ABA Application during Flowering and Fruit Set Reduces Berry Number and Improves Cluster Uniformity

Abstract

The major drawback of the early ripening Vitis vinifera cv. Early Sweet is its cluster compactness and significant variability in berry size. Here, we tested the ability of abscisic acid (ABA) to reduce the number of berries per cluster and improve berry size uniformity. Data collected from a commercial vineyard over three successive growing seasons suggest that ABA application significantly reduces berry number per cluster in a concentration-dependent manner, improves uniformity of berry size within clusters, and has no significant effect on rachis length. The results also suggest that the thinning effect of ABA depends on the bloom status of the inflorescence, with limited or no effect when applied at preanthesis and with a significant effect when applied at full bloom and soon after anthers are detached. Accordingly, the effects of a semicommercial application of ABA on berry number and size was significantly affected by asynchronous blooming. Completion of bloom may serve as a clear phenological marker for optimal timing of ABA application for uniform and effective thinning.