TECHNICAL BULLETIN No. 43 JUNE 1961

EFFECTS OF GROWTH REGULATORS ON FRUIT SET AND GROWTHOF THE ACEROLA (NIALPIGHIA GLABBA L.)

G. M. YAMANEand

H. Y. NAKASONE

HAWAII AGRICULTURAL EXPERIMENT STATION, UNIVERSITY OF HAWAII

CONTENTS

I N TROD U C TI ON .

R EVI E W O F L ITERATURE

MATERIALS AN D M ET H O DS

Materials

Methods

R E SU L T S AND DISCUSSION

Select ion of Growth Regul at ors

Comparison between the Acid and Salt Forms of PCA

Comparative Effect s of Six Derivatives of thePhenoxyacetic Acid Series .

Compara t ive Te st between PCA and IBA under Field Conditions

Receptivity at D ifferent Stages of BlossomDevelopment to Grewth Regulator (PCA )

Translocation of Growth Regulator .

Effect of PCA Spraying on Flowering Cycle

SUMMARY

LITERAT URE CIT ED

P AGE

3

3

4

4

4

5

8

8

10

11

12

13

16

17

ACKNOWLEDGMENT

The authors acknow ledge with gratitude the generous financial aid and facil ­ities provided by the Hawaiian Acero la Company, a Division of Nutrilite Products,Inc ., Buena Park, Cal ifornia, during the course of this investigation.

THE AUTHORS

G. M. YAMANE was Assistant in Horticult ure at the Hawaii Agricultural Experi­ment Station, and is presently Assista nt Supervisor, Do le Corporation, LanaiCity, Lanai.

DR. H. Y. NAKASONE is Assistant Horticu lturist at t he Hawaii AgriculturalExperiment Station and Assistant Professor of Agriculture, University of Hawaii.

EFFECTS OF GROWTH REGULATORS ON FRUIT SET AND GROWTH

OF THE ACEROLA (MALPIGHIA GLABRA L.)

G . .vI. 'Yn ntan c and H . Y. Nah.asone

INTRODUCTION

Pr evious studies ( ma nusc ript in pr ess) I on pollination and fruit set of acerolashowed t hat fl owers are produced in grea t abundance, bu t actua l f ruit set is low,th e percen tage of f ruit set rang ing f rom 1.3 to 11. 5 percent. The commo nlykn own ca uses of low f ruit set such as d ich ogamy, inviab le pollen, and pecu liariti esin f lora l struct ures such as he terostyly were ru led out in th e previous st udy . In­co mpa ti bility, another commo n ca use of low f ruit set, was found to be pre sentto some degree. H and pollinati on was show n to increase yields, w ith cross pol ­linat ion effec t ing signif ica n tly grea te r yields th an self-pollina t ion, indi cat ing ahigh er degree of self -incompatibi lity. T he ineffec t iveness of pollen-d isseminatingage n ts such as wind and insects was found to be th e maj or cause of low frui t setof the ace ro la in Hawaii. Ledin's report ( 15) ind ica tes that in Florida largepopulat ions of bees are found working amo ng the acerola fl owers and natural fru itset is relati vely high.

The abov e fac tors sugges ted th e need to indu ce f ruit set by some artificial means.This st udy was ini ti at ed to determine some chemica l means of achiev ing high erf rui t set and to st udy th e effects of suc h chem ica ls on th e f rui ting and fl oweringbehavior of the acerola plan t.

REVIEW OF LITERATURE

Pr even t ion of f lor al abscission and induc tio n of ovary swelling wi th wa terex trac ts of pollen by Fitting (9) was th e fi rst indica t ion th at speci fic subs ta nceswere in volved in parthcnocarpic f ruit set. Yasuda (36) pr oduced parrhcnocarpicegg plan t frui ts wit h poll en extrac ts . T hima nn (32) found that aux ins werein volved and Gustafso n ( 11, 12) induced f ru it set and development in to mato,pepp er , petuni a, and salpig lossis by the applica tio n of syn thet ic grow t h reg ulato rs.T he increase in auxi n conten t in the ovaries of N ico t iana tahacu m im mediate lyafter pollinat ion as reported by Muir ( 19) an d the fi ndings of G us tafson h aveshow n th at it is possible to ind uce f ruit set artific ially by applica tio n of additiona laux ms ,

1 Manuscri pt en titled Pollina tion IIml Fruit Set 0/ Acerola submitt ed to Proc. Amer. Soc. Hort. Sci .

4 HAWAII AGRI C U LTURA L E XPER IME NT ST ATIO N

O ne of the f irst cro ps in which grow th regulators were used to set fruits whencond itions of pollination and ferti lization were un fa vorable was to matoes. U ndercertain environmenta l cond itions, such as low ligh t in tensit y (26, 33 , 3 5) andhigh day te mpera t ures (28), elongat ion of the pisti ls t akes place and ex tendsbeyond the st amens. In these cases, growth regulator s were effect ive and resultedin increased yie ld and increased f ruit size. Others (13, 14, 21 , 22, 27 ) have alsoreported these beneficia l result s by using growth regulators in fr uit sett ing ofto matoes.

In addition to inc reased fruit set and prod uction of seed less tomatoes, plan tgrowth regulator sprays have had t hese effects on vegetable crops such as snap­beans (2 3, 34 ) , on fruit c rops such as certain variet ies of pears (2, 10, 24 ),Calmyra fi gs (5), and avo cado (3 0 ) , and on ornamenta l crops suc h as Americanholl y (3).

MATERIALS AND METHODSMaterials

Acerola plan ts used in th is study were d raw n fro m various sources. Seedlingsfrom Ace. No. 6663 (sour ty pe) were derived fro m seeds acquired f rom the Uni­versity of Florida in J une, 1955. C lone Maunawi li (sour t ype) was obta inedf rom the Hawaiian Suga r Planters' Association Experiment Station. Clo ne 269-2(sour ty pe) is a select ion from 10 seed ling s (P .I. 2·09269) received from theUnited Sta tes P lan t In t rodu ction Garden, G lenda le, Mary land. C lones H- 5(sweet type) and 13 Yz ( sour type) arc designat ions of the Hawaiian Ace rolaCo mpany.

Growth reg ulators util ized were in crystalli ne forms. Where the ac id fo rmswere used, 95 perce nt et hy l alcoho l was used as the brid ging solven t. A sma llamount of commercia l wetting agen t (T ween 20) was added to t he d ist illed wa tersolution.

MethodsA ll prelim inary field and labora to ry experiments were con ducted on th e Manoa

campus of the H awaii Agricult ural Experiment Station , Universit y of H awa ii.Ex per iments requi ring large numbers of clonal plan ts were con ducted at the Pu naorc hard of the Hawaiian Ace rola Co mp any at Pu na, H awaii.

The fo llow ing orga nic compounds were ut ilized to det ermine the ir efficacyin prom oting f ru it set: I-naphthaleneacet ic acid (NAA ) and its sodium salt(NaNAA), 2- naphthoxyacetic acid (NOAA) and its sodium salt (NaNOAA) ,3-indoleace t ic acid (I AA ) , 3-indo lebutyric acid (lB A), and -l-chlorophcnoxyacet icacid ( para-c hlorophcnoxyacetic acid, PCA ) .

A queous growth reg ulator solut ions were applied to branch trea tments wi th asma ll hand ato mizer, and a 4 Yz -gallon kna psack sprayer was used for whole-treet reat men ts. W here necessary, a plast ic sc reen, 6x12 feet, was emp loyed to min i­m ize drift to adjacent trees.

Fr uit set was determined 6 days after applicat ion of treat ments, frui ts at thisage being approximately 3/ 16 inc h in diameter.

EFFECTS OF GROWTH R E G U L A T ORS ON ACE ROLA

A fter it w as ascertained chroma tographica lly f ollow in g methods ou tli ned byStrohec ker, 1'1 (/1. (3 I ) that there were no interfering su bsta nces, the iodate ti tra­tion method after Ballan t ine ( I) was used for asco rbic acid dete rminat ion .

For ana lys is of data fo llow ing a bin omial distribu tion, confidence in ter valta bles from Snedecor ( 29 ) we re used. Yie ld dat a we re su m m arized by an alysisof v ar ianc e as ou tl ined b y Snedecor. The D u ncan's mul ti p le ran ge test ( 6 ) w asused to delimi t differenc es bet ween me an s. Mean weigh ts of fruits w ere est ima te dfro m ra ndom samples draw n from eac h treatment at eac h harvest.

