Implications of the mesophyll conductance to CO2 for photosynthesis and water‐use efficiency during long‐term water stress and recovery in two contrasting Eucalyptus 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 Eucalyptus 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 CO₂. The diffusive limitations to CO₂, 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 (iWUE) and reduces the leaf oxidative stress, highlighting the importance of g_m as a target for breeding/genetic engineering.