The role of leaf water potential in the temperature response of mesophyll conductance

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψ_leaf ), which may influence mesophyll conductance (g_m ). However, the effects of Ψ_leaf on g_m have not yet been considered in models of the g_m response to temperature. Temperature responses of g_m and Ψ_leaf and the response of g_m to Ψ_leaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψ_leaf was incorporated into an existing g_m -T model. In wheat, Ψ_leaf was dramatically decreased with increasing T, whereas in rice Ψ_leaf was less sensitive or insensitive to T. Without taking Ψ_leaf into account, g_m for wheat showed no response to T. However, at a given Ψ_leaf , g_m was significantly higher at high temperature compared with low. After incorporating the function of Ψ_leaf into the g_m -T model, we suggest that the g_m -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψ_leaf gradient between temperatures, and the sensitivity of g_m to Ψ_leaf , below a threshold (Ψ_leaf,0 ). The data presented here suggest that Ψ_leaf plays an important role in the g_m -T relationship and should be considered in future studies related to the temperature response of g_m and photosynthesis.