RESULTS AND DISCUSSION

Selection of growth regulatorsD at a pr esen ted in table 1 sho w that PC A and IBA were effect ive in promoting

f ru it set , the fo rmer ov er a w ide ran ge of conce n t rat ions and IBA onl y at 100ppm. N A A , N OAA, and NaN OAA were ineffect ive, alt ho ug h N aNOAA wassligh tly effec t ive at 5000 ppm. A lt ho ug h in crease in fruit set w as ob tain ed withh igh co nce ntrat ions, t he f ru its were deform ed and reduced in size . Fruits set wi thPC A at 500 and 1000 ppm were h ighl y ridged and sma ll. At lower co ncen t ra ­tio ns, th e f rui t s were less ridge d and larger in size , t ho ug h sma ller tha n th ose ofthe contro ls.

TABLE I. Mean percent f ruit set of ace rola treated with five acid Jnd two sod ium saltforms of );ro wt h reg ulator s (summary o f different experiments)

GRO W T l l It EGU LATORSTREATM ENTS

(PPM)._ - - -

NAA IlIA IAA NaNAA PCA Na NOAA NOAA

0 0 .5 19. 5 4.8 4 .8 0 4 . >

10 2.4 .6 16.0 7 .> 33.8 3.8 0

50 2. 1 2.7 15.8 6 .2 88 .2 0 0

10 0 0 6 1.5 15.2 13.1 89 .0 0 U

500 9.0 86.7

1000 0 82.4

5000 0 4 .0 12. 8 22.0 13.7._ - - - -

A ltho ugh tremendo us increase in fr uit set was realized with PCA, undesirabl eeffects in the form of leaf curling and yellowing of veget at ive term in als resulted .Figures 1 and 2 show the curling effect of PC A on the term inal leaves. Matureleaves sprayed with PCA showed no effects. O n ly yo ung leaves and leaves w hichappeared after ap plica tion of the chemica l were affec ted (fig. 2) . In severecases the terminals t u rned yellow and ult ima tely died . T he severity of t heseconditions wa s related to the concentration used . At 10 and 50 ppm , leaf curlingwa s less pronounced and less t ime was required for leave s to return to normal.

6 HAWA II AG R ICU LTU RA L EX PER IMEN T ST AT ION

NoAA SNA I AA

FIG. I . Lea f c url ing effec ts of fi ve grow t h reg ula tors. O nly PCA caused leaf c url ing .

F IG. 2 . C heck group of bran ches o n t he lef t . Seve re effects fro m 10 0 pp m PC A o n yo u ngexpan di ng shoots.

E F F ECTS O F GROWT H REG ULATORS ON A CEROl.A 7

In the case of IBA , no leaf c ur ling or yellowi ng of te rmina ls was pr esent.Fruits, on the other hand, sho wed ridging and slight def ormity. T he other g row thregul ators tested d id not p roduce any visib le ph yto to xic effects as in the caseof PCA.

Growt h reg u lato r treatments appeare d to im prove the firmness of the fr uit s.Fruits set by treatments were solid and did not bruise as easily as th ose fro mnatu ral set . T his may be an im portant considerat ion from the sta nd point of post ­harvest ha nd ling of fruit s when acerola is grow n fo r ascorb ic ac id production .Pon t ing and J oslyn (25 ) have rep orted th at asco rbic acid is mor e readily ox idizedwh en th e exocarp of apples is bruised or b rok en.

As has been dem onst rated with to matoes (26), the use of growth reg ulatorsprays did not alter th e asco rbic aci d con tent of th e ace rola frui t. T his is clearl yshown in ta b le 2. T here was no rea l d ifferen ce in the ascorbic acid con te n tbet ween open- po llinated fru it s and PC A-treated fruits.

TABL E 2. Ascorbic acid content (mg. per 10 0 gm .) of acerola fr u itsfro m open po llination and p e A treatment ( 10 0 ppm)

P AR A- CH LO ROPI I ENOXYAC ET ICO PEN POI.LlNATION

PLANT( MG. 10 0 GM.)

/t.. CJDPER

( MG. 100 GM.)PER- --- _.

R 3-TI 1998 17 9 5

R4 -T 2 2038 19 61

R 4-T 4 1989 2073

R 4-T6 19 36 1980

R 4 -Ts 2 193 2 138

4 -14 2 1 56 20 46

8-1 I 1894 1771

12B -1 5 1510 161 0

13-26 2 195 22 10

14 -22 2276 2028

20 -46 1635 171 0

2 1-26 173 0 1672

2 3-33 1908 2064

25 - 14 15 12 172 5

Since PC A was most effective fo r f ruit set, furth er st udies were co nduc tedwi th PCA and th e der ivatives of the phenox yacctic ser ies. IBA was also used 111

fi eld tes ts for compara ti ve purposes and wi ll be reported in a later sec t ion.

