Quality and Quantity Improvement of Citrus: Role of …ndpublisher.in/admin/issues/IJAEBV8N211.pdfQuality and Quantity Improvement of Citrus: Role of Plant Growth Regulators Harsimrat

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators

Harsimrat K. Bons1*, Nirmaljit Kaur2 and H.S. Rattanpal11Department of Fruit Science, Punjab Agricultural University, Ludhiana, Punjab, India.2Department of Botany, Punjab Agricultural University, Ludhiana, India.

*Corresponding author: [email protected]

Paper No. 335 Received: 18 April 2014 Accepted: 22 May 2015 Published: 29 June 2015

Abstract

Citrusisoneofthemostimportantfruittreespeciesintheworld,asthefruitsareavaluablesourceofnutrients,vitaminsandotherantioxidantcompounds.Thecitrusproductivitydependsonvariousfactors, among these the plant growth regulators holds a prime position. The use of plant growthregulatorshasbecomeanimportantcomponentinthefieldofcitriculturebecauseofthewiderangeof potential roles they play in increasing the productivity of crop per unit area. The plant growthregulatingcompoundsactivelyregulatethegrowthanddevelopmentbyregulationoftheendogenousprocessesandthereexogenousapplicationshavebeenexploitedformodifyingthegrowthresponse.Plantgrowthregulatorshavebeenusedincitrusfruitproductionforinfluencingflowering,fruitsetandfruitdropandplayamajorroleinfruitgrowthandabscission.Theseregulatorshavealsobeenusedtoinfluencefruitqualityfactorslikepeelqualityandcolour,fruitsize,juicequalityandtoimprovetotalsolublesolidsindifferentcitrusspecies.Thisreviewmayserveasacompletetreatiseonthepossiblerolesofgrowthpromotingsubstancesonthephysiologicalprocessesofcitrusplant.

Highlights

• Impactofplantgrowthregulatorsonfruitqualityandproductivityofcitrus

Keywords : citrus,plantgrowthregulators,flowering,fruitgrowth,fruitquality

Thegrowthanddevelopmentincitruslikeallotherfruitcropsisgovernedbyboththeintrinsicaswellasextrinsicfactors.Theintrinsicfactorscompriseofthegenotypeandtheendogenoushormonebalance,whereas,theextrinsicfactorsaretheenvironmentalconditions encountered by the plant. There areother factors, viz., planting density, irrigation andfertilization, which may affect the growth anddevelopment of the plant. Since the discovery ofthe plant growth regulators, they have been usedto manipulate plant growth and development forthe improvement of quality and quantity of theproduceinordertoenablethefruitgrowerstomeet

to pressure of increasingdemand for food of highquality.

In citrus industry, a largenumberofplant growthregulatorshavebeenevaluatedfortheircommercialapplication. Plant growth regulating chemicals areusedincitrusproductioninallthecitrusproducingareas.Theyareusedforproductiontechnologyandquality improvement. In production technology,emphasis is primarily on nursery production,crop regulation by manipulation of flowering,improvement of fruit set and fruit growth andmanipulationofthetimeofharvest.Theapplicationof plant growth regulators for improvement of

HORTICULTURE

International Journal of Agriculture, Environment and BiotechnologyCitation: IJAEB: 8(2): 433-447 June 2015DOI Number: 10.5958/2230-732X.2015.00051.0©2015 New Delhi Publishers. All rights reserved

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internalfruitqualityandpostharvestfruitstorageisofequalimportance.

Plant growth regulators have been used in citrusfruitproduction for influencingflowering, fruit setandfruitdrop(Berhow2000).Theseregulatorshavealsobeenusedtoinfluencefruitqualityfactorslikepeelqualityandcolour,fruitsize,juicequalityandto improve total soluble solids in different citrusspecies.AmongPGRs,theauxinshavedirecteffectonabscissionbycausingadelayofabscissionresultingin improvement infruitqualityandyieldincitrus.Napthylaceticacid,2-4DichlorophenoxyaceticacidandGibberellic acid have been tried for reductionof physiological drop (Ullah et al. 2014 Salicylicacid is consideredapotentplanthormonebecauseof itsdiverse regulatory roles inplantmetabolism.It is well established that salicylic acid potentiallygenerates awide array of responses in plants andalso affects the photosynthetic parameters whichenhanceyield(Mahdiet al.2012)

Thepoortreehealthalsoplaysasignificantrole inenhancingoffruitdropduetodepletionofnutrientsupply to thedemanding sinks.Citrus ishowever,a relatively high nutrient demanding crop (Wanget al.2006)anddeficiencyorexcessofnutrientscanleadtoinferiorfruitquality.Thereisaneedtoboostupyieldthroughpropernutritionandmaintaininginternalhormonalbalance.Suitablecombinationofmicronutrients and growth regulatorsmay controlexcessive fruit drop for the improvement of fruityieldandquality. Althoughmanyeffortshavebeenmade to study (Baldwin 1993) the physiologicaland biochemical aspects of citrus, there is still anenormous unexplored potential in the study ofregulation of metabolites associated with citrusphysiology.

