Determinant of photosynthetic capacity in rice leaves under ambient air conditions

At the grain-filling stage, net photosynthetic rate (P-N), stomatal conductance (g(s)), and ribulose-1,5-bisphosphate carboxylation efficiency (CE) were correlated in order to find the determinant of photosynthetic capacity in rice leaves. For a flag leaf, P-N in leaf middle region was higher than in its upper region, and leaf basal region had the lowest P-N value. The differences in g, and CE were similar. P-N, g(s), and CE gradually declined from upper to basal leaves, showing a leaf position gradient. The correlation coefficient between P-N and CE was much higher than that between P-N and g(s) in both cases, and P-N was negatively correlated with intercellular CO2 concentration (C-i). Hence the carboxylation activity or activated amount of ribulose-1,5-bisphosphate carboxylase/oxygenase rather than g(s) was the determinant of the photosynthetic capacity in rice leaves. In addition, in flag leaves of different tillers P-N was positively correlated with g(s), but negatively correlated with C-i. Thus g(s) is not the determinant of the photosynthetic capacity in rice leaves.