8 HAWAII AG RI CULTURAL E X P E R IME N T STATION

Comparison between the acid and salt forms of PCATo determine whether or not the salt form of PCA was equally effective as

the acid form in fruit set t ing and to ascertain its phytotoxic properties, both formswere tested on open flowers. The result s tabulated in tab le 3 show that the acidfo rm wa s superior to the salt form. T hese result s, which arc not obvious ly ex­pla inable on the basis of comparable molarities of acid equivalents, arc in agree­ment with those reported by Grigg s, ct al. (J 0), using 2,4,5 - t richlorophcnoxy­propionic acid and its alkonalarnine salt on the fruit set of pears. In ph ytotoxiceffects, visua l observations of vegetat ive plant parts showed the occurrence ofleaf curling on trees sprayed with both forms and the degree of curling was relatedto concentration.

T h Ul.E 3 . Percen t fru it set of acero la treated with ac id and sod iu m salt formso f para-chlor op hcn oxyaccric acid (PCA) ( Ma un awi li C lone )

A C ID SO lHUM SALT-- - ~-

TREA TMENTSConfi dence Co nfidenc e

( PPM)pH

PercentInterv als. pH

PercentInterval s,

Fruit Set9 5 Per cen t

Fruit Set95 Per cent

0 5. 6 3.33 0-13 1.6 3.33 0- 13

10 5.1 3 5.0 0 25 -52 1.7 50 .00 40 - 67

50 4.5 58 .0 0 50-76 5.8 53 .3 3 45 - 70

100 4 .0 91.66 8 1-97 5.8 63 .33 55-79

50 0 3 .4 81. 00 72-92 5.9 6 0.0 0 51-74

1000 3 .2 6 3. 0 0 62 -83 6.1 6 1.6 6 53- 78

50 00 I 2.8 35 .00 2 5- 52 6.2 13.33 7-27

Comparative effects of six derivatives of the phenoxyacetic acid seriesMuir, ct al. (20 ), using the A vena st raigh t -g rowth test to assess the relative

activity of some of t he ch lorine-substituted phenoxyacctic acids, reported thatchanging the number and the positions of the chlorine atoms on th e phenoxyring drastica lly changed th eir act ivit ies. To determine the types of effectsproduced by some of the chloro-der iva t ivcs of the phcnoxyacctic acid series,six compound s were used at 100 ppm, and the different subst it uen ts showedgreatly different activities as shown in table 4 and f ig . 3. The data show thatphcnoxyacetic acid and 2,6 -dichlorophenoxyacetic acid showed no activity at all,and 2-ch lorophenoxyacetic ac id fai led to set fruits and produced but a sligh tcurling of the leaves. Terminals spray ed with th ese three chemicals are shownin fig. 3, 13, C, and G . On the other hand, 4-chlorophenoxyacetic acid (P CA),

EFFECTS OF GR OWTH REGULATORS ON A CE RO LA 9

sA B c D E F G

FIG. 3. Terminal leaf def ormat ion by spraying six compounds of th e phe nox yacet ic acid ser ies.A. control; B. phenoxyacet ic acid ; C. o-ch lo rophenoxya ceti c acid; D . p- chlorophcnoxy­aceti c acid; E. 2,4-d ichl orophcnoxy acct ic acid ; F. 2,4,5-trichlorophcnoxyacctic acid ; andG . 2.6- d ichlorophenoxyacetic acid,

FIG. 4. Te rmin al leaf cu rl ing an d st un t ing of growt h when three con cent rati on s of PCA solu­t ions were fed to seedling s t hrou gh the roots.

10 HAWAII AGRICULTURAL E XPERIM E N T STATION

TABLE 4. Percent fru it set of acer ola treated with growt h regulatorsof t he phcnoxyaceric acid ser ies ( 100 ppm)

