This study was conducted to estimate the effects of nitrogen fertilizer application at low, moderate, and high level on photosynthesis, embryo area, and grain yield of a rice giant embryo mutant line (MGE13) in comparison with the originated rice variety Mizuhochikara (Miz). As increasing nitrogen fertilizer application, photosynthetic rate in term of the carbon dioxide exchange rate (CER) increased at the tillering, heading, and dough-ripening stages in both rice genotypes. Especially at the heading stage, the increase of CER was much higher in MGE13 than that in Miz as increasing nitrogen fertilizer from low to high level. The embryo area at both the dough-ripening and harvesting stages increased more in MGE13 than in Miz as increasing nitrogen fertilizer application level. The embryo area was significantly correlated with CER at the heading stage and with the plant growth rate during the ripening stage in both rice genotypes. Grain yield was similar in MGE13 and in Miz under low and moderate nitrogen fertilizer levels, but it was much higher in MGE13 than in Miz under the high nitrogen level, mainly due to the greater CER at heading stage, the higher dry matter weight and the larger number of panicles per plant.
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