Interaction of applied water amounts and leaf removal in the fruiting zone on grapevine water relations and productivity of Merlot

A study was conducted in the San Joaquin Valley of California on Merlot to determine the interaction of applied water amounts [at 0.4, 0.8, and 1.2 of estimated vineyard evapotranspiration (ET c )] and leaf removal (at berry set or veraison) in the fruiting zone on productivity. Shaded area was measured beneath the canopy of the 1.2 irrigation treatment at solar noon throughout the study to provide an estimate of seasonal crop coefficients ( K c ). Vine water status was assessed across treatments and years by measuring midday leaf water potential (Ψ l ). The maximum K c determined from the percent shaded area was 0.7 at the row spacing of 3.66 m and canopy type that developed a “California Sprawl.” Irrigation treatment had a significant effect on midday Ψ l and no such effect for leaf removal. Clusters exposed to direct solar radiation had significantly higher temperatures and lower cluster Ψ than clusters in the shade. Irrigation treatment had a significant effect on berry weight, soluble solids, and titratable acidity. Yields of vines significantly increased as applied water amounts increased. In this wine grape production area, profitability is dependent upon yield. This study provided a reliable estimate of ET c and applied water amounts to maximize yield.