Stomatal responses in grapevine become increasingly more tolerant to low water potentials throughout the growing season.

The leaf of a deciduous species completes its life cycle in a few months. During leaf maturation, osmolyte accumulation leads to a significant reduction of the turgor loss point (Ψ ), a known marker for stomatal closure. Here we exposed two grapevine cultivars to drought at three different times during the growing season to explore if the seasonal decrease in leaf Ψ influences the stomatal response to drought. The results showed a significant seasonal shift in the response of stomatal conductance to stem water potential (g ~Ψ ), demonstrating that grapevines become increasingly tolerant to low Ψ as the season progresses in coordination with the decrease in Ψ . We also used the SurEau hydraulic model to demonstrate a direct link between osmotic adjustment and the plasticity of g ~Ψ . To understand the possible advantages of g ~Ψ plasticity, we incorporated a seasonally dynamic leaf osmotic potential into the model that simulated stomatal conductance under several water availabilities and climatic scenarios. The model demonstrated that a seasonally dynamic stomatal closure threshold results in trade-offs: it reduces the time to turgor loss under sustained long-term drought, but increases overall gas exchange particularly under seasonal shifts in temperature and stochastic water availability. A projected hotter future is expected to lower the increase in gas exchange that plants gain from the seasonal shift in g ~Ψ . These findings show that accounting for dynamic stomatal regulation is critical for understanding drought tolerance.