Optimizing Budburst and Yield in Superior Seedless Grapevines Using Supplementary Dormancy-Breakers

In this study, superior Seedless grapevines were pruned during the winter to retain 10 fruit canes (FCs), each with 10 buds. On 8th of January, all vines were treated with a 5% hydrogen cyanamide (HC) spray, serving as the control treatment. For the four additional treatments, the vines received the same HC spray, supplemented by an additional dormancy-breaking agent (SDBA) applied on 16th of January to the basal five buds of each FC to promote budburst. The SDBA treatments comprised either HC (2.5%), potassium nitrate (KNO₃; 2%), ammonium nitrate (NH₄NO₃; 2%), or zinc sulfate (ZnSO₄; 2%). By mid-March, shoot and cluster thinning was conducted to retain the six most vigorous shoots per FC and the largest 30 clusters per vine. The results indicated that all supplemental dormancy-breaking treatments enhanced budburst and bud fertility compared to the control (5% HC alone). These treatments also increased yield per vine and cluster weight. Furthermore, they significantly improved 100-berry weight, berry firmness, detachment force, juice TSS/acid ratio, leaf number per vine and per shoot, leaf-to-cluster ratio, as well as the length and thickness of new canes relative to the control.