Effects of Plant Growth Regulators on Fruit Set and Fruit Shape of Parthenocarpic Apple Fruits

Abstract

We applied auxin, cytokinin, gibberellins, and gibberellin synthetic inhibitor to flowers of ‘Ohrin’, which is parthenocarpic, and ‘Fuji’, which is non-parthenocarpic, to elucidate the relationship between plant growth regulators and parthenocarpy in apple. The percentage of fruit set in gibberellin A₃ (GA₃) was about 60% in ‘Ohrin’ and about 7% in ‘Fuji’. Forchlorfenuron (CPPU) combined with GA₃ treatment increased the fruit retention rate more than 14 days after treatment (DAT) than GA₃ alone. Dichlorprop (2,4-DP) combined with GA₃ slightly enhanced the gibberellin effect of inducing parthenocarpy. In ‘Ohrin’, GA₃ and gibberellin A_{4 + 7} (GA_{4 + 7}) showed a higher fruit retention rate before flowering than after. Parthenocarpic fruits were induced even when uniconazole (UCZ) was sprayed after flowering, but not before flowering. The calyx end was large in GA_{4 + 7}, and in GA₃ and CPPU singly, and GA₃ and CPPU combined treatments, which increased the percentage of fruit set. These results suggest that gibberellins before flowering trigger parthenocarpic apple fruits. Subsequent fruit growth requires cytokinins and auxin.