During maturation of the crop, abscission of the ripe fruit occurs. This preharvest drop is undesirable
as far as hand picking is concerned. It is common practice to prevent it by using NAA sprays in pears and
apples and with 2,4D [167,172] and NAA in grapefruit and oranges [31]. Trials with NAA have also been
conducted to try to reduce the loss of grapes from bunches before and after harvest [191]. Spraying with
AVG 1 month before harvest delayed the preharvest drop of apples [89].
Harvesting can be carried out using machines that beat, shake, or blow fruit from plants. The greater
the force needed to separate the fruit, the greater the damage to the fruit and the plant. Ethylene-releasing
sprays have been used in trials to accelerate and synchronize abscission prior to harvest of grapes [192],
oranges [165], apples [172], olives [193], raspberries [194], and blackberries. Ethephon promotes both
abscission and reddening in peppers and its feasibility is being assessed as an aid to once-over harvesting
[195]. Although these methods have been successful as an aid to mechanical harvesting [196], they can
produce unfortunate side effects, such as the shedding of leaves [196], shoot dieback, gummosis
[165,196], and excessive fruit drop prior to harvest [192]. The problem of leaf loss has been overcome
successfully by the use of calcium acetate sprays [197,198]. Ethephon can give inconsistent results [164]
because ethylene production is very dependent on ambient temperature [196] and the pH of the cell sap
[163,168]. 2-Chloroethylmethylbisphenylmethoxysilane (CGA) [196] and Etacelasil (164) [2-
chlorotris(2-methoxyethoxy)silane] may prove more reliable ethylene-releasing agents.
It is important to realize that using ethylene-generating sprays will accelerate abscission only if nat-
ural ethylene production is subsaturating. For this reason, adding more ethylene to ripening raspberries is
counterproductive as it has no effect on abscission of ripe fruit but instead causes immature green fruit to
redden and abscise [85].
D. Leaf Loss
Leaf fall is induced in several crops where the foliage interferes with the mechanical harvesting of fruit.
In cotton both desiccants and ethylene-releasing sprays are employed prior to boll harvest [166,167]. De-
foliation of nursery trees is also practiced prior to shipping [152,164,174].
Leaf fall is a problem in some display plants, such as Ficus benjamina[199].Radermachera[200],
Philodendron[201], and potted roses. It is induced by low light intensities, water stress [199], and ethy-
lene pollution during retailing. These difficulties are usually solved using silver thiosulfate [200], but the
loss of holly leaves is treated using the auxin analogue NAA [202].
E. Other Uses
The ability to control abscission has been put to a number of rather unusual uses. During the storage of
lemons the short stem (button) left attached to the fruit abscises and allows entry of the fungus Alternaria
via the scar. This problem is overcome by adding the isopropyl ester of 2,4D [203]. The reverse problem
is encountered with bananas, where the failure of perianth abscission makes some varieties of fruit less
attractive [204]. Ethephon sprays have been used to reduce mistletoe infestations of Black Spruce by
causing the abscission of 90 to 100% of the mistletoe shoots [205]. Twig abscission has been induced in
white oak in an attempt to improve timber quality [206].
V. SUMMARY
Abscission occurs at genetically determined abscission zones, where the induction of wall-degrading en-
zymes such as cellulase and polygalacturonase weakens a restricted band of cells called the separation
layer. It seems likely that the separation layer cells are specifically preprogrammed to respond to the in-
ductive stimuli. The generation of mechanical forces to facilitate separation of the loosened separation
zone cells and rupture the xylem is an important component of the process. After abscission, the wound
is protected from microbial attack by the formation of chitinase and -1,3-glucanase, division and suber-
ization of the surface cells, and blockage of the vascular traces.
The control of abscission seems to depend on the relative concentrations of IAA and ethylene. Auxin
is an inhibitor of abscission, and ethylene accelerates and synchronizes the process. It is not clear if one
or another of these regulators or the relative concentrations of both are responsible for the induction of
abscission. Other hormones, such as ABA, probably have indirect effects via ethylene and IAA.
ABSCISSION 223