CitrusfruitsuselargeamountofKascomparedtoothermacronutrients(AlvaandTucker1999)becauseKisinvolvedinseveralbasicphysiologicalfunctionsi.e. formation of sugars and starch, synthesis ofproteins, cell division, growth and neutralizationof organic acids (Liu et al. 2000). It improves fruitqualitythroughenhancingfruitcolour,sizeandjuiceflavor(Tiwari2005).Reportsindicatethedeficiencies

ofmicronutrients likeZn,Cu,Fe andMn in citrusorchards of India and among them Zn is moreacute.LiteratureindicatesthattheapplicationofZnincreasesthefruityieldandquality(Rodriguezet al.2005).Thesuitablecombinationsofmacronutrients,micronutrientsandgrowthregulatorscouldcontroltheexcessivefruitdropandimprovethecitrusfruityieldanditsquality(DobermanandFairhurst2000).

The literature pertaining the role of plant growthregulators during different periods of growth anddevelopmentisreviewedunderthefollowingheads:

Role of Plant Growth Regulators

Flowering

Citrus trees, once past the juvenile phase, bloomeveryyear.Annualfloweringofadulttreesisaffectedby several exogenous and endogenous factors. Incitrus,cooltemperaturecaninduceflowering,asinmosttropicalandsubtropicaltrees(Inoue1990,Lenz1967,Moss1976,Nishikawaet al.2007,Wilkie et al. 2008).InSatsumamandarin,floralinductionoccursin trees exposed to15°C formore than1.5months(Inoue1990b,Nishikawa et al. 2007).Treesgenerallyremaininthevegetativegrowthphaseuntilthetreesareexposedtotemperaturesoflessthan25°C(InoueandHarada1988).Underfieldconditions,thetreesareexposedtocooltemperatureduringfall,duringwhichfloral inductionproceeds. Theplant growthregulatorsstimulatetheabscissionintheflowersthatcauses heavy flower drop. According to Martinez et al. (2004)floweringofHernandinagetsreducedby25%andofOrograndeby60%whenGA3(20-50mg/l)whengivenasa foliar spray (6Lper tree) toallthecitrus trees.Duringbuddevelopment incitrus,theapplicationofGA3hasshowntoinhibitflowerproduction(Guardiola et al. 1982),leadingtogreaterratio of terminal flowers in the leafy shoots thushigher development of fruits (Iglesias et al. 2007).Theseresultswerealsoshownbyuseofethychlozateand GA3 for flower induction in citrus fruits butGA3caused inhibitoryeffect (Takahara et al. 2001).According to Ben-Cheikh et al. (1997) gibberellinsare the factors responsible for ovary transition. Invegetativeorgans,gibberellinsactivatetheprocessof


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celldivisionandcellenlargement(Talon et al. 1991)thus are also associated with initiation of growth(Talon and Zeevart 1992). Gibberellins reduce theflower production resulting in higher productivityof better quality fruits. It acts like a thinner agentbut it also showed the ability to retain theflowers(Iglesias et al. 2007)whereas2,4-Ddelayorstimulatethe abscission. Talon and Zeevart (1992) reportedthatincitrusthereproductiveprocessesareaffectedbyplantgrowthregulatorsshowingthatregulatorymechanism being controlled by critical hormonalcomponent.

The phenomenon of flowering in plants is underthe control of many factors, viz., cultivar, geneticmakeup, the environment and cultural practices.Mostofthecitruscultivarsflowerprofuselysubjectto optimum environmental conditions and therearemorethan100,000flowerspertree(Agusti et al. 1982).Dependingonthecultivar,mostoftheflowersabsciseleadingtolessthan1%fruitset(El-Otmani et al. 1992).Theapplicationofplantgrowthregulatorsis to regulate tree productivity either by reducingvegetativegrowthandenhancingflowering(during‘off-year‘)orbyenhancingvegetativegrowthandreducing flower initiation and development ( ‘on-year ‘).Monselise andHalevy (1964) reported thatgibberellicacidcouldinhibitfloweringin‘Shamouti’orange. Similar inhibitory effect having beenreportedbyDavenport(1990).However,thetriazolecompoundsthatinhibitgibberellicacidbiosynthesishave been reported to promote inflorescenceproduction (Delgado et al. 1986 1986b; Harty andvan Staden 1988). It is the time of application andconcentration which determines the action of aparticular growth promoter (Guardiola et al. 1982,Lord and Eckard 1987). In subtropical climates ofthe northern hemisphere floral induction occursduringtheperiodfromNovemberthroughJanuary.For sweet oranges and somemandarins inductionoccurs early during this period (Guardiola et al. 1982), whereas for ‘Satsuma’ mandarins it occurslater (lwahori and Oohata 1981). The optimumconcentration for promotion of flowering are GA3( 25 µg/ml ) for ‘Navelate’ sweet orange with theapplication inmid-December and for ‘Washington

Navel’ with the appropriate time of applicationbeing mid-November in Spain (Guardiola et al. 1977).Optimumconcentrations(100mg/L)andtimeofapplication(lateJanuary)for‘Satsuma’mandarinin Japan isreportedby lwahoriandOohata (1981).Agusti et al. (1981) concluded that GA3 (10 mg/l)applied in late November could reduce floweringto40%in‘Clementine’mandarinirrespectiveofthedate of harvest (i.e., the presence of the crop wasnot inhibiting flowering). Low GA3 concentrationreduced flowering by 37 and 70% when it wasapplied to ‘Satsuma’ mandarin trees in mid-December(Garcia-Luis et al. 1986)andlateDecember(Guardiola et al. 1982), respectively. However, thedurationofsensitivity isveryshortandtreatmentsmust be applied before the developing shoots aremorethan1mmlong(Guardiola et al. 1982),whichcorrespondstothestageoffullsepaldevelopmentoftheapicalflower(LordandEckard1987).

