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 (g_sc) at the panicle initiation stage and to the greater mesophyll conductance (g_m) at the grain-filling stage in both current and elevated atmospheric CO₂ concentrations [CO₂]. Takanari had a higher level of leaf nitrogen content (N_l) compared with Koshihikari at the grain-filling stage, which led to greater g_m and maximum carboxylation rate (V_c,max), but N_l alone did not explain the variations of g_m within the variety. A clear correlation was found between V_c,max and N_l. Calculating V_c,max taking g_m into consideration removed the artifact of V_c,max25 in relation to N_l that was observed when g_m was assumed to be infinite. Our results emphasize the need to separate the roles of V_c,max and g_m to accurately understand the ecophysiological processes that control leaf photosynthesis in Takanari.