Introduction

Lenticel damage (LD) is a major fruit appearance problem in many mango cultivars (Hofman et al.

2010b). Recent studies have shown that commercial picking and packing practices can constitute

cumulative stressors that increase LD on mango fruit (Cronje 2009b, Oosthuyse 2002, Self et al.

2006). In Australia, B74 fruit picked and packed commercially are harvested from the tree, dipped

in detergent, treated with fungicide, dried under forced air, brushed and sorted on conveyors,

packed into single fruit layer trays, and ripened in due course with ethylene at around 20°C.

Commercial practices as mentioned above often increase the risk of LD on B74 fruit; especially

when the fruit reaches the ripe stage (Hofman et al. 2010a). Accordingly, the impact of key

postharvest activities, like solution contact and brushing, on LD warrants better understanding.

Exposure to water or water-based solutions; e.g. rain, pesticides, detergents, fungicides can increase

LD in many mango cultivars (Jacobi and Giles 1997, O’Hare et al. 1999, Shorter and Joyce 1998,

Willis and Duvenhage 2002). Water uptake by lenticel cells can also render lenticels vulnerable to

other external stresses, such as mechanical injuries (Self et al. 2006). Contact with water and / or

solutions is largely unavoidable in current commercial mango handling because of high labour costs

in Australia and the attendant need to achieve highly efficient picking and packing systems.

Applying solutions with osmotic potentials approximately equal to the water potential of lenticel

cells potentially may work to inhibit water uptake into cells inside the lenticel chambers. That is,

the proposition is to balance the physio-chemical potentials of solutions and fruit cells so that cells

in the lenticel cavities neither loose nor gain water. The equilibrium potential of the solution was

proposed to reduce free water entry into cells of the lenticel cavity (Cronje 2009b).

As a compatible osmoticum, polyethylene glycol (PEG) is used to establish varying osmotic

potential solutions. High molecular weight PEG is not absorbed into plant tissues, as compared to

sugars and salts (Mexal et al. 1975). The osmotic potential of individual epidermal cells is ~ -0.8

MPa (Shackel 1987). The osmotic potential of PEG 6000 can be varied from -0.2 MPa to -1.7 MPa

over concentrations of 100 to 400 g.kg-1 H2O at 25 oC (Michel and Kaufmann 1973). Thus,

85

equilibrium balance between fruit water potential and the osmotic potential of the surrounding

solution with PEG may prevent an increase in turgor pressure of cells inside lenticel cavities, and

possibly minimise LD.

Brushing is an important step in a commercial packing line as it removes pesticide residues, dust,

and debris from the mango fruit surface (Hofman et al. 2010b). However, recent studies have

shown that brushing contributes to the discolouration of lenticels on mango fruit. The surface of

mango fruit is mostly covered by an outer epicuticular wax layer (Hess and Foy 2000). Brushing

may disrupt the fruit cuticular wax layer and, thereby, permit water entry into lenticels (Du Plooy et

al. 2002, Tamjinda et al. 1992). Considered overall, optimising postharvest solution and brushing

treatments may help to reduce the risk of LD.

Given its profound commercial importance, the ways in which postharvest variables increase LD on

B74 mango fruit merit further investigation. The aim in this chapter was to document the impact on

LD of commercial practices at points along the packing line. The study also evaluated various

solution treatments and fruit brushing with a view to reduce LD. Histological observations were

used in an attempt to better characterise lenticel cavity changes at the anatomical level.


6.2.1 Fruit

B74 mango fruit were harvested at the green-mature stage (~ 14 % dry matter) from a commercial

farm at Dimbulah (17°11’S, 145°10’E), North Queensland in mid-December, 2011. Fruit free of

skin damage were selected and assigned randomly to treatments within experiments.

6.2.2 Experiments

Experiment 1. Effects of key commercial steps on lenticel damage severity, wetting angle,

lenticel external feature and morphology

This experiment aimed to investigate the effects of the main postharvest handling practices on LD.

Eight fruit were sampled at each of four sequential points during commercial handling; viz.directly

off the tree, after detergent treatment (0.25% Mango Wash® for 1 minute), after brushing, and from

the end of the commercial packing line. An extra 10 fruit were collected at the end of the

commercial packing line to study lenticel external features and morphology changes during

86

ripening. The fruit were placed into single layer cardboard trays and transported within 2 days by

plane and by car to the Maroochy Research Facilities (MRF) laboratories near Nambour in South-

East Queensland. All fruit were treated with 10 µL.L-1 ethylene at 20 °C for 2 days and then

allowed to ripen at 20 °C until fully ripe. Fruit were sampled for lenticel morphology during

ripening as described below. One hundred similar green lenticels (10 fruit) were selected and

marked on the fruit surface. During ripening, external features and morphology of lenticels were

examined over time.

Experiment 2. Effects of solution treatments on firmness, lenticel damage and morphology

The hypothesis tested was that balancing the osmotic potential of postharvest washing solutions

with that of plant cells would reduce the impact of the solutions on LD on mango fruit. Fruit were

harvested directly from 15 trees (10 fruit per tree) without exposure to water or detergent. Fifteen

other fruit harvested from the same trees were passed through a commercial packing line. These

fruit served as the ‘end packing line’ controls. After sampling, fruit were placed into single layer

trays with plastic inserts and transported by car within 30 minutes to research laboratories at

Mareeba in North Queensland. The treatments were applied within 1 hour of harvest. There were

eleven treatments in total and 15 single fruit replicates per treatment (Table 6.1).

Table 6.1. Solution treatments to investigate the effect of solutions on lenticel damage and fruit

quality attributes of B74 mango fruit. Fruit were treated with DI water, DI water +agitation, 0.1%

surfactant, NaCl at 1, 2, and 3%, and PEG 6000 at 150, 300, and 400 g.kg-1 H2O.

Treatment name Description

Off-tree control Fruit were harvested directly from the tree into trays.

End packing line

control

Fruit from the same batch were run over a commercial packing line: viz.,

treated with detergent and fungicide, brushed, dried, and packed into trays.

DI Water Off-tree, dipped in deionised (DI) water for 2 minutes (min).

DI Water agitation Off-tree, dipped in DI water plus agitation for 2 min.

Surfactant Off-tree, dipped in 0.1% surfactant (Agral®) for 2 min.

NaCl 1% Off-tree, dipped in 1% NaCl for 2 min (osmotic potential, OP = -0.8 MPa).

NaCl 2% Off-tree, dipped in 2% NaCl for 2 min (OP = -1.3 MPa).

NaCl 3% Off-tree, dipped in 3% NaCl for 2 min (OP = -2.2MPa).

PEG150 Off-tree, dipped in PEG 6000 at 150 g.kg-1 H2O for 2 min (OP = -0.3 MPa).



87

Agitation consisted of gently shaking the bucket containing fruit and DI water for 2 minutes. No

agitation was the counter treatment. PEG 6000 (class PG) and NaCl (class AR) were supplied by

Chem Supply Pty Ltd. Agral® (non-ionic surfactant) was supplied by Crop Care Australia Pty Ltd.

After treatments, the fruit were air freighted from Cairns (North Queensland) to Brisbane (South-

East Queensland) within 4 hours. Within 2 days of harvest, fruit were exposed to 600 to 800 Gy at

the Steritech Pty Ltd facility in Narangba, Brisbane. The fruit were then transported by car to the

MRF laboratories. There they were treated with ethylene and allowed to ripen as described above

in experiment 1. Before and during ripening, fruit were assessed for LD, firmness, and lenticel

morphology.

Experiment 3. Effects of brushing treatments on lenticel damage

The objective of this experiment was to investigate effect of various brushing treatments on LD on

B74 mango fruit while aiming to ensure a shiny lustre. A total 225 fruit at ~ 14 % dry matter were

collected from afield bin after harvest from 10 trees on the same farm and day as for experiment 2.

Fruit from field bins had been commercially picked and dipped in a 0.25% Mango Wash® detergent

solution ) for 1 minute. However, they had not entered the packing line for fungicide treatment, air

drying, brushing, or packing. An extra 25 fruit were collected from the same bin and were passed

through the commercial packing line. Fruit were placed into single layer trays with plastic inserts

and then transported by car to Cairns airport within 3 hours. Fruit were air freighted to Brisbane

and then taken by car to MRF. They were brushed for 1 or 2 minutes with a brushing unit

(LenzeACTech Ltd., Bundaberg, Queensland, Australia) at 84 revolutions.minute-1 with either soft

brush (blue poly propylene bristles) or hard brush (black nylon bristles) with or without water spray

as described in Table 6.2.

88

Table 6.2. Brushing treatments to investigate the effects of brushing with or without water spray

for 1 or 2 minutes on lenticel damage and fruit quality attributes of B74 mango fruit.

Treatment name Description

Field bin

Fruit collected from the field bin after dipping in detergent without

further treatment.

End packing line

Fruit from the same batch were run over the commercial packing line

(dipped in detergent, treated with fungicide, brushed, dried by

forced air, and packed into single layer trays with plastic inserts).

Soft brush 1 min dry Fruit were brushed with soft brush for 1 min with no water spray.

Soft brush 2 min dry Fruit were brushed with soft brush for 2 min with no water spray.

Soft brush 1 min wet Fruit were brushed with soft brush for 1 min with water spray.

Soft brush 2 min wet Fruit were brushed with soft brush for 2 min with water spray.

Hard brush 1 min dry Fruit were brushed with hard brush for 1 min with no water spray.

Hard brush 2 min dry Fruit were brushed with hard brush for 2 min with no water spray.

Hard brush 1 min wet Fruit were brushed with hard brush for 1 min with water spray.

Hard brush 2 min wet Fruit were brushed with hard brush for 2 min with water spray.

Immediately after brushing, 10 fruit per treatment were used to measure wetting angle and to assess

skin gloss. They were then used for dye uptake. Another 15 fruit per treatment were treated with

ethylene and allowed to ripen as per experiment 1 and then assessed for quality.

6.2.3 Assessments


Lenticel damage severity on each fruit was rated at full colour on a 0 - 5 scale based on both the

degree of damage of the lenticels and the overall area of skin affected. Rating scales for LD

severity on the skin of B74 fruit was described in Table 3.1 (Hofman et al. 2010b).

Subjective firmness

Subjective firmness was rated for individual fruit using hand pressure with the scale of 0 - 4 (0:

hard, no ‘give’ in the fruit; 1: rubbery, slight ‘give’ in the fruit; 2: sprung, flesh deforms by 2 - 3

mm with extreme thumb pressure; 3: firm soft, whole fruit deforms with moderate hand pressure;

and 4: soft, whole fruit deforms with slight hand pressure (Hofman et al. 2010b).

89

Objective firmness

The firmness of individual fruit was measured was described in chapter 5 (the section on Objective

firmness in 5.2.3 Assessment).

Wetting angle

Wetting angle was measured to assess the capacity of a water droplet to wet the cuticle and enter

into a stomata / lenticel cavity (Eichert and Burkhardt 2001, Peschel et al. 2003, Ruckenstei and

Lee 1975, Schonherr and Bukovac 1971). The wetting angles of 5 μL DI water droplets on the fruit

surface were measured in the middle between the two ‘cheeks’ and at two other points around the

equator of each fruit. Fruit were firmly positioned, with the selected test point in the horizontal

direction and the droplet carefully placed on the fruit. At equilibrium, the contact angle between the

droplet and the fruit surface was photographed with a Canon DOS40D camera fitted with a Canon

macro-lens EF-S 60 mm (Canon Inc., Tokyo, Japan). The contact angles (ɤ) from the images were

measured using Image-J software version 1.48 (National Institutes of Health, Maryland, USA). The

contact angles were measured between tangent and droplet base (Fig. 6.1; Lamour and Hamraoui

2010, Peschel et al. 2003).

Figure 6.1. A wetting angle of a 5 μL DI water droplet on the surface of a mango fruit

Skin colour

Skin colour of individual fruit was rated as the proportion (%) of yellow area on fruit surface

(excluding the red blush area) using a 1 - 6 scale: 1: 0 - 10%, 2: 11 - 30%, 3: 31 - 50%, 4: 51 - 70%,

5: 71 - 90% and 6: 91 - 100% ( Hofman et al. 2010b). A rating of 6 indicates full yellow colour and

is the stage at which mature B74 fruit reach an acceptable eating quality.

Skin browning

Skin browning was assessed for individual fruit using a 1 - 6 scale: 0: nil, 1: less than 1 cm², 2:

between 1 - 3 cm², 3: between 3 - 12 cm², 4: from 12 cm² (about 10%) to 25%, and 5: more than

25% of the surface (Hofman et al. 2010b).

ɤ

90

Glossiness

Glossiness or lustre was visually assessed by rating the proportion of the fruit surface area with a

glossy appearance using a 1 - 6 scale: 1: less than 10%, 2: between 11 - 30%, 3: between 31 - 50%,

4: between 51 - 70%, 5: between 71 - 90%; and 6: over 90 of the skin surface with glossiness.

Dyed lenticel index and blotchy dye index

Fruit were submerged in a 1 g.L-1 Brilliant Blue dye solution (All Colour Supplies Pty Ltd., New

South Wales, Australia) at 25°C for the proxy assessment of water entry into lenticels (dyed lenticel

index) and infiltration into through cuticle (blotchy dye index), The dye bath with the fruit was

then moved into a 12 °C cold room for 16 hours. Thereupon, the fruit were rinsed three times in

running tap water and blotted dry with paper towels. The dyed lenticel index was rated using a 1 - 6

scale: 1: less than 10%; 2: between 11 - 30%; 3: between 31 - 50%; 4: between 51 - 70%; 4:

between 71 - 90%; and 6: over 90% of fruit surface area with dye lenticels. The extent of blotchy

dye index (diameter > 1 mm) was determined using a 1 - 6 scale: 1: less than 10%; 2: between 11 -

30%; 3: between 31 - 50%; 4: between 51 - 70%; 4: between 71 - 90%; and 6: over 90% of fruit

surface area with blotchy dye.

Lenticel external feature

The diameters of lenticel openings were quantified in sections from skin explants comprised of the

epidermis and exocarp tissue. Explants (~ 1 mm depth) containing lenticels were excised using a

surgical blade (Swann-Morton Co. Ltd., Shelfield, England) from five random positions on each of

five fruit. The thin sections were placed onto a glass slide with the skin surface facing up to the

objective lens. Plan features were viewed under the dissecting light microscopy (TYP 376788

Wild, Heerbrugg, Switzerland) and photographed using a digital camera (Canon DOS40D, Canon

Inc., Tokyo, Japan).

Lenticel morphology


The examination of fresh tissues and fixed tissues were described in chapter 3 (the section on

Lenticels morphology in 3.2.3 Assessment).


Data were analysed using one-way ANOVAs using the ‘General Analysis of Variance’ model of

Genstat 16 for Windows (VSN International Ltd., Hertfordshire, UK) for sampling points,

91

solutions treatments or brushing treatments as the treatment structures. The protected least

significant difference (LSD) procedure at P = 0.05 was used to test for differences between

treatment means for skin colour, objective firmness, wetting angle, glossiness, lenticel damage

severity and dyed lenticel index.

6.3 Results

6.3.1 Effects of commercial operations

Effects of commercial packing line steps on wetting angles and lenticel damage severity

Wetting angles on fruit decreased and LD severity increased significantly (P < 0.05) with

harvesting and packing activities along the commercial packing line (Table 6.3). With a lower

wetting angle, water droplets can spread more readily over the fruit surface. This wetting can

facilitate water infiltration into lenticel cavities.

Table 6.3. Effects of various serial commercial packing line steps on wetting angle (°) of 5 µl DI

water droplets on the fruit surface at harvest and lenticel damage severity (0 = no damage to 5 =

severe damage) of B74 fruit at full yellow. If followed by different letters, treatment means for

wetting angle or lenticel damage severity within each column are significantly different (P < 0.05)

by LSD (P = 0.05).

Fruit sampling point Wetting angle (°) Lenticel damage severity (0 - 5) at full yellow

Off-tree 117.4 d 0.6 a

After detergent treatment 105.1 c 0.7 ab

After brushing 100.1 b 1.1 b

End of the packing line 91.9 a 1.7 c

LSD 0.05 4.10 0.48

Typical sections through lenticels tissues confirmed the increase in LD on the fruit sampled from

the tree, after detergent treatment, after brushing and at the end of the commercial packing line (Fig.

6.2). Slightly brown tissues in and around the lenticel cavity were observed on the fruit harvested

directly from the tree. Darker brown tissues were observed on fruit collected after brushing. The

darkest brown tissues were found on fruit collected at the end of a commercial packing line.

92

Figure 6.2. Unstained transverse tissues sections of B74 mango fruit at full yellow skin. Fruit

were sampled along a commercial packing line at directly off the tree (A), after detergent treatment

(B), after brushing (C), and after packing (D). Lenticel morphology was observed following the

hand sectioning method. Brown tissues (Bt) surrounded the discoloured lenticel. Scale bars

represent 50 μm.

Changes in lenticel external appearance and morphology during ripening

No signs of discolouration were evident on selected lenticels during initial stages of ripening (Fig.

6.3). During the later stages of ripening, brown colour developed around the lenticel cavity. At 6

days after full colour, the discolouration of lenticels became obvious on the fruit surface.

A B

C D

Bt Bt

Bt Bt

93

Figure 6.3. Changes in appearance of the same lenticel from 2 days to 15 days after harvest on a

typical B74 fruit picked and packed under commercial conditions. Lenticels at 2 days (A); 7 days

(B); 9 days (full yellow, C); 12 days (D); and 15 days after harvest (E). Brown tissues (Bt)

surrounding discolouring lenticels. Scale bars represent 50 μm.

Tissue sections through lenticels affirmed very little brown pigment in the surrounding cells at 2

days after harvest (Fig. 6.4). During ripening, there was an expansion of darkened tissue around

lenticels.

Figure 6.4. Changes in lenticel morphology of typical lenticels from 2 days to 15 days after harvest

on a B74 fruit. The fruit were picked and packed under commercial conditions. Unstained

transverse 5 µm-thick tissue sections were sampled at: 2 days (A); 7 days (B); 9 days (full yellow,

C); 12 days (D); and, 15 days after harvest (E). Brown tissues (Bt) form around the discolouring

lenticels (B, C, D, E). Scale bar present 50 μm.

6.3.2 Effects of solution treatments

Effect of solution treatments on lenticel damage severity and firmness of fruit

LD severity on fruit from the off-tree control and the DI water treatment was relatively low at both

full yellow and 7 days later (Table 6.4). However, the DI water plus agitation treatment resulted in

higher LD severity on fruit at full yellow as compared to either the off-tree control (no water

contact) or fruit treated with water without agitation. Fruit exposed to the surfactant solution

displayed greater LD severity. Fruit collected at the end of the commercial packing line had

significantly (P < 0.05) higher again LD at full yellow and at 7 days after full yellow as compared

A B C E D

A B C E D

Bt Bt Bt Bt

Bt Bt

Bt Bt Bt

94

to fruit harvested directly from the tree. At the full colour stage, fruit that received a 1% NaCl

solution treatment had similar LD severity to fruit that received a DI water treatment with agitation

and further increasing NaCl concentrations worsened LD.

