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Aspects of Nitrogen Metabolism in the Rice Seedling ^1,2

^a Department of Chemistry, The International Rice Research Institute, Los Baños, Laguna, Philippines

The effects of nitrogen source NO₃^– or NH₄⁺ on nitrogen metabolism during the first 2 weeks of germination of the rice seedling (Oryza sativa L., var. IR22) grown in nutrient solution containing 40 µg/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 NH₄⁺-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 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.

¹ This work was supported in part by Contract NO1-AM-7-0726, National Institute of Arthritis, Metabolism and Digestive Diseases, National Institutes of Health.

² This paper is in part the thesis of R. S. M. submitted to the Indian Agricultural Research Institute, New Delhi, India, in partial fulfillment of the Ph.D. degree.