Nitrogen-Metabolizing Enzymes: Effect of Nitrogen Sources and Saline Irrigation

Abstract

Indian mustard (Brassica juncea L. cv. RH-30) was grown under different types and levels of nitrogen (N) sources, i.e. nitrate, ammonical, and nitrate plus ammonical, at 40, 80, and 120 kg ha^{− 1} under green house conditions. The plants were salinized with 8 and 12 dSm^{− 1} at 35 and 55 days after sowing. A progressive inhibition of the activity of enzymes of N metabolism, i.e., nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH), was observed with increasing level of salinity. However, the magnitude of such reductions was lowest at the highest level of N (120 kg ha^{− 1}) as compared with the lowest level (40 kg ha^{− 1}) irrespective of N source. The activity of nitrate-assimilating enzymes (NR and NiR) was maximum with nitrate fertilizer, and minimum with the ammonical form. The alleviation of detrimental effects of salinity on NR and NiR was better with the highest level of N (120 kg ha^{− 1}) in nitrate form as compared with the lowest level of N (40 kg ha^{− 1}). In contrast, the maximum activity of ammonium-assimilating enzymes (GS, GOGAT, and GDH) was observed with the highest level of N (120 kg ha^{− 1}) and the minimum with the nitrate form of N under salinity. These results indicate that despite the high salinity, an increase in the concentration and uptake of N stimulates the activities of nitrate-assimilating enzymes (NR and NiR) as well as of the ammonia-assimilating enzymes (GS, GOGAT, and GDH).

Keywords:

• salinity
• nitrate reductase
• nitrite reductase
• glutamine synthetase
• glutamate synthase
• glutamate dehydrogenase

ACKNOWLEDGMENTS

Thanks are due to the Indian Council of Agricultural Research for awarding a senior research fellowship to the first author during his Ph.D. program.