Leaf Photosynthetic Rate and Mesophyll Cell Anatomy Changes During Ontogenesis in Backcrossed Indica × japonica Rice Inbred Lines

The high-yielding indica rice variety, 'Takanari', has the high rate of leaf photosynthesis compared with the commercial japonica varieties. Among backcrossed inbred lines from a cross between 'Takanari' and a japonica variety, 'Koshihikari', two lines, BTK-a and BTK-b, showed approximately 20% higher photosynthetic rate than that of 'Takanari' for a flag leaf at full heading. This is a highest recorded rate of rice leaf photosynthesis. Here, the timing and cause of the increased leaf photosynthesis in the BTK lines were investigated by examining the photosynthesis and related parameters, as well as mesophyll cell anatomy during ontogenesis. Their photosynthetic rate was greater than that of 'Takanari' in the 13th leaf, as well as the flag leaf, but there were no differences in the 7th and 10th leaves. There were no consistent differences in the stomatal conductance, or the leaf nitrogen and Rubisco contents in the 13th and flag leaves. The total surface area of mesophyll cells per leaf area (TA) in the 13th and flag leaves increased significantly in the BTK lines due to the increased number and developed lobes of mesophyll cells compared with in 'Takanari'. The mesophyll conductance (g ) became greater in the BTK lines compared with 'Takanari' in the flag leaves but not in the 10th leaves. A close correlation was observed between TA and g . We concluded that the increased mesophyll conductance through the development of mesophyll cells during the reproductive period is a probable cause of the greater photosynthetic rate in the BTK lines.