Post-véraison irrigation frequency, rather than amount, better preserves vascular function and delays late-season dehydration in Cabernet Sauvignon grapes.

Late-season dehydration (LSD) reduces grape yield and quality in semi-arid regions, yet the roles of post-véraison irrigation amount (IA) and frequency (IF) in regulating berry vascular function remain unclear. This study investigated how IA and IF influence berry physicochemical properties, phenolic composition, and vascular function in Vitis vinifera L. cv. Cabernet Sauvignon. Irrigation frequency exerted a stronger influence than amount on berry composition, phenolic accumulation, and vascular continuity. Significant four-way interactions among developmental stage, irrigation amount, IF, and berry condition (p < 0.001) indicated that vascular function was regulated in a stage-dependent manner rather than through additive water-supply effects. Under the experimental conditions of this study, the moderate irrigation amount combined with the highest IF (T938-5) most effectively maintained berry quality, preserved bidirectional xylem transport, and delayed phloem back-flow loss. Xylem transport progressively declined from the rachis-pedicel interface, with faster hydraulic disruption in shriveled than in healthy berries. Earlier loss of phloem back-flow suggests that phloem dysfunction might be an early event in vascular isolation during LSD. Overall, irrigation frequency emerged as the primary determinant of berry vascular integrity, delaying hydraulic failure and mitigating late-season dehydration under semi-arid conditions.