Effects of plant growth regulators and floral cluster thinning on fruit quality of ‘shine muscat’ grape

‘Shine Muscat’, recently introduced into Korea, is a seedless grape (Vitis vinifera) with an edible skin and high sugar content. Despite the rapid increase in its cultivation area, there is no research on how to apply plant growth regulators (PGR) for seedlessness and floral cluster thinning of ‘Shine Muscat,’ which is the basis of this study. Five different PGR treatment methods were applied. Streptomycin (SM) at 200 mg·L^-1 was applied 7 days before blooming in most of the treatment groups, and an untreated control was included. Primary treatments were applied 1-3 days after full bloom (DAFB) and involved the application of 12.5 or 25.0 mg·L^-1 gibberellic acid (GA₃) and 2.5 or 5.0 mg·L^-1 thidiazuron (TDZ) or 5 mg·L^-1 forchlorfenuron (CPPU), and one treatment involved a second application of 200 mg·L^-1 SM. Secondary treatments were applied 12-15 DAFB and involved the application of 12.5 or 25.0 mg·L^-1 GA₃ or no PGR treatment. Three different flower cluster treatments were applied. Flower clusters were thinned by hand to 3, 4, and 5 cm. Fruit quality was investigated through the measurement of the seedlessness rate, cluster weight, berry weight, berry length, berry diameter, soluble solids contents, acidity, and aroma. The results showed that the use of SM is advantageous for seedlessness when applied 7 days before blooming, and fruit quality differed between the CPPU and TDZ treatments. Floral cluster thinning to 3 and 4 cm resulted in higher quality grapes compared with thinning to 5 cm. We hope these findings will contribute to establishing standards for producing seedless ‘Shine Muscat’ grapes with high fruit quality.