Attempts to promote flowering using growthretardants that are reported to inhibit synthesis ofgibberellins viz., CCC and paclobutrazol have notbeenabletoprovideconclusiveresultsasreportedbyHartyandvanStaden(1988)andDavenport(1990).Greenberg et al. (1993)reportedthattheapplicationof paclobutrazol (foliar spray or soil application)shifts the balance of shoot types toward the pureleafless inflorescences, which is just the reverse ofGA3,whichpushes thebalance towardvegetative,flowerlessshoots.

There are reports of role of auxins in the processof flowering by Zeevart (1978). Application of2,4-D during mid-November to mid-Decemberto ‘Navelate’ sweet orange reduced flowering byapproximately 30% (Guardiola et al. 1977). Higherconcentrations of synthetic auxins did not provideanyadditionaleffect (Agusti1980)and lower ratesdidnotshowinhibitoryeffect(Garcia-Luis et al. 1986).Abscisicacidmayalsobeinvolvedintheregulationofflowering (YoungandCooper1969).The roleofcytokinins in the regulation of flowering in citrushas received little attention, although a correlationbetweenendogenouscytokininlevelsandbudbreakandgrowthhasbeenreported(Davenport1990). It

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has been argued by Davenport (1990) that lack offloweringduring(off-bloom)whentreesarebearingtheheavy‘on-yearcrop’forseverelyalternatebearingcultivars,islikelyduetolackofcytokinins.Thelowlevelofcytokininsareduetolackofcarbohydratesandothernutrientsnecessaryforactiverootgrowthandfortheproductionofcytokinins.

Gibberellins appear to be the only plant growthregulators that consistently inhibit flowering, buttheuseofgibberellicacidasacommercialregulatorsforreducingflowernumbertoincreasefruitsizeorinanexpected‘on-year’toreducealternatebearingis limited. Climatic conditions, i.e., temperatureextremes that might prevail during floweringinvocationandinductionmayresultinpoorfruitsetorexcessivefruitdropresultinginlowyield.

Crop Regulation

Consumer preferenceworldwide is for large sizedandhealthyfruits(Gilfillan1987,MillerandHofman1988). Fruit juice acidity (percent citric acid) andjuice volume are verymuch affected by fruit size.Smallfruithaveasubstantiallyhigherpercentacidthan larger fruit and total soluble solids per fruitis considerably lower in small fruit. Reducing theproportionofsmallfruitharvestedincreasesgrowerreturn both for fresh fruit (because there aremorefruit of commerciallyvaluable export size) and forjuice(becauseofincreasedjuiceandTSS).Smallfruitsize results due tomany factors, viz., competitionbetween fruit-lets (Hirose 1981). Usually, after anextremelyheavycrop,alternatebearingcanresultintreedeclineandevencollapseinsomeextremecasesas‘Kinnow’mandarin(Jones et al. 1975).Competitionforcarbohydratesreservesamongsubsequentcropsis probably one of themajor cause but results areinconclusive (Monselise and Goldschmidt 1982).There is an inverse relationship reported betweenfruitsizeandflowernumberandfruitnumberpertree (Guardiola 1992). Consequently, flower andfruit thinning,bothmanuallyandchemically,havebeen used to improve fruit size (Zaragoza et al. 1992). Hand thinning requires a lot of labour andtime.Whereas, chemical thinninghasbeendesired

strategyforcultivarsthatbearprofuselyinordertoavoid the effects of alternate bearing (Hirose 1981;Monselise et al. 1981; Gallasch 1984).A significantincreaseinfruitsizeoccursonlyif fruit thinningisconsiderable and performed sufficiently early infruitdevelopment(Zaragoza et al. 1992).

Themostwidelytestedgrowthregulatorsusedforthinningareethephon(Wheaton1981,Gallasch1988),NAA (Hirose 1981,Monselise et al. 1981,Wheaton1981,Gallasch1988,Guardiola et al. 1988).Inordertoavoidoverthinning,treatmentsshouldbeappliedearlyinthemorningorlateintheafternoonandonlyduring a period in which excessive temperatures(>30°C)arenotcommon(Hirose1981).Inadditiontoanincreaseinfruitsize,advantagesofthinningalsoinclude prevention of tree collapse,which ensurestreesurvival.Inthiscase,evenalossinimmediateprofitcanbejustified(Monselise et al. 1981).

Fruit-Set

After successful fertilization the early changes inflowerarefollowedbyfruit-set(Petho1993).Theearlychangesoftheflowerafterasuccessfulfertilizationare the signs of fruit set. In some cases, the merefact of pollination may also initiate the growth ofthe ovary. Without fertilization, the degenerationof the ovary is expected,which is followed by thedeathandabscissionoftheflowertoo.Flowerdropis caused by the appearance of ethylene-producedautocatalytically.

The pollinated flower develop to a fruit, and thefertilized ovules grow to seeds due to an intensesynthesis of growth substances. The intense celldivisionandgrowthof the tissues absorbs a lot oforganicmatter of the reserves competingwith thevegetative organs, consequently, an interactionbetweenthedifferentpartsoftheorgansofthetreeis building up. Not only the fate of the growingfruit, itssizeandqualitybutalso thephysiologicalpotentialofthewholetreeisinfluencedbyrelationsofsourcesandsinks,whichinturnmayimpairthemaintenanceofthefruitsset.Thegrowthsubstancesinduce the growth and thickening of the peduncletoo(Krezdon1973,Saleemetal,2008,Jain et al. 2014)


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Fruit Growth

Fruitsizeisadesirablecharacterforthemarketabilityofcitrus.Smallfruitsizeisverycommoninmandarins.Thevariationinfruitsizeresultsfromdifferencesincellnumberorcellsizeoracombinationofboth.Thepotential for increasing fruit sizebyenhancingcelldivisionearlyisarelativelyundevelopedapproach.Treatment with plant growth regulators such asgibberellins and cytokinins during or shortly afterflower opening enhances early fruit growth (EI-Otmani et al. 1992;Guardiola et al. 1993)butdoesnot always result in an increase in fruit weight atharvest.Thisearlyincreaseinfruitsizeisduemainlyto a transient increase in cell division in the ovarywall.Earlyapplicationofauxinsincreasesfinalfruitsizemoreconsistently(Agusti et al. 1992;1996,1998;EI-Otmani et al. 1993;Aznar et al. 1995).

Theratesofapplicationoftheplantgrowthregulatorvarieswithcultivar,stageoffruitdevelopmentandclimatic region. It is important to ensure optimumtree nutrition and irrigation as well as to providefull tree coverageduringapplication.Themodeofaction of these plant growth regulators has beensummarizedbyAgustiet al.1996,butitisimportantto note that they increase vesicle size, not number(Agusti et al. 1992, 1996; EI-Otmani et al. 1993),and that their effect on vesicle size is through anenhancement of cell enlargement, not cell division(EI-Otmaniet al.1993;Agustiet al.1996).Developingfruit are important sinks for water and photoassimilates;theirsinkstrengthisincreasedasaresultof auxin treatment,withall fruit tissues increasingin proportion. Auxin application increases fruitpedunclesizethroughanincreaseinphloemcellsize(Agusti et al. 1996) consistent with increased sinkstrengthandcarbohydratemovementintothefruit. Reduced fruit growth rate during the early stagesoffruitdevelopmentwaspositivelycorrelatedwithfruitabscission,particularlywhenfruitweretreatedduringtheJunedropperiod(Agustiet al.1995).

Fruit Drop

AccordingtoSoostandBurnett(1961)theexogenousapplicationofGA3improvestheparthenocarpicfruit

set andgrowthof self-incompatiblegenotypes likeClementine that shows negligible parthenocarpicfruit set in the absence of cross-pollination El-Sese(2005)confirmed that thenumberof fruitsper treeandthetotalyieldincreasedwithGA3ascomparedtocontrol.ThefruitsetwassignificantlyaffectedbytheGA3treatmentindividuallyandalsoincombinationwithauxinandgavethemaximumfruitsetof32.3per cent (Saleem et al. 2005). The GA3 and 2,4-Dareusedinabscissiontocontroltheoverloadingoffruitforbetterqualityoffruits(Iglesiaset al.2007).The application of 2,4-D after fruit set results inaccelerationofabscission(Lee2003).Theroleof2,4-Ddependsonexternalandinternalfactors.AccordingtoIglesiaset al.(2007),decreaseinGAandincreasetheABA level shows that both these plant growthsubstancescontroltheinitialfruitsetandJunedropalong with other essential component. Randhawaet al.(1961)reportedthat2,4-Dreducedpre-harvestdrop in Jaffa oranges. Bajwa et al. (1971) reportedthat in sweet orange cv. pineapple when sprayedwith 2,4-D (20ug/ml) or NAA, both plant growthsubstancecontrolthepre-harvestdrop.AccordingtoRandhawaet al. (1961)applicationof2,4-D(15and20µg/ml) and 2,4,5-T (5 and 10µg/ml) reduced thefruitdropinLahorelocalandNagpurmandarin.

TheenvironmentalconditionslikehightemperatureandlowhumidityinsoilandinairgreatlyinfluencetheJunedrop(Levitt1964;Davieset al. 1981).Knapp(1996)reportedthattheapplicationof2,4-Disopropylester acid at 60 to 70g /ha forduring 6 to 8weeksafterbloomreducedthesummerdrop.Summerfruitdropusually occurs frommonthofmid-August toOctobertillmaturity.

The abscission of fruits is generally coupled withstructural changes in theplant (Baird andWebster 1996), which means that several abscission zonesare formed within the same inflorescence. It hasbeen postulated that there is appearance of twoor three abscission zones around the same fruit. Fruitsareabscisedbecausethecellsgetlooseintheabscissionzone.Pectin,hemicellulosesandcelluloseare dissolved by the respective enzymes, and themechanical stressdetach the fruits. Sometimes, the

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deadxylemelementskeepthefruithangingawhile.Asexternalagents,thewindmayhelpthedrop,butthedehydrationofsenescentcellscausestensionandcontributestotheprocess.Thescarofthedetachedfruitisgenerallysuberisedorlignified.

Theabscissionzoneisdistinctfromtherestoftissuesnotonlyinitsanatomybutalsoinmetabolicterms.Abscission zone excels by intense cell division,synthesisofproteinsandRNA,highO2consumptionas well as peroxidase-activity. Important role isattributed to the middle lamella and the attachedprimary cell wall, as being dissolved. The middlelamella is softened by the enzyme pectin-esterase,which demethylate the pectin making it solubleandthecellsareeasilyseparated.Anotherenzyme,cellulaseisalsoactivatedtogetherwithgalacturonase.

Duringthephaseofabscission,thegenesresponsiblefor inducing the synthesis of hydrolytic enzymesdissolving polysaccharides of the cell wall mainlyas cellulases and pectinases are activated. Thoseenzymes dissolve themiddle lamella and also thecellwallsandthecohesionofcellsisweakened.

Followingtheflowerdrop,fruitdropensuingbeforematurityisattributedtothecollapseofthehormonalbalance in the growing fruits, where the growthsubstancesbeingactiveinfavourofgrowthlosttheirinfluence against abscisic acid causing abscission.TheroleofABAbecomesprevalentwhentheyoungfruit-lets drop, and, when fruits are almost ripe.ThedifferencebetweenvarietiespronetofruitdropcorrespondedtotheirABAcontenttoo.

Theabscissionofyoungfruitsseemstodependonauxin as a correlative dominance signal (Bangerth 1990). The investigations of Luckwill (1953) madeearly reports in searching the causal relationsbetween auxin production of the developing fruitandfruitdrop. TheyexplainedthatIAAmayspeeduporalternativelyinhibittheprocessofabscission.

Lackofnutrients,whichdoesnotdependsolelyonthemeagresoil,butratheronthecompetitionbetweenthevegetativeorgansandthegrowingfruitsoftheplant.Thefateofayoungfruitisoftenimpairedbythedominanceof another fruitorvigorous shoots.

The abscission of fruits is generally coupled withstructural changes in theplant (Baird andWebster1996),whichmeansthatseveralabscissionzonesareformedwithinthesameinflorescence.

Gibberellins areknown for their ability to increasecell enlargement, thus enhancing fruit growth incertain species suchas citrus (Eman et al. 2007,El-Sese2005),guava(El-SharkawyandMehaisen2005)andpear. In all species so far studied, gibberellinshad the potential for increasing fruit size. Salicylicacid is considered to be a potent plant hormone(Raskin1992)becauseofitsdiverseregulatoryrolesinplantgrowthregulatorand its role isevident infruit yield (Klessig andMalamy1994).Applicationof plant growth regulators like gibberellins andsalicylic acidaloneor coupledwithmicronutrientsmayimprovecroppingpotential.

Early reproductive processes in citrus are stronglyaffectedbyplantgrowth regulators indicating thattheregulatorymechanismsthatcontrolfruitsetandabscissionofovariesandfruitletspossessesapivotalhormonalcomponent(Talonet al.1990).Gibberellinsand cytokinins are generally considered to bepositiveregulatorsoffruitgrowthwhileauxinshavebeen reported to act as stimulators of growth andalso as abscission agents (El-Otmani andOubahou1996).Abscisic acid (ABA) and ethylenehavebeenimplicatedinseveralwaysinabscission.Gibberellinsarethoughttobepivotaleffectorsresponsiblefortheovary-fruit transition(Talonet al.1992,Ben-Cheikhet al. 1997). They activate cell division and cellenlargement processes in vegetative organs (Talonet al. 1991) and therefore are generally associatedwith the initiation of growth (Talon and Zeevaart1992).The intensityof abscissionduring the initialphasesofgrowthisalsorelatedtothephenologyofflowering.

Salicylicacidandothersalicylatesareknowntoaffectvarious physiological and biochemical activities ofplants andmayplay a key role in regulating theirgrowthandproductivity(Arberg1981).Exogenousapplication of salicylic acid may also influenceseed germination and fruit yield. Sivakumar et al.(2002) determined that the application of salicylic


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acid increased the content of protein in grains ofpearlmillet.Kalarani et al. (2002) carriedoutapotcultureexperiment todetermine theeffectof foliarapplication of different concentrations of salicylicacidonphysiological andbiochemical constituentsas well as yield and quality of tomato and foundthat Salicylic acid at all concentrations showed itsefficiencyininducingearlyfloweringandincreasedfruitsetpercentage.

Besides, hormonal induction of fruit growth,nutrients may have regulatory functions on theirown and/or through the maintenance of adequatehormonal levels (Gillaspy et al. 1993). The averagenumber of flowers produced in a normal citrustree is by far extremely high in comparison to thenumber of fruits that the same tree can supportuntil ripening. Hence, many fruits are abscisedduring growth apparently due to competition fornutrients especially photo-assimilates. During theinitialmoments of phase I, citrus fruitlets functionas carbohydrateutilization sinksbut over thefinalstagesofthisperiodandduringthetransitionfromcell division to cell enlargement, developing fruitsshifttheirmetabolismandstarttobehaveasstoragesinks (Mehouachi et al. 1995). Defoliation duringphaseIreducescarbohydrateamounts,arrestsfruitletgrowthandpromotesmassiveabscission(Mehouachiet al. 2000)whereas,defoliationafter theJunedropalso arrests growthbutdoesnot induce abscission(Lenz 1967). The link between carbohydrates andfruitgrowthiscurrentlysupportedbyawidebodyofevidenceincludingseveralstudiesonsource-sinkimbalances, defoliation, girdling, shading, sucrosesupplementation, de-fruiting and fruit thinning(Goldschmidt and Koch 1996, Iglesias et al. 2003,Syvertsen et al. 2003). First, the enhancement ofcarbohydrate availability was associated with animprovement of fruit set and yield of citrus trees(Goldschmidt 1999). Later, a strong relationshipwas demonstrated between carbohydrate levelsavailabletofruitletsandtheprobabilityofabscission(Gomez-Cadenas et al. 2000, Iglesias et al. 2003).Thisphenomenon that has also beendescribed forother tree species is also supported by studies ontranslocationof14CmetabolitesandCO2-enrichment

