Light-saturated photosynthetic rate in high-nitrogen rice (Oryza sativa L.) leaves is related to chloroplastic CO2 concentration

To identify the effect of nitrogen (N) nutrition on photosynthetic efficiency and mesophyll conductance of rice seedlings (Oryza sativa L., cv. 'Shanyou 63' hybrid indica China), hydroponic experiments with different concentrations of N were conducted in a greenhouse. Although leaf N concentration on a dry mass basis increased with increasing supply of N, no significant differences in seedling biomass were observed. A higher light-saturated CO2 assimilation rate (A) with a high concentration of supplied N was associated with a higher carboxylation efficiency (CE), but not a higher apparent quantum yield (alpha). Based on classic photosynthetic models, both the Rubisco content and the ribulose bisphosphate (RuBP) regeneration rate were sufficient for light-saturated photosynthesis in rice seedlings; the estimated chloroplastic CO2 concentration (C-c) and mesophyll conductance (g(m)) demonstrated that a low C-c was the ultimate limiting factor to photosynthetic efficiency with a higher N supply. Due to a greater chloroplast size (i.e. a shorter distance to the plasma membrane) with a higher supply of N, the CO2 transport resistance in the liquid phase (g(liq)) in high-N leaves was lower than that in low-N leaves, which resulted in higher g(m) and C-c in high-N leaves. Although CEA/Ci was higher with a high supply of N, there were no differences in CEA/Cc between plants grown with different concentrations of N, indicating that the carboxylation capacity of Rubisco between plants grown at different N concentrations was constant. The enhanced photosynthetic rate with supply of a high N concentration was attributed to a higher CO2 concentration in the chloroplasts, related to a higher mesophyll conductance due to an increased chloroplast size.