Impact of Prohexadione Calcium and Mepiquat Chloride on Vegetative Growth and Fruit Quality in ‘Shine Muscat’ Grapevines

The vigorous growth of new shoots can significantly reduce grape yield and compromise fruit quality. In order to explore the effects of prohexadione calcium (Pro-Ca) and mepiquat chloride (MC) on the control effect of new shoot growth and fruit quality of grape, ‘Shine Muscat’ grapevine (Vitis labruscana × V. vinifera) was used as the test material, and different concentrations of Pro-Ca and a combination of Pro-Ca and MC were sprayed four times before flowering of ‘Shine Muscat’ grapevines, and the effects of the different treatments on the new shoot growth and fruit quality of ‘Shine Muscat’ grape were analyzed and evaluated. The results demonstrated that low concentrations of Pro-Ca had limited efficacy in controlling shoot growth. However, the combined treatment of Pro-Ca 300 mg/L + MC 300 mg/L not only effectively inhibited shoot elongation but also significantly enhanced the chlorophyll content of the leaves opposite to the clusters and increased branch density. Additionally, this treatment improved berry size (single berry weight, vertical and horizontal diameter) and elevated the soluble solids content (SSC). These findings suggest that the combined application of Pro-Ca (300 mg/L) and MC (300 mg/L) is the most effective strategy for balancing vegetative growth and enhancing fruit quality in ‘Shine Muscat’ grapevines.