NUMBER CONFIDENCEN U M B ER PER CE N T

OF IN T ER VA LS,TREATMENTS O F FR UIT

FRUITS 9 5F LOWERS SET

SET P ERCE NT

Control 158 7 4.43 1.0 - 9.0

Phenoxyacetic 162 5 3.08 1.0-8 .0

2-chlorophenoxya cet ic 153 3 1.96 0.0-7.0

4-chlorophenox yacetic ( PC A) 147 106 72 . 10 63.0-80.0

2,4-d ichlorophenoxyacetic 162 107 66.04 57.0-74.0

2,4,5 - t ric hloroplrenoxy acet ic 179 83 46 .36 37.0 -55.9

2,6 -di chlorophenoxyace ric 189 11 5.82 2.0- 12.0

- Normal+ Slight leaf cu rl

++ Heavy leaf c url

PHYTO­

TOXIC

EFFECTS

+++++

+

2,4 -d ichlorophenoxyacetic acid (2,4-D), and 2,4,5 - t richlorophcnoxyncet ic acid(2,4,5 -T) were highl y active, although the latter was least active in setting fruitsand in inducing leaf curling. Figure 3, D , E, and F show the leaf curling effectsof PCA, 2,4 -D, and 2,4,5 -T, respectively. In the case of the former two com­pounds, leaves two to five nodes below the terminal were also affected whensprayed with 100 ppm.

Studies with tomatoes (2 8, 35), f igs (5), and pears (I 0) have shown thatimmature leaves appearing after applicat ion of growth regu lators (PCA, 2,4-D,2,4,5 -T) resulted in abnormalities, while fully expanded leaves were not affected.

Comparative test between PCA and ISA under field conditionsSince flowering and vegetative flu shing occur at approximately th e same time

in acero la, increasing fruit set with PCA has cau sed undesirab le effects upon theyoung expanding shoots. Since mA, at levels effective in fruit set , did not showthese concomitant ill effects, a fie ld comparison of the two chemicals was initiatedat the Puna orchard.

T reat ment s consisted of the control, PCA, and mA in randomized blocks,replicated three t imes. Eac h treatment consisted of three 3-ycar-old trees of cloneH-5 . When yields for all treatments were compiled and summarized, resultsshowed that significant differences existed among the treatments (mean y ields:control, 45 gm.; IBA, 1402 gm.; and PCA, 2840 gm .) . PCA treatment gave twiceas many fruits as the IBA treatment.

Differences in the mean weight of fruits were also obtained (table 5). Theinverse relationship between mean fruit weight and total yield was apparent inall treatments.

E FFE CT S O F GROWTH REGULA TORS ON ACE ROLA

TAB LE 5. R ank ed order o f m ean wei gh t (g m . ) per fruit of done H - 5 set by openpollin ation, indolcbut y r ic ac id and 4 -c hloro phc l1oxya cct ic aci d

TREAT"ENTS ( 10 0 PI' '' )

11

M EA NS

PCA

2.30

lIl A

3.62

C on trol

5.46

1\;' 0 / (' : D if feren ce between an y two mean s not under scored by the same lin e issig ni f icant at the 1 percent leve l.

Examination of terminal leaves on trees of both t reatments showed that whileyoung exp and ing leaves of PCA t reat ment were curled with some tips turningyellow, similar aged leaves an d terminals of th e IBA treatment we re completelyfree from thes e sympto ms.

Receptivity at different stages of blossom dev elopmentto growth regula tor (p e A)

The receptivit y of the flowers over a wide range of developmen t deter­min es to a great extent th e yield and the ti me interval bet ween spray applications,if more than one is requ ired. Roberts and St ruckmeyer (27) and Leopold andScott (1 6) reported tha t au xin sprays were effec tive in induc ing f ru it set over awide range of blossom dev elopment in tomatoe s, i.e., fr om small unopened budsto abscission of flow ers.

In preliminar y spray st udies w ith PCA, it was noted that greater numbers offruit s were reach ing matu rit y over lon ger periods tha n th at fo und in untreatedtrees. Th is seems to indica te tha t fl ow ers in diff eren t stages of development wereaffected. To est abli sh def ini tel y wh ether acerola blossoms reac ted to grow thregulators as did tomato blossom s, PC A at 100 and 500 ppm was appl ied toblossoms at different stages of development. Flowe r bud s 2 days and 1 day beforean the sis, and fl owers at anthcsis and 1 and 2 days after anthesis wer e used inthis st udy.

When the results ( ta ble 6) were analyze d, signif icant differences in fru it setwere shown with the differen t stages of blossom developm ent and w ith th e con­centrati on s of PC A. F lowe rs at ant hesis were signif icantly more receptive toPCA than flo wer buds and f lowe rs af te r an thc sis, and th ere were no differencesin response to 100 and 500 ppm t reatments. H owever, differen ces in responsesto different con centrati ons at other st ages of development indicate that fl owersbefore and after anthcsis requ ire greater st imulat ion th an flow ers at an the sis.

