Citrus Fruit Abscission
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CITRUS FRUIT ABSCISSION J. K. BURNS Citrus Researchand EducationCenter Lake Alfred, FL 33850 I. Whatis abscission? 130 II. Citrusfruitabscission 130 A. Anatomyof citrusfruit abscission.. 13l B. Physiology of maturecitrus fruit abscission. .. .. 131 C. Growthregulatoreffects ... 132 m. Abscissionand fresh fruit quality. .. 133 IV. Literaturecited 135 I. What is abscission? The tenD 'abscission' is used to describe a distinct process that culminates in the shedding of plant parts (Sexton and Roberts 1982). Fruit, flowers, flower parts, leaves, stems, and various other vegetative structures are shed at different periods throughout the life of a plant. Plant organs can be shed by mechanical tearing or as a result of tissue death and decay, such as occurs with the shedding of roots. Abscission, however, is distinct from loss of organs by mechanical tearing or tissue death becauseit occurs in well-defined areasof the plant known as abscission zones. As plant organs abscise,a series of physiological and biochemical events in the abscission zone lead to cell wall breakdown in the few rows of cells on either side of the fracture line and ultimately to detachment of the organ. II. Citrus fruit abscission Citrus fruit abscissionand its regulationhave attracted the attentionof Florida researchers andthe citrus industryfor many years(Wilson 1966;Wilson et alI977). Maturecitrus fruit require unusuallylarge forcesto removethem from the tree (break-strength),and this hasbeen an obstacle to efficient and economical mechanicalharvesting (Goren 1993; Whitney 1995). The unique anatomy and physiology of the citrus fruit abscissionzone before, during and after abscission significantly affectsthe break-strengthof the fruit. 130 II. A. Anatomy of citrus fruit abscission Citrus in generalhas four abscissionzones. Fruit laminar have two abscission zones: the first abscissionzone is AZ located betweenthe branch and the fruit peduncle(AZ-A) " and the secondis found in the fruit calyx (AZ-C). Leaves havetwo abscissionmIleS: the fust is located~ thebranch Branch and the petiole (branch AZ), and the second is found AZ betweenthe petiole and the leaf blade(laminar AZ) (Figure AZ-A 1). During the first 8 weeksof fruit development,fruit AZ-C normally absciseat AZ-A. After this time, AZ-A losesit's ability to readily abscise. Fruit older than 8 weeksabscise only at AZ-C, however,young fruit also havethe ability to absciseat AZ-C. Although older fruit do not absciseat AZ- A, microscopy studieshave shown that abscission-related.. eventsare actually occurring in isolatedpockets of tissuein Flgu~e~. Citrus leaf and fruit AZ A H frui ~_:I b . thi ab . abscIssionzones. -. owever, t J.Wl to a SClseat s scIssionzone because of extensive lignification and secondary wall formation of abscissionzone cells that surroundthe vasculartissue and inner cortex (Goren and Hubennan 1976; Greenberget a11975). Cells of the AZ-C undergo numerous anatomical changes that lead to abscission. Prior to the onset of abscission, the cell layers that will be involved in the abscission process can readily be distinguished from surrounding tissues becauseof the presence of starch grains. Evidence of cell divisions are observed early in the abscission process and abscission zone cell walls become thickened. Cell wall thickening is a result of breakdown of the middle lamella followed by hydration of cell wall components. As abscission progresses,the primary cell wall is degraded. Eventually, a fracture develops across the abscission zone resulting in fruit separation from the peduncle (Hubennan et a11983; lwahori and Van Steveninck 1976). II. B. Physiology of mature citrus fruit abscission Abscissionis a naturalplant processand many aspectsof its physiology and regulationare unknown. Howevert it is well establishedthat plant hormonesare involved in control of abscission (Sextonand Roberts1982). Ethyleneaccelerates abscission, and this property has beenutilized to develop abscissionmaterials for the purposeof looseningfruit of various agricultural crops for harvest. Normally auxin delaysabscissiont but the effect of auxin on abscissionis relatedto the endogenoushormonal balance that changesas abscissionproceeds. Marked physiological and biochemical changesoccur during abscissionof mature citrus fruit. When ethylene is used to accelerate abscission, increased amounts of mRNA and protein are measured in AZ-C tissue (Kazokas 1997). We do not know the function of most of this mRNA and protein, but some mRNA and proteins are associated with the enzymes cellulase and polygalacturonase (pO)(Burns et all995). The activities of cellulase and PO greatly increase during 131 the abscission process in AZ-C (Table I). These enzymes are thought to function