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.