Nitrate reductase and molybdenum cofactor in annual ryegrass as affected by salinity and nitrogen source

Abstract

The influence of salinity on the activity of nitrate reductase (NR, EC 1.6.6.1) and the level of the molybdenum cofactor (MoCo) as affected by the source and concentration of nitrogen was studied in annual ryegrass (Lolium multiflorum cv. Westerwoldicum). Plants grown in sand were irrigated with nutrient solution with an electrical conductivity of 2 or 11.2 dS m−1, containing nitrogen (0.5 or 4.5 mM) in the form of NH4NO3 or NaNO3 Salinity‐treated (11.2 dS m−1) plants produced less biomass and more organic nitrogen while accumulating more NO−3 than control plants. Increased nitrogen concentration in the irrigation solutions enhanced biomass and organic nitrogen production as well as NO−3 accumulation irrespective of the electrical conductivity. Salinity inhibited shoot growth and increased shoot NR activity of plants receiving 4.5 mM NH4NO3 or NaNO3. Similar effects were observed in roots of plants grown in 4.5 mM NaNO3. Nitrate added to a complementation medium containing ryegrass MoCo and the NR apoprotein of Neurospora crassa mutant nit‐1 stimulated the activity of the reconstituted NR (NADPH‐nitrate reductase, EC 1.6.6.3). Increased salinity and nitrogen in the nutrient solutions caused an increase of MoCo content in roots and shoots. Similar results were observed for NR activity in the shoots. The increase of MoCo in response to salinity was more pronounced than that of NR, especially in the roots. We conclude that the pool size of MoCo in ryegrass is not constant, but varies in response to nutritional and environmental factors.