A lthough th e dat a indi cate th at 500 ppm was super ior to 100 ppm in fruitsetting ability, use of high er concentrations is not wa rra nte d because of in creaseddetrimen tal effec ts to th e trees and to th e fr uits. As wi ll be shown below,maximum yields fr om each fl owering cy cle can be obtained w ith one appl icationof growth regulator spr ay, provided the spra y is app lied during peak flo wering.

12 HAWAII AGRICULTURAL EXPERIMENT STATION

TABLE 6. Relative effectiveness of growth regulator spray (PCA) on fruit setting of ace ro laat different stages of blossom development (Maunawili Clone, 60 flowers per treatment)

NUMBER OF FRUITS SET AT T H EPERCENTAGE CONFIDENCE

FOLLOWING CONCE N TR ATIO N SOF INTERVALS,ANTHESIS TI ME

OF GROWTH REGULATORFRUIT SET 95 PERCENT

Control 100 ppm 500 ppm

2 days before 1 6 28 19.64 13-26

1 day bef or e 2 25 34 33.88 27-42

An th esis 4 52 5\ 51.66 44-60

1 d ay after 0 24 31 30 .55 22-39

2 days after 0 0 0 0.00 0-3

Percent set 2.33 35.66 48.00Co nfidence intervals, 95 percent 1-5 30-42 42-54

Translocation of growth regulatorThe use of growth regulators to induce fruit set in commercial orchards raises

the question of whether or not each flower must be sprayed directly for setting.Absorption and translocation of growth regulators within the plant are suggestedas being conditioned by certain factors (4 , 7, 17). The direct ion of translocationappears to be controversial with Mitchell and Brown (17) and Mitchell and Ham­ner (18), reporting that only downward movement took place in the stem; where­as Ennis and Boyd (8) stated that translocation, both upward and downward,was influenced by the concentration of the growth regulator and by the additionof Carbowax (polyethylene glycols) .

To determine whether growth regulators app lied to one flower were absorbedand translocated to another, one of a pair of flowers in each of several inflo­rescences was treated with PCA at 100 ppm. The pronounced difference in fruitset between treated (76.3 percent) and untreated (6 .3 percent) indicated clearlythat the growth regulator was not translocated to the untreated flower of the pairto effect fruit set.

To determine whether growth regulator was absorbed by the leaves and trans­located to the flowers to effect fruit set, all expanding and mature leaves onseveral tagged branches were swabbed with 100 ppm PCA at the onset of flower­ing. Other branches of similar age and size on the same trees were designated ascontrol. The PCA solution resu lted in leaf curling on ly on expa nd ing leaves ofthe t reated branches and did not induce any fruit to set. These results indicatedthat t he growth regulator was absorbed by the leaves but was ineffective in in­duc ing frui t set .

When three concentrations of PCA (10, 10'0, 1000 ppm) solutions were fedto seedlings through the roots, leaf curl ing and stunting of growth resulted.Fig ure 4 shows plants treated with three concentrations compared with the con­trol. Growth was inhibited in relation to the concentration used. The terminals

EFFECT S O F GRO WTH R E GULATORS ON AC EROLA 13

and leaves at the apex of the plants treated with 100 and 1000 ppm showedyellowing and eventually died.

When PCA at the same three concentrations used abo ve was fed to flower ingplants growing in cans, fl owers were not st imulated to set by syste mic mean s,although translocation took place as indi cated by curl ing and ye llowing of ter­mina l leaves. For induction of fruit set , each flower mu st receive the growthregulator solut ion directly.

Effect of peA sprayinog on flowering cycleIt is well known in fruit crop s such as appl es, pears, and man gos that high

yield s in one year affect the yield s of the followin g year. W ith a heav y crop setartificiall y with growth regu lators in the acero la, the qu estion ari ses as to theyields in subsequen t flow eri ng cycles. Undoubtedly, not onl y w ill th e qua nti tyof f lowe rs be affec ted bu t the frequency of fl owering may also be change d.

