Lime Sulfur–Boric Acid Synergy in Subtropical Viticulture: Temporal Regulation of Budbreak and Nutrient Remobilization

The rapid development of viticulture in subtropical regions represents a significant achievement in China’s table grape industry over the past two decades. However, insufficient winter chilling in these areas often leads to inadequate dormancy, which compromises nutrient translocation and storage in grapevine. Insufficient chilling accumulation results in asynchronous budbreak and reduced cane quality. In this study, ‘Shine Muscat’ grapevines were used to systematically evaluate how different defoliant agents affect budbreak characteristics from the perspective of nutrient translocation and storage. The results indicated applications of ethephon or urea alone, as well as their combinations with boric acid, yielded unstable effects, often causing primary bud necrosis, decreased flower formation rates, and phytotoxicity. In contrast, the combination of lime sulfur and boric acid exhibited a remarkable synergistic effect, significantly promoting dry matter and starch accumulation in the canes, while enhancing budbreak speed, uniformity, and flower cluster formation rate. Further experiments with varying concentrations of lime sulfur combined with 0.2% boric acid revealed 2% lime sulfur combined with 0.2% boric acid produced the most pronounced effects. This combination achieved the highest dormancy-breaking efficacy under conventional cultivation conditions and was used for the first time to produce a second crop in off-season cultivation. The dual effects of dormancy release and bud promotion offered by this approach provide a reliable solution for high-quality and efficient grape production in subtropical regions.