experiments(Mosset al.1972,Downtonet al.1987).Hence,photosynthesisactivityhasbeenprovedtobecrucialsincehighcarbohydraterequirementsduringfruitsetincreasesphotosyntheticrate(Iglesiaset al.2002).Thissuggestionalsoimpliesthatareductionin net CO2 assimilation results in lower sugarproduction and fruit set. The sugar concentrationinleavesmightbethesignalthatregulatesthefeed-back mechanism stimulating photosynthesis inresponsetofruitsugardemand.Thus,oncecarbondemands are fulfilled, carbohydrate accumulationmay elicit end-product feedback control ofphotosynthesis.ThepositiveeffectofexogenousGAson fruit set andgrowthmay alsopartially operatethrough the induction of a stronger mobilizationof14Cmetabolitestoovaries(PowellandKrezdorn1977). The exogenous GAs have also been shownto stimulategrowthand increase carbon supply invegetativetissues(Mehouachiet al.1996).Thusthesugars are deeply implicated in the regulation offruitlet growth and that overall carbon deficiencyinducesfruitabscission.

Although the specific mechanism involved in theresponse of fruit growth to carbohydrates has notbeen studied at the molecular level, observationssuggest that sugars may act not only as essentialnutrientfactorsbutalsoassignalstriggeringspecifichormonalresponses(Roitsch1999).Theobservationlinkingcarbohydrateandabscissionwasconfirmedwiththefindingthatcarbonshortageduringovaryand fruitlet drop increasedABA and ethylene andbothareinvolvedintheinductionofearlyabscission(Gomezet al.2000).

The alterations in the nutrient balance that areaccompanied with increased fruitlet abscissionduring the June drop provoke an unambiguoustendency to both increase nitrogen content and toreduce carbon shortage. Abscission intensity maybecorrelatedpositivelywithcarbohydrateshortage.The two main Conclusions that can be extractedare that the fruit fall that takes place during Juneisvery likelydue to thecarbohydrate insufficiencycaused by an increased carbon demand of a hugepopulation of expanding fruitlets; and second,

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carbondeficiencyisagainassociatedwithABArise,ethylene release and massive fruitlet abscission.ThisideathatcitrusfruitabscissionisconnectedtocarbohydrateavailabilitywasinitiallyanticipatedbyGoldschmidt andMonselise (1977) who suggestedthatcitrusmightpossessaninternalself-regulatorymechanism that adjusts fruit load to the ability ofthe tree to supplymetabolites. The above findingsidentify leaf sugar content, ABA and ethylene asmajor components of the self-regulatory adjustingmechanismofabscission.Recently, ithasalsobeenproposedthatinadditiontoJunedrop,earliestovaryandfruitletfallsthatoccurthroughabscissionzone,are also dependent upon nutritional factors suchsugars(Iglesiaset al.2006).

Literatureindicatesthattheapplicationofnutrientsand plant growth regulators increase the fruityield and quality, hence the suitable combinationof macronutrients, micronutrients and growthregulatorscouldcontroltheexcessivefruitdropandimprovethecitrusfruityieldanditsquality.

Fruit Quality

Fruitqualityisaconceptthatvariesaccordingtothefinaluseofthefruitandatwhatpointfromorchardto consumer the fruit is evaluated.For thegrower,anyfruitthatcanbesoldatareasonablygoodpriceis of good quality. For the fruit packing industry,fruitsthatareofuniformsize,freeofblemishesoffergoodmarket. The harvested fresh fruit requires tobeseedless(lowseednumber),highTSS:Acidratio and excellent color, shape and firmness.Whereas,the requirementof the juice industry is fruitswithhighjuiceandsugarcontent.Parametersimportanttoboththefreshfruitandjuiceindustrieshavebeenshowntorespondtoplantgrowthregulators.

Thejuicecontentandorganoleptictasteareimportantparametersanddesirableforfreshfruitconsumptionofcitrus.Organolepticqualityistheresultofsugarand acid content, and the presence of volatiles inthe juice. Nutritional quality includes sugar, acid,vitamins,etc.Forfruitprocessing,internalqualityisfarmoreimportantthantheexternalappearanceofthe fruit. Plant growth regulators that canbeused

tomanipulatetheseparametersincitrusarealmostnon-existent.Althoughnotaplantgrowthregulator,leadarsenatewasusedformanyyearsinFloridatoimprove fruit internal quality (Knapp 1996). Thiscompound causes a reduction in total acidity andconsequently,anincreaseinthesugar-to-acidratio.

