Temperature response of mesophyll conductance in cultivated and wild Oryza species with contrasting mesophyll cell wall thickness

A critical component of photosynthetic capacity is the conductance of CO ₂ from intercellular airspaces to the sites of CO ₂ fixation in the stroma of chloroplasts, termed mesophyll conductance (g _m). Leaf anatomy has been identified as an important determinant of g _m. There are few studies of the temperature response of g _m and none has examined the implications of leaf anatomy. Hence, we compared a cultivar of Oryza sativa with two wild Oryza relatives endemic to the hot northern savannah of Australia, namely Oryza meridionalis and Oryza australiensis. All three species had similar leaf anatomical properties, except that the wild relatives had significantly thicker mesophyll cell walls than O.sativa. Thicker mesophyll cell walls in the wild rice species are likely to have contributed to the reduction in g _m, which was associated with a greater drawdown of CO ₂ into chloroplasts (C _i-C _c) compared with O.sativa. Mesophyll conductance increased at higher temperatures, whereas the rate of CO ₂ assimilation was relatively stable between 20 and 40°C. Consequently, C _i-C _c decreased for all three species as temperature increased.