CITRUS FRUIT ABSCISSION A. Anatomy of citrus fruit abscission.

CITRUS FRUIT ABSCISSION
J. K. BURNS
Citrus Researchand EducationCenter
Lake Alfred, FL 33850
I. Whatis abscission?
II. Citrusfruitabscission
130
130
A. Anatomyof citrusfruitabscission.
..
.
.
13l
B. Physiologyof maturecitrus fruit abscission. . . . . . .. . . . . .. 131
C. Growthregulatoreffects
... 132
m. Abscissionandfreshfruit 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 abscissionzones. 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 regulationhaveattractedthe attentionof Florida researchers
andthe citrus industryfor manyyears(Wilson 1966;Wilson et alI977). Maturecitrus fruit require
unusuallylargeforcesto removethem from the tree (break-strength),and this hasbeenan obstacle
to efficient and economicalmechanicalharvesting(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
have two abscission zones: the first abscissionzone is
locatedbetweenthe branchand the fruit peduncle(AZ-A)
and the secondis found in the fruit calyx (AZ-C). Leaves
havetwo abscission
mIleS:thefust is located~
thebranch
and the petiole (branch AZ), and the second is found
betweenthe petiole and the leaf blade(laminar AZ) (Figure
1).
laminar
AZ
"
Branch
AZ
AZ-A
AZ-C
During the first 8 weeksof fruit development,fruit
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 AZA, microscopy studieshave shown that abscission-related..
.
eventsareactually occurring in isolatedpocketsof tissuein Flgu~e~. Citrus leaf and fruit
AZ A H
frui ~_:I
b .
thi ab . .
abscIssion
zones.
-.
owever, t J.Wl to a SClse
at s scIssionzone
because of extensive lignification and secondary wall
formation of abscissionzone cells that surroundthe vasculartissueand inner cortex (Goren and
Hubennan1976;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 presenceof 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 hormonesareinvolved in control of abscission
(SextonandRoberts1982). Ethyleneacceleratesabscission,and this propertyhasbeenutilized to
develop abscissionmaterialsfor the purposeof looseningfruit of various agricultural crops for
harvest. Normally auxin delaysabscissiontbut the effect of auxin on abscissionis relatedto the
endogenoushormonalbalancethat changesasabscissionproceeds.
Marked physiological and biochemical changesoccur during abscissionof mature citrus fruit.
When ethylene is used to accelerate abscission, increased amounts of mRNA and protein are
measuredin 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 increaseduring
131
the abscission process in AZ-C (Table I). Theseenzymes are thought to function in the degradation
of adhesivecell wall componentsthat 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.
Ethyleneandauxin interactduring abscissionof maturecitrus fruit. Whenethyleneis first
applied,thereis a periodof time in the AZ-C when changesin cellulaseand PG activities and fruit
break-strengthcannotbedetected.As the durationof ethyleneexposureincreases,enzymeactivities
increase(Kazokas 1997) and a reduction of fruit break-strengthoccurs. Auxin is found in all
portionsof the citrus plant, including AZ-C (Gorenand Goldschmidt1970). Its presenceprevents
the inductionof abscissionby ethylene. Continuedexposureto ethyleneresultsin 1) inhibition of
auxin transport from the leavesand stem to the AZ-C, and 2) destructionof auxin in the AZ-C.
Abscissioncanthenproceed.Onceabscissionhascommenced,however,auxin applicationdoesnot
inhibit abscissionbut in fact promotesit by acceleratingendogenousethyleneproduction (Sexton
andRoberts1982). Gibberellinsand cytokinins appearto haveno effect on the abscissionprocess
of maturecitrus fruit Abscissicacid canenhanceabscission,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,
andits objectionableside-effectssuchas gumming
of the trunk when appliedin warm weather(Cooperet al1968).
It is well establishedthat the production of ethyleneis a physiological consequenceof
woundingplant tissues(Sextonand Roberts1982). The majority of abscissionmaterialstestedin
Florida have beenthosethat produceethyleneby tissueinjury (Wilson et al1977). ACTI-AID(i),
Pick-O~ and Release(i)are examplesof abscissionmaterials that produce ethylene by this
mechanism. When applied as a spray to fruit, peel blemishesoften appeared,reducing their
marketabilityasfreshfruit. Propertiming of applicationand uptakewere also problemswith these
materials.For example,the active ingredientof ACTI-AID(i), cycloheximide,is a potent inhibitor
of protein synthesis.Whenappliedat timesof limited uptake,suchasin conditionsof cold weather,
the majority of activeingredientwould reachthe abscissionzoneafter abscissionhad commenced.
This actuallyresultedin inhibition of abscissionby reducingthe synthesisof cellulaseandPO in the
abscissionzone. Non-specificity and cost were also problemswith theseabscissionmaterials.
'Transfer' is a new abscissionmaterialthat haspotential for useon citrus destinedfor the
processedmarket. In a recentreportto the InternationalSocietyof Citriculture (Wilcox and Taylor
1996), 'Transfer' applied at low concentrationswas reportedto reducefruit break-strengthover a
threeweekpost-applicationperiod. The modeof action,specificityand 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
ratherbeginswherethetissuesof the buttonandflavedojoin anddipsthrougha small portion of the
albedo (Figure 2). Vasculartissuesthat carry water and
sugarspassthroughthe AZ-C. Theventralvasculartraces
mainpeduncle.
of the centralfruit coreandthe dorsalandlateralvascular
vascular
trace
peduncle
-
tracesthat encirclethe fruit mustbe severedat abscission.
These vascular traces significantly contribute to the
tensilestrengthandrigidity of AZ-C (Cooperet al 1968).
calyx
i::::~{
flaved
dorsal and lateral
traces
5'"
Plugging of mature citrus fruit results from the albedo
{
incompletemechanical
ruptureof AZ-C duringharvest.
AZ-C
~:~::;-:;;;;Iar trace
An abrupt transition of cell sizes, shapes,and types
through the AZ-C createsa localized, structurally weak
region (Webster 1968). When fruit are harvested,the
.
.
.
centralfruit core~
mechanical rupture of the tissue or 'plug' WIll occur
through the structurally weakestareaof this region. If
I
fruit are harvestedbeforea well-formed abscissionzone.
...
d 1
.f suscep tI.bl e thin -pee 1 e d fru I' t suc h as Figure 2. Longitudinal
diagram
of
the
stem.
eve
ops,
or
I
.t
.
.
end 0f an orangef rul.
tangerInesare not carefully harvested,a portIon of the
flavedo is tom away from the fruit. This can exposethe albedo and segmentsof the fruit to
,
1~3
desiccation and decay. Plugged fruit will be eliminated from the fresh fruit stream in the
packinghouse.In somecases,pluggedfruit can accountfor approximately11% of total harvested
fruit (Gaffney et at 1976).
Harvested fruit are commonly degreenedfor marketing PllrlX>sesduring the early part of the
season. In the degreening process fruit are treated with ethylene to cause the destruction of
chlorophyll.
Bannore and Brown (1985) have demonstrated that degreening enhances the
susceptibility of fruit to stem-end rot caused by Diplodia natalensis. Although the growth and
penetration of Diplodia natalensis through the fruit's stem-end is stimulated by ethylene treatments,
recent work suggests that the enzymatic digestion of AZ-C induced by ethylene degreening
treatments can ease the physical barrier to fungal penetration and therefore contribute to the
development of the disease (Brown and Burns, 1995).
Abscission materialssuch as 'Transfer' may have potential for harvestingof fresh fruit.
Tangerineswould greatly benefit from a reductionin plugging due to a reduction in fruit breakstrength. Freshfruit harvestingratescould potentially increasewithout an increasein mechanical
damage.'Transfer' mustbe thoroughlytestedfor its effectson postharvestquality, however,before
its potential for harvestingof fresh fruit canbe evaluated.
