NITRATE REDUCTASE AND MOLYBDENUM COFACTOR IN ANNUAL RYEGRASS AS AFFECTED BY SALINITY AND NITROGEN-SOURCE

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 ryeg rass (Lolium multiflorum cv. Westerwoldicum). Plants grown in sand wer e 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 les s biomass and more organic nitrogen while accumulating more NO3- than control plants. Increased nitrogen concentration in the irrigation sol utions enhanced biomass and organic nitrogen production as well as NO3 - accumulation irrespective of the electrical conductivity. Salinity i nhibited shoot growth and increased shoot NR activity of plants receiv ing 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 mediu m containing ryegrass MoCo and the NR apoprotein of Neurospora crassa mutant nit-1 stimulated the activity of the reconstituted NR (NADPR-ni trate reductase, EC 1.6.6.3). Increased salinity and nitrogen in the n utrient solutions caused an increase of MoCo content in roots and shoo ts. Similar results were observed for NR activity in the shoots. The i ncrease 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 an d environmental factors.