Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent. I: GA₃ increases the yield of commercially valuable ‘Nules’ Clementine Mandarin fruit only in the off-crop year of an alternate bearing orchard
2011
Chao, Chih-Cheng T. | Khuong, Toan | Cheng, Yuk-shing | Lovatt, Carol J.
Worldwide, gibberellic acid (GA₃) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA₃ concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2mm) of ‘Nules’ Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard. GA₃ 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 GA₃ treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA₃ in the high yield on-crop year had fewer abscised fruit than untreated control trees (P=0.0188) and GA₃ 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 25mgL⁻¹ GA₃ 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 GA₃: no treatment increased total yield or fruit size and five of seven GA₃ treatments tested reduced total yield as kilograms and number of fruit per tree (P=0.0003). The results provide strong evidence that GA₃ efficacy is crop load-dependent and dictate that crop load should be considered when using GA₃ to increase fruit set or fruit size of mandarins.
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