Deficit irrigation alters grapevine growth, physiology, and fruit microclimate

A deficit irrigation trial was conducted with field-grown Cabernet Sauvignon grapevines in the Columbia Valley of southeastern Washington. Four irrigation regimes were applied in four replicated blocks to replace various fractions of crop evapotranspiration (ET_c) between fruit set and harvest. These treatments were designated ET₁₀₀ (100% ET_c), ET₇₀ (70% ET_c), ET₂₅ (25% ET_c), and ET_25/100 (25% ET_c before veraison and 100% ET_c thereafter). Leaf water status and gas exchange, canopy growth and microclimate, and yield formation were evaluated over three years. Despite yearly variation in growing season temperatures, irrigation treatment effects were consistent among years. Overall, deficit irrigation did not enhance water-use efficiency. The ET₁₀₀ and ET₇₀ regimes rarely differed in vine physiology and performance. The ET₂₅ regime, however, strongly limited gas exchange and led to a decline in vine capacity and productivity, suggesting that this degree of water deficit was economically unsustainable. In addition, this treatment was associated with small berries on small clusters, very high fruit-zone sunlight exposure, and elevated cluster temperature. The ET_25/100 regime was generally intermediate in vine physiology, growth, and yield components. This treatment resulted in open canopies and small berries without the penalty in vine capacity and yield that was incurred with ET₂₅. Potential effects of water deficit on fruit composition may be related to altered canopy size and microclimate, in addition to decreased berry size.