Granulation of fruit is a physiological conditionin which the juice sacs become gelled with littleextractable juice It develops pre-harvest (Smoot et al. 1971) as well as post-harvest (Gilfillan andStevenson1977).AreductionofthedisorderbyGA3 sprayshasbeenreported,butthiseffecthasnotbeenreproducedconsistently

AccordingtoKauret al.(2000)fruitweightincreasedwithincreaseinamountof2,4-DintreesofKinnowmandarin.The fruitweight andpeel thicknesshasbeen increasedwith theapplicationofZnaloneorin combination with GA3 in ‘Washington Navel’orange(Emenet al.2007).TheapplicationofZnandKincreasedfruitweightascomparedwithcontrol.According to Sourour (2000) both Zn and EDTAincreasedthenumberandweightoffruitspertree.According to Chundawat and Randhawa (1972)GA increased the fruit size in Saharanpur specialgrapefruittrees.Theapplicationof2,4-Dat20µg/mlalongwithCuSO4at0.25or0.50percent increasedthefruitsize(SinghandMishra1986).

ChundawatandRandhawa(1972)reportedthatGA3 and 2,4-D increased peel thickness in Saharanpurspecial variety of grapefruit. Also Dinar et al. (1977) observed that both gibberellic acid and 2,4-D increased peel thickness in Marsh grapefruit.Applicationofnutrients increasedthe juicecontent(Ram and Bose 2000). Maximum juice from fruitswasobservedfromthe treessprayedwith1%ureaand 0.8% Zinc sulphate (Malik et al. 2000). Treestreated with gibberillic acid yield juice more than10%thanthecontrolones(Davieset al.2001).SuchincreasewaseconomicallybeneficialtoFloridacitrusgrowersbecauseprocessedfruitvalueincreasewithjuiceyield(Braddock1999).

The applicationofZn alone andwith combinationwithFeandMnincreasedtheTSSvalueascompared


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tocontrol.AccordingtoMongaandJosan(2000)thehighest TSS was obtained from tree sprayed withZn (0.3%) alone. Treatment with 2,4-D at 20µg/mlandgibberllicacidat20µg/mlincreasedtheTSSingrapefruitjuice(Llaneset al.1991).

Foliar application of Zn alone and along with FeandMnonKinnowmandarin resulted indecreaseacidity as compared to control (Monga and Josan2000).According to SinghandMishra (1986) 2,4-Dincreased the acidity level inKinnow fruit.Age oftreeandtypeofcultivaralsoinfluencedtheacidityoforanges(FrometaandEchazabal,1988).

Foliar application of Zn was observed to increasetheascorbic acid contentsof juice invarious citrusvarieties (Dawood et al. 2001). Increased contentof ascorbic acid was observed with application ofZnalone ,Zn+MnorZn+B(Tariqet al.2007).Highascorbic acid was obtained by sprays of Zinc andgibberellicacid (Eman et al. 2007).ChundawatandRandhawa (1973) reported that vitamin C contentincreasedwithsprayof2,4-DinDuncancultivarofgrapefruit. Immature citrus fruits have the highestamountofascorbicacidwhereas,ripenedfruithavethe least as reported by Nagy (1980). However,ascorbic acid content increased with ripening offruitsinapricot,peachandpapayasbutdecreasedinapplesandmangoes(LeeandKader2000).

The TSS:Acid ratio of juice was increased withtheapplicationofSA,ZnandKaloneandalsobycombination of both. Abd-Allah (2006) reportedthat the application of K in combination withmicronutrientsimprovedTSS:acidratiowhileZn+Kand other different combinationswas observed byAshrafet al.(2012).

Among sugarderivatives; sugarnucleotides, sugarphosphates, glycosides and polyles are important.D-galacturonicacid ,D-Glucuronicacid,L-ascorbicacidaresugaracidswerefoundincitrusfruits.Sugaralcoholssuchasmyo-inositolwasfoundinorangesand grapefruits in the range of 88-170mg/100g ofjuice Lemons had 56-76 mg myo-inositol/100g ofjuice. The reducing sugars, non- reducing sugarsand total sugars increase as the fruit start to ripe.

Fructose,alphaandbeta–glucoseandsmallamountofgalactosewerealsofoundinValenciaorangejuiceBy using 14C labelled compounds, Sawamura andOsajima(1973)observedthattranslocationofsugarsfromleaftofruitoccursintheformofglucoseandfructosewhich are further changed into sucrose inthefruit.Kuraokaet al.(1976)foundchangesinsugarcontent of flavedo tissue of Satsuma mandarinsgrown in Japan.TingandAltaway (1971) reportedthattheratioofFructose:Glucose:Sucroseas1:1:2inValenciaoranges.

Production

Optimumyieldisayieldthatissustainableyearafteryearandthatutilizesthefullpotentialoftheavailablelandalongwiththeenergyandnutritionalresourcesof the tree.Thisyield isa functionof treeplantingdensity,canopydevelopment,intensityofflowering,fruitset,fruitgrowthandnumberoffruitsharvestedat maturity. In young trees, a high proportion ofphotosynthateisallocatedtovegetativedevelopmentandgrowth.Usually,onlya fewflowersandfruitsareproducedduringthe2to3yearsafterplantinga nursery tree. Canopy development and growthcontinues and reaches an optimumbetween 5 and12 years, depending on planting density, atwhichtimeyieldisatitshighestvalue(Boswellet al.19701975).At this point a natural equilibrium betweenvegetative growth and reproductive developmentisestablishedwithvariationsduetoenvironmentalconditions and genotype of the cultivar, providedthatnutritionandlightinterceptionareoptimal.

