By Tripti Vashisth
In a multi-year field trial, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers have seen beneficial effects of gibberellic acid (GA) on Valencia sweet oranges. Monthly application of GA (September to January) on Valencia improved yield on average by 30%, reduced fruit drop and elicited enhanced plant defense response.
These benefits are possibly due to the unique advantages of this application-use pattern. September to January application coincides with the floral induction period as well as influences the growing fruit. However, efficacy of GA in early-maturing sweet orange varieties is still to be evaluated due to a shorter fruit-growth period (as compared to Valencia) and the need to stop GA application by October or November to allow sufficient time for color/Brix development.
In a new field trial on Hamlin oranges, half of a 20-acre block received two GA applications (Oct. 7 and Nov. 5). The other half was left untreated. On Nov. 15, a week after the second spray, the GA-treated Hamlin trees showed higher fruit detachment force (FDF) and less abscission (the physiological process of fruit separation) than the control. Upon pulling the fruit, most of the oranges in GA-treated trees were breaking at the calyx-stem junction rather than smoothly coming off as observed in the control, suggesting readiness of untreated fruit to drop versus GA-treated fruit hanging tightly to branches.
Ethylene, a plant hormone, is the natural cause of fruit abscission in citrus. HLB and a number of other biotic and abiotic factors can contribute to increase in ethylene production, thus resulting in increased fruit drop. GA is known to counteract ethylene.
To display efficacy of GA in reducing stress and fruit drop in HLB-affected Hamlin, a field tour took place on Dec. 14. For demonstration, a small number of GA-treated and untreated trees in the above-mentioned trial were sprayed with ethylene to simulate a stress situation. As expected, the untreated trees dropped 30% more fruit as compared to trees that received two GA applications.
The effect of GA on Hamlin is quite encouraging. GA seems to reduce stress and fruit drop with fall application. Moreover, the Brix in untreated and GA-treated trees is similar with a marginal difference in fruit color.
However, this trial and other trials in Florida are still in their preliminary stages and have not been harvested yet. Researchers need at least two years of data to develop recommendations for Hamlin. Moreover, a postharvest GA spray to affect the floral induction period needs careful scrutiny.
Tripti Vashisth is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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