To ascer tain these factors, field experiments were conduc ted in the Punaorc hard. Treatmen ts con sisted of sprayi ng different t rees wi th PCA (100 ppm )at one fl owering cy cle and also at two success ive f lowering cy cles. The trees usedwere approx ima tely 3 ~/z years old (clone 13 Yz ). Each trea t ment plot consistedof three t rees, rep lica ted five times. Since it wa s not pract ical to co unt th e numb erof f lowers ap pearing at each f lowering cycle, the fru it y ields were used to indica teflor al production . A ll fru its harvested duri ng the period of th is st udy (June 26to September 25 , 195 9) were recorded on the basis of indiv idual t rees, and in theana lysis of data , a sing le mean f rom each t reatment was der ived for each harvestperiod. A summary of the dat a is pr esen ted in ta ble 7.

T he resul ts showed that high est yields were obtained from the t reatmen t inwhic h t wo successive fl owering cycles were sprayed, followed by th ose sprayedat one flow ering cycle. T otal yield of the control plot s was very low in com­parison to th e t reated plot s.

It was also fo und that subseq uent f lowering cycles of trees fro m the treatedplot s were delayed considera bly . Norma lly in Hawaii, acerola f lowe ring cy clesocc ur approxima tely eve ry 20 to 28 days, as shown by th e in tervals bet ween cy clesfor th e control t rees. Peak fl owering occ urs 3 to 6 days after th e init ial f lower­ing. O ne sprayi ng of PCA at the fi rst fl oweri ng cy cle induced a t remendo usincrease in frui t set and delayed th e second fl owering cy cle by approxi ma te ly 18days, when compare d to the interval bet ween fi rst and second cycles of the con ­trols. On the second fl owering cy cle, the .yield of t he treatment receiving onlyone spraying at the initial cy cle was reduce d to approximate th e yield of the con­t rol. This seems to indica te that residua l effects of the gro wth regu lator were nolonger presen t. Trees in this treatment also reverted to normal fl ow ering cy clein the th ird and fo urth cycles, wit h yie lds comparable to the co ntrol t rees.

T he treatment receivi ng another spray of PCA at t he seco nd f lowe ring cycleaga in showed a high yield, tho ug h on ly half of that produced in the first flower­ing cy cle. This reduc tion in yield in th e seco nd fl ower ing cy cle resulted from arath er heav y decrease in f lowering after gro w th reg ulator application. T he delay

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E F F E CT S OF G RO W T H REGULA TORS ON ACEROLA 15

in the seco nd f lowering cy cle wa s the same as th at of th e one-spray treat me nt.With t he app lication of PCA spray at the seco nd cycle , th e third f lowe ring cycleappeared 49 days later , a delay of 25 days from normal f lowering. Because of th edela y in the appear ance of the third cy cle, no y ield reco rd s were obtained.

T he number of f low ering cycle s for the three lots of t rees from Ju ne 26 toSepte m ber 27 is worth y of note, especially when related to th e tota l yields . T hecon trol t rees growing under norma l condi t ions prod uced fi ve flow erin g cyclesduri ng t his period. W hen trees were sprayed once du ring the init ia l floweringcy cle, there were fo ur cycl es prod uced w it hin the same period. W hen th e firstand second cycles were sprayed, the n umb er of cycles wa s red uced to on ly th ree.Wi th on ly three f lowe ri ng cycles durin g the period of th e in vesti gati on, t he tota lyield was almo st twice that of the onc e-sprayed lot and more t han 11 t imes thatof the contro l lot .

ln a separa te experime nt, lower co ncentrat ions of PC A (25, 50, 75, and 100ppm ) were t ried und er f ield co nditions for a period of one f low eri ng cy cle todeterm ine differen tia l effects on f lowering and f ruiting . I n table 8 are p resen teddata on mean yields and mea n weights of fru it s derived from the various t reat ­men ts.

TAIlLE 8. Ranked ord er of mean y ields an d f rui t weig ht s of acero la sprayedwith fo ur co ncen tratio ns o f pe A

Cont ro lT REATME NTS ( P P M )

25 50 75 10 0

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M EAN W EIGHT PER FRU IT ( G hl.)

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2 .6

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2 84 5.8

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Con trol

4 .6

T here was no realt hey were superior

No l l': Di f fe renc e between any me an s no t un d er scored by the same lines an.' signi ficant a t t heI pcrccn t level.

As expec ted, 100 ppm treatment ga ve the high est yield.difference in yields between 50 and 75 ppm treatments, butto th e yields of the 25 ppm treatmen t and the cont rol.

As in previou s t rials, mea n fru it we ight decreased with inc rease in yield.Figure 5 show s the redu ction in size of f r uits set with four conce nt ratio ns ofPC A and the lower h alf of tab le 8 shows the mean weights in rank ed order.A mong the three high er concentratio ns, t here were no sig nif ica n t differen ces inmean wei ght of f ruits.

