Protein metabolism in leaves and developing grains of rices differing in grain protein content

Four semi-dwarf rices (Oryza sativa L.) differing in percentage of grain protein, grown in a flooded field receiving basal N fertilization, had a peak activity of root glutamate dehydrogenase 4 weeks after transplanting. A lower peak occurred during panicle formation 10 weeks after transplanting. The percentage of N of the active leaf blades was also highest 4 weeks after transplanting. The activity of nitrate reductase in the leaf blades was low and decreased after transplanting.Among the three rices with similar grain yield, the rice with high percentage of protein tended to translocate more leaf N to the developing grains than the rices with average grain protein content. The leaf blades of the former also had lower rates of leucine incorporation during grain development but higher protease activity than leaves of the rices with average protein content. Developing grains of the rices with high percentage of protein tended to have higher levels of soluble protein, free amino N, and protease, and a faster rate of leucine incorporation than grains of the IR3 rice with average percentage of protein, regardless of grain yield.