The temperature response of mesophyll conductance, and its component conductances, varies between species and genotypes.

The temperature response of mesophyll conductance to CO diffusion (g) has been shown to vary considerably between species but remains poorly understood. Here, we tested the hypothesis that increases in chloroplast surface area with increasing temperature, due to the formation of chloroplast protrusions, caused observed positive responses of g to temperature. We found no evidence of chloroplast protrusions. Using simultaneous measurements of carbon and oxygen isotope discrimination during photosynthesis to separate total g (g) into cell wall and plasma membrane conductance (g) and chloroplast membrane conductance (g) components, we explored the temperature response in genotypes of soybean and barley, and sunflower plants grown at differing CO concentrations. Differences in the temperature sensitivity of g were found between genotypes and between plants grown at differing CO concentration but did not relate to measured anatomical features such as chloroplast surface area or cell wall thickness. The closest fit of modelled g to estimated values was found when cell wall thickness was allowed to decline at higher temperatures and transpiration rates, but it remains to be tested if this decline is realistic. The temperature response of g (calculated from the difference between 1/g and 1/g) varied between barley genotypes, and was best fitted by an optimal response in sunflower. Taken together, these results indicate that g is a highly complex trait with unpredictable sensitivity to temperature that varies between species, between genotypes within a single species, with growth environment, between replicate leaves, and even with age for an individual leaf.