Effects of Irrigation and Crop Load on Leaf Gas Exchange and Fruit Composition in Red Winegrapes Grown on a Loamy Sand

Abstract

Effects of reducing irrigation from fruit set to veraison or harvest combined with crop-load adjustment by cluster or shoot thinning were determined for Merlot and Cabernet Sauvignon grapevines cultured on a coarse loamy sand. Geographic information system tools were used to develop maps of moisture distribution in the soil profile, which revealed reductions in total moisture levels and increased spatial variation in response to reduced emitter density. Stomatal conductance and leaf gas exchange decreased in response to reduced irrigation but also declined across all treatments during the lag phase of berry development and then increased postveraison. Pruning mass was affected little by treatments in Merlot but was reduced by either shoot or cluster thinning in Cabernet Sauvignon. Berry mass and anthocyanin and tannin contents were affected little and inconsistently by irrigation and crop-load adjustment and varied mostly among years, indicating a dominant influence of seasonal climate on berry development and composition.