Portable photosynthetic gas analysis system LI-COR 6400 was used to measure in situ of net photosynthetic rate (Pn), stomatal conductance (Gs), and RuBP carboxylation efficiency (CE) at the grain-filling stage. A correlation analysis was made to explore the determinant of the photosynthetic capacity in rice leaf. In flag leaf,Pn in middle region was higher than that in its upper region,and leaf basal region had a lowest Pn value. Also there were some similar differences in Gs and CE between these different regions. In different positiongs of those leaves, Pn,Gs,and CE were gradually declined from upper to basal showing a leaf position gradient.The results of correlation analysis of these data indicates that the correlation coefficient between Pn and CE was much higher than that between Pn and Gs in two cases, and Pn was negatively correlated with intercellular CO_2 concentration (Ci). It is concluded that the carboxylation activity or activated amount of Rubisco rather than stomatal conducatance is a determinant of photosynthetic capacity in rice leaves. In addition, while Pn in flag leaves of different tillers was positively correlated with Gs but negatively correlated with Ci demonstrates that stomatal conducatance is not a determinant of potosynthetic capacity in rice leaves.
Net photosynthetic rate