lilies [154,177], geraniums [155], Pelargonium[153], cyclamen [24], Impatiens, Bougainvillea, del-
phiniums, and foxgloves [25]. There are many other delightful flowers that are not marketed commer-
cially because of these difficulties. The problems usually occur during transit and retailing, when low
light intensities, water stress, high temperatures, and ethylene buildup contribute to the problem. The con-
centration of ethylene in mixed cool stores may reach high levels, particularly if ethylene-generating fruit
are enclosed with the flowers. Motor exhausts (0.25% ethylene) have been shown to raise ethylene levels
to inductive concentrations in auction halls [152] and the trucks used for transport [179,180].
Silver thiosulfate (0.5 to mM) has been extremely effective at reducing this loss, either when sprayed
directly on the plants or pulsedthrough the transpiration stream of cut flowers [152,174]. There are ru-
mors that its use may be banned because of pollution problems. Auxin analogues have been used with
flowers [167,178], but petals seem rather unresponsive.
C. Fruit Thinning and Harvesting
Some plants produce vastly more flowers than will mature into fruit. This is particularly true of many fruit
trees, such as orange [31], apple, apricot, mango [33], avocado [34,181], and cherry, where a natural thin-
ning process occurs. Only 0.2% of Washington Navel orange flowers develop into fruit [31]. Floral and
young fruit abscission is also a particular problem in leguminous crops such as soybean [37,182], field
bean (Vicia) [166], lupins (38), French bean (Phaseolus) [183], and cowpeas [184]. In other crops, such
as pistachio, alternate bearing is a difficulty, where a heavy crop of fruit appears to cause excessive ab-
scission of the subsequent year’s buds [108]. Flowers and fruit are lost in a succession of abscission
episodes [31]. These are classified as follows:
- Bud dropthat occurs before the flowers reach anthesis
- Flower or young fruit dropimmediately after anthesis
- Enlarging fruit or June drop
- Mature fruit or preharvest drop
The loss of flower buds can be considerable. In oranges, it can be up to 33% and is attributed to nu-
tritional causes such as zinc deficiency [31]. In apricots, a failure to fulfill chilling requirements in areas
such as South Africa, Turkey, and Israel results in almost complete loss of flowers. This can be overcome
by spraying the trees with GA to break dormancy or by growing varieties with a short chilling require-
ment [31].
Loss of opened flowers can usually be attributed to a failure of flower development or failure of fer-
tilization. In Shamouti orange, 77% of the flowers that were shed had abnormalities, often with aborted
pistils [31]. Benzyladenine has been used to prevent flower and young fruit drop [182,185].
The June drop of enlarging fruit can be very significant in citrus crops, being as high as 21% in
lemons, 45% in Shamouti oranges, and 60% in clementines [31]. The main causes [31] of this drop are:
- Abnormalitiesin or lack of fertilization and zygote abortion or degeneration.
- Competitionfor photosynthates and mineral nutrients between fruit and vegetative apices; fruit
with fewer seeds on weaker branches tend to be shed first. - Water stresscan be a major cause of abscission [102] in arid areas because fruit desiccate first,
having a higher (less negative) water potential than leaves. Hail and wind damage are also cli-
matic factors. - Invasion of fruit by fungal pathogensor by herbivorous larvae can cause abscission. Runoffof
blackcurrants, which results in the premature abscission of apparently healthy fruitlets, can re-
duce yield by 50% [90]. Evidence suggests that symptomless infection with Botrytiscauses el-
evated ethylene production, which, in turn, induces abscission [75].
Aminoethoxyvinyl glycine (AVG), the ethylene synthesis inhibitor, has been used to prevent fruit
thinning in apples [170,171]. When natural thinning is not vigorous enough and too many fruit are set,
sprays are used to reduce the crop and get fewer bigger fruit. Apples have been thinned with ethephon
[167,186], carbaryl [167], oxamyl [169], and NAA [173]. Ethephon has also been used with pecans [187],
peaches [188], prunes [189], and pears [190].
222 SEXTON