suffer from bud drop and flower shattering during transit and marketing [152]. This can be reduced by
avoiding the inductive conditions that lead to abscission: high temperatures [153], low light intensities
[154], ethylene pollution [152], fertilization of flowers [24,25], and mechanical perturbation [155]. In
many cases the physiological basis of agriculturally important abscission is not yet fully understood, the
extent of young fruit thinning being an important example.
- Genetics of abscission behavior. Geneticists have been able to breed varieties with different ab-
scission characteristics. For instance, among the many raspberry cultivars, there is wide variation in the
extent to which fruit abscission has progressed at the time the fruit are ready for harvest [156]. Cultivars
that do not drop ripe berries when the bushes are shaken are suitable for hand picking, while more easily
detached varieties are selected for mechanical harvesting. - Recombinant DNA technologies and the manipulation of abscission. Being able to manipulate
abscission using transgenic plant technologies is a very real prospect. Oeller et al. [92] have already pro-
duced nonripening transgenic tomato plants in which the synthesis of ethylene is blocked. This was
achieved by expressing antisense RNA for ACC synthase. One would predict that this strategy should
produce slow or nonabscising plants that would abscise to order if treated with ethylene-generating
sprays. Similarly, ethylene production has been reduced in plants producing antisense ethylene-forming
enzyme RNA [157]. An alternative approach is to overexpress in plants the ACC deaminase gene from
bacteria, destroying the ethylene precursor ACC as it is formed [158,159].
It might prove possible to manipulate abscission by producing more IAA in the distal tissues of
transgenic plants. The Ti plasmid IAA synthesis genes have already been cloned and there are leaf- and
fruit-specific promoters to drive them. An alternative strategy for preventing abscission could be to pro-
duce transgenic plants expressing antisense RNA for the wall-degrading enzymes cellulase and poly-
galacturonase. This approach has been used to reduce softening in fruit [160,161]. - Accelerating abscission by increasing ethylene production. Ethylene-releasing sprays such as
Ethephon [(2-chloroethyl)phosphonic acid] [162,163] and Etacelasil [164], which release ethylene spon-
taneously, are very effective at causing abscission. Their use is not always straightforward, as they can
have unfortunate side effects. For instance, ethephon used to promote fruit abscission often induces un-
desirable leaf fall. Attempts have been made to increase ethylene production using the plant’s natural sub-
strate ACC [164], although the method is not widely used.
Ethylene production can be increased by wounding the plant. The protein synthesis inhibitor cyclo-
heximide has been employed to damage and induce consequent abscission of oranges prior to mechani-
cal harvesting. Initial trials with ethylene-releasing sprays were unsuccessful because they damaged the
trees by defoliating them, so a method of localizing ethylene production in the fruit was sought. It was
found that cycloheximide damaged the peel of the fruit, causing wound ethylene formation, which in turn
induced abscission [165]. The abscission of cotton leaves prior to boll harvest is also achieved by dam-
aging the leaves and inducing ethylene formation [166,167]. The mechanism of action of some thinning
agents, such as insecticides carbaryl and oxamyl, which are used to thin apples [168,169], is not fully un-
derstood. - Inhibiting abscission by reducing ethylene production and sensitivity. Abscission can be inhib-
ited by reducing natural ethylene production or interfering with ethylene action. A reduction of natural fruit
thinning has been reported using AVG and AOA to inhibit ethylene formation [89,90,170,171]. Silver thio-
sulfate [152] has been widely employed to prevent abscission of ornamental flowers to such an extent that
it is a pollution hazard in some horticultural areas. The ethylene antagonist DACP [77] also seems ex-
tremely effective (Figure 8), but unfortunately, it is an explosive gas, which may limit its field use! - Auxin and control of abscission in the field. Auxin sprays have been employed to prevent ab-
scission. Indoleacetic acid is not used because it is rapidly degraded in the plant and synthetic auxin ana-
logues such as 1-naphthaleneacetic acid (NAA), picloram, and 2,4D are preferred. Preharvest drop of ap-
ples has been treated in this way [172]. Rather perversely, NAA sprays are used to thin young apple fruit
[169]. In this case, the effect seems to be caused indirectly by interfering with sugar translocation from
sprayed leaves to the developing fruit [168,173].
B. Flower Shattering
The abscission or shattering of whole buds or floral parts is a major problem in a number of ornamental
plants [152,174]. These include Zygocactus[175],Fuschia, Calceolaria, snap-dragon, sweet peas [176],
ABSCISSION 221