Nitrogen is an essential element required for plant growth and development. However, over-use of nitrogen fertilizer is not only costly but also environmentally unsafe. Therefore, identification of genotypes possessing higher nitrogen use efficiency (NUE) at sub-optimal dose of nitrogen is the key challenge. In this study, effect of nitrogen on the activities of nitrogen assimilatory enzymes, total soluble proteins and amino acids was studied in wheat genotypes of diverse physiology (viz., GLU 1101, GLU 1356, GLU 2001, GLU 700, PH132-4836, PH132- 4840). Nitrogen was applied at recommended dose (RDN), RDN-50%, RDN-25% and RDN+25% using urea as a source. With increase of nitrogen rate, significant increase in the activities of nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase and glutamate dehydrogenase was noted, which caused an increase in protein and amino acid contents in all the genotypes. Activity pattern of studied enzymes revealed an increasing trend from tillering to anthesis stage and thereafter declined in parallel with decrease in protein and amino acid contents. Conversely, nitrogen and chlorophyll content showed a consistent decline with leaf growth. Genotypes GLU 1356 and GLU 2001 maintained higher activities of nitrogen assimilatory enzymes in parallel with higher NUE at RDN-50% and RDN-25%, while genotypes PH132-4836 and PH132-4840 were found to be promising at RDN+25%. A positive correlation of nitrogen assimilatory enzymes (nitrate reductase and glutamine synthetase) with NUE and nitrogen content was also observed indicating that these might be the rate limiting enzymes in nitrogen metabolism. In crux, 25% less than the recommended dose of nitrogen had significant influence on nitrogen metabolism especially in GLU 1356 and GLU 2001 due to high activities of nitrate reductase and glutamine synthetase. These nitrogen efficient genotypes may be exploited in enhancing wheat crop productivity under lower dose of nitrogen to save environment and input cost.
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