PREHARVEST APPLICATIONS OF GIBBERELLIC-ACID DELAY SENESCENCE OF FLORIDA GRAPEFRUIT

Peel senescence of 'Marsh' grapefruit (Citrus paradisi Macf.) was redu ced by gibberellic acid (GA) treatment at three production sites in Fl orida as indicated by resistance to puncture and the retention of gree n colour. Applications of GA at 49 g ha(-1) plus 0.05% Silwet L-77 wer e equivalent to 25 g ha(-1) plus 0.1% Silwet L-77. GA at 49 g ha(-1) p lus 0.05% Silwet L-77 was more effective than the maximum label amount of GA (145 g ha(-1)) without surfactant. GA applications just prior t o colourbreak in August and September were found to be most effective. GA treatments increased peel oil compared with controls, but did not affect fruit quality. Chlorophyll and carotene concentrations in Octob er and November were higher in GA-treated fruit than fruit harvested i n the remainder of the season. In contrast, there was only a small eff ect of GA on time to degreen fruit. Leaf drop was accelerated by GA tr eatment, but was not different from controls at the end of season in t wo of the three sites tested. The data suggest that GA efficacy is not dependent on application method, but on spray volume and GA dose. Gra pefruit susceptibility to fruit fly attack is correlated with peel col our, firmness and oil content, and GA enhances these attributes.