Theproductionperunitareaisthekeyfactorforthecitriculturists. However, the productivity dependsuponfactorsviz.,lightinterception,carbondioxidefixation, water availability, and mineral nutrientuptake.Besides,theproductivityisalsoaffectedbycompetitionamongsimilarordifferentplantorgansfor photo assimilates and nutrients. Plant growthregulatorshavebeenusedto improveproductivitywherever feasible. Unfavourable environmentalconditions may restrict many of the processesrelated to production from flower initiation (Moss1969; Southwick and Davenport 1986; Lovatt et

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al. 1988) to the development of inflorescences andflowersormaycauseexcessivedropofflowerbuds,openflowers,entireinflorescences,anddevelopingfruitlets.Plantpathogensmaycauseexcessive fruitdrop andmaydeteriorate fruit quality (Lima et al.1980).Imbalanceoffertilizersthatmaysupplyexcessofnitrogenleadtoexcessivevegetativegrowthandlow flower number. On the other hand, nitrogendeficiency leads to low flower initiation and thusreducedyields(Davenport 1990).

Plant growth regulators are used to reduce theseasonalfluctuationsinyieldandalsotomaximizeenergy allocation to harvestable fruit in order toincrease fruit number and fruit size rather than toenhanceexcessivevegetativegrowth. Plantgrowthregulators can be used to hasten or delay fruitcoloring and fruit maturation so as to shorten orextend theharvest season toget thebesteconomicproductivity.(Jainet al.2014)

Yield (Kg per tree) includes both fruit numberand fruit size.Fruitnumber isa functionofflowerintensityandfruitset.Fruitsizeisafunctionofcelldivisionandcellenlargementprocesses.Thenumberoffruitthatsetandpersisttoharvestinfluencesfruitsize.Citrusfruitgrowthisacontinuousprocess,fruitweightandfruitdiameteris representedbyasinglesigmoidal growth curve (Guardiola and Lazaro 1987).Thedevelopmentof theovary intoamaturefruitproceedsthroughthreestages:celldivision,cellenlargement, and maturation. During Stage I, celldivisionpredominates.Celldivisionisessentialforthe formationof theovaryduringflower initiationand continues past petal fall to approximatelymid-JunetolateJuly(Lovatt 1999).Atthistimecelldivision,whichoccursinallthetissues,iscompletedexcept in the flavedo. Thus, during the periodencompassingfruitsetandJunedrop,fruitgrowthisdominatedbycelldivision.StageIIistheperiodofmaximumfruitgrowthoccurringoverthefollowing3 months for the early maturing cultivars (e.g.,‘Satsuma’mandarin)andthenext7to10monthsforthe latematuringcultivars (e.g., ‘Valencia’orange).This stage of fruit development is dominated bycell differentiation and cell enlargement. The juice

vesiclesandloculesincreaseinsizewiththeuptakeof water, and the flavedo continues cell division,whereas the cells of the albedo must expand andstretch to accommodate the growth of the locules.Drymatterandwateraccumulateinthevacuolesatahighrate(Guardiola1992).Fruitthatpersistonthetree through the Junedropperiodarenot likely toabsciseintheabsenceofpestdamage,physiologicaldisorders such as splitting, or stress such aswaterdeficit and excess temperatures. Stage III is theperiodoffruitmaturation.Fruitcontinuetoincreasein size predominately by cell expansion, but at aslower rate. Peel thickness may increase in somecultivarsandpuffinessmayoccur,particularlyinthemandarintypes.

According to Bengal et al. (1982), the combinedapplication of NAA (25ug/ml) and urea (1%)increasedtheseedyield.FoliarapplicationofNAAonpaddy (Oryza sativaL.)under low levelofN (0and60kg/ha)gavethebeneficialeffectsasreportedby Grewal and Gill (1986). The beneficial effectwasobservedbyincreaseinnumberofear-bearingshoots/plant,grainweight,numberoffilledgrains/panicle.Thesufficientnitrogensupplywasessentialfor increasing theyieldof ryeandbarleybymonoethanolamine(BergmannandEckert1990).

Conclusion

Plantgrowthregulatorsarecommerciallyexploitedinthefieldofhorticulturebecauseofthewiderangeofpotentialrolestheyplayinincreaseofproductivityper unit area. Their application in citrus industryduringdifferentphasesofgrowthanddevelopmentiscurrentlyinpracticeasisevidentfromthepresentliterature.Theadvantageofplantgrowthregulatorsis their use at very low concentration because ofwhichtheydonotlayanyhealthhazards.However,itisimportanttounderstandthebasicmechanismsunderlying the citrus growth and development inordertomanipulatethekeyphysiologicalprocessesand make use of the plant growth regulators atthe appropriate stage of development and at theoptimum dose. Therefore it is desirable to persueresearchonthemechanismofactionofplantgrowth


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regulators on the physiological, biochemical andgeneticregulationofgrowthanddevelopment.

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