Effect of Preveraison Water Deficits on the Yield Components of 15 Winegrape Cultivars

Accurate information regarding crop reproductive development and yield formation in response to water deficits is needed for informed vineyard irrigation management decisions, particularly when water supply is limiting. Fifteen red winegrape cultivars grown in the San Joaquin Valley of California were subjected to two regulated deficit irrigation (RDI) treatments for four years to determine yield component responses to water deficits (no applied water) preveraison (ED) and postveraison (LD). In the fifth year, the cultivars were kept well-watered to determine carryover effects. In the first four years, early water deficits (ED) consistently and significantly reduced yields compared to the control (sustained deficit, SD; applied water at 50% of estimated crop evapotranspiration (ETc) throughout the growing season) across all years and cultivars, but the late (postveraison) deficit (LD) treatment vines were not different from the control. The reduction in yield with ED was primarily due to a significant reduction in berry fresh weight (FW) and clusters per vine, with little change in berries per cluster. Neither flowers per cluster nor percent berry set were affected by the treatments, although flowers per cluster varied significantly among cultivars. Berries per cluster increased linearly with flowers per cluster until saturation at similar to 150 berries per cluster as percent berry set declined at similar to 250 flowers per cluster. In the fifth year, yields of the two RDI treatments recovered somewhat because of increases in berry FW and a small, but significant, increase in clusters per vine. These results show that berry size, because of a reduction in FW, is the most sensitive yield component to water deficits, followed by clusters per vine and berries per cluster, with sensitivity maximum preveraison and few differences among cultivars.