HORMONAL REGULATION OF PLUM CROPPING

In years favouring severe abscission of fruitlets of plum cv. Victoria (Prunus domestica L.) applications of high-volume sprays of gibberellic acid (GA₃) and 2, 4, 5-trichlorophenoxy propionic acid (2,4,5-TP) can lead to dramatic increases in yield.

The optimum timing for spraying is prior to the onset of the pit-hardening stage of fruit development. Over several years the best responses in prevention of abscission and ultimate fruit yield have been obtained with sequential sprays at 4 and 6 weeks after 50% petal-fall (PF). In certain seasons application of GA₃ alone has increased yield, whereas individual sprays of 2,4,5-TP have usually proved ineffective.

In years of average blossom density only 6-8% of the blossoms need to set and fruits to be retained for a full crop. Often, more than 50% set occurs and, if natural fruitlet abscission is completely prevented by hormone spray application, overcropping can result which in turn significantly reduces fruit size and affects return bloom in the subsequent season. Applications of GA₃ per se also reduce flower bud number in the subsequent season.

Although cumulative yields over the period 1978–80 were highest on GA/TP-sprayed trees, treatments were only significant in 2 of the 3 seasons. All the spray treatments in 1978 reduced flower bud numbers in the following year and this reduction was maintained but not increased in subsequent years. Despite this effect, yields were still improved where spray applications were continued.

Seed and fruit flesh of Victoria plum were shown to contain free and conjugated forms of both indolyl-3-acetic acid and abscisic acid. Changes in their total contents and of components with stage of fruit development indicate that free IAA can be derived from the aspartate ester conjugate. Single and combined applications of GA₃ and 2,4,5-TP at 3 stages of fruitlet development modulated endogenous hormone levels, which suggests their involvement in control of abscission.