134
IV. Literature cited
B8rDlore,C. R. andBrown, G. E. 1985. Influenceof ethyleneon increasedsusceptibilityof
orangesto Dip/odia nata/ensis.Plant Dis. 69:228-230.
2
Brown, G. E. and Burns, J. K. 1995(Abstr.). Factorsaffecting susceptibilityof ethylene.
degreenedorangesto Diplodia stem-endrot. Phytopathology85:1123.
3
Bums, J. K., Nairn, C. J. and Lewandowski, D. J. 1995. Cell wall hydrolase activity and
cellulase gene expressionduring abscissionof Valencia citrus fruit and leaves. Proc. Fla. State
Hort. Soc. 108:254-258.
4
Cooper,W. C., Rasmussen,
G. K., Rogers,B. J., Reece,P. C. andHenry,W. H. 1968. Control
of abscissionin agricultural cropsand its physiologicalbasis. Plant Physiol. 34:1560-1576.
s.
Gaffney,J. J., Miller, W. M. andCoppoc~G. E. 1976. Citrusfruit injury as relatedto
mechanicalharvestingwith limb shaker-catch
framesystems.Proc.Fla. StateHort. Soc.
89:179-182.
6,
Goren, R. 1993. Anatomical, physiological,and hormonalaspectsof abscissionin citrus.
From: Horticultural Reviews. Vol. IS. pp.14S-182.JohnWiley and SonsInc., N. Y.
7.
Goren,R. andGoldschmidt,E. E. 1970. Regulativesystemsin the developingcitrus fruit. I
The hormonalbalancein orangefruit tissues. Physiol.Plant.23:937-947.
8. Goren,R. andHubennan,M. 1976. Effectsof ethyleneand2,4-0 on the activity of cellulase
isoenzymesin abscissionzonesof the developingorangefruit. Physiol. Plant. 37:123-130.
9
Greenberg,J., Goren,R. and Riov, J. 1975. The role of cellulaseand polygalacturonasein
abscissionof young and matureShamoutiorangefruits. Physiol. Plant. 34:1-7.
10. Hube~
M., Goren, R. and Zamski, E. 1983. Anatomical aspectsofhonnonal regulation
of abscission in citrus - the shoot-peduncleabscissionzone in the non-abscising stage. Physiol.
Plant. 59:445-454.
Iwahori, S. and Van Steveninck,R. F. M 1976. Ultrastructuralobservationof lemon fruit
abscission. ScientiaHort. 4:235-246.
12. Kazokas, W. 1997. Activities of cell wall hydro lases and cellulase gene expression in
Valencia orange and Tahiti lime calyx abscissionzonesduring intennittent ethylene treatments.
M. S. Thesis, University of Florida.
13. Sexton, R. and Roberts,J. A
33:133-162.
14
1982 Cell biology of abscission. Ann. Rev. Plant Physiol
Webster,B. D 1968. Anatomicalaspectsof abscission.Plant Physiol.43:1512-1544
135
15. Whitney, J. D. 1995. A review of citrus harvestingin Florida. Citrus Eng. Conf.41:33-60
16. Wilcox, M. and J. B. Taylor. 1996. Transfer: a new abscission agent. VIIJ1bCongress of the
International Society of Citriculture (in press).
7. Wilson, W. C. 1966. The anatomyand physiology of citrus fruit abscissioninduced by
iodoaceticacid. Ph. D. Thesis,University of Florida.
18. Wilson, W. C. and Hemdershott,
C. H. 1968. Anatomicaland histochemicalstudiesof
abscission
of oranges.Proc.Amer.Soc.Hort.Sci.92:202-210.
19. Wilson, W. C., Kenney, D. S. and Holm, R. E. 1977. The Florida Departmentof Citrus
cooperativechemicalscreeningprogramsfor citrus. Proc.Int. Soc.Citriculture 2:692-696.
136