in the degradation of adhesive cell wall components that link abscissionzone cells together, and this increasein enzyme activities is highly correlated with reduction in fruit break-strength (Goren and Huberman 1976; Greenberg et al 1975). Table 1. Total protein, total mRNA, cellulase and PG activities of Valencia orange calyx abscission zones (AZ-C). AZ-C were either removed from freshly harvestedfruit explants or removed from explant previously treated with 5 ppm ethylene, 30OC,95% RH. Ethyleneand auxin interactduring abscissionof maturecitrus fruit. When ethyleneis first applied,there is a periodof time in the AZ-C when changesin cellulaseand PG activities and fruit break-strengthcannot be detected.As the durationof ethyleneexposure increases, enzyme activities increase(Kazokas 1997) and a reduction of fruit break-strengthoccurs. Auxin is found in all portionsof the citrus plant, including AZ-C (Gorenand Goldschmidt 1970). Its presenceprevents the inductionof abscissionby ethylene. Continuedexposure to ethyleneresults in 1) inhibition of auxin transport from the leavesand stem to the AZ-C, and 2) destructionof auxin in the AZ-C. Abscissioncan then proceed. Once abscission has commenced, however, auxin applicationdoes not inhibit abscissionbut in fact promotesit by acceleratingendogenous ethylene production (Sexton and Roberts1982). Gibberellinsand cytokinins appearto haveno effect on the abscissionprocess of maturecitrus fruit Abscissicacid can enhanceabscission, but only in situationswhen ethylene is also releasedas a result of tissueinjury (Goren 1993). II. C. Growth regulatoreffects The promotive effect of ethylene on mature citrus fruit abscission has been utilized to develop abscission materials that facilitate mechanical harvesting. In general, these abscission materials releaseethylene either by I) chemical breakdown, 2) metabolism by the tissue, or 3) tissue injury. Abscission materials that releaseethylene by chemical breakdown produce ethylene at a very rapid rate. The presence of fruit and other plant parts are not required. However, these chemicals were totally ineffective at promoting abscission in the field becauseof rapid dissipation of ethylene under field conditions. The most notable abscission material that releases ethylene as a result of tissue metabolism is ethephon, or ethrel. Ethephon is absorbedby the fruit and surrounding tissues, and metabolized to ethylene under the slightly alkaline conditions that exist in cells. Ethephon was shown to have limited utility for inducing abscission in mature citrus fruit because of its non- 132 specificity,its low activity at low tempemtures,and its objectionableside-effects such as gumming of the trunk when applied in warm weather(Cooper et al1968). It is well establishedthat the production of ethylene is a physiological consequenceof woundingplant tissues(Sexton and Roberts1982). The majority of abscissionmaterials tested in Florida have beenthose that produceethylene by tissueinjury (Wilson et al1977). ACTI-AID(i), Pick-O~ and Release(i)are examples of abscissionmaterials that produce ethylene by this mechanism. When applied as a spray to fruit, peel blemishesoften appeared,reducing their marketabilityas fresh fruit. Propertiming of applicationand uptakewere also problemswith these materials.For example,the active ingredientof ACTI-AID(i), cycloheximide,is a potent inhibitor of protein synthesis.When applied at times of limited uptake,such as in conditionsof cold weather, the majority of active ingredientwould reachthe abscissionzone after abscissionhad commenced. This actuallyresulted in inhibition of abscissionby reducingthe synthesisof cellulaseand PO in the abscissionzone. Non-specificity and cost were also problemswith theseabscission materials. 'Transfer' is a new abscissionmaterial that has potential for use on citrus destinedfor the processedmarket. In a recentreport to the InternationalSociety of Citriculture (Wilcox and Taylor 1996), 'Transfer' applied at low concentrationswas reportedto reducefruit break-strengthover a threeweek post-applicationperiod. The modeof action,specificity and phytotoxicity of 'Transfer' is not known at this time, since the material remainsproprietary and has not been releasedfor experimentaltrials. 111. Abscission and fresh fruit quality AZ-C is embeddedin the remnantsof the floral calyx or 'button' of mature citrus fruit (Wilson and Hendershott1968). AZ-C doesnot traversethe button in straight cross-section,but ratherbegins where the tissuesof the button andflavedo join anddips througha small portion of the albedo (Figure 2). Vasculartissues that carry water and sugarspass through the AZ-C. The ventralvascular traces mainpeduncle. of the centralfruit coreand the dorsaland lateralvascular vasculartrace - peduncle tracesthat encirclethe fruit must