Recovery of flo wering cycle and affec ted leaves was related to th e concen­tration of PCA. Trees of the 25 ppm treatmen t reverted to norma l fl oweri ngw it h moderat e to heavy f lowering 4 to 6 da ys af te r harvest in g t he c ro p set withPCA 22 da ys before. D ur in g th is period , trees treated w ith 50 ppm produced

16 HAWAII AGRI C U LTURAL E X PE R IMENT STATION

F IG . 5. R edu ction of frui t size w it h inc reasing con centrat ion of IleA.

scattered flowers on ly on branches that had not flow ered during the previouscycl e when spr ays were applied. Trees spray ed with 7 5 and 100 ppm did notproduce any flowers for thi s period and hea vy flowering did not occur until 28da ys af ter harvest ing the crop set by spray ing . At this t ime all treatments wereagain in peak f lowering.

Leaf curling effects were re latively ligh t at conc entrations below 5'0 ppm.Twenty-four days after application of spr ays , only a few leaves were sligh tlycurled on these trees, whereas those sprayed with 7 5 an d 10 0 ppm were s t illseverely curled. Further observations made 4 1 da ys af ter spray ing showed thatleaves of all sprayed trees had recovered, exce pt those sprayed with 100 pp mwhic h showed slight leaf cu rlin g.

The lar ger yields from fewer harvests as shown by the different experimentsdisc ussed above wo uld undou bted ly reduce operationa l cost s an d increase efficiency.However, t he effects of suc h a harvest pattern on other horticu ltu ra l practicesan d the effec ts of longer terms of application of growth regulators will req ui refurther study .

SUMMARYT he low natural fru it set of acerola in Hawaii, attributed to absence of pollen ­

transferr ing agen ts, indicated the fea sib ilit y of growth regulator ap plication toind uce fruit set. Among the several gro wth regulators tried initia lly, 4-ch loro ­phenox yacetic acid (PCA) and indolebut yri c acid ( IBA) were found to be effec­ti ve in promoting f ru it set, t he fo rmer bein g more effic ien t over a wider ran ge

EFFECTS OF GROWTH REGULATORS ON ACEROLA 17

of co nce n t ra t ion. PC A exhibited some phytotoxic effec ts in the natu re of leafcu rli ng, yellowing, and even death of you ng vegetat ive ti ps. Ph yto toxic effectswere reduced in severity with decrease in conce ntra tio n . lilA at 100 pp m effec tedapproximately 60 percen t fruit set without an y apparent phytotoxic effec ts.

PC A treatmen ts did not affect the ascorbic acid content of the fruits. How­ever, there wa s a red uc tion in fruit size an d weight when growth reg ulators wereused. T he other effects on the f ruits were increased firm ness and ridg in g of thef ru it sur f ace.

W hen six deri vat ives of the phenoxyacetic acid series we re tested for theirf ruit set t ing abi lity an d ph yt oto xic propert ies, it was fo u nd t hat 4 -c h lorophenoxy­acetic ac id and 2,4-d ich lorophenoxyacet ic acid induced t he h igh est fr uit setpercentage as we ll as ph yto to xic effects. 2,4,5 -trichl orophenoxyacet ic acidwas ap proxi mately two- thirds as effective as PCA and 2,4 - D in f ruit set and lesssevere in phytotoxic effec ts. The other th ree compounds , ph en oxy-, 2-ch loro-,and 2,6-dich lorophenoxyace tic ac ids, were ineffect ive in both f ru it sett ing andleaf curl ing.

Under fie ld co nditions the sat isfac tory fruit set and lack of phytotoxic effectswi th IBA at 100 ppm make th is a highl y desirabl e co m po und, but th e wid e useof it m ay be pr ohibited by it s h igh cost. O n the other hand, PCA at 50 p pmcan ind uce as mu ch f ruit set as l il A at 100 pp m, and phy to to xic effects are mi ldwith early recovery. Furthermore, t he vel")' low cost of this materi al mak es it suse eco no mica lly feasibl e.

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UNIVERSITY OF HAWAIICOLLECE OF TROPICAL ACRICULTURE

HAWAII ACRICULTURAL EXPERIMENT STATIONHONOLULU, HAWAII

LAURENCE H. SNYDE'RPresident of the University

MORTON M. ROSENBERGDean of the College and

Director of the Experiment Station