Table 6.4. Effect of solution treatments to B74 mango fruit on lenticel damage severity (0 = no

damage to 5 = severe damage), subjective firmness (0 = hard to 4 = soft) and objective firmness

(Aweta reading). If followed by different letters, means for lenticel damage severity, subjective

firmness, or objective firmness within each column are significantly different (P < 0.05) by LSD (P

= 0.05).

Treatment

Lenticel damage

severity (0 - 5)

Subjective firmness

(0 - 4)

Objective firmness

(Aweta)

At full

yellow

At 7 days after

full yellow

At full

yellow

At 7 days after

full yellow

At full

yellow

At 9 days after

full yellow

Off-tree control 0.8 ab 2.1 a 2.5 3.8 cd 20.0 bc 13.6

End packing line control 3.2 f 3.2 b 2.8 3.9 d 22.3 bc 16.9

DI Water 0.6 a 1.6 a 2.6 3.8 cd 20.3 abc 15.4

DI Water+agitation 1.2 c 2.0 a 2.7 3.7 bcd 16.4 a 16.5

Surfactant 0.1% 2.1 e 2.8 b 2.6 3.7 bcd 23.5 c 15.9

NaCl 1% 1.2 bc 2.0 a 2.6 3.7 bcd 22.6 bc 17.0

NaCl 2% 1.7 d 2.1 a 2.8 3.5 ab 23.0 c 18.1

NaCl 3% 2.0 de 2.1 a 2.6 3.5 ab 18.8 ab 16.7

PEG150 3.6 g 3.9 c 2.5 3.6 abc 21.8 bc 20.3

PEG300 4.0 g 4.3 cd 2.7 3.6 abcd 23.5 c 17.2

PEG400 4.0 g 4.5 d 2.6 3.5 a 21.8 bc 18.6

LSD 0.05 0.44 0.52 n.s. 0.24 4.07 n.s.

All PEG treatments resulted in much higher LD severity at full colour and at 7 days after full colour

as compared to water or NaCl treatments or the controls (Table 6.4). Sticky PEG residue was

observed on the fruit surface for all three PEG treatments. At 7 days after full colour, no significant

(P > 0.05) treatment differences were evident among the off-tree control, water, and NaCl

treatments. However, the surfactant and PEG treated fruit still had higher LD severity than did DI

water or NaCl treated fruit. There was no treatment effect on objective firmness at full colour.

However, there were significant (P < 0.05) differences in subjective firmness at 7 days later. When

95

measured by Aweta, there were significant (P < 0.05) differences at full yellow, but not at 9 days

later. In general, the effects of the solution treatments on objective firmness were inconsistent.

Effect of solution treatments on lenticel morphology

Lenticels on fruit from the off-tree control, DI water, and 1% NaCl treatments had almost no brown

cells in their cavities (Fig. 6.5). Fruit sampled from the packing line control displayed greater areas

of discoloured tissue than did fruit from either the off-tree control or water treatments. The

surfactant treated fruit had more damaged cells than did fruit from the off-tree control and water

treatments. The PEG treatments resulted in highly darkened tissue around the lenticel.

96

Figure 6.5. Unstained transverse 5 µm-thick tissue sections of lenticels on B74 mango fruit at full

yellow for the treatments: off-tree control (A), end of packing line control (B), DI water (C),

surfactant 0.1% (D), NaCl 1% (E), NaCl 3% (F), PEG 150 g.kg-1 H2O (G), and PEG 400 g.kg-1

H2O (H). Brown tissues (Bt) surrounded discoloured lenticels (B, D, E, G, H). Scale bars

represent 50 μm.

G H

A B

C D

E F

Bt

Bt

Bt

Bt

Bt

97

6.3.3 Effect of brushing treatments

Effect of brushing on lenticel damage severity, glossiness and dyed lenticel index

At full colour, LD severity on fruit from all brushing treatments was similar to that of the field bin

control. Moreover, it was lower than that for the end packing line control (Table 6.6). At 7 days

after full colour, most brushed fruit had similar or lower LD severity to fruit from the field bin

control. The exception was for fruit exposed to hard brushing without water for 2 minutes, which

elicited higher LD severity than seen in the field bin control. Fruit from the end of packing line

control had the highest LD severity at both full yellow and 7 days after full colour. There was no

consistent evidence that the addition of water with brushing, the type of brushing (i.e. soft, hard),

and / or duration (i.e. 1 or 2 minutes) increased LD severity. Soft brushing with or without water

for 1 or 2 minutes and hard brushing with water for 1 minute were observed to give adequate

chemical residue or dust removal and polishing without increasing LD severity.

In general, brushed fruit had either a similar or higher degree of glossiness than did commercial

control fruit sampled from the end of a commercial packing line (Table 6.5). Without brushing,

fruit from the untreated field bin control had the lowest level of glossiness. In most brushing

treatments, hard brushing resulted in fruit with higher gloss than did soft brushing. A longer

duration (1 or 2 minutes) of brushing had no effect on glossiness in most treatments, except with

soft dry brushes; which increased fruit glossiness.

There were significant (P < 0.05) brushing effects on dyed lenticel index and blotchy dye index

(Table 6.5). Longer brushing increased both dyed lenticel index and blotchy dye index. Exposure

of fruit to water increased the blotchy dye index when accompanied by hard brushing for 2 minutes

and soft brushing for 1 minute. However, exposure of fruit to water alone had no effect on dyed

lenticel index.

98

Table 6.5. Effects of brushing treatments to B74 mango fruit on lenticel damage severity (0 = no

damage to 5 = severe damage), skin glossiness (1 = less glossiness to 6 = almost glossiness), dyed

lenticel index (1 = almost no dyed lenticels to 6 = almost dyed lenticels), and blotchy dye (1 =

almost no blotchy area to 6 = almost blotchy area). If followed by different letters, means for

lenticel damage severity, glossiness, dyed lenticel index, or blotchy dye index within each column

are significantly different (P < 0.05) by LSD (P = 0.05).

Treatment

Lenticel damage severity (0 - 5) Glossiness

(1 - 6)

at harvest

Dyed lenticel

index

(1 - 6)

Blotchy

dye index

(1 - 6)

At full

yellow

At 7 days after

full yellow

Field bin (untreated) 1.8 ab 3.1 bc 0.6 a 4.1 abc 4.7 cd

End packing line 3.2 c 4.6 e 2.7 b 4.9 def 4.1 bc

Soft dry brushing 1 min 1.3 a 2.5 a 2.9 b 4.3 bcd 3.4 a

Soft dry brushing 2 min 1.8 ab 2.7 ab 4.6 cd 5.2 ef 5.4 e

Soft wet brushing 1 min 1.7 ab 3.1 bc 3.0 b 3.6 ab 4.2 bc

Soft wet brushing 2 min 1.7 ab 3.0 abc 3.4 b 4.5 cde 5.1 de

Hard dry brushing 1 min 2.1 b 3.4 cd 4.6 cd 3.4 a 3.7 ab

Hard dry brushing 2 min 1.8 ab 2.5 a 5.1 d 4.6 cdef 4.1 bc

Hard wet brushing 1 min 1.9 ab 3.2 bcd 4.3 c 4.1 abc 4.3 bc

Hard wet brushing 2 min 1.9 ab 3.7 d 4.5 cd 5.3 f 5.7 e

LSD 0.05 0.64 0.53 0.47 0.77 0.69

Effect of brushing treatments on skin browning and wetting angle

There were effects of the brushing treatments on skin browning and wetting angle (Table 6.6). Soft

brushing caused similar skin browning severity to fruit sampled from the end of packing line and

assessed at full colour and at 7 days after full colour. Hard dry brushing of fruit produced the most

severe skin browning of the treatments in fruit at full colour. Hard wet brushing resulted in similar

skin browning severity to the two controls. Longer duration of brushing did not increase skin

browning at full colour.

99

Table 6.6. Effects of brushing treatments to B74 mango fruit on wetting angle (°) of 5 µl DI water

on the fruit surface and skin browning (0 = no skin browning to 5 = severe skin browning) at full

yellow (eating ripe) and at 7 days after full colour. If followed by different letters, means of wetting

angle or skin browning within each column are significantly different (P < 0.05) by LSD (P = 0.05).

Treatment

Wetting angle (°) of DI water

droplet on the fruit surface

Skin browning (0 - 5)

At full colour At 7 days after full colour

Field bin (untreated) 104.2 d 0.5 a 1.6 ab

End packing line 98.9 c 0.8 ab 1.8 abc

Soft dry brushing 1 min 95.6 b 1.1 abc 1.4 a

Soft dry brushing 2 min 89.6 a 1.0 abc 1.8 abc

Soft wet brushing 1 min 94.4 b 0.6 a 1.3 a

Soft wet brushing 2 min 98.3 c 0.7 ab 2.5 cd

Hard dry brushing 1 min 89.4 a 1.5 c 2.7 d

Hard dry brushing 2 min 93.2 b 1.5 c 1.8 abc

Hard wet brushing 1 min 94.8 b 0.8 ab 1.9 abcd

Hard wet brushing 2 min 99.7 c 0.7 ab 2.3 bcd

LSD 0.05 2.57 0.61 0.81

The wetting angle formed by 5 µL droplets of water on fruit from all brushing treatments was lower

than that on fruit from the field bin control (Table 6.7). There were no clear differences in the

wetting angle on fruit exposed to hard and soft brushes. A longer duration of brushing had

inconsistent effects on wetting angle. Brushing plus water increased the wetting angle as compared

to brushing without water in most cases, except for soft brush for 1 minute. The end packing line

positive control resulted in lower wetting angle as compared to field bin negative control.

Effect of brushing treatments on lenticel morphology

The different brushing treatments did not affect the structure of lenticels (Fig. 6.6). Lenticels on

fruit from hard dry brushing for 1 or 2 minutes and soft dry brushing for 1 or 2 minutes had similar

degrees of browned tissue in their cavities.

100

Figure 6.6. Unstained transverse sections of B74 mango fruit at 7 days after full yellow skin

(eating soft) for treatments of: hard dry brushing for 1 minute (A), hard wet brushing for 1 minute

(B), soft dry brushing for 1 minute (C), and soft wet brushing for 1 minute (D). Tissues were fixed

in fixative solution (FAA) and embedded in paraffin wax. Brown tissues (Bt) around discoloured

lenticels (A, B, C, D). Scale bars represent 50 μm.

6.4 Discussion

Various studies indicate that exposure of mango fruit to commercial picking and packing practices

can increase LD (Cronje 2009b, Hofman et al. 2010a, Oosthuyse 2002, Self et al. 2006). In the

current study, postharvest operations in a commercial packing line tended to reduce the wetting

angle of DI water on the surface of B74 mango fruit (Table 6.3). This measure suggests that

wettability of the fruit was increased by commercial handling practices. Du Plooy et al. (2002)

showed that commercial packing house operations caused irreversible changes to the superficial

structure on the surface of Kent mango fruit. Such changes may well lead to a reduced wetting

angle of water or aqueous solutions on fruit and, thereby, increased potential for water to enter

lenticels. In turn, hydration may increase cell turgor and probably increase sensitivity of lenticels

to other stressors known to exacerbate LD severity (Self et al. 2006). This proposition can explain

A

C

B

D

Bt

Bt

Bt Bt

101

why LD severity on B74 mango fruit was increased by commercial harvest and postharvest

handling activities (Table 6.3). The proposed mechanism of action accounts for the results of

Hofman et al. (2010a) in that B74 fruit collected from the end of commercial pack line had 2 - 3

times higher LD severity than did fruit harvested directly from the tree; i.e., without postharvest

treatments. Cronje (2009b) and Dietz et al. (1988b) also ascertained that packing house operations

increased LD on mango fruit. In the present study, tissue sections through lenticels revealed

serially progressive browning of cells surrounding lenticel cavities with passage over the packing

line (Fig. 6.2; Fig. 6.3; Fig. 6.4). Self et al. (2006) also observed that commercial postharvest

operations increased LD in an accumulative manner. Tissue browning is likely a consequence of

oxidation of polyphenols in cells surrounding lenticel cavities (Du Plooy et al. 2006). These

responsive cell layers may play a role as a protective barrier in their hypersensitive response

against external stressors (Du Plooy et al. 2004, Tamjinda et al. 1992).

The contact between water or solutions and mango fruit is unavoidable in current Australian

commercial mango handling practices. A range of recent studies have affirmed that water

contributes to increased LD severity on mango fruit (Hofman et al. 2010b, O'Hare and Prasad 1992,

Self et al. 2006, Whiley et al. 2006, Willis and Duvenhage 2002), and also on avocado fruit (Everett

et al. 2008). Water dipping alone without agitation or any postharvest operations did not increase

LD severity on B74 fruit (Table 6.4). However, water dipping plus agitation increased LD severity

at full colour as compared to control fruit harvested directly off the tree. Agitation may remove air

entrapped over and / or inside lenticel cavities and thereby enable water to more easily penetrate

into the lenticels. In this same vein, adding a surfactant increased LD severity on B74 fruit more

than did water treatments or with the off tree control. The surfactant likely reduced the liquid

surface tension and increased wettability of fruit so as to facilitate water entry into lenticels and

increase the turgor of cells lining the cavities. However, it is also possible that chemicals in the

surfactant (Agral) may directly damage cell membranes. As with previous studies cited above, fruit

sampled from the end packing line had high LD severity.

Treatment of B74 fruit with NaCl at 1% resulted in a similar level (P < 0.05) of LD severity as for

the off-tree control treatment (Table 6.4). This similarity suggests that the 1% NaCl solution had no

or little effect on cell turgor within lenticel cavities presumably because the osmotic potential of this

solution (~ -0.8 MPa at 25 ºC) was similar to the water or osmotic potential of fruit cells.

Treatments with 2 and 3% NaCl involve lower osmotic potentials than those expected in cells

within lenticel cavities. Lower osmotic potential solutions are expected to draw water from cells.

Osmotic dehydration evidently caused a relatively higher severity of LD on B74 at full colour.

102

Treatment with all PEG concentrations resulted in significantly (P < 0.05) higher LD severity on

B74 relative to the untreated control and the end packing line control. PEG 6000 at high

concentration has a low osmotic pressure that potentially withdraws water from cells (Mexal et al.

1975). Moreover, PEG treatment resulted in viscous residue on the fruit surface. Hydrated residues

with low osmotic potentials may absorb moisture from the air environment and / or, more likely, the

fruit during fruit ripening of 20 ºC. Mango fruit have a water content of ~ 85% (Hofman et al.

2010b). This proposition constitutes another stress on lenticels that causes LD. Alternatively or as

well, Na+ and Cl- from NaCl and possibly PEG6000, should it enter cells, may be phytotoxic and,

therefore, elicit LD as chemical stressors.

In general, there was little difference among brushing treatments in regard to LD severity on B74

fruit (Table 6.5). At full colour, fruit exposed to either wet or dry brushing treatments for either 1

or 2 minutes developed LD at similar levels to that seen for fruit from the field bin control. At 7

days after full colour, only the treatment of a hard wet brushing for 2 minutes increased LD severity

as compared to the field bin control with no brushing. This observation is contrary to that of

Hofman et al. (2010b). They found that B74 fruit picked from the tree and exposed only to the

brushing step on a commercial packing line had more LD severity than did fruit not exposed to

brushing. In this study, the duration of soft brushing did not affect LD severity and skin browning

on B74 fruit. Similarly, brushing of Keitt mango fruit for 30 seconds and for 2 or 5 minutes after

wetting them with water did not increase LD (Cronje 2009b). The brushing treatment in the present

study was similar to a commercial brushing operation in terms of brush types with either soft brush

(blue poly propylene bristles or black nylon bristles), brush rotation speeds (84 revolutions.minute-

1) and durations from 1 - 2 minutes. However, it is possible that brushes employed under real world

industry conditions become more abrasive from wear and contaminants in the packing line. All

brushing treatments employed in the present study produced fruit with similar or greater glossiness

to fruit sampled from the end packing line (end packing line control; Table 6.5). This similarity

indicated that all brushing treatments in this study were sufficient to meet industry expectations of

glossiness without increased LD severity on fruit at full colour. Nonetheless, there was an

increased risk of skin browning with brushing, especially with hard dry brushing (Table 6.6).

Therefore, treatment with soft brushes for 1 or 2 minutes with or without water or treatment with

hard wet brushes for 1 minute are relatively safe for removing residues and polishing the fruit

without increasing either LD or skin browning.

103

Overall, the results confirmed the negative impacts of selected commercial postharvest activities in

increasing LD severity on B74 fruit. Water plus agitation and aqueous solutions, which had a more

negative osmotic potential than the typical osmotic potential of plant cells, also increased LD

severity. PEG, non-ionic osmoticum, increased LD severity as it possibly created high moisture

levels in and over the lenticels. Brushing with or without water for 1 or 2 minutes had no effect on

the severity of LD that developed on B74 mango fruit at the ripe stage. At 7 days after, only fruit

exposed to hard brushing with no water for 2 minutes had higher LD severity than the field bin

control fruit. Soft brushing ± water for 1 or 2 minutes and hard wet brushing for 1 minute ensured

adequate residue removal, and polishing without increasing LD severity or skin browning.

104

Chapter 7

EFFICACY OF BAGGING AND COATING TREATMENTS TO REDUCE LENTICEL

DAMAGE ON B74 AND CAT CHU MANGO FRUIT

7.1 Introduction

Mango (Mangifera indica L.) is a tropical fruit crop of considerable economic importance

(Sivakumar et al. 2011). It is widely cultivated in many countries, including in Australia and

Vietnam (Do et al. 2006, Johnson 2000). In 2011, ~ 38,500 tonnes of fresh mango fruit were sold

on the Australian markets (AMIA 2011). In the same year, ~ 596,000 tonnes of mango fruit were

produced in Vietnam on ~ 88,000 hectares (VietnamStat 2011). The annual mango harvest season

is from October to March in Australia and from March to May in Vietnam. B74 fruit have a highly

attractive appearance with red blush (Hofman et al. 2010a). However, they are very sensitive to

lenticel damage (LD). In Vietnam, Cat Chu is the major commercial mango cultivar. Its fruit are

typically high quality with good flavour (Do et al. 2006). LD has also been reported to strongly

afflict this cultivar (Do et al. 2006). Recent studies showed that exposure of mango fruit to water

based solutions during the pre-harvest and postharvest periods can increase LD expression (Hofman

et al. 2005, Rymbai et al. 2012, Tamjinda et al. 1992).

Covering mango fruit with plastic or paper bags before harvest may reduce fruit contact with water

and, thereby, reduce LD. Bagging of mango fruit has been studied mainly to reduce diseases

(Hofman et al. 1997, Oosthuyse 2007a). Also, placing plastic bags over Namdokmai fruit was

reported to improve fruit skin glossiness (Chonhenchob et al. 2010). Plastic bagging was shown to

increase postharvest weight loss and shrivelling and to shorten the storage life of Sensation mango

fruit (Joyce et al. 1997). In contrast, paper bagging had no clear effects on either fruit weight or

quality of Keitt (Hofman et al. 1997) or Sensation (Joyce et al. 1997) mangoes. Moreover, effects

on LD were not reported. Fruit bags were considered by Joyce et al. (1997) to create a barrier to

water vapour transfer and so reduce transpiration by the fruit. By preventing physical damage to

fruit (e.g. branch rub), bagging can prevent damage of the fruit surface wax layer and, thereby,

reduce effects of rain on lenticels. Hence, covering fruit with paper bags should minimise rainwater

contact and, consequently, reduce LD.

105

Surface coatings applied to fruits either before or after harvest may also reduce the entry of water

into lenticels. Pre-harvest coatings with 2.5% EpiShield™ (an emulsifiable mixture of plant

extracts and vegetable esters), or 5% NaturalShine™ 9000 (a carnauba wax) reduced LD on Gala

apples by 50 - 70% after 3 months of cold storage (Curry et al. 2008). Similarly, immersing

Tommy Atkins mango fruit in 100% PolymerCoat at 6 weeks before harvest reduced LD

(Oosthuyse 2007b). Lipophilic coatings may reduce LD by covering micro-cracks in the cuticle.

For example, four weekly applications before harvest with 5% RainGard™, a cracking suppressant,

significantly (P < 0.05) reduced cuticle cracking in Bing cherry fruit (Schrader and Sun 2006).

Coating Fuji apple fruit with Raynox®, a sunburn protectant, also reduced LD, presumably by

reflecting solar radiation and reducing heat on the fruit surface (Schrader et al. 2008).

It may, however, be more commercially efficient in terms of time and labour to apply coatings

postharvest rather than pre-harvest. Postharvest coatings with carnauba wax, pectin-based

emulsion, or chitosan were shown to improve appearance, delay colour development and prolong

the storage life of Ataulfo, KP and Tainong mango fruit (Dang et al. 2008, Moalemiyan et al. 2010,

Zhu et al. 2008b). Postharvest coating with Natural Shine™ TFC210, an emulsifiable carnauba

wax, reduced weight loss and firmness of Keitt mango fruit (Baez-Sañudo et al. 2005). Oosthuyse

(1998) found that coating with shellac reduced LD on Sensation mango fruit after 4 weeks of

storage at 12.5 ºC. The effectiveness of fruit coatings to reduce LD may be influenced by their

ability to coat cells lining the lenticel cavity. Increasing the wax layer covering lenticel cavity cells

has been associated with reduced LD in mango fruit (Du Plooy et al. 2009b). The capacity of

coating compounds to enter into the lenticel cavity may be affected by wettability of the solutions

on the fruit surface. Wettability is a significant factor influencing liquid penetration into stomata

(Schonherr and Bukovac 1971) and, presumably, similar cavities; including lenticels on the fruit

skin. Hence, addition of surfactants to coating solutions could improve wettability by the solutions

and potentially improve their entry into lenticel cavities.

In the present study, the efficacy of pre-harvest bagging and coating treatments to reduce water

contact and LD on B74 mangoes was investigated. Postharvest coating treatments were also tested

for their capacity to reduce LD on B74 and Cat Chu mango fruit. The potential for these treatments

to enhance the wax layer within the lenticel cavity was examined. The effects of treatments on LD

and fruit quality attributes were assessed using histology and dye infiltration.

106


7.2.1 Fruit

For pre-harvest bagging and coatings, eight year old mango (Mangifera indica) B74 trees growing

on a commercial farm near Childers (25°14’S, 152°37’E) in South-East Queensland were used

during the 2011/12, 2012/13 and 2013/14 seasons. Ten trees with similar canopy size and fruit

yields in two adjacent rows were selected in each season. Thirty fruit hanging in full sunlight

outside the canopies over all orientations and with no skin damage or other defects were otherwise

randomly selected from each tree in early December, this being ~ 2 months before the predicted

commercial harvest date. In each fruiting season, mango trees were utilised from the same two

rows.

For postharvest coatings, 120 green-mature B74 fruit with the criteria described above were

harvested from 10 trees (20 fruit per tree) at a commercial farm near Katherine (14°60’S,

132°00’E), Northern Territory in November, 2014. In Vietnam, 60 green-mature Cat Chu mango

fruit were collected at the Long Bien wholesale market in Hanoi, Vietnam, at 4 days after harvest.

7.2.2 Fruit bags and coating compounds

For the pre-harvest bagging treatment, Japanese made commercial white paper fruit bags with

dimensions of 220 mm wide x 310 mm high were used. The bags were distributed by Palmwoods

Farm and Garden Supplies, Australia. For on-tree fruit coatings over the three seasons, Natural

Shine™ TFC210 (TFC, a carnauba-based coating for humidity and breakdown resistance),

RainGard™ (RainGard, a cracking suppressant) and Raynox® (Raynox, a carnauba-based coating

for UV protection) were tested. All coatings are manufactured by Pace International Ltd., USA and

distributed by Colin Campbell Chemicals Ltd., Australia. In the 2013/14 season, a surfactant

(Maxx Organosilicone Surfactant™, Sumitomo Chemical Australia Ltd., Australia) was added into

TFC solutions to improve on-tree coating. In the 2014/15 season, TFC at various concentrations

plus the surfactant were tested as a postharvest treatment.

107

7.2.3 Treatments

The hypothesis tested in this study was that preventing fruit contact with water by bagging or

coating treatments would reduce LD on mango fruit. The research was carried out over four mango

harvest seasons. The treatments that reduced LD in the previous year were re-tested in the

following year with some modification.

Experiment 1: Pre-harvest bagging and coatings in 2011/12 season

The effects of bagging individual fruit with a white paper bag at 2 months before harvest and of

coatings with different TFC concentrations and application times on LD on B74 fruit were

examined. The pre-harvest treatments were: (i) non-bagged or non-coated control; (ii) bagging at 2

months before commercial harvest; (iii) coating with 5% TFC in three repeat applications at 3, 2

and 1 week before harvest; (iv) coating with 5% TFC at 3 weeks before harvest; (v) coating with

5% TFC at 1 day before harvest; and, (vi) coating with 2.5% TFC at 1 day before harvest.


This study re-tested promising treatments from the 2011/12 season and also included an evaluation

of RainGard and Raynox coating materials. The pre-harvest treatments for B74 fruit in the 2012/13

season were: (i) non-bagged or non-coated control; (ii) bagging at 2 months before commercial

harvest; (iii) coating with 5% RainGard at 2 months, 1 month, and 1 day before harvest; (iv) coating

with 5% RainGard at 2 months, 1 month, and 1 day before harvest with, at ~ 2 hours after RainGard

application ,the fruit being immersed for 15 minutes into a polyethylene bag filled with deionised

water to simulate a rain event; (v) coating with 5% Raynox at 2 months, 1 month, and 1 day before

harvest; and, (vi) coating with 2.5% TFC at 1 day before harvest..


The pre-harvest treatments that were the most effective in reducing LD on B74 fruit in the 2012/13

season were re-tested. Additionally, surfactant was added into coating solutions to potentially

improve the capability of the coatings solutions to enter into lenticel cavities. A series of

concentrations were tested to strengthen (e.g. thicken) the coating layer on the fruit surface and also,

potentially, inside the lenticel cavity while ideally maintaining normal aerobic fruit metabolism.

The pre-harvest treatments were: (i) non-bagged or non-coated control; (ii) bagging at 2 months

before commercial harvest; (iii) coating with 2.5% TFC; (iv) coating with 2.5% TFC plus 0.1%

surfactant; (v) coating with 5% TFC plus 0.1% surfactant; (vi) coating with 10% TFC plus 0.1%

surfactant; and, (vii) coating with 20% TFC plus 0.1% surfactant. All coating treatments in the

108

2013/14 season were applied at 1 week and re-applied at 1 day before harvest.

Experiment 4: Postharvest coatings in Vietnam in 2014/15 season

This experiment aimed to extend the benefits of the pre-harvest coating treatments to a potential

postharvest control strategy for LD on Cat Chu mango fruit in Vietnam. The TFC concentrations

which most effectively reduced LD in the 2013/14 season were investigated for harvested mango

fruit. The postharvest coating treatments for Cat Chu fruit were: (i) non-coated control; (ii) dipping

in water for 15 seconds; (iii) dipping in 20% TFC plus 0.1% surfactant for 15 seconds (one

application); and, (iv) dipping in 20% TFC plus 0.1% surfactant for 15 seconds, allowing to dry at

room temperature for 1 hour and re-applying for 15 seconds (two applications).

Experiment 5: Postharvest coatings in Australia in 2014/15 season

The more or less corresponding postharvest coating treatments for B74 fruit in Australia were: (i)

non-coated control; (ii) dipping fruit into a solution of 15% TFC (no surfactant added) for 1 minute,

dried in shade for 30 minutes, then re-applied for 1 minute, and dried 30 minutes; (iii) dipping fruit

into the solution of 15% TFC plus 0.1% surfactant for 1 minute, air dried for 30 minutes, then re-

applied for 1 minute, and dried 30 minutes; and, (iv) dipping fruit into a solution of 30% TFC plus

0.1% surfactant for 1 minute, air dried for 30 minutes, then re-applied for 1 minute, and dried 30

minutes. The treatments applied over the four harvest seasons are summarised in Table 7.1.

109

Table 7.1. Treatments applied in the 2011/12, 2012/13 and 2013/14 seasons to test effects of bagging and coatings applied before harvest on LD and

fruit attributes of mango fruit B74. Postharvest coatings were also applied in the 2014/15 season to test effects of coatings for harvested B74 and Cat

Chu fruit on quality, including lenticel damage.

Pre-harvest bagging and coating of B74 fruit Postharvest coatings of Cat Chu Postharvest coatings of B74

2011/12 season 2012/13 season 2013/14 season 2014/15 season 2014/15 season

Control Control Control Control Control

Bagging at 2 months Bagging at 2 months Bagging at 2 months DI water for 15 seconds 15% TFC for 15 seconds

5% TFC at 3 weeks,

2 weeks and 1 week

5% RainGard at 2 months,

1 month and 1 day

2.5% TFC at 1 week and

1 day

20% TFC + 0.1% surfactant for

15 seconds

15% TFC + 0.1%

surfactant for 15 seconds

5% TFC at 3 weeks

5% RainGard + water at 2

months, 1 month and 1 day

2.5%TFC + surfactant at

1 week and 1 day

20% TFC + 0.1% surfactant for

15 seconds (two applications)

30% TFC + 0.1%

surfactant for 15 seconds

5% TFC at 1 day

5% Raynox at 2 months, 1

month and 1 day

5% TFC + surfactant at 1

week and 1 day

2.5% TFC at 1 day

2.5% TFC at 1 day

10% TFC + surfactant at

1 week and 1 day

20%TFC + surfactant at 1

week and 1 day

110

7.2.4 Fruit harvest and processing

For the pre-harvest bagging treatment, individual fruit were enclosed in bags. Then, the bags were

stapled closed around the fruit stem. At harvest, the bags were removed from each fruit. Pre-

harvest coatings were administered by spraying the whole fruit surface to runoff using a low

pressure handgun. Postharvest coatings were applied out by dipping the whole fruit into the coating

solutions.

For pre-harvest treatments in the 2011/12, 2012/13, and 2013/14 seasons, 40 fruit per treatment

(four fruit per treatment per replicate tree) were harvested at the green-mature stage. All fruit were

de-sapped in a detergent (0.25% Mango Wash®, Septone Ltd., Queensland, Australia) for 1 minute

and placed in the shade to dry. They were then carefully placed into single layer cardboard trays

and transported by air conditioned (ca. 22 ºC) car to the Maroochy Research Facility (MRF), near

Nambour, Queensland within 5 hours of harvest.

For postharvest treatments in the 2014/15 season in Australia, 30 green-mature fruit per treatment

were harvested at the green-mature stage. All fruit were de-sapped in 0.25% Mango Wash® for 1

minute and placed in the shade to dry. When dry, they were carefully placed in single layer trays

with plastic inserts. Fruit were then transported by car from Katherine to the airport in Darwin,

Northern Territory within 4 hours. Within 24 hours, fruit were air freighted from Darwin to

Brisbane, Queensland and then transported by car to the MRF. Finally, postharvest coatings were

applied to the fruit at the MRF.

At the MRF for all experiments in Australia, the fruit were treated with a fungicide (0.55 mL.L-1

Sportak®, a.i. prochloraz, Bayer Crop Science Ltd., Victoria, Australia) for 30 seconds and then air

dried for 30 minutes. The fruit were then placed onto a brushing unit (Adds Up Engineering Ltd.,

Bundaberg, Queensland, Australia) that consisted of dry soft brushes rotating at 84 revolutions.min-

1 for 1 minute in line with commercial practice. Fruit were treated with 10 µL.L-1 ethylene at 20 °C

and 80 - 85% relative humidity (RH) for 2 days to trigger ripening as per commercial practice.

They were then allowed to ripen at 20 °C and 80 - 85% RH. Before and during ripening at 20 °C,

fruit weight loss, skin colour, subjective firmness, LD, skin colour, lenticel morphology, titratable

acid (TA), total soluble solid (TSS), colour of flesh and sensorial evaluations were undertaken as

below.

111

For the experiment in the 2014/15 season in Vietnam, 60 green-mature fruit were divided into four

groups of 15 fruit per treatment. After fruit were treated with either water or 20% TFC, they were

triggered to ripen by dipping into 0.5% Ethrel® solution for 5 minutes. The dipped fruit were kept

at 20 °C and ~ 85% RH. During ripening, fruit skin colour, subjective firmness, LD, weight loss

and sensory evaluations were conducted every 2 days as described below.

7.2.5 Assessments


LD severity on each fruit was rated at full colour on a 0-5 scale based on both the degree of damage

of the lenticels and the overall area of skin affected. Rating scales for LD severity on the skin of

B74 fruit was described in Table 3.1 (Hofman et al. 2010b).

Dyed lenticel index and proportion (%) of dyed lenticels

Fruit were immersed in a food dye solution (1 g.L-1 Brilliant Blue, All Colour Supplies Ltd., NSW,

Australia) in order to test the potential of lenticels to take up water (Shorter and Joyce 1998).

Briefly, the dye solution and fruit were equilibrated separately at 25°C. The fruit were then

submerged into the dye solution and held at 12 °C for 16 hours. Upon removal, the fruit were

rinsed three times in running tap water and then blotted dry with paper towel. The dyed lenticels on

each fruit were rated using a dyed lenticel index of 1 = 0 - 10%; 2 = 11 - 30%; 3 = 31 - 50%; 4 = 51

- 70%; 5 = 71 - 90% and 6 = 91 - 100% of the fruit area affected by dyed lenticels. Dyed and non-

dyed lenticels were then counted in four 2 cm-diameter circles on the fruit equator (on the two

cheeks and the two far sides of the cheeks) under a magnifying glass (Maggylamp model M.L.

2141, Newbound Pty. Ltd, NSW, Australia). The proportion (%) of dyed lenticels = 100 x count of

dyed lenticels / count of total lenticels.

Wetting angle of the deionised water and coating solutions

Wetting angle has been used to assess the capacity of a water droplet to wet the cuticle and enter

into a stomata / lenticel cavity (Eichert and Burkhardt 2001, Peschel et al. 2003, Ruckenstei and

Lee 1975, Schonherr and Bukovac 1971). In this chapter, wetting angle was measured to assess the

entry capacity of DI water and TFC coating solutions associated with a surfactant into lenticel

cavities. Glass microscope slides were selected as a standard surface to initially study the

characteristics of solutions: deionised (DI) water, DI water plus 0.1% surfactant, 15% TFC, 15%

TFC plus 0.1% surfactant, and, 30% TFC plus 0.1% surfactant. The wetting angles of these

solutions on mango fruit harvested directly from tree to tray without any postharvest treatment were

112

also measured. The wetting angles of 5 μL solutions on the fruit surface were measured at two

cheeks, at convex face and at beak face of the fruit and in the middle of these points around the fruit

equator (Fig. 7.1). Fruit were firmly positioned with the selected test point in the horizontal

direction and the droplet was carefully placed on the fruit using a micropipette. At equilibrium, the

contact angle between the droplet and the fruit surface was photographed with a Canon DOS40D

camera fitted with a Canon macro-lens EF-S 60 mm (Canon Inc., Tokyo, Japan). The contact

angles from the images were measured using Image-J software version 1.48 for Window (National

Institutes of Health, Maryland, USA). The contact angles were measured between tangent and

droplet base (Lamour and Hamraoui 2010).

Figure 7.1. The locations on the surface of mango fruit used to measure wetting angles.

Proportion of lenticels taking up the solution

The proportion (%) of lenticels taking up solution was also used to assess the potential for lenticels

to uptake coating solutions. The process was by immersing fruit into the coating solution plus 0.1%

Brilliant Blue dye. The dye was added into DI water, DI water plus 0.1% surfactant, and the

coating solutions of 15% TFC, 15% TFC plus 0.1% surfactant, and 30% TFC plus 0.1% surfactant.

The fruit were submerged into the above solutions for 1 minute at room temperature (ca. 25°C).

Upon removal, the fruit were rinsed three times in running tap water and then blotted dry with paper

towel. Dyed and non-dyed lenticels were enumerated as described above.

Skin colour

Skin colour of individual fruit was rated as was described in chapter 6 (the section on Skin colour in

6.2.3 Assessment).

Colour measurement

Fruit skin colour was objectively measured using a Konica Minolta CR-400 (Konica Minolta

Sensing Inc., Japan). The colour of the mango skin was expressed as L* (lightness), a* (redness or

greenness), and b* (yellowness or blueness) (Moalemiyan et al. 2010).

Beak

face

Convex

face Cheek

face

113

Subjective firmness

Subjective firmness of individual fruit was rated subjectively by hand pressure as described in

chapter 6 (the section on Subjective firmness in 6.2.3 Assessment; Hofman et al. 2010b).

Objective firmness

The firmness of individual fruit was measured objectively as described in chapter 5 (the section on

Objective firmness in 5.2.3 Assessment).

Fruit weight loss

Individual fruit were individually weighed at harvest and every 2 - 3 days during ripening to enable

calculation of weight loss.

Fruit dry matter

For each treatment, a tissue sample from 10 replicate green-mature fruit was cut from the cheek to a

depth of 3 cm. Fruit samples were peeled and the flesh samples were diced and placed in plastic

trays. Each sample was weighed and recorded as fresh weight (FW). The samples were dried at 65

°C until the sample weight was constant. The proportion (%) of dry matter (DM) of the fruit pulp

was determined using the formula: DM (%) = 100 x DM/FW (Lechaudel et al. 2002, Hassan et al.

2009).

Total soluble solids

Total soluble solids content of the flesh of individual fruit was measured using a pocket

refractometer PAL-1 (Atago Co., Ltd, Tokyo, Japan).

Titratable acidity

Titratable acidity of the flesh of individual fruit was determined by the titrimetric method using a

G20 Compact titrator (Mehler-Tolodo International Inc., Greifensee, Switzeland) with 0.1M NaOH

as the titrant.

Fruit flavour

Fruit flavour of the flesh samples was evaluated by a panel of nine staff members at the MRF. The

panellists were familiar with mango flavour. Flesh from the cheeks of five fruit per treatment at 9

days after full yellow were diced and mixed. Fruit flavour was rated using the 9-point hedonic

scale, where the anchor points were: 1 = dislike very much; 5 = neither dislike nor like; and, 9 =

like very much.

114

Lenticel external features

Mango fruit tissue explants (~ 1 mm depth) containing lenticels were excised using a surgical blade

(Swann-Morton Co. Ltd., Shelfield, England) from five random positions on each of five fruit; i.e.

25 samples. The thin sections were placed onto a glass slide with the cuticular surface facing up.

Plan features were viewed under a dissecting light microscope (TYP 376788 Wild, Heerbrugg,

Switzerland) and were photographed using a digital camera (Canon DOS40D, Canon Inc., Tokyo,

Japan).

Lenticel morphology


The examination of fresh tissues and fixed tissues were described in chapter 3 (the section on

Lenticels morphology in 3.2.3 Assessment).

7.2.6 Experiment design and statistical analyses

For pre-harvest bagging and coatings in the 2011/12, 2012/13 and 2013/14 seasons, 10 replicate

trees were used. Each treatment was applied to four fruit per tree (40 fruit per treatment). For

postharvest coatings in the 2014/15 season, 30 replicate fruit in Australia and 15 replicate fruit in

Vietnam were used for each treatment. The fruit were assigned in completely randomised design

arrangements to trays during postharvest dipping and brushing and also during ripening at 20 ºC.

Data were statistically analysed as one way ANOVAs using the ‘General Analysis of Variance’

model of Genstat16 for Windows (VSN International Ltd., Hertfordshire, UK). Linear

regressions of dyed lenticel at harvest and LD of fruit at the green-mature stage and at the full

yellow stage were analysed. The regressions can be considered significant if P value is less than

0.05. The protected least significant difference (LSD) procedure at P = 0.05 was used to test for

differences between treatment means for LD, lenticel water uptake index, wetting angle, proportion

of lenticels taking up the solution and weight loss.

7.3 Results

7.3.1 Experiment 1: pre-harvest bagging and coatings of B74 fruit in 2011/12

Effects of bagging and coating treatments on lenticel damage severity and dyed lenticel index

Lenticel damage severity of fruit from either bagging or coating treatments was similar to the non-

bagged or non-coated control at 7 days after harvest (full yellow or eating ripe, Table 7.2).

115

However, at 14 days after harvest (eating soft), bagging, coating with 5% TFC at 3 weeks, and

coating with 2.5% TFC at 1 day before harvest reduced LD severity, as compared to the control.

Multiple applications of TFC coatings did not reduce LD severity. Thus, bagging and coating with

2.5% TFC at 1 day before harvest reduced LD severity of B74 fruit by 48% and 43% at eating ripe,

respectively, as compared to the non-bagged or non-coated control. The controlled fruit had the

highest dyed lenticel index and proportion of dyed lenticels at harvest as compared to fruit given the

bagging and coating treatments.

Table 7.2. Effects of pre-harvest bagging and coatings of B74 mango fruit on lenticel damage

severity (0 = no damage to 5 = severe damage), dyed lenticel index (1 = almost no dyed lenticels to

6 = almost dyed lenticels) and proportion (%) of dyed lenticels in the 2011/12 season. If followed

by different letters, means for lenticel damage severity, dyed lenticel index, or proportion of dyed

lenticels within each column are significantly different (P < 0.05) by LSD (P = 0.05).

Treatments

Lenticel damage severity (0 - 5) Dyed lenticel

index (1 - 6)

at harvest

Proportion (%) of

dyed lenticels at

harvest

At 7 days after

harvest (eating ripe)

At 14 days after

harvest (eating soft)

Control 0.8 abc 1.9 d 5.1 c 49.3 c

Bag 0.7 ab 1.0 a 4.3 b 40.8 bc

5% TFC 3, 2, 1 weeks 0.6 a 1.6 cd 3.2 a 26.9 a

5% TFC 3 weeks 0.9 bc 1.3 abc 4.4 bc 30.2 a

5% TFC 2 day 1.0 c 1.5 bcd 4.3 bc 25.3 a

2.5% TFC 1 day 0.9 bc 1.1 ab 3.9 ab 32.5 ab

LSD 0.05 0.24 0.47 0.76 9.91

This increase in dyed lenticel index with the control relates to the higher LD severity of the control

fruit at eating ripe (at full yellow) and at eating soft (7 days after full yellow; Table 7.2). However,

there were no significant (P > 0.05) correlations between LD severity at eating ripe or eating soft

and dyed lenticel index. There were also no significant (P > 0.05) correlations between LD at

eating ripe or eating soft and proportion of dyed lenticels at harvest.

Effect of bagging and coatings on objective firmness during ripening

There were no significant (P > 0.05) differences in firmness of fruit among treatments and the non-

bagged or non-coated control as judged by the Aweta firmness tester (data are presented in

Appendix 7). The average firmness was 35.5 Aweta units at 7 days after harvest and 31.3 Aweta

116

units at 7 days later.

Effects of bagging and coatings on lenticel morphology

The six treatments examined in this study produced similar lenticel morphology at 7 days after

harvest (full yellow colour). In terms of LD, two contrasting lenticels are shown in Fig. 7.2.

Figure 7.2. Transverse 5 µm thick tissue sections through non-discoloured lenticels (A and C) and

discoloured lenticels (B and D) of B74 mango fruit at full yellow colour in the 2011/12 season.

Sectioned tissues were either left unstained (A and B) or stained with Toluidine Blue (C and D).

Blue stained tissue (circle in D) highlights accumulation of phenolic compounds. Brown tissues

(Bt) surrounded the discoloured lenticel (B and D). The lenticel cavity (Lc) and resin canal (Rc) are

also evident. Scale bars represent 50 μm.

Brown tissues surrounding lenticel cavities were observed in sections of discoloured lenticels (Fig.

7.2B and D). No brown tissues were observed in non-discoloured lenticel cavity (Fig. 7.2A and C).

There was also no consistent evidence for spatial proximity between resin canals and the cavities of

discoloured lenticels. Moreover, no natural cuticular wax layer nor wax coating from coating

treatments were observed in light micrographs (Fig. 7.2). Presumably, they were removed by the

A B

C D

Rc Rc

Rc

Rc Rc

Lc Lc

Lc Bt

Lc

Bt

117

imbedding process.

7.3.2 Experiment 2: pre-harvest bagging and coatings of B74 fruit in 2012/13

Effects of pre-harvest bagging and coating treatments on lenticel damage, dyed lenticel index,

weight loss and firmness

Bagging and coating with 2.5% TFC were the most effective treatments for reducing LD severity

(Table 7.3). For example, bagging significantly (P < 0.05) reduced LD severity by 55% at harvest

and 43% at eating ripe, while 2.5% TFC significantly (P < 0.05) reduced LD severity by 30% at

harvest and 39% at eating ripe (8 days after harvest).


severity (0 = no damage to 5 = severe damage), dyed lenticel index (1 = almost no dyed lenticels to

6 = almost dyed lenticels), and weight loss (% of initial fresh weight). If followed by different

letters, means for lenticel damage severity, dyed lenticel index or weight loss within each column

are significantly different (P < 0.05) by LSD (P = 0.05).

Treatments

Lenticel damage severity (0 - 5) Dyed lenticel

index (1 - 6)

at harvest

Weight loss (% of

initial fresh weight)

at 16 days after

harvest (eating soft)

At harvest

(green mature)

At 8 days after

harvest (eating ripe,

colour rating ~ 6)

Control 2.0 c 2.8 c 5.1 a 95.9 a

Bagging 0.9 a 1.6 a 4.4 a 96.0 ab

RainGard 1.7 bc 1.6 a 4.8 a 96.8 c

RainGard + Water 1.9 bc 2.5 bc 5.2 a 96.5 bc

Raynox 1.5 bc 2.0 ab 5.0 a 96.6 c

TFC 1.4 ab 1.7 a 4.8 a 96.0 ab

While 5% RainGard and 5% Raynox did not reduce LD severity in green-mature fruit, they did

result in lower LD severity in ripe fruit. Fruit from RainGard coating associated water dipping after

coating significantly (P < 0.05) increased LD severity at eating ripe, as compared to fruit from

RainGard coating without water dipping. Immersing RainGard-treated fruit in water to simulate a

rain event, negated the benefits of RainGard. There was no significant (P > 0.05) effect of bagging

and coating treatments on the dyed lenticel index ratings at harvest (Table 7.3). However, there

were significant positive linear correlations (y = y0 + ax) between LD on fruit at harvest (y) and

118

their dyed lenticel index (x) at harvest: y = -4.612 + 1.265x, r² = 0.824, P < 0.05 (Fig. 7.3). There

was also a significant positive linear correlation (y = y0 + ax) between LD on fruit at the eating ripe

stage (y) and the dyed lenticel index (x) at harvest: y = -4.943 + 1.429x, r² = 0.644, P < 0.05.

Treatments with a high dyed lenticel index rating at harvest had high LD at harvest and eating ripe.

Dyed lenticel index (1-6)

4.2 4.4 4.6 4.8 5.0 5.2 5.4

Le

ntice

l d

dam

ag

e s

eve

rity

(0

-5)

0

1

2

3

4

5

Dyed lenticel index vs. LD severity at eating ripe

y = -4.943 + 1.429x; r = 0.802; P < 0.05

Dyed lenticel index vs. LD severity at harvest

y = -4.612 + 1.265x; r = 0.908; P < 0.05

Figure 7.3. Lenticel damage severity (0 = no damage to 5 = severe damage) in B74 mango fruit at

harvest and at 8 days after harvest (eating ripe) versus dyed lenticel index (1 = almost no dyed

lenticels to 6 = almost dyed lenticels) at harvest. Each data point in the graph represents the

average means of each treatment.

There was a significant (P < 0.05) treatment effect on fruit weight at 16 days after harvest (eating

soft; Table. 7.3). Coating fruit with RainGard or Raynox resulted in less weight loss at the later

stages of ripening as compared with the non-bagged or non-coated control. The effects of bagging

and coating with 2.5% TFC alone on weight loss were similar. There was no significant (P > 0.05)

difference on objective firmness as measured by Aweta (Appendix 8). The average firmness

decreased from eating ripe (33.1 Aweta units) to 7 days later (24.9 Aweta units).


There were no clear treatment effects on lenticel morphology (Fig. 7.4). Discoloured lenticels had

distinct browning tissues inside their cavity. No discoloured resin ducts adjacent to lenticels were

observed. This finding on lenticel morphology was consistent with the previous year’s results.

119

Figure 7.4. Unstained transverse 5 µm-thick tissue sections of B74 fruit at full colour in 2012/13

for the control (A), bagging (B), 5% RainGard coating (C), 5% RainGard coating + water (D), 5%

Raynox (E), and 2.5% TFC coating (F). Brown tissues (Bt) surrounded the discoloured lenticel (B,

D). The lenticel cavity (Lc) and resin canal (Rc) are also evident. Scale bars represent 50 μm.

A

Rc

Lc

C

Rc

Lc

D Rc

Lc

E

Lc

F

Rc

Lc

Lc

Bt

Bt

Bt Bt

Bt Bt

B

120

7.3.3 Experiment 3: Pre-harvest bagging and coatings of B74 fruit in 2013/14

Effect of surfactant on deionised water and coating solutions

There were significant (P < 0.05) treatment effects on the wetting angle and proportion (%) of

lenticels taking up the solution (Table 7.4). DI water had the largest wetting angle on the glass

slides and on the fruit surface. Fruit from this treatment had the smallest proportion of dyed

lenticels.

Table 7.4. Wetting angle (º) on glass slides and on the skin surfaces of B74 fruit and proportion (%)

of lenticels taking up solutions of DI water plus surfactant, 15% TFC plus surfactant, or 30% TFC

plus surfactant. If followed by different letters, means for wetting angle on the glass slide, wetting

angle on the fruit surface, or proportion of dyed lenticels within each column are significantly


Solutions Wetting angle (º) of solutions Proportion (%) of lenticels taking

up the solution On slide glass On off-tree fruit

DI water 45.2 e 122.3 e 5.6 a

DI water + 0.1% Surfactant 4.3 a 3.6 a 91.0 c

15% TFC 38.1 d 83.6 d 5.6 a

15% TFC + 0.1% Surfactant 19.1 b 54.3 b 15.6 b

30% TFC + 0.1% Surfactant 22.3 c 60.2 c 23.1 b

When 0.1% surfactant was added to DI water, the wetting angle was the smallest on both the glass

slide and fruit surface (Table 7.4). This solution was associated with the largest proportion of dyed

lenticels developing on fruit; viz., 91% of lenticels took up the solution. Coating with 15% TFC

plus 0.1% surfactant had a smaller wetting angle to15% TFC solution alone. Nonetheless, fruit

from this treatment had a three times higher proportion of dyed lenticels. The wetting angle formed

from a droplet of 30% TFC plus 0.1% surfactant was slightly larger than that for 15% TFC plus

0.1% surfactant. However, the proportion of dyed lenticels on fruit coated with 30% TFC plus

surfactant and 15% TFC plus surfactant was not significantly (P > 0.05) different.

Effects of bagging and coating treatments on lenticel damage severity and dyed lenticel index

In the 2013/14 season, bagging or coating B74 fruit with 10% or 20% TFC coating plus surfactant

reduced LD severity in green-mature fruit as compared to the non-bagged or non-coated control

(Table 7.5). However, all the treatments reduced LD severity on fruit at 7 days after harvest (eating

121

ripe) and 16 days after harvest (eating soft) relative to the non-bagged or non-coated control. LD

severity was reduced most by bagging fruit or treating them with 2.5% TFC without surfactant or

20% TFC with surfactant. Treatment with 20% TFC plus surfactant reduced LD severity by 43% at

15 days after harvest as compared to the control.


severity (0 = no damage to 5 = severe damage) and dyed lenticel index (1 = almost no dyed lenticels

to 6 = almost dyed lenticels) in the 2013/14 season. If followed by different letters, means for

lenticel damage severity or dyed lenticel index within each column are significantly different (P <

0.05) by LSD (P = 0.05).

Treatment Lenticel damage severity (0 - 5) Dyed lenticel

index (1 - 6) at

harvest

At harvest

(green-mature)

At 7 days after

harvest (eating ripe,

colour rating ~ 6)

At 15 days

after harvest

(eating soft)

Control 1.1 b 1.9 c 3.2 d 4.5 b

Bagging 0.6 a 0.8 a 2.2 ab 4.0 a

2.5% TFC - surfactant 0.6 a 0.7 a 2.1 ab 4.1 a

2.5% TFC + surfactant 1.0 b 1.4 b 2.7 c 5.5 c

5% TFC + surfactant 1.0 b 1.4 b 2.7 c 5.4 c

10% TFC + surfactant 0.6 a 0.9 a 2.3 bc 5.6 c

20% TFC + surfactant 0.7 a 0.9 a 1.8 a 5.5 c

Bagging fruit or treatment with 2.5% TFC without surfactant resulted in the lowest dyed lenticel

index rating at harvest, while all TFC plus surfactant treatments resulted in higher dyed lenticel

index ratings as compared with the non-bagged or non-coated control (Table 7.5). There was no

correlation between treatment means of the dyed lenticel index and treatment means of LD at eating

ripe when surfactant was added. Bagging fruit and coating with 2.5% TFC produced lower dyed

lenticel ratings and also reduced LD. This trend was consistent with the results of 2012/13 season.

However, this trend was not observed for the other TFC coatings when a surfactant was added.

Adding the surfactant into TFC solutions resulted in lower LD, but higher dyed lenticel ratings,

relative to the non-bagged or non-coated control.

122

Effects of bagging and coatings treatments on weight loss and skin colour

There was no significant difference in weight loss during ripening among the non-bagged or non-

coated control, bagged, or 2.5% TFC (no surfactant) treatments (Fig. 7.5A). However, increasing

concentrations of TFC significantly (P < 0.05) reduced weight loss after harvest, as compared to the

control.


0 2 4 6 8 10 12 14 16

Fru

it w

eig

ht

loss (

% o

f in

itia

l fr

esh

weig

ht)

97.0

97.5

98.0

98.5

99.0

99.5

100.0

Control

Bag

TFC 2.5%

TFC 2.5% + surfactant

TFC 5% + surfactant

TFC 10% + surfactant

TFC 20% + surfactant


0 2 4 6 8 10 12 14 16

Skin

co

lour

(1-6

)

1

2

3

4

5

6

Control

Bag

2.5% TFC

2.5% TFC + surfactant

5% TFC + surfactant

10% TFC + surfactant


Figure 7.5. Effects of pre-harvest bagging and coatings of B74 mango fruit on fruit weight loss (%

of initial fresh weight) and skin colour (1 = green to 6 = yellow) during ripening in the 2013/14

season. The vertical bars with the symbols represent the standard error for the mean values where

they exceed the symbol size.

Coating with 20% TFC plus surfactant resulted in less yellow colour on the skin at 7 days after

harvest and they required a further 6 days for all fruit to reach colour rating 6 (Fig. 7.5). All other

treatments did not significantly (P > 0.05) affect skin colour development.

Effects of bagging and coatings treatments on dry matter, fruit firmness and fruit flavour

There was no treatment effect on fruit dry matter (DM, %) at harvest (Appendix 9). The average

DM content was 17.8%. No significant (P > 0.05) treatment effects on objective firmness were

noted at 15 days after harvest (eating soft), as measured by the Aweta (Appendix 9). The average

Aweta reading was 29.4 units. There were no significant (P > 0.05) treatment effects on sensorial

evaluation (flavour) at the full yellow (ripe) stage (Appendix 9) and no off flavours were noted.


There were no obvious effects of bagging and coating treatments on lenticel morphology.

However, there was expected difference between non-discoloured and discoloured lenticels (Fig.

7.6). Brown tissue and cells surrounding lenticel cavities were observed in sections through

123

discoloured lenticels. Cells lining lenticel cavities did not stain with Sudan IV, suggesting an

absence of cuticular waxes. There was again no evidence of proximal juxtaposition between resin

canals and the cavities of discoloured lenticels. The natural waxy layer of the cuticle and the

artificial wax coatings from applied treatments were likely removed in the course of tissue

preparation for sectioning.

Figure 7.6. Transverse 5 µm thick tissue sections through a non-discoloured lenticel (A) and a

discoloured lenticel (B) of B74 mango fruit at full yellow colour in the 2013/14 season. Sectioned

tissues were stained with Sudan IV. Red stained tissue highlights the location of lipid-rich cuticle

layers (CI). Brown tissues (Bt) surround the discoloured lenticel (B). The lenticel cavity (Lc) and

resin canal (Rc) are also evident. Scale bars represent 50 μm.

There were white deposits of presumably coating material inside lenticel cavities of fruit coated

with 20% TFC plus surfactant as compared to in the lenticels of non-bagged or non-coated control

fruit (Fig. 7.7A and B). It is probable that TFC wax penetrated into lenticel cavities (arrows in Fig.

7.7B and C) in fruit coated with 20% TFC plus surfactant. When 0.1% Brilliant Blue, a food dye,

was added into 20% TFC plus surfactant, it in presumed admixture was observed as a distinct blue

colour inside the lenticel cavity.

A B

Rc Rc

Cl Lc Cl Lc

Bt

124

Figure 7.7. External features of a typical lenticel on B74 mango for non-bagged or non-coated

control fruit (A), for a 20% TFC plus surfactant coated fruit (B), and for a 20% TFC plus surfactant

in admixture with 0.1% Brilliant Blue dye in the 2013/14 season. The arrows show TFC wax filling

lenticel cavity. Scale bars represent 50 μm.

