High mesophyll conductance in the high-yielding rice cultivar Takanari quantified with the combined gas exchange and chlorophyll fluorescence measurements under free-air CO2 enrichment

An effective strategy for increasing crop production is increasing the rate of photosynthesis. In this study, we conducted gas exchange and chlorophyll fluorescence measurements for a high-yielding rice cultivar, Takanari, to identify the leaf physiological properties that contribute to high capacity for photosynthesis of the uppermost leaves before (panicle initiation stage) and after heading (grain-filling stage) in the Tsukuba free-air CO2 enrichment (FACE) facility. The higher photosynthesis rate of Takanari compared with that of the commonly cultivated cultivar, Koshihikari, was mainly attributed to the greater stomatal conductance for CO2 (gsc) at the panicle initiation stage and to the greater mesophyll conductance (gm) at the grain-filling stage in both current and elevated atmospheric CO2 concentrations [CO2]. Takanari had a higher level of leaf nitrogen content (Nl) compared with Koshihikari at the grain-filling stage, which led to greater gm and maximum carboxylation rate (Vc,max), but Nl alone did not explain the variations of gm within the variety. A clear correlation was found between Vc,max and Nl. Calculating Vc,max taking gm into consideration removed the artifact of Vc,max25 in relation to Nl that was observed when gm was assumed to be infinite. Our results emphasize the need to separate the roles of Vc,max and gm to accurately understand the ecophysiological processes that control leaf photosynthesis in Takanari.