Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent. I: GA 3 increases the yield of commercially valuable ‘Nules’ Clementine Mandarin fruit only in the off-crop year of an alternate bearing orchard

Abstract

Worldwide, gibberellic acid (GA 3) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA 3 concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2 mm) of ‘Nules’ Clementine mandarin ( Citrus reticulata Blanco) was determined in a commercial orchard. GA 3 applied during the period of intense flower abscission failed to reduce the total number of abscised flowers in both the light off- and heavy on-bloom years. No GA 3 treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA 3 in the high yield on-crop year had fewer abscised fruit than untreated control trees ( P = 0.0188) and GA 3 applied 10 days after 75% petal fall and in July increased the number of fruit retained on tagged branches >20% compared to control trees ( P = 0.0005). Maximum air temperature was not related to flower or fruit abscission. In the off-crop year (548 fruit per untreated control tree), it was necessary to apply 15 or 25 mg L −1 GA 3 at 60% bloom, 90% bloom, 75% petal fall and 10 days after 75% petal fall to significantly increase the number of fruit per tree and yield of commercially valuable fruit (kilograms and number per tree) ( P < 0.0001) above that of control trees, with no reduction in total kilograms per tree. In the following on-crop year, it was better not to apply GA 3: no treatment increased total yield or fruit size and five of seven GA 3 treatments tested reduced total yield as kilograms and number of fruit per tree ( P = 0.0003). The results provide strong evidence that GA 3 efficacy is crop load-dependent and dictate that crop load should be considered when using GA 3 to increase fruit set or fruit size of mandarins.