When lenticel morphology was examined by hand sectioning, TFC wax was observed on the fruit

cuticle surface and inside the lenticel cavity of 20% TFC plus surfactant (Fig.7.8).

Figure 7.8. Transverse sections of B74 fruit at full colour stage in the 2013/14 season. The typical

sections of the non-bagged or non-coated control lenticels (A) and of the pre-harvest coatings with

20% TFC plus surfactant lenticels (B) were obtained by hand sectioning. Sudan IV stained lipids of

the cuticle layer, turning a red colour. Coating with 20% TFC plus surfactant (Wl) covered the fruit

surface and entered the lenticel chamber. Red stained tissue highlights the location of lipid rich

cuticle layers (CI). Brown tissues (Bt) surround the lenticel cavities (Lc). Scale bars represent 50

μm.

A B C

A

Lc

Bt

B

Lc Bt

Wl

Cl

Cl

Wax Wax

Lc Lc Lc

125

7.3.4 Experiment 4: postharvest coatings of Cat Chu fruit in 2014/15

Effects of postharvest coatings on lenticel damage severity and weight loss

Coating with 20% TFC plus surfactant reduced LD severity as compared to the non-coated control

(Fig. 7.9A). There was no difference in LD severity between one application and two applications

with 20% TFC plus surfactant. This was possibly due to high LD severity on Cat Chu fruit. LD

severity on the skin of the non-coated control fruit and water dipped fruit were similar. There was a

significant (P < 0.05) treatment effect on fruit weight loss after harvest (Fig. 7.9B). There was no

difference in weight loss between the non-coated control and the fruit treated with water. However,

coating with 20% TFC significantly (P < 0.05) reduced fruit weight loss during ripening, relative to

the non-coated control. Treatment with a second application of 20% TFC further reduced weight

loss.


4 6 8 10 12

Le

ntice

l da

ma

ge

se

ve

rity

(0

-5)

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0Control

Water dipping

20% TFC, two applications

20% TFC, one application


4 6 8 10 12

Fru

it w

eig

ht

loss (

% o

f in

itia

l fr

esh w

eig

ht)

92

94

96

98

100

Control

Water dipping

20% TFC, 2 applications

20% TFC, 1 application

Figure 7.9. Effect of postharvest coatings of Cat Chu mango fruit on lenticel damage severity (0 =

no damage to 5 = severe damage) and weight loss (% of initial fresh weight) during ripening at 20

°C. The vertical bars with the symbols represent the standard error for the mean values where they

exceed the symbol size.

Effects of postharvest coatings on subjective firmness and skin colour

Coating with 20% TFC plus surfactant in one or two applications reduced Cat Chu skin colour at 8

and 10 days after harvest (Fig. 7.10). However, at 2 days after harvest, no significant (P > 0.05)

difference in skin colour was recorded among treatments and the non-coated control. Coating with

20% TFC plus surfactant retarded skin colour at 8 and 10 days after harvest. However, coated fruit

developed similar skin colour to the uncoated fruit at the eating soft stage.

126


4 6 8 10 12

Skin

co

lour

(1-6

)

2

3

4

5

6

Control

Water dipping

20% TFC, 2 applications

20% TFC, 1 application

Figure 7.10. Effects of postharvest coatings of Cat Chu mango fruit on skin colour (1 = green to 6

= yellow) during ripening at 20 °C. Non-coated fruit served as the control. The vertical bars with

the symbols represent the standard error for the mean values where they exceed the symbol size.

There were no significant (P > 0.05) differences in subjective firmness as determined by hand at 8

and 12 days after harvest (Appendix 10). The average subjective firmness at 8 days and 12 days

after harvest were 1.8 (firm) and 3.9 (soft), respectively.

7.3.5 Experiment 5: postharvest coatings of B74 fruit in 2014/15

Effects of postharvest coatings on lenticel damage severity

There was a significant (P < 0.05) coating effect on LD severity at 10 days (eating ripe) and 18 days

(eating soft) after harvest (Table 7.6). The non-coated control fruit had the highest LD severity at

both stages. Coating with 30% TFC plus surfactant reduced LD severity about 43% and 50% as

compared to the control at eating ripe and eating soft, respectively. Coating with 15% TFC plus

surfactant had similar LD severity to 15% TFC alone.

127

Table 7.6. Effects of postharvest coatings of B74 mango fruit on lenticel damage severity (0 = no

damage to 5 = severe damage) in the 2014/15 season. If followed by different letters, means for

lenticel damage severity within each column are significantly different (P < 0.05) by LSD (P =

0.05).


At 10 days after harvest (eating ripe) At 18 days after harvest (eating soft)

Control 1.6 c 2.6 c

15% TFC 1.3 bc 2.0 b

15% TFC + surfactant 1.0 ab 1.7 ab

30% TFC + surfactant 0.9 a 1.3 a

Effects of postharvest coatings on weight loss

Coating with TFC resulted in less weight loss during ripening as compared to the non-coated

control (Fig. 7.11). There was little difference in weight loss between fruit treated with 15% TFC

plus surfactant and 15% TFC alone. Coating fruit with 30% TFC plus surfactant reduced weight

loss the most.


0 2 4 6 8 10 12 14 16Fru

it w

eig

ht

loss (

% o

f in

itia

l fre

sh w

eig

ht)

98.0

98.5

99.0

99.5

100.0

Control

15% TFC



Figure 7.11. Effects of postharvest coatings of B74 mango fruit on fruit weight loss (% of initial

fresh weight) during ripening at 20 °C, 85% RH in the 2014/15 season. The vertical bars with the

symbols represent the standard error for the mean values where they exceed the symbol size.

128

Effects of postharvest coatings on skin colour ratings and skin colour measurement

There was a treatment effect on skin colour from 6 days after harvest (Fig. 7.12A). Coating

treatments delayed the skin colour development as compared to the non-coated control, especially

the 30% TFC plus surfactant treatment. The non-coated control reached completely full yellow

skin colour at day 12, while 30% TFC plus surfactant reached completely full yellow skin colour at

16 days after harvest; i.e. about 4 days later.

0 2 4 6 8 10 12 14 16

Skin

co

lour

(1-6

)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Control

15% TFC



(A)

0 2 4 6 8 10 12 14

L*

valu

e

72

73

74

75

76

77

78

79

80

Control

15% TFC



(B)


0 2 4 6 8 10 12 14

a*

valu

e

-15

-10

-5

0

5

10

Control

15% TFc



(C)


0 2 4 6 8 10 12 14

b*

valu

e

40

45

50

55

60

Control

15% TFC



(D)

Figure 7.12. Effects of postharvest coatings of the B74 mango fruit on skin colour, as measured

subjectively (1 = green to 6 = yellow) or objectively (L*, a* and b* value) during ripening at 20 °C

in the 2014/15 season. The vertical bars with the symbols represent the standard error for the mean

values where they exceed the symbol size.

From 4 to 8 days after harvest, coatings resulted in decreased lightness (L*) as compared to the non-

coated control. However, there was no treatment effect in lightness from 10 to14 days after harvest

(Fig. 7.12B). TFC-coated mango fruit showed lower a* and b* values as compared to non-coated

fruit (Fig. 7.12C and D). Fruit treated with 30% TFC coating plus surfactant showed the most

reduced a* and b* values. There were no clear differences in a* and b* values between fruit treated

129

with 15% TFC plus surfactant and those treated with 15% TFC alone. Lower a* and b* values

indicated a delay in fruit colour development. The a* and b* values consistently reflected skin

colour where the 30% TFC coating retarded yellow colouration.

Effects of postharvest coatings on subjective firmness, total soluble solid, titratable acid and

fruit flavour and flesh colour measurement

There were no significant (P > 0.05) effects of postharvest treatments on subjective firmness of fruit

a s assessed by hand pressure at 10 or 16 days, total soluble solids (TSS) assessed at 21 days, or

titratable acidity (TA) assessed at 21 days after harvest (Appendix 11). The average subjective

firmness score at 10 days and 16 days after harvest were 2.4 and 4.0, respectively. The average

TSS and TA for fruit from these treatments were 17.3 ºBrix and 2.8% at 21 days after harvest

(eating soft), respectively. There were no differences in flesh colour at 21 days after harvest among

the treatments. The averaged L*, a*, b* values were 67.7, 9.5, and 52.6, respectively (Appendix

11). Coatings did not negatively affect flavour as measured on a hedonic scale of 1 - 9 at eating soft

(Appendix 11), and no off-flavours were noted.

Effects of postharvest coatings on lenticel morphology

When viewed under light microscopy, TFC coated fruit had wax deposition on the fruit surface and

onto cells lining lenticel cavities (Fig. 7.13B, C and D). No wax layers were evident in transverse

sections of the non-coated control fruit (Fig. 7.12A).

130

Figure 7.13. Lenticel morphology of B74 mango fruit during ripening at 20 °C in the 2014/15

season. Harvested fruit were coated with 15% TFC (B), 15% TFC plus surfactant (C), or 30% TFC

plus surfactant (D). Non-coated fruit served as the control (A). Transverse sections were obtained

by cutting fresh samples (hand sectioning) and then stained with Sudan IV. The dye stained the

lipids of the cuticle layer (Cl) a red colour, but did not stain the TFC wax layer. Arrows show a

TFC wax (Wl) cover on the fruit surface and into the lenticel cavity (Lc). Scale bars represent 50

μm.

7.4 Discussion

Bagging and coating treatments both pre-harvest and postharvest could reduce LD severity in

ripened B74 mango fruit. Consistent reduction in LD severity with pre-harvest bagging may be

associated with a reduction in physical damage to fruit surfaces as previously suggested for bagged

Keitt mango fruit (Hofman et al. 1997). Joyce et al. (1997) similarly found that covering Sensation

mango fruit with paper bags reduced levels of fruit blemish. In the present study, there did not

appear to be a bagging effect in terms of lenticel morphology on B74 fruit, as compared to the

untreated fruit (Fig. 7.2; Fig. 7.4; Fig. 7.6; Fig. 7.8; Fig. 7.13).

C

B A

D

Wl

Wl Wl

Lc Cl Lc Cl

Lc Cl Lc Cl

131

During the first season, a pre-harvest coating treatment of B74 fruit with 2.5 % TFC most markedly

reduced LD severity relative to the non-bagged or non-coated fruit (Table 7.2). This was possibly

because 2.5% TFC has evidently high potential for infiltration into the lenticel cavity. In the

2012/13 season, treatment of B74 fruit with 2.5% TFC, 5% RainGard, and 5% Raynox reduced LD

at the eating ripe stage as compared to the control. Notably, only 2.5% TFC reduced LD at both

harvest and eating ripe (Table 7.3). This observation affirms the efficiency of this coating solution.

The wettability of a liquid on a surface is characterised by its wetting angle (Ruckenstei and Lee

1975). The wetting angle of 5 µl DI water droplets on the surface of B74 mango fruit was 122.3º

(Table 7.4), which is slightly larger than the wetting angle of 1 µl DI water droplets on cherry fruit

(92.4º; Peschel et al. 2003). Differences in cuticular wax chemistry and structure are more likely

the reason. Fruit treated with DI water and which had a very high wetting angle exhibited a low

proportion of dyed lenticels (5.6%). This association suggests that the DI water had a relatively low

capacity to infiltrate lenticel chambers.

In the 2013/14 season, addition of the surfactant Maxx reduced the wetting angle of DI water on the

fruit surface from 122.3º to 3.6º (Table 7.4). This surfactant treatment was associated with a

relative increase in numbers of lenticels taking up DI water from 5.6% to over 90%. When the

surfactant was mixed with 15% TFC, the proportion of lenticels taking up the coating solution was

3 times higher as compared to fruit exposed to 15% TFC alone. However, the proportion of

lenticels taking up the TFC plus surfactant solution was still low (15.6%). This is possibly because

TFC solution has relatively high viscosity that reduces its capacity to enter into the lenticel cavity.

Adding the surfactant into the TFC solution did not improve the coating efficiency in terms of

reducing LD severity for fruit treated at low TFC concentration of 2.5 or 5% (Table 7.5). This lack

of effect may be due to the coating layers at these concentrations having relatively low resistance to

disturbance by postharvest practices, including detergent dipping, fungicide treatment and brushing.

However, increasing the concentration of TFC to 10 and 20% plus surfactant in the pre-harvest

period had a positive effect in reducing LD severity. Postharvest coatings with 15%, 20%, and 30%

TFC plus surfactant reduced LD. In particular, fruit treated with 30% TFC had reduced LD severity

by 50% at eating soft as compared to the control. When viewed under light microscopy, fruit

coated with TFC at 15% or higher had wax coating deposition on the cuticle and also into lenticel

cavities. This coating likely forms a barrier to protect lenticels from external stressors.

132

Water is considered a key trigger for LD (Tamjinda et al. 1992, Self et al. 2006). That treatment of

B74 fruit with RainGard followed by water dipping to simulate a rain event after each coating

application increased LD severity at eating ripe as compared to treatment with RainGard alone

affirmed the important critical role of free water in LD (Table 7.3). This proposition is consistent

with observations of Everett et al. (2008), who found that Hass avocado fruit that imbibed water

had a high incidence of LD manifested as skin spotting. These findings highlight the latent

potential to reduce LD on mango by minimising water contact. In this context, further investigation

into the physics and chemistry of water and solutions on mango fruit surfaces and in regard to

coatings is warranted.

The pre-harvest bagging treatments did not significantly (P > 0.05) affect B74 mango fruit weight

loss and skin colour during ripening at 20 °C (Table 7.3; Fig. 7.5). This finding is consistent with a

report by Joyce et al. (1997), who found that paper bags did not affect weight loss and skin colour

of Sensation during storage. Treatment of B74 or Cat Chu fruit with increasing TFC concentrations

plus surfactant in both pre-harvest and postharvest periods markedly reduced fruit weight loss (Fig.

7.5; Fig. 7.9; Fig. 7.11). Coating B74 fruit with high TFC concentrations (e.g. 20% TFC pre-

harvest; 15% or higher postharvest) was relatively effective in reducing weight loss. This was in

line with the findings of (Baez-Sañudo et al. 2005) who found that postharvest coating with Natural

Shine™ TFC210 reduced 50% of fruit weight loss of Keitt mango fruit as compared to uncoated

control. The TFC coatings may lessen fruit transpiration and respiration rates. However, the also

delayed yellow skin development by about 4 days, as compared to the control. Meanwhile, pre-

harvest coating of B74 fruit with 10% TFC or less had no effect on yellow skin development. The

result of a high concentration of TFC at 15% or higher affecting yellow skin colour development is

consistent with the findings of Dang et al. (2005), who found that applying a carnauba based 50%

coating to KP mango retarded skin yellowing formation during ripening at 21 °C. While treatment

with TFC at 15% or higher affected skin colour, the coating treatments did not affect sensory

quality at eating soft.

In the 2011/12 season, non-treated control B74 fruit displayed a high LD severity. They also

exhibited a high dyed lenticel index and a high proportion of dyed lenticels at harvest (Table 7.2).

However, there were no correlations between LD and a high dyed lenticel index or the proportion of

dyed lenticels at harvest. The lack of a relationship was possibly because LD severity in all

treatments and also the control was relatively low and because difference in LD among treatments

was small. There was a linear correlation between dyed lenticel index ratings at harvest and LD at

133

harvest or full yellow skin in fruit harvested in the 2012/13 season (Fig. 7.3). Thus, lenticels which

take up the dye can have higher potential to exhibit discolouration symptoms in ripened fruit as

compared with lenticels which fail to absorb the dye. Curry and Kupferman (2004) found a positive

link between the proportion (%) of dyed lenticels at harvest and damaged lenticels on Royal Gala

apple fruit after storage. A previous study on B74 fruit also found a positive correlation between

dye infiltration at harvest and LD at the full colour stage for fruit harvested at different maturities

(Marques et al. 2013). In the present study, this relationship did not hold when fruit were pre-

treated with coatings plus surfactant. This is likely because the surfactant may facilitate infiltration

of food dye into lenticels. Nevertheless, the dye test at harvest is a promising indicator to predict

LD of B74 fruit at the eating ripe stage, at least when a surfactant is not used.

The observation that browned cells and tissues are adjacent to the cavity of discoloured lenticels

affirms that LD involves accumulation of condensed phenolics (Fig. 7.2; Fig. 7.4; Fig. 7.6, Fig. 7.8;

Fig. 7.13). The mechanism is proposed to involve endomembrane damage, release of phenolics

from sub-cellular compartments, and their mixing with oxidative enzymes (Beckman 2000,

Bezuidenhout et al. 2005). Du Plooy et al. (2009a) reported that the formation of high polymeric

phenolic compounds around lenticels was probably a physiological stress response as a barrier to

protect underlying mesophyll tissues. The absence of any evidence for consistently immediate

proximity between resin ducts and discoloured lenticels suggests that resin phytotoxicity (e.g.

resorcinol; Loveys et al. 1992) does not play a direct role in discolouration. Nonetheless,

Bezuidenhout et al. (2005) and Du Plooy et al. (2006) raised this scenario as a possible mechanism,

particularly under conditions that generate high fruit turgor; such as consistent rain. Other external

stressors can also contribute to LD, including water, irradiation (Hofman et al. 2010a, Hofman et al.

2010b, Self et al. 2006).

The natural epicuticular wax layer can play an important role in minimising water loss and also

stress on lenticels. This natural waxy layer is readily damaged when fruit pass through a packing

line (Du Plooy et al. 2002). Accordingly, postharvest practices contribute to increasing the

potential for LD on fruit. The natural wax and deposition of coating materials inside lenticel

cavities was not clearly observed in the current study (Fig. 7.2; Fig. 7.4; Fig. 7.6). One reason is

that xylene, a wax solvent, was necessarily used in the histological paraffin wax embedding

method. Nevertheless, in observing lenticel external features and internal morphology by the hand

sectioning method, coating with TFC plus surfactant and the waxes infiltration into lenticel cavities

was evident (Fig. 7.8; Fig. 7.13). Thus, it is probable that a protective wax coating from the TFC

treatment also penetrated into lenticel cavities to minimise disruption to the function of surrounding

134

cells.

In conclusion, bagging and coatings with 5% RainGard, 5% Raynox and 10% or higher TFC plus

surfactant were demonstrably promising approaches to reduce LD severity on B74 mango fruit.

However, bagging fruit would require substantial additional labour and so this treatment may only

be useful for high value markets; such as premium lines for export. The potential of RainGard and

Raynox to reduce mango LD should be further investigated. The efficacy of the TFC coating in LD

reduction was improved when surfactant was included. However, TFC concentrations at 15% or

higher slightly adversely affected fruit skin development. However, use of TFC at up to 15% might

be further trialled with a view to harmonising the positive and negative aspects of this approach

towards as commercial treatment.

135

Chapter 8

GENERAL DISCUSSION

Mango is an important fruit crop in Australia (Johnson 2000). One of the main mango cultivars is

B74, which has a reliable high yield and an attractive fruit blush. However, B74 has a postharvest

problem with lenticel damage (LD) (Hofman et al. 2010a). LD exhibits as a darkened area

surrounding the lenticel cavity. It reduces the cosmetic value of B74 and many other mango

cultivars (Bezuidenhout et al. 2005, Du Plooy et al. 2006, Hofman et al. 2010a, Rymbai et al.

2012). Recent research has shown that LD is a major barrier to marketability of B74 fruit,

especially on export markets (Hofman et al. 2010b). Current commercial handling practices (e.g.

detergent treatments for sap removal, irradiation for disinfestation) exacerbate LD on B74 mango

fruit (Whiley et al. 2006, Hofman et al. 2010b).

The origin of lenticels and how they relate to LD on B74 mango fruit was unknown and so there

was no clear reason why lenticels discoloured as the fruit reach maturity, even though they are

present early in fruit development. In the literature covering a range of cultivars, there is somewhat

inconsistent explanation concerning the mechanism/s of LD (Rymbai et al. 2012) and as to which

internal and / or external stressors are causal to LD (e.g. Bezuidenhout et al. 2005, Self et al. 2006).

Some mango cultivars are relatively resistant, or at least tolerant, and others relatively sensitive to

LD. A better understanding of cultivar sensitivity and LD mechanism/s and either control or

management strategies would contribute to further enhancing the marketability of mango fruit,

including expanding the commercial prospects of B74 mango.

Water is possibly an initial mediating stressor which triggers LD in excess (viz. high turgor) and

perhaps in deficiency (viz. turgor loss). When present in excess, water may penetrate into lenticel

cavities, increase the turgor of adjacent cells and create physical stress in the walls of adjacent cells,

potentially leading to their forceful separation. In the context of heightened cell and tissue water

status, the working hypothesis adopted in the present study is that high cell turgor within the lenticel

cavity predisposes the fruit to LD that is exacerbated by other physical and / or chemical stressors.

This stress mechanism may lead to strains, including damage to the cell membranes and the allied

release and mixing of phenolic substrates and browning enzymes from their sub-cellular

compartments.

136

8.1 Lenticel damage sensitivity, genetic inheritance and lenticel damage mechanisms of B74

fruit

Studies have been undertaken into lenticel ontogeny for some mango cultivars. Moreover, different

cultivars have different susceptibility to LD (Bally et al. 1997, Cronje 2009a, Du Plooy et al.

2009b). Tamjinda et al. (1992) explained that cultivar differences in LD exist because of the

thickness of cell walls at the boundary in the lenticel cavity. Du Plooy et al. (2004) reported that

cultivar sensitivity to LD varied because of spatial differences in lenticel structure and the thickness

of the wax-cutin layer covering and / or in the lenticel cavity.

In the present work (Chapter 3), there were no clear differences in lenticel structures between B74

and the comparator cultivars Kensington Pride (KP), Honey Gold (HG) and Sensation. However,

B74 fruit had 2 to 7 times higher lenticel density at harvest than did KP, HG and R2E2 fruit.

Cultivars with a relatively high density of lenticels may have higher potential for increased density

of damaged lenticels. The higher density may explain the predisposition of B74 fruit to expressing

commercially significant LD severity. This distinctive characteristic was likely inherited from

Sensation, one of the two parent cultivars of B74. Nonetheless, when exposed to postharvest

operations, including detergent, fungicide and brushing treatments, B74 fruit significantly (P <

0.05) had less LD severity and a lower proportion of damaged lenticels than did KP fruit, the other

parent cultivar of B74. Accordingly, it appears that pronounced browning severity in B74 fruit may

be inherited from KP. It was also revealed (Chapter 3) that lenticels on B74 were formed at a very

early fruit age, at ~ 1 cm long. At this stage, lenticels were akin to stomata in terms of their external

features and anatomical structure in cross section. However, no guard cells were observed in

lenticels at this young fruit age and lenticels even manifested damage as browning of tissue in the

lenticel cavity. This indicated stomata ruptured at this stage to form lenticels. This finding is

consistent with those of Bally (1999), Bezuidenhout et al. (2005) and Scora et al. (2002). They

determined that lenticels on mango fruit developed from stomata. However, a high proportion of

B74 lenticels also arose from cracks in the epidermis during later stages of fruit development.

Similarly, Dietz et al. (1988) suggested that lenticels also originated from cracking of the fruit

epidermis. A larger proportion of lenticels on B74 fruit had smaller chambers as compared to KP

fruit, perhaps because of their formation from cracks later during fruit development.

Bezuidenhout et al. (2005) and Du Plooy et al. (2006) proposed that terpenoids in mango fruit

contributed to LD via their mobilisation from resin ducts into lenticel cavities. In the present work,

137

however, no link between sub-cuticular resin ducts and LD was evident in any of the light

micrographs. Therefore, it would seem that external stressors may likely be the more critical LD

causing agents. That there was accumulation of condensed phenolics around damaged lenticels

suggests activation of natural protective host defence barriers against external stressors. This

scenario is consistent with that proposed by Du Plooy et al. (2009a), who characterised phenolics in

affected lenticels. Natural waxes may play a pivotal role in creating a protective barrier over and

inside lenticel cavities against external stressors, such as free water. Understanding the effects of

external stressors in causing LD is important with regard to devising methods for managing and,

ideally, controlling the disorder.

8.2 Improvement of pre- and postharvest management through irrigation, solution

treatments and brushing to minimise lenticel damage on B74 fruit

The impact of an important pre-harvest practice (i.e. irrigation) and of postharvest practices (i.e.

aqueous solutions and brushing) on fruit attributes, LD and lenticel morphology were investigated

in the current work to better understand their apparent roles in LD. Studies suggested that

withholding irrigation before harvest can reduce LD on Keitt (Cronje 2009a) and also on KP

(Simmons et al. 1998) mango fruit. However, in the present research (Chapter 5), B74 fruit

harvested after 3 - 8 weeks of no irrigation had similar or even higher LD than fruit from standard

irrigation treatments. This was the case even though withholding irrigation also resulted in

significantly reduced soil moisture content for 2 weeks before harvest and leaf stomatal

conductance at harvest. On the positive side, this lack of effect on LD and other fruit quality

parameters such as fruit size suggests that cost savings can be realised by withholding irrigation

before harvest, at least for fruit that are not subsequently irradiated for disinfestation (Chapter 5).

Under typical Australian commercial practice, B74 mango fruit are harvested from the trees, de-

stemmed, dipped into detergent solution to minimise sap-related damage, placed into 450 kg field

bins and transported to the packing house. At the packing house, the fruit are treated with

fungicide, dried under forced air, brushed, sorted on conveyors and packed into single-layer

cardboard trays with plastic inserts. Thereafter, they are usually cooled to 14 - 16 ºC, transported to

wholesale markets or retail distribution centres, ripened with ethylene and stored at 20 °C (Hofman

et al. 2010b). Pre-harvest and postharvest water and aqueous solution contact with B74 fruit

(Hofman et al. 2010a) and fruit of other mango cultivars such as KP and Tommy Atkins (Bally et

al. 1997, Jacobi and Giles 1997, Oosthuyse 1998, Self et al. 2006, Shorter and Joyce 1998) has

been shown to increase LD severity. One of the key parameters for solution penetration into

138

lenticel cavities is the wettability of the fruit surface, which can be characterised by the wetting

angle (Eichert and Burkhardt 2001, Peschel et al. 2003, Ruckenstei and Lee 1975, Schonherr and

Bukovac 1971). Reducing the wetting angle of DI water or coating solutions by adding 0.1%

surfactant (Maxx Organosilicone Surfactant™) was demonstrated to increase the entry capacity of

DI water and solutions into lenticel cavities (Chapter 7). Moreover, dipping fruit in DI water with

agitation increased LD severity in ripe fruit as compared to DI water dipping without agitation

(Chapter 6). In commercial practice, all fruit and solution contact is associated with agitation, such

as washing in the field upon harvest, dipping in detergent solution on a moving harvest truck and

treating with fungicide on conveyors in the pack shed. Agitation likely facilitates water entry into

lenticels by disturbing the air film otherwise trapped by the fruit cuticle.

Solution entry into cells lining the lenticel cavity could cause cell turgor increases depending on the

balance between the osmotic potential of the solution and the water potential of the cells. In

adjusting solution osmotic potential, B74 fruit treated with 1% NaCl expressed similar LD severity

as did the off-tree control fruit (Chapter 6). The osmotic potential of 1% NaCl at -0.8 MPa is

approximately equal to that of plant cells. The equilibrium may restrict water uptake by the cell and

thereby reduce the potential for increases in cell turgor. Solutions of 2 - 3% NaCl had a more

negative osmotic potential than the typical osmotic potential of plant cells, and further increased LD

severity. It is possible that the Na+ and / or Cl- ions had phytotoxic effects. Adding a surfactant,

Agral®, also resulted in greater LD severity. The surfactant presumably facilitated solution entry

into lenticels and may have further damaged lenticels by virtue of its chemical composition.

Knoche (1994) also found that solutions containing a silicone surfactant could penetrate open

stomata in many plants. Fruit treated with polyethylene glycol 6000 also had high LD severity.

This non-ionic osmoticum was observed to concentrate with drying on the fruit surface and possibly

retained high moisture levels in and over the lenticels. Overall, these findings confirmed the

negative impacts of water plus agitation and aqueous solutions in increasing LD severity on B74

fruit.

Brushing the fruit surface has been shown to contribute to LD on mangoes (Cronje 2009b, Du

Plooy et al. 2002, Oosthuyse 2002). On Kent mango fruit, brushing perturbed lenticel structure and

modified the cuticular wax layer structure (Du Plooy et al. 2002). However, under the conditions of

the present work, brushing had no significant (P > 0.05) effect on the severity of LD that developed

on B74 mango fruit at the ripe stage, as compared to fruit from the field bin (untreated) control. At

7 days after full colour, most of brushed fruit had similar or lower LD severity to fruit from the field

bin control, except for fruit exposed to hard brushing with no water for 2 minutes which contributed

139

to higher LD severity than the field bin control. Soft brushing with or without water for 1 or 2

minutes and hard wet brushing for 1 minute ensured adequate residue removal and polishing

without increasing LD severity or skin browning.

8.3 Potential for bagging and coatings to reduce lenticel damage on B74 fruit

Water and aqueous solutions were shown to be triggers of LD in this (Chapter 6) and previous

studies (Bally et al. 1997, Everett et al. 2008, Hofman et al. 2010a, Jacobi and Giles 1997,

Oosthuyse 1998, Self et al. 2006, Shorter and Joyce 1998). Preventing fruit and solution contact

through bagging and coatings before harvest or after harvest was shown in Chapter 7 to reduce LD

on B74 mango fruit. A low concentration of a carnauba wax coating, provided as 2.5% Natural

Shine™ TFC210 (TFC), was more effective than a high concentration (5%), possibly because a

lower concentration entered into the lenticel cavity more readily. Mixing TFC with a surfactant

(Maxx Organosilicone Surfactant™) reduced the wetting angle of the solution and facilitated the

entry of coating material into lenticel cavity (Chapter 7). In Chapter 6, the outcome of adding a

surfactant, Agral®, was considered. This action resulted in higher LD severity, possibly because of

its active chemical component/s. In contrast, Maxx surfactant may be less or not harmful to cells.

Adding a surfactant into 15 - 30% TFC during both pre- and postharvest coating evidently

improved coating efficiency by presumably increasing the filling of lenticel cavities with the

coating material. Thereby, a relatively high concentration of TFC may supplement the natural

lenticel coating and thereby reduce LD from postharvest handling operations.

8.4 Directions for future research

The physical characteristics of B74 mango fruit lenticels are inherited from its parent cultivars, KP

and Sensation. In particular, the distinctive high density and susceptibility to browning of B74

lenticels appear to be inherited from Sensation and KP, respectively. Together, these traits may

explain the apparently high LD sensitivity of B74. To gain a complete understanding of the genetic

inheritance, there is a need to comparatively characterise the relevant biochemistry and anatomy in

B74 fruit and those of its parents. In particular, changes in phenolic compounds, activities of

browning enzymes (viz., polyphenoloxidases and peroxidases) in and around lenticels at harvest

and during ripening on these three cultivars warrant investigation. It is also important to

anatomically elucidate cell level responses of organelle decompartmentation, phenolics

condensation and cell necrosis. Perception and signalling processes in lenticel cells are of allied

interest in regard to understanding apparently highly functional defence responses.

140

In relation to more applied management of LD on B74 mango fruit, it could be opportune to

examine on-tree coating applied as whole tree sprays, including their effects on tree performance,

fruit yield and fruit quality in following seasons. Postharvest coatings may be a more promising

management approach compared with bagging because it could be more economically applied on a

large scale. However, the effects of coating fruit should be evaluated through commercial packing

lines and supply chains, including irradiation, transport and ripening. Also, altering the wetting

angle of coating solutions with surfactants may increase the efficiency of entry of the coating

material into the lenticel cavities and reduce the negative effects of water on the surrounding cells.

Finally, several approaches could be taken to develop early indicators of low LD sensitivity to assist

breeding programs. These could include counting lenticel density in developing fruit, determining

if a relationship exists between stomatal density on leaves and lenticel density on mature fruit, and a

rapid and reliable test (e.g. solute treatment or irradiation) to determine the potential for browning

of lenticels.

141

REFERENCES

Agusti, M., Almela, V., Juan, M., Alferez, F., Tadeo., F.R. and Zacarias, L. (2001) Histological and

physiological characterization of rind breakdown of ‘Navelate’ sweet orange. Annals of Botany

88, 415-422.

Ahuja, S., Kumar, M., Kumae, P., Gupta, V.K., Singhal, R.K., Yadav, A. and Singh, B. (2014)

Metabolic and biochemical changes caused by gamma irradiation in plants. Journal of

Radioanal Nuclear Chemistry 300, 199-212.

Ajila, C.M., Jaganmohan Rao, L. and Prasada Rao, U.J.S. (2010) Characterization of bioactive

compounds from raw and ripe Mangifera indica L. peel extracts. Food and Chemical

Toxicology 48, 3406-3411.

Alberti, G. and De Simone, A. (2005) Wetting of rough surfaces: a homogenization approach.

Proceeding of Royal Society A 461, 79-97.

Al-Yahyai, R. (2012) Managing irrigation of fruit trees using plant water status. Agricultural

Sciences 3(1), 35-43.

AMIA (2011) Mango production data in Australia. Accessed online on 2nd December 2011:

http://www.mangoes.net.au.

Baez-Sañudo, M., Siller-Cepeda, J., Contreras-Martinez, R., Contreras-Angulo, L., Velez, R. and

Muy-Rangel, D. (2005) Commercial film coatings reduce weight loss and improve

appearance of `Keitt' mango fruits (Mangifera indica L.). HortScience 40, 994. Abstract.

Bally, I.S.E., O’Hare, T.J. and Holmes, R.J. (1997) Detrimental effects of detergent in the

development of mango skin browning Acta Horticulturea, 455, 612-621.

Bally, I.S.E. (1999) Changes in the cuticular surface during the development of mango (Mangifera

indica L.) cv. Kensington Pride. Scientia Horticulturae 79, 13-22.

Beckman, C.H. (2000) Phenolic-storing cells: keys to programmed cell death and periderm

formation in wilt disease resistance and in general defence responses in plants? Physiological

and Molecular Plant Pathology 57, 101-110.

Berardini, N., Knodler, M., Schieber, A. and Carle, R. (2005) Utilization of mango peels as a source

of pectin and polyphenolics. Innovative Food Science and Emerging Technologies 6, 442-452.

Bezuidenhout, J.L.J., Robbertse, P.J. and Kaiser, C. (2005) Anatomical investigation of lenticel

development and subsequent discolouration of ‘Tommy Atkins’ and ‘Keitt’ mango (Mangifera

indica L.) fruit. Journal of Horticultural Science and Biotechnology 80(1), 18-22.

Bolarin, M.C., Estan, M.T., Caro, M., Romero-Aranda, R. and Cuartero, J. (2001) Relationship

between tomato fruit growth and fruit osmotic potential under salinity. Plant Science 160, 1153-

1159.

http://www.mangoes.net.au/

142

Boyers, J.S., James, R.A., Munns, R., Condon, T.A.G. and Passioura, J.B. (2008) Osmotic

adjustment leads to anomalously low estimates of relative water content in wheat and barley.

Functional Plant Biology 35, 1172-1182.

Brown, K. and Considine, J. (1982) Physical aspect of fruit growth: stress distribution around

lenticels. Plant Physiology 69, 585-590.

Burdon, J.N., Moore, K.G. and Wainwright, H. (1991) Mineral distribution in mango fruit

susceptible to the physiological disorder soft-nose. Scientia Horticulturae 48, 329-336.

Burdon, J. and Clark, C. (2001) Effect of postharvest water loss on ‘Hayward’ kiwifruit water

status. Postharvest Biology and Technology 22, 215-225.

Byers, R.E., Carbaugh, D.H. and Presley, C.N. (1990) Stayman fruit cracking as affected by

surfactants, plant growth regulators, and other chemicals. Journal of American Society for

Horticultural Science 115(3), 405-411.

Chonhenchob, V., Kamhangwong, D., Kruenate, J., Khongrat, K., Tangchantra, N., Wichaid U. and

Singhe S.P. (2010) Preharvest bagging with wavelength-selective materials enhances

development and quality of mango (Mangifera indica L.) cv. Namdokmai #4. Journal of

Science of Food and Agriculture 91, 664-671.

Chidtragoola, S., Ketsaa, S., Bowend, J., Fergusond, I.B. and Van Doorn, W.G. (2011) Chilling

injury in mango fruit peel: Cultivar differences are related to the activity of phenylalanine

ammonia lyase. Postharvest Biology and Technology 62, 59-63.

Cochrane, T.T and Cochrane T.A. (2005) Osmotic potential properties of solutes common in the

soil-plant solution continuum. Soil Science 170(6), 433-444.

Cronje, R.B. (2009a) Effect of harvesting practices and pre-packing storage on lenticel

discolouration of mango. Acta Horticulturae 820, 653-664.

Cronje, R.B. (2009b) Effect of packhouse procedures on lenticel discolouration of mangoes. Acta

Horticulturae 820, 673-679.

Cronje, P.J.R., Barry, G.H. and Huysamer, M. (2011) Postharvest rind breakdown of ‘Nules

Clementine’ mandarin is influenced by ethylene application, storage temperature and storage

duration. Postharvest Biology and Technology 60, 192-201.

Curry E. and E Kupferman (2004) Predicting susceptibility of Gala apples to lenticel breakdown

disorder: Guidelines for using the dye uptake test. Postharvest Inform Network. Washington

State University. http://postharvest.tfrec.wsu.edu/aniline-blue.pdf.

Curry, E.A., Torres, C. and Neubauer, L. (2008) Preharvest lipophilic coatings reduce lenticel

breakdown disorder in ‘Gala’ apples. HortTechnology 18(4), 690-696.

http://postharvest.tfrec.wsu.edu/aniline-blue.pdf

143

Dang, T.H.K., Singh, Z. and Swinny, E.E. (2008) Edible coatings influence fruit ripening, quality,

and aroma biosynthesis in mango fruit. Journal of Agricultural and Food Chemistry 56, 1361-

1370.

Dietz, T.H., Raju, K.R.T. and Joshi, S.S. (1988a) Structure and development of cuticle and lenticels

in fruits of certain cultivars of mango. Acta Horticulturae 231, 457-460.

Dietz, T.H., Raju, K.R.T. and Joshi, S.S. (1988b) Studies on loss of weight of mango fruit as

influenced by cuticle and lenticels. Acta Horticulturae 231, 685-687.

Do C.B. and Cormier, F. (1990) Accumulation of anthocyanins enhanced by a high osmotic

potential in grape (Vitis vinifera L.) cell suspensions. Plant Cell Reports 9, 143-146.

Do, M.H., Nguyen, T.T. and Huynh, V.V. (2006) Evaluation report for mango industry in Tien

Giang and Dong Thap provinces. Final report, Southern Fruit Research Institute, Vietnam.

Dogan, S., Turan, P., Dogan, M., Arslan, O. and Alkan, M. (2007) Variations of peroxidase activity

among Salvia species. Journal of Food Engineering 79, 375-382.

Du Plooy, G.W., Van der Merwe, C.F. and Korsten, L. (2002) Changes to the epicuticular wax

layer of mango (cv. Kent) due to handling along a commercial packline. South African Mango

Growers’ Association Research Journal 32-37.

Du Plooy, G.W., Van der Merwe, C.F. and Korsten, L. (2004) Differences in the surface structures

of three mango cultivars and the effect of kaolin on these structures. South African Mango

Growers’ Association Research Journal 24, 29-36.

Du Plooy, G.W., Van der Merwe, C.F. and Korsten, L. (2006) Lenticel discolouration in mango

(Mangifera indica L.) fruit - a cytological study of mesophyll cells from affected tissue. Journal

of Horticulture Science and Biotechnology 81, 869-873.

Du Plooy, G.W., Combrinck, S., Regnier, T. and Botha, B.M. (2009a) Linking lenticel

discolouration of mango (Mangifera indica L.) fruit to reversed-phase HPLC profiles of

phenolic compounds. Journal of Horticultural Science and Biotechnology 84(4), 421-426.

Du Plooy, G.W., Combrinck, S., Botha, B., Van der Merwe, C.F. and Regnier, T. (2009b)

Development of discolouration in mango lenticels. Acta Horticulturae 820, 665-672.

Durigan, J.F., Teixeira, G.H.d.A., Castanharo, N.M. and Domarco, R.E. (2004) Postharvest

conservation of 'Tommy Atkins' mango fruit influenced by gamma radiation, wax, hot water,

and refrigeration. Acta Horticulturae 645, 601-604.

Duvenhage, J.A. (1993) The influence of wet picking on postharvest diseases and disorders of

avocado fruit. South Africa Mango Growers’ Association Research Journal 16, 77-79.

Eichert, T. and Burkhardt, J. (2001) Quantification of stomatal uptake of ionic solutes using a new

model system. Journal of Experimental Botany 52(357), 771-781.

144

Eigenbrode, S.D. and Espelie, K.E. (1995) Effects of plant epicuticular lipids on insect herbivores.

Annual Review of Entomology 40, 171-194.

Elsheery, N.I. and Cao, K.F. (2008) Gas exchange, chlorophyll fluorescence, and osmotic

adjustment in two mango cultivars under drought stress. Acta Physiologiae Plantarum 30, 769-

777.

El-Siddig, K., Inanaga, S., Ali, A.M., An, P., Gebauer, J. and Ebert, G. (2004) Response of

Tamarindus icdica L. to iso-osmotic solutions of NaCl and PEG during germination. Journal of

Applied Botany 78, 1-4.

Everett, K.R., Hallett, I.C., Rees-George, J., Chynoweth, R.W. and Pak H.A. (2008) Avocado

lenticel damage: The cause and the effect on fruit quality, Postharvest Biology and Technology

48, 383-390.

Ferguson, I.B., Thorp, T.G., Barnett, A.M., Boyd, L.M. and Triggs, C.M. (2003) Inorganic nutrient

concentrations and physiological pitting in `Hayward' kiwifruit. Journal of Horticultural

Science and Biotechnology 78(4), 497-504.

Feygenberg, O., Keinan, A., Kobiler, I., Falik, E., Pesis, E., Lers, A. and Prusky, D. (2014)

Improved management of mango fruit though orchard and packinghouse treatments to reduce

lenticel discoloration and prevent decay. Postharvest Biology and Technology 91, 128-133.

Feys, M., Naesens, W., Tobback, P. and Maes, E. (1980) Lipoxygenase activity in apples in relation

to storage and physiological disorders, Phytochemistry 19, 1009-1011.

Franck, C., Lammertyna, J., Ho, Q.T., Verboven, P., Verlinden, B. and Nicola, B.M. (2007)

Browning disorders in pear fruit. Postharvest Biology and Technology 43, 1-13.

Gazzola, R., Alves, R.E., Filgueiras, H.A.C. and Soares, A.A. (2004) Permeability and microscopic

structure of the cuticle in 'Tommy Atkins' mangoes skin. Acta Horticulturae 645, 589-593.

Grove, T., Steyn, W.P. and De Beer, M.S. (2004) Irradiation and the effect on mango fruit quality.

South African Mango Growers’ Association Research Journal 24, 22-25.

Haminiuk, C.W.I., Maciel, G.M., Plata-Oviedo, M.S.V. and Peralta, R.M. (2012) Phenolic

compounds in fruits – an overview. International Journal of Food Science and Technology 47,

2023-2044.

Hassan, M.K., Irving, D.E., Dann, E.K., Coates, L.M. and Hofman, P.J. (2009) Sap properties and

alk(en)ylresorcinol concentrations in Australian-grown mangoes. Annals of Applied Biology

154, 419-427.

Hatfield, S.G.S. and Knee, M. (1988) Effects of water loss on apples in storage. International

Journal of Food Science and Technology 23, 575-583.

Hess F.D. and Foy C.L. (2000) Interaction of surfactants with plant cuticles. Weed Technology 14,

807-813.

145

Hernandez-Sanchez, N., Hills, B.P., Barreiro, P. and Marigheto, N. (2007) An NMR study on

internal browning in pears. Postharvest Biology and Technology 44, 260-270.

Hofman, P.J., Smith, L.G., Joyce, D.C., Johnson, G.I. and Meiburg, G.F. (1997) Bagging of mango

(Mangifera indica cv. ‘Keitt’) fruit influences fruit quality and mineral composition.

Postharvest Biology and Technology 12, 83-91.

Hofman P.J. and Ledger, S.N. (2005) Commercialising new cultivar requires a supply chain

approach. Acta Horticulturae 694, 387-392.

Hofman, P.J., Marques, J.R., Taylor, L.M., Stubbings, B.A., Ledger, S.N. and Jordan, R.A. (2010a)

Skin damage to two new mango cultivars during irradiation and cold storage. Acta Horticulturae

877, 475-481.

Hofman, P.J., Whiley, A., Marques, J.R., Stubbings, B.A., Taylor, L. and Stirling, L.J. (2010b)

Development of best practice pre- and postharvest of B74 mango: Phase II. Final report

MG06005. Horticulture Australia Ltd., Sydney.

IMS (2010) The report of the 9th International Mango Symposium in China. Accessed online on

22nd April 2011: http://www.mango(2010).cn.

Iwanami, H., Yamada, M. and Sato, A. (2002) A great increase of soluble solids concentration by

shallow concentric skin cracks in Japanese persimmon. Scientia Horticulturae 94, 251-256.

Jacobi, K.K. and Giles, J.E. (1997) Quality of ‘Kensington’ mango (Mangifera indica Linn.) fruit

following combined vapour heat disinfestation and hot water disease control treatments.

Postharvest Biology and Technology 12, 285–292.

Jacobi, K.K., MacRae, E.A. and Hetherington, S.E. (2001) Loss of heat tolerance in ‘Kensington’

mango fruit following heat treatments. Postharvest Biology and Technology 21, 321-330.

Janave, M.T. and Sharma, A. (2008) Spongy tissue development in Alphonso mango: association

with Staphylococcus xylosus. European Journal of Plant Pathology 122, 335-348.

Jiang, Y., Duan, X., Joyce, D., Zhang, Z. and Li, J. (2004) Advances in understanding of enzymatic

browning in harvested litchi fruit. Food Chemistry 88, 443-446.

Jiang, Y., Lib, J. and Jiang, W. (2005) Effects of chitosan coating on shelf life of cold-stored litchi

fruit at ambient temperature. Food Science and Technology 38, 757-761

Jobling, J.J., Patterson, B.D., Moradi, S. and Joyce, D. (1997) A non-destructive method for

measuring the water potential of fruit and vegetables. Postharvest Biology and Technology 10,

l-8.

Johnson, G.I. (2000) Introduction of the mango to Australia. Proceedings of the Royal Society of

Queensland 109, 83-90.

146

Johnson, G.I., Boag, T.S., Cooke, A.W., Izard, M., Panitz. M. and Sangchote, S. (1990) Interaction

of postharvest disease control treatments and gamma irradiation on mangoes. Annals of Applied

Biology 116, 245-257.

Johnson, G.I. and Hofman, P.J. (2009) Postharvest technology and quarantine treatments’ in Litz

R.E. (ed.), The Mango, 2nd Edition - Botany, production and uses, CABI, Wallingford, UK,

529-605.

Jongdee, B., Fukai, S. and Cooper, M. (2002) Leaf water potential and osmotic adjustment as

physiological traits to improve drought tolerance in rice. Field Crop Research 76, 153-163.

Joyce, D.C., Beasley, D.R. and Shorter, A.J. (1997) Effect of preharvest bagging on fruit calcium

levels, storage and ripening characteristics of Sensation mangoes. Australian Journal of

Experimental Agriculture 37, 383-389.

Joyce, D.C., Shorter, A.J. and Hockings, P.D. (2001) Mango fruit calcium levels and the effect of

postharvest calcium infiltration at different maturities. Scientia Horticulturae 91, 81-99.

Kajiura, I., Yamaki, S., Omura, M. and Shimura, I. (1976) Watercore in Japanese pear (Pyrus

Serotina var. ‘Culta’ Rehder). I. Description of the disorder and its relation to fruit maturity.

Scientia Horticulturae 4, 261-270.

Kakani, V.G., Reddy, K.R., Zhao, D. and Mohammed, A.R. (2003) Effect of ultraviolet-B on cotton

(Gossypium hirsutum L.) morphology and anatomy. Annals of Botany 91, 817-826.

Kays S.J. and Paull R.E. (2004) Postharvest Biology. (Exon Press: Athen, USA).

Ketelaere, B.D., Howarth, M. S., Crezee, L., Lammertyn, J., Viaene, K., Bulens, I. and

Baerdemaeker, J.D. (2006) Postharvest firmness changes as measured by acoustic and low-

mass impact devices: a comparison of techniques. Postharvest Biology and Technology 41,

275-284.

Ketsa, S. Phakawatmongkol, W. and Subhadrabhandhu, S. (1999) Peel enzymatic activity and

colour changes in ripening mango fruit. Journal of Plant Physiology 154, 363-366.

Khanal, B.P., Grimm, E. and Knoche, M. (2011) Fruit growth, cuticle deposition, water uptake, and

fruit cracking in jostaberry, gooseberry, and black currant. Scientia Horticulturae 128, 289-296.

Kim, H., Moon, J.Y., Kim, H., Lee, D.S., Cho, M., Choi, H.K., Kim, Y.S., Mosaddik, A. and Cho,

S.K. (2010) Antioxidant and antiproliferative activities of mango (Mangifera indica L.) flesh

and peel. Food Chemistry 121, 429-436.

Knoche, M., (1994) Organosilicone surfactants: performance in agricultural spray application: a

review. Weed Research 34, 221-239.

Lamour, G. and Hamraoui, A. (2010) Contact angle measurements using a simplified experimental

setup. Journal of Chemical Education 87(12), 1403-1407.

147

Landrigan, M., Morris, S.C., Eamus, D. and McGlasson, W.B. (1996) Postharvest water

relationships and tissue browning of rambutan fruit. Scientia Horticulturae 66, 201-208.

Lechaudel, M. M., Lescourrer, G. F., Urban, L. and Jannoyer, M. (2005) Leaf-to-fruit ratio affects

water and dry-matter content of mango fruit. Journal of Horticultural Science & Biotechnology

77(6), 773-777.

Lin, D. and Zhao, Y. (2007) Innovations in the development and application of edible coatings for

fresh and minimally processed fruits and vegetables. Comprehensive Food Science and Food

Safety 6, 60-75.

Loveys, B.R., Robinson, S.P., Brophy, J.J. and Chacko, E.K. (1992) Mango sapburn: components

of fruit sap and their role in causing skin damage. Australian Journal of Plant Physiology 19,

449-457.

Lu, P., Chacko, E.K., Bithell, S.L., Schaper, H., Wiebel, J., Cole, S. and Müller, W.J. (2012)

Photosynthesis and stomatal conductance of five mango cultivars in the seasonally wet-dry

tropics of northern Australia. Scientia Horticulturae 138, 108-119.

Marques, R., Nguyen, M., Hofman, P. and Joyce, D. (2013) Do late-harvested mangoes develop

more lenticel damage after harvest. Australia Mango Industry Association Report 29-31.

Masibo, M. and He, Q. (2008) Major mango polyphenols and their potential significance to human

health. Comprehensive Reviews in Food Science and Food Safety 7, 309-319.

Mavroudis, N.E., Dejmek, P. and Sjoholm, I. (2004) Osmotic-treatment-induced cell death and

osmotic processing kinetics of apples with characterised raw material properties. Journal of

Food Engineering 63, 47-56.

Mc Lauchlan, R.L., Johnson, G.I., Mitchell, G.E. and Wills, P.A. (1990) Irradiation of Kensington

Pride mangoes. Acta Horticulturae 269, 469-476.

Mexal, J., Fisher, J.T., Osteryoung, J. and Patrick Reid, C.P. (1975) Oxygen availability in

polyethylene glycol solutions and its implications in plant-water relations. Plant Physiology 55,

20-24.

Michel, B.E. and Kaufmann, R. (1973) The osmotic potential of polyethylene glycol 6000. Plant

Physiology 51, 914-916.

Mitra, S.K. and Baldwin, E.A. (1997). Mango. In: Mitra S.K.(Ed), Postharvest Physiology and

Storage of Tropical and Subtropical Fruits. CAB International. New York, 85-122.

Moalemiyan, M., Ramaswamy, H.S. and Maftoonazad, N. (2010) Pectin-based edible coating for

shelf-life extension of Ataulfo mango. Journal of Food Process Engineering 35, 572-600.

Nair, S., Singh, Z. and Tan, S.C. (2003) Aroma volatiles emission in relation to chilling injury in

'Kensington Pride' mango fruits. The Journal of Horticultural Science and Biotechnology 78,

866-873.

148

Ndiaye, C., Xu, S.Y. and Wang Z. (2009) Steam blanching effect on polyphenoloxidase, peroxidase

and colour of mango (Mangifera indica L.) slices. Food Chemistry 113, 92-95.

O’Brien, T.P. and McCully, M.E. (1981) The study of plant structure principles and selected

methods. Termarcarphi Pty. Ltd., Melbourne, Australia.

O’Hare, T.J. and Prasad, A. (1992) The alleviation of sap-induced mango skin injury by calcium

hydroxide. Acta Horticulturae 321, 372-381.

O’Hare, T.J., Bally, I.S.E., Dahler, J.M., Saks, Y. and Underhill, S.J.R. (1999) Characterisation and

induction of ‘etch’ browning in the skin of mango fruit. Postharvest Biology and Technology

16, 269-277.

Oosthuyse S.A. (1998) Effect of environmental conditions at harvest on the incidence of lenticel

damage in mango. South African Mango Growers’ Association Research Journal 18, 15-17.

Oosthuyse, S.A. (1999) Effects of each of the stages in the pack-line on the incidence of lenticel

damage in Keitt and Tommy Atkins mango. South African Mango growers' Association

Research Journal 19, 37-39.

Oosthuyse, S.A. (2002) Lenticel damage in mango: Influence of harvest date and cold-storage

conditions. South Africa Mango Growers’ Association Research Journal 20-27.

Oosthuyse, S.A. (2007a) Effect of placing paper sleeves over Tommy Atkins, Kent or Keitt

mangoes, prior to harvest, on diseases incidence and physico-chemical characteristics after

extended cold-storage. South African Mango Growers’ Association Research Journal 27, 75-79.

Oosthuyse S.A. (2007b) Effect of pre-harvest polymer coating of Tommy Atkins, Kent or Keitt

mangoes on diseases incidence and fruit physico - chemical characteristics after extended cold-

storage. South African Mango Growers’ Association Research Journal 27, 80-83.

Ortiz, I.G., Sugaya, S., Sekozawa, Y., Ito, H., Wada, K. and Gemma, H. (2005) Efficacy of 1-

methylcyclopropene 1-MCP in prolonging the shelf-life of Rendaiji persimmon fruits

previously subjected to astringency removal treatment. Journal of the Japanese Society and

Horticultural Science 74(3), 248-254.

Palafox-Carlos, H., Yahia, E.M. and González-Aguilar, G.A. (2012) Identification and

quantification of major phenolic compounds from mango (Mangifera indica, cv. Ataulfo) fruit

by HPLC–DAD–MS/MS-ESI and their individual contribution to the antioxidant activity during

ripening. Food Chemistry 135, 105-111.

Park, M.Y. and Lee, Y.J. (2006) Ripening responses and quality changes of Fuyu persimmon fruit

as influence be exogenous ethylene and subsequent short-term storage temperature. Korean

Journal of Horticultural Science and Technology 24(2), 216-221.

Payasi, A., Mishra, N.N., Chaves, A.L.S. and Singh, R. (2009) Biochemistry of fruit softening: an

overview. Physiology and Molecule Biology of Plants 15(2), 103-113.

149

Pereira, A.V. and Calbo, A.G. (2000) Elastic stresses and plastic deformations in ‘Santa Clara’

tomato fruits caused by package dependent compression. Pesquisa Agropecuária Brasileira 35,

2429–2436.

Peschel, S., Beyer, M. and Knoche, M. (2003) Surface characteristics of sweet cherry fruit: stomata-

number, distribution, functionality and surface wetting. Scientia Horticulturae 97, 265-278.

Peschel, S., Franke, R., Schreiber, L. and Knoche, M. (2007) Composition of the cuticle of

developing sweet cherry fruit. Phytochemistry 68, 1017-1025.

Pesis, E., Aharoni, D., Aharon, Z., Ben-Arie, R., Aharoni, N. and Fuchs, Y. (2000) Modified

atmosphere and modified humidity packaging alleviates chilling injury symptoms in mango

fruit. Postharvest Biology and Technology 19, 93-101.

Pesis, E. (2005) The role of the anaerobic metabolites, acetaldehyde and ethanol, in fruit ripening,

enhancement of fruit quality and fruit deterioration. Postharvest Biology and Technology 37, 1-

19.

Petit-Jimenez, D., Gonzales-Leon, A., Gonzales-Aguilar, G., Sotelo-Mundo, R. and Baez-Sanudo,

R. (2009) Permeability of cuticular membrane during the ontogeny of Mangifera indica L. Acta

Horticulturea 820, 213-220.

Prabhu, K.N., Femades, P. and Kumar, G. (2009) Effect of substrate surface roughness on wetting

behaviour of vegetable oils. Materials and Design 30, 297-305.

Prinsloo, L.C., Du Plooy, W. and Van der Merwe, C. (2004) Raman spectroscopic study of the

epicuticular layer of mature mango Mangifera indica. Journal of Raman Spectroscopy 35, 561-

567.

Purvis, A.C. (2004) Regulation of oxidative stress in horticultural crops. HortScience 39, 930-932.

Queiroz, C., Lopes, M.L.M., Fialho, E. and Valente-Mesquita, V.L. (2008) Polyphenol oxidase:

characteristics and mechanisms of browning control. Food Reviews International 24(4), 361-

375.

Raison, J.K. and Lyons, J.M. (1986) Chilling injury: a plea for uniform terminology. Plant Cell and

Environment 9(9), 685-686.

Reyes, L.F. and Cisneros-Zevallos, L. (2007) Electron-beam ionizing radiation stress effects on

(Mangifera indica L.) antioxidant constituents before during postharvest storage. Journal

Agricultural Food Chemistry 55, 6132-6139.

Ribeiro, S.M.R., Barbosa, L.C.A., Queiroz, J.H., Knodler, M. and Schieber, A. (2008) Phenolic

compounds and antioxidant capacity of Brazilian mango (Mangifera indica L.) varieties. Food

Chemistry 110, 620-626.

Robards, K., Prenzler, P.D., Tucker, G., Swatsitang, P. and Glover, W. (1999) Phenolic compounds

and their role in oxidative processes in fruits. Food Chemistry 66, 401-436.

150

Robinson, S.P., Loveys B.R. and Chacko, E.K. (1993) Polyphenol oxidase enzymes in the sap and

skin of mango fruit. Australian Journal of Plant Physiology 20, 99-107.

Ruckenstei, E. and Lee, P.S. (1975) The wetting angle of very small and large drops. Surface

Science 52, 298-310.

Rymbai, H., Srivastav, M., Sharma, R.R. and Singh, S.K. (2012) Lenticels on mango fruit: origin,

development, discoloration and prevention of their discoloration. Scientia Horticulturae 135,

164-170.

Saftner, R. A. and Conway, W. S. (1998) Effect of postharvest calcium chloride treatments on

tissue water relation, cell wall calcium levels and postharvest life of Golden Delicious apple.

Journal of American Horticultural Science 123(5), 893-897.

Samuels, L., Kunst, L. and Jetter, R. (2008) Sealing plant surfaces: cuticular wax formation by

epidermal cells. Annual Review of Plant Biology 59, 683-707.

Salvador, A., Carvalho, C.P., Monterde, A. and Martìnez-Jávega, J.M. (2006) Note. 1-MCP Effect

on Chilling Injury Development in 'Nova' and 'Ortanique' Mandarins. Food Science and

Technology International 12, 165-171.

Sauco, V.G. (2009) Physiological Disorders in RE Litz (ed.), The Mango: Botany, production and

uses, CAB International Publishing, Wallingford, UK, 303-312.

Scandalios, J.G. (1993) Oxygen stress and superoxide dismutase. Plant Physiology 101, 7-12.

Schonherr, J. and Bukovac, M.J. (1971) Penetration of stomata by liquids, dependance on surface

tension, wettability and stomata morphology. Plant Physiology 49, 813-819.

Schotsmans, W., Verlinden, B.E., Lammertyn, J. and Nicola, B.M. (2004) The relationship between

gas transport properties and the histology of apple. Journal of the Science of Food and

Agriculture 84, 1131-1140.

Schrader, L. and Sun, J. (2006) Cheery cracking: cause and suspension. Yearly Proceeding of

Oregon Horticultural Society. Accessed online on 2nd December 2012:

http://www.mangoes.net.au.

Schrader, L., Sun, J., Zhang, J., Felicetti, D. and Tian, J. (2008) Heat and light-induced apple skin

disorders: causes and prevention. Acta Horticulturae 772, 51-58.

Scora, R.W., Wolstenholme, B.N. and Lavi, U. (2002) Taxonomy and botany’ in Whiley, A,

Schaffer, B. and Wolstenholme, B. (eds.) The Avocado: Botany, Production and Uses. CAB

International Publishing, Wallingford, UK, 15-37.

Self, Q., De Assis, J.S. and Caron, V.C. (2006) Effects of postharvest handling on lenticel spotting

of Tommy Atkins mangoes from northeast Brazil. Acta Horticulturae 712, 543-550.

http://www.mangoes.net.au/

151

Shackel A.K (1987) Direct measurement of turgor and osmotic potential in individual epidermal

cells – Independent confirmation of leaf water potential as determine by in Situ psychrometry.

Plant Physiology 83, 719-722.

Sharma, R.R. and Singh, R. (2009) The fruit pitting disorder - A physiological anomaly in mango

(Mangifera indica L.) due to deficiency of calcium and boron. Scientia Horticulturae 119, 388-

391.

Shivashankara, K.S. and Mathai, C.K. (1999) Relationship of leaf and fruit transpiration rates to the

incidence of spongy tissue disorder in two mango (Mangifera indica L.) cultivars. Scientia

Horticulturae 82, 317-323.

Shorter, A.J. and Joyce, D.C. (1998) Effect of partial pressure infiltration of calcium into

Kensington mango fruit. Australian Journal of Experiment Agriculture 38, 287-294.

Simmons, S.L., Hofman, P.J. and Hetherington, S.E. (1995) The effects of water stress on mango

fruit quality. Proceedings of the 2000 Mango Production Workshop 191-197.

Simmons, S.L., Hofman, P.J., Whiley, A.W. and Hetherington, S.E. (1998) Effects of leaf-fruit

ratios on fruit growth, mineral concentration and quality of mango (Mangifera indica L. cv.

Kensington Pride). Journal of Horticultural Science and Biotechnology 73, 367-374.

Sivakumar, D., Jiang, Y. and Yahia, E.M. (2011) Maintaining mango (Mangifera indica L.) fruit

quality during the export chain. Food Research International 44, 1254-1263.

Spreer, W., Nagle, M., Neidhart, S., Carle, R., Ongprasert, S. and Muller, J. (2007) Effect of

regulated deficit irrigation and partial rootzone drying on the quality of mango fruits (Mangifera

indica L., cv. ‘Chok Anan’). Agricultureral Water Management 88, 173-180.

Spreer, W., Ongprasert, S., Hegele, M., Wunsche, J. N. and Muller, J. (2009) Yield and fruit

development in mango (Mangifera indica L. cv. Chok Anan) under different irrigation regimes.

Agricultureral Water Management 96, 574-584.

Swati, Z.A., Imtiaz, M., Ali, S. and Raziuddin (2000) Effect of moisture stress on leaf water

potential and relative leaf water content in wheat (Triticum aestivum L.). Pakistan Journal of

Biological Sciences 3(1), 87-90.

Tamjinda, B., Siriphanich, J. and Nobuchi, T. (1992) Anatomy of lenticels and the occurrence of

their discolouration in mangoes (Mangifera indica cv. Namdokmai). Kasetsart Journal 26, 57-

64.

Toivonen P.M.A. and Brummell D.A. (2008) Biochemical bases of appearance and texture changes

in fresh-cut fruit and vegetables. Postharvest Biology and Technology 48, 1-14.

Underhill S.J.R. and Simons, D.H. (1993) Lychee (Litchi chinensis Sonn. ) pericarp desiccation and

the importance of postharvest micro-cracking. Scientia Horticulturae 54, 287-294.

152

VietnamStat (2011) Mango production data in Vietnam. Government Statistic Organisation.

Accessed online on 2nd December 2014: http://www.gso.gov.vn/.

Whiley, A.W., Mayers, P.E., Saranah, J.B. and Bartley, J.P. (1993) Breeding mangoes for

Australian conditions. Acta Horticulturae 341, 136-145.

Whiley, A.W. (2001) Mango (Mangifera indica L) ‘B74’. Australian Plant Varieties Journal 14,

45-46.

Whiley, A.W., Hofman, P.J., Christiansen, H., Marques, R., Stubbings, B. and Whiley, D.G. (2006)

Development of best practice pre- and postharvest protocols for production of B74™ mango.

Final report FR02049, Horticulture Australia Ltd., Sydney.

Willis, A. and Duvenhage, J.A. (2002) Evaluation of orchard dip for reduction of sapburn, lenticel

damage and postharvest diseases on mango- Final report. South African Mango growers’

Association Research Journal 46-48.

Winston, E.C., Hofman, P.J., Marques, R., Stubbings, B., Taylor, L., Scurr, G. and Whiley, A.W.

(2010) Agronomic and market development of Honey Gold mango. Final report MG06022,

Horticulture Australia Ltd., Sydney.

Yamada, M., Yamane, H. and Hirabayashi, T. (1987) Yearly fluctuation of two types of fruit

cracking in seedling populations of Japanese persimmon (Diospyros kaki Thunb.). Journal of

the Japanese Society for Horticultural Science 56(3), 287-292.

Yeats, T.H., Martin, L.B.B., Viart, H.M.F., He, Y., Zhao, L., Matas, A. J., Buda, G. J., Domozych,

D. S., Clausen, M. H. and Rose, J. K. C. (2012) The identification of cutin synthase: formation

of the plant polyester cutin. Nature Chemical Biology 8(7), 609-611.

Zhu, X., Cao, J., Wang, Q., Jiang, W. (2008a) Postharvest infiltration of BTH reduces infection of

mango fruits (Mangifera indica L. cv. Tainong) by Colletotrichum gloeosporioides and

enhances resistance inducing compounds. Journal of Phytopathology 156, 68-74.

Zhu, X., Wang, Q., Cao, J. and Jiang, W. (2008b) Effects of chitosan coating on postharvest quality

of mango (Mangifera indica L. cv. Tainong) fruits. Journal of Food Processing and

Preservation 32, 770-784.

http://www.gso.gov.vn/

153

APPENDICES

Appendix 1. Field site of B74 mango production for the irrigation experiments

Figure A1. The location of two experiment sites at the commercial farm in Katherine, Northern

Territory. In 2012, withholding irrigation for 3 weeks before harvest and standard irrigation were

carried out in the site 1. In 2013, withholding irrigations for 4 weeks and 8 weeks, and standard

irrigation were carried out at the site 1; withholding irrigation for 4 weeks and standard irrigation

were carried out at the site 2.

Appendix 2. Soil compositions of the two sites for irrigation experiments

Table A1. Differences in soil compositions between site 1 and site 2 at the farm where the irrigation

experiments took place in 2012 and 2013 in Katherine, Northern Territory.

Analysis Result of

site 1

Result of

site 2

Analysis Result of site

1

Result of

site 2

pH [1:5 CaCl2 7.4 7.8 Magnesium (meq/100g) 1.47 1.74

CEC (meq/100g) 6.6 7.3 Copper (ppm) 0.8 0.5

NO3-N (ppm) < 1.0 2.0 Manganese (ppm) 21.8 8.1

Phosphorus (ppm) 12 27 Zinc (ppm) 0.7 39.1

Chloride (ppm) 22 <1 Aluminium (meq/100g) 0.32 0.13

Source: Analysis results on soil components analysed by Phosyn Analytical, Queensland in Dec

2012 and Jul 2013, as required by the farm manager

Site 2

Site 1

154

Appendix 3. Irrigation schedule of the standard irrigation

Table A2. Irrigation schedule of the standard irrigation served as the control at the commercial farm

where the irrigation experiments took place in 2012 and 2013 seasons. The schedule was provided

by the farm manager.

Day Time on

(pm)

Time off

(pm)

Time length

(hour)

Rate of water

delivery

Water mount

L/tree/day

Monday 17.00 18.30 1.5 70 Lh-1 105

Tuesday 17.30 18.30 1 70 Lh-1 70

Wednesday 17.00 18.30 1.5 70 Lh-1 105

Thursday 17.30 18.30 1 70 Lh-1 70

Friday 17.00 18.30 1.5 70 Lh-1 105

Saturday Nil

Sunday 17.00 18.30 1.5 70 Lh-1 105

Appendix 4. Effect of withholding irrigation for 3 weeks on leaf water status in 2012

Table A3. Effects of irrigation treatments of B74 mango tree on leaf relative water content (%) and

leaf water content (%). Mango trees were grown on a commercial farm where the trees were either

given standard irrigation or withholding irrigation for 3 weeks in 2012. If followed by different

letters, means for leaf relative water content or leaf water content within each column are


Treatment Leaf relative water content (%) Leaf water content (%)

Standard irrigation 69.8 a 41.9 a

Withholding irrigation 63.8 a 36.1 a

Average of means 66.8 39.0

155

Appendix 5. Effect of withholding irrigation for 3 weeks on fruit pericarp relative water

content in 2012

Table A4. Effect of withholding irrigation on fruit pericarp relative water content (%) of B74 fruit.

Fruit were collected from the trees either given standard irrigation or withholding irrigation for 3

weeks in 2012. The tests were done on the day of harvest for the no-irradiation treatments, or 3

days after harvest for the irradiation treatments. If followed by different letters, means for relative

water content are significantly different (P < 0.05) by LSD (P = 0.05).

Parameter Treatment No water Water

No irradiation Irradiation No irradiation Irradiation

Fruit pericarp relative

water content (%)

Standard irrigation 94.5 a 95.1 a 94.1 a 94.2 a

Withholding irrigation 94.4 a 94.9 a 94.7 a 94.9 a

Average of means 94.6

Appendix 6. Effect of withholding irrigations for 4 and 8 weeks on dry matter and fruit

firmness in 2013

Table A5. Effect of withholding irrigations on flesh dry matter (%) at harvest, objective firmness

(Aweta) at full colour and 7 days after full colour of B74 mango fruit in 2013. If followed by

different letters, means for dry matter or objective firmness within each column at the same site are


Site Treatment

Dry matter (%)

at harvest

Objective firmness (Aweta)

At full yellow At 7 days after full yellow

Site 1 Standard irrigation 17.9 a 22.2 a 18.3 a

Withholding irrigation for 4 weeks 18.3 a 20.9 a 16.8 a


Average of means 18.4 21.4 18.3

Site 2 Standard irrigation 17.3 a 26.8 a 20.8 a



156

Appendix 7. Effect of pre-harvest bagging and coatings of B74 mango fruit on fruit firmness

in 2011/12

Table A6. Effects of pre-harvest bagging and coatings of B74 mango fruit on objective firmness

(Aweta) at 7 days (eating ripe) and at 14 days after harvest (eating soft) in 2011/12 season. If

followed by different letters, means for objective firmness within each column are significantly


Treatment At 7 days after harvest At 14 days after harvest

Control 33.3 a 33.2 a

Bagging 30.6 a 28.3 a

Coating 3 weeks 39.7 a 33.7 a

Coating 3, 2, 1 weeks 31.4 a 32.5 a

Coating 5% 1 day 39.7 a 29.6 a

Coating 2.5% 1 day 38.3 a 30.2 a


Appendix 8. Effect of pre-harvest bagging and coatings of B74 mango fruit on fruit firmness

in 2012/13

Table A7. Effects of pre-harvest bagging and coatings of B74 mango fruit on objective fruit

firmness (Aweta) at 8 and 15 days after harvest in 2012/13 season. If followed by different letters,

means for objective firmness within each column are significantly different (P < 0.05) by LSD (P =

0.05).

Treatment At 8 days after harvest At 15 days after harvest

Control 32.1 a 25.7 a

Bagging 27.8 a 20.5 a

RainGard 36.0 a 24.8 a

RainGard +Water 34.2 a 25.0 a

Raynox 32.0 a 25.2 a

TFC 2.5% 36.5 a 27.9 a


157

Appendix 9. Effect of pre-harvest bagging and coatings of B74 mango fruit on dry matter,

fruit firmness and sensorial evaluation in 2013/14

Table A8. Effects of pre-harvest bagging and coatings of B74 mango fruit on dry matter (%) at

harvest (green-mature), objective firmness (aweta) and sensorial evaluation (1 = extremely dislike to

9 = extremely like) in 2013/14 season. If followed by different letters, means for dry matter,

objective firmness or sensorial evaluation within each column are significantly different (P < 0.05)

by LSD (P = 0.05).

Treatment

Dry matter (%)

at harvest

Firmness (aweta)

at 15 days after harvest

Sensorial evaluation (1-9)

at 15 days after harvest

Control 17.3 a 28.1 a 6.4 a

Bagging 18.7 a 27.8 a 4.8 a

2.5% TFC - surfactant 17.4 a 31.7 a 5.8 a

2.5% TFC +surfactant 17.3 a 30.3 a 6.2 a

5% TFC +surfactant 18.3 a 28.3 a 6.1 a




Appendix 10. Effect of postharvest coatings of Cat Chu mango fruit on fruit firmness of fruit

in 2014/15

Table A9. Effects of postharvest coatings on subjective firmness (0 = hard to 4 = soft) at 8 and 12

days after harvest of Cat Chu mango fruit in the 2014/15 season. If followed by different letters,

means for subjective firmness within each column are significantly different (P < 0.05) by LSD (P

= 0.05).

Treatment Subjective firmness (0 - 4)

At 8 days after harvest At 12 days after harvest

Control (no waxing) 2.1 a 3.8 a

Water dipping 1.9 a 4.0 a

20% TFC +surfactant (one application) 1.4 a 3.9 a

20% TFC + surfactant (two applications) 1.8 a 3.9 a


158

Appendix 11. Effect of postharvest coatings of B74 mango on fruit attributes in 2014/15

Table A10. Effects of postharvest coatings on subjective firmness (0 = hard to 4 = soft) at 10 days

and 16 days, total soluble solid (TSS, °Bx), titratable acid (TS, %), sensorial evaluation (1:

extremely dislike and 9: extremely like) and flesh colour measurement (L*, a*, b*) at 21 days after

harvest of B74 fruit in 2014/15 season. If followed by different letters, means for subjective

firmness, TSS, TS, sensory evaluation or flesh colour (L*, a*, b*) within each column are


Treatment Subjective firmness

(0 - 4)

TSS (ºBx)

at 21

days

TS (%)

at 21

days

Sensorial

evaluation

(1-9) at 21

days

Flesh colour at 21

days after harvest

At 10

days

At 16

days L* a* b*

Control (no waxing) 2.6 a 4.0 a 17.1 a 2.8 a 5.9 a 68.4 a 10.4 a 49.7 a

15% TFC 2.4 a 4.0 a 17.4 a 2.7 a 5.8 a 66.3 a 9.6 a 53.0 a

15% TFC + surfactant 2.3 a 3.9 a 17.4 a 2.7 a 5.8 a 67.8 a 9.1 a 52.9 a

30% TFC + surfactant 2.4 a 3.9 a 17.3 a 2.8 a 6.0 a 68.3 a 9.0 a 54.6 a

Average of means 2.4 4.0 17.3 2.8 5.9 67.7 9.5 52.6

Documents

LENTICEL DAMAGE ON ‘B74’ MANGO FRUIT372944/s4247519...i LENTICEL DAMAGE ON ‘B74’ MANGO FRUIT Tuan Minh Nguyen B.Sc Food Technology M.Sc Agriculture Food Production A thesis