Implications of the mesophyll conductance to CO 2 for photosynthesis and water‐use efficiency during long‐term water stress and recovery in two contrasting E ucalyptus species

Water stress ( WS ) slows growth and photosynthesis ( A n ), but most knowledge comes from short‐time studies that do not account for longer term acclimation processes that are especially relevant in tree species. Using two E ucalyptus species that contrast in drought tolerance, we induced moderate and severe water deficits by withholding water until stomatal conductance ( g sw ) decreased to two pre‐defined values for 24 d, WS was maintained at the target g sw for 29 d and then plants were re‐watered. Additionally, we developed new equations to simulate the effect on mesophyll conductance ( g m ) of accounting for the resistance to refixation of CO 2 . The diffusive limitations to CO 2 , dominated by the stomata, were the most important constraints to A n . Full recovery of A n was reached after re‐watering, characterized by quick recovery of g m and even higher biochemical capacity, in contrast to the slower recovery of g sw. The acclimation to long‐term WS led to decreased mesophyll and biochemical limitations, in contrast to studies in which stress was imposed more rapidly. Finally, we provide evidence that higher g m under WS contributes to higher intrinsic water‐use efficiency (i WUE ) and reduces the leaf oxidative stress, highlighting the importance of g m as a target for breeding/genetic engineering.