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