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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 NO3- than control plants. Increased nitrogen concentration in the irrigation solutions enhanced biomass and organic nitrogen production as well as NO3- 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. [ABSTRACT FROM AUTHOR]

Positive influences of high concentrations of dