Response of Muscat of Alexandria table grapes to post-veraison regulated deficit irrigation in Japan

The effects of post-veraison regulated deficit irrigation (RDI) on vine water status, ripening, and quality of table grapes, cv. Muscat of Alexandria grown under a polyhouse and root-zone restriction condition were investigated in the Okayama University Experimental vineyard, Japan. From bud break to veraison all vines were irrigated to a soil moisture tension of 3 kPa at a depth of 15 cm, and were re-irrigated when soil moisture tension approached 15 kPa. Starting 10 d after veraison, 3 irrigation regimes were imposed: (1) Control (C): Re-irrigation immediately when soil moisture tension reached 15 kPa; (2) Moderate Deficit Irrigation (MDI): Re-irrigation 2 d after reaching a soil moisture tension of 15 kPa; and (3) Severe Deficit Irrigation (SDI): Re-irrigation 4 d after reaching a soil moisture tension of 15 kPa. Treatments were continued for 6 weeks until harvest. By the end of the experiment, as the vine water status decreased, only SDI vines were wilted or necrotic in the fruit-zone. In SDI vines, the cumulative effect of increased vine water deficit indicated by lower Ψ₁, resulted in berries that were lower in firmness and acidity, with a small increase in aroma, and a higher TSS than control at harvest. The decrease in vine water status in the MDI treatment had a slight effect on berry ripening as compared with control while RDI had no effect on berry weight or juice pH at harvest.