Control Of Preharvestdrop Of Fruit

Decreasing preharvest drop is probably the most cost-effective use of PGRs in apple production. As with chemical thinning, its use is widespread in the pome fruit industry. As fruit mature, the force required to remove them from the tree decreases. To prevent this, one of two methods is employed. The first, an older approach, is the ap-

TABLE P3.2. Potential time of thinning for stone fruit crops, materials used, and their strengths and weaknesses

Timing

Materials Used

Type of PGR

Strengths

Weaknesses

Summer

Gibberellin

Plant hormone

Greatest influence

Difficult to predict timing, cultivar specific

Bloom

Ammonium thiosulfate

Fertilizer

Inexpensive

Erratic performance

Postbloom

Ethephon

Releases ethylene

Later application

Gummosis, leaf abscission

plication of a synthetic auxin such as naphthalene acetic acid (NAA). This application is generally made during the harvest period and leads to a temporary suppression of fruit drop. This synthetic auxin directly affects the abscission zone, suppressing the enzymes needed to promote abscission. At the same time, auxin application leads to the production of "auxin-induced ethylene." Treated fruit remain attached but color and soften faster than untreated fruit. Consequently, storability of NAA-treated fruit is reduced. The other approach is to suppress the synthesis or action of ethylene in the attached fruit. Suppression of ethylene biosynthesis and preharvest drop began with the use of daminozide. This chemical was used successfully for about 20 years, until its registration was cancelled in the late 1980s.

About the same period daminozide registration was cancelled, scientists in S. F. Yang's laboratory at the University of California elucidated the ethylene-biosynthesis pathway (Adams and Yang, 1979). From that work, two PGR approaches emerged, one for suppressing ethylene biosynthesis and the other for suppressing its action in maturing fruit. The most widely used ethylene-biosynthesis inhibitor is aminoethoxyvinylglycine (AVG). With AVG usage, maturation is delayed and preharvest drop suppressed. As fruit can continue growing for an additional one to two weeks prior to the onset of endogenous ethylene production, AVG application can enhance fruit size.

Suppression of ethylene action occurs through treatment with 1-methylcyclopropene (MCP), which blocks ethylene's attachment to its binding site, thereby preventing ethylene action and ripening.

MCP usage is currently limited to postharvest applications to enhance storability, although it seems likely that an MCP formulation will be developed that can be used in the orchard.

The use of synthetic auxin as a chemical thinner, and again later in the season to prevent preharvest drop, presents an interesting conundrum. When a synthetic auxin is used to chemically thin, it is applied when fruit set is heavy. Since cell division is occurring, carbohydrate requirements are high in each fruitlet. Auxin application leads to a dramatic increase in fruitlet ethylene and epinasty (leaf curling and shoot bending below the tip) on the day of treatment (Walsh, Swartz, and Edgerton, 1979). In the next few days, decreased carbohydrate levels in the fruit are measurable along with a decrease in growth rate. These are followed by seed abortion and fruitlet drop, which typically occur about two weeks after application. In this situation, PGR usage builds on the naturally occurring waves of fruit abscission. When applied to control preharvest drop, growth-promoting hormones in the fruit are relatively low and size is much greater. Consequently, the major effect of an auxin application is to replace the growth-promoting signal needed to maintain an intact abscission zone. Although auxin-induced ethylene is produced, its role at that time is merely an effect on fruit quality. Figure P3.1 shows the differences that occur when auxin is applied for chemical thinning versus preharvest drop of apple.

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