Merlot vines were drip-irrigated with 100, 70, or 35% of estimated crop evapotranspiration (ETc) or 35% ETc until veraison followed by 70% ETc until harvest. Midday leaf water potential (Ψ) differed among irrigation regimes and ranged from −0.9 to −1.7 MPa, with a 0.3 to 0.5 MPa difference between 35 and 100% ETc. Decline in shoot growth, berry size, cluster weight, yield, trunk growth, cluster number, and berry titratable acidity corresponded with a decline in Ψ. Increasing irrigation from 35 to 70% ETc at veraison consumed 30% less water producing similar yield and quality as the 70% ETc treatment.
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