Effects of GA₃ and streptomycin on seedless fruit formation in blackberry

This study was carried out to determine the effects of various application times and concentrations of GA₃ with streptomycin (SM) on fruiting, the rate of seedless drupelet formation, and fruit quality in ‘Super’ blackberry. Flower clusters were sprayed with GA₃ at 12.5, 25.0, 50.0, and 100 mg·L^-1 combined with 200 mg·L^-1 SM. Single treatments were conducted using a hand sprayer at 30% and 50% of full bloom (FB), and duplicate treatments were conducted by spraying twice at 30% and 50% FB, respectively. The application of GA₃ + SM on flower clusters had no effect on fruit set, harvest time, or various fruit qualities (fruit firmness and SSC). The rate of seedless drupelet formation for control fruit was approximately 2.5%. Under single treatment, the rate of seedless drupelet formation was higher at a spray time of 50% FB compared with 30% FB treatment, and the rate was highest at a GA concentration of 25.0 mg·L^-1. The highest seedless drupelet rate was 56.7% for fruits sprayed twice at 30% and 50% FB with GA + SM. Single treatment at 30% FB with 100 mg·L^-1 GA₃ + SM induced 22.2-35.2% seedless drupelet formation, whereas single treatment at 50% FB and duplicate treatments at 30 and 50% FB led to the formation of seedless drupelets at a rate of 31.6-42.2% and 22.6-56.7%, respectively. Among GA concentrations, 25.0 mg·L^-1 GA₃ treatment led to the highest rate of seedless drupelet formation (35.3-56.7%). The occurrence of malformed berries was higher in the GA + SM treatment group than in the control, and the incidence increased with increasing GA concentration. GA₃ + SM treatment slightly increased fruit length but had no effect on fruit diameter.