Yonatan Sorek  - Institute of Soil, Water and Environmental Science, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet-Dagan, 50250, Israel.; The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, 76100, Israel.
 Smadar Grinshtein  - Institute of Soil, Water and Environmental Science, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet-Dagan, 50250, Israel. 
Yishai Netzer  - Department of Chemical engineering, Ariel University, Ariel, 40700, Israel.; Agriculture and Oenology Department, Eastern R&D Center, Ariel, 40700, Israel.

Ilana Shtein  - Agriculture and Oenology Department, Eastern R&D Center, Ariel, 40700, Israel. 
Steven Jansen  - Institute of Systematic Botany and Ecology, Ulm University, Albert-Einstein-Allee 11, D-89081, Ulm, Germany. 
Uri Hochberg  - Institute of Soil, Water and Environmental Science, Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet-Dagan, 50250, Israel.

Although xylem embolism resistance is traditionally considered as static, we hypothesized that in grapevine (Vitis vinifera) leaf xylem becomes more embolism resistant along the growing season. We evaluated xylem architecture, turgor loss point (Ψ_TLP ), and water potentials leading to 25% of maximal stomatal conductance (g_s25 ) or 50% embolism in the leaf xylem (P₅₀ ) in three irrigation treatments and at three time points during the growing season, while separating the effects of leaf age and time of season. Hydraulic traits acclimated over the growing season in a coordinated manner. Without irrigation, Ψ_TLP , g_s25 , and P₅₀ decreased between late May and late August by 0.95, 0.77, and 0.71 MPa, respectively. A seasonal shift in P₅₀ occurred even in mature leaves, while irrigation had only a mild effect (<0.2 MPa) on P₅₀ . Vessel size and pit membrane thickness were also seasonally dynamic, providing a plausible explanation for the shift in P₅₀ . Our findings provide clear evidence that grapevines can modify their hydraulic traits along a growing season to allow lower xylem water potential, without compromising gas exchange, leaf turgor, or xylem integrity. Seasonal changes should be considered when modeling ecosystem vulnerability to drought or comparing data sets acquired at different phenological stages.