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

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 A4 + 7 (GA4 + 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 GA4 + 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.