Contrasting water stress responses of Chardonnay and Sauvignon blanc grapevines: Implications for postveraison irrigation

Understanding how stomatal regulation relates to plant hydraulic traits and whole-plant performance is critical for improving water management in grapevines. This study evaluated whether (i) cultivar differences in stomatal sensitivity are linked to leaf hydraulic traits, (ii) these differences translate into integrative responses such as yield and water-use efficiency, and (iii) acclimation processes modulate these responses under field conditions. A field experiment was conducted on Vitis vinifera cvs. Chardonnay and Sauvignon blanc under three irrigation treatments (R100, R50, R0) across two growing seasons. Measurements included gas exchange, plant water status (Ψ pd , Ψ leaf ), pressure–volume curve traits, hydraulic vulnerability, carbon isotope discrimination (Δ¹³C), yield, and water productivity. Logistic and linear models were used to characterize functional responses to water deficit. Sauvignon blanc exhibited higher stomatal sensitivity, with a less negative Ψgs₅₀ (−0.21 MPa) than Chardonnay (−0.48 MPa), whereas Chardonnay maintained higher stomatal conductance under declining water potential. These differences were associated with higher leaf hydraulic capacitance (0.060 vs 0.053) and lower relative water content at turgor loss (88.1% vs 90.6%), but not with petiole anatomy or xylem embolism resistance. Despite these contrasting physiological strategies, no consistent cultivar differences were detected in yield, carbon isotope discrimination, water productivity, or final soluble solids, indicating that leaf-level responses were largely decoupled from whole-plant performance under moderate post-veraison water deficit. Moreover, no evidence of hydraulic, anatomical, or photosynthetic acclimation was detected. Overall, grapevines responded primarily through dynamic and reversible stomatal regulation rather than structural acclimation, indicating that although stomatal sensitivity reflects cultivar-specific hydraulic behaviour, it does not reliably predict final agronomic performance during berry ripening under moderate post-veraison water deficit.