The effects of nitrogen source NO3-or NH41 on nitrogen metabolism during the first 2 weeks of germination of the rice seedling (Oryza sativa L., var. IR22) grown in nutrient solution contang 40 ,ug/ml N were studied. Total, soluble protein, and free amino N levels were higher in the NH4+-grown seedling, particularly during the 1st week of germination. Asparagine accounted for most of the difference in free amino acid level, in both the root and the shoot. Nitrate and nitrite reductase activities were present mainly in the shoot and were higher in the NO3--grown seedling, whereas the activity of glutamate dehydrogenase and glutamine synthetase in the root tended to be lower than that of the NH41-grown seedling during the 1st week of germination. Glycolate oxidase and catalase activities were present mainly in the shoot. Maximum activity of the above five enzymes occurred 7 to 10 days after germination. Differences in the zymograms of nitrate reductase, glutamate dehydrogenase, and catalase were mainly between shoot and root and not from N source. Nitrite reductase bands were observed only in plants grown in NO3-.Ten-day-old seedlings of three rices differing in level of grain protein did not differ in the level of N fractions and of enzyme activities, which were consistent with their differences in grain protein content.Rice (Oryza sativa L.) is capable of growing in both flooded and upland culture and can grow well in nutrient culture containing either NH4+ or NO3-N, even without aeration (29). Although several papers have been published on the subject of growth and N metabolism of rice plants grown in different sources of N, they only consider particular aspects of interest to the investigators. In plants grown in NH4+, absorbed inorganic N is converted in the roots by reductive amination of a-keto acids (21). By contrast, in NO3--grown plants, NO3-is absorbed by the roots and is largely translocated into the leaves, where it undergoes reduction to NH4+, and, subsequently, reductive amination of a-keto acids (22). Nitrate reductase, which limits the rate of conversion of NO3-to NO2-in plants, is present in the leaf and requires light for activity (1, 10). In addition, Mitsui et al. (18,19) A positive relationship has been observed in some upland cereal crops between leaf nitrate reductase activity and grain protein production (1, 5). No such relationship was noted among four rices that differed in grain protein content for 1-month-old seedlings transplanted and grown under flooded conditions to maturity (24). Seedlings of three rices differing in grain protein content were tested for possible differences in levels of nitrogenous compounds and of various enzymes to check if younger plants show an index of grain protein content. MATERIALS AND METHODSSeeds of IR22, IR8, and IR480-5-9 rice (Oryza sativa L.) were obtained from the plant breeding department of the Institute. Seeds were sterilized by soaking in 0.64% HCHO for 15 min, rinsed with distilled H20, and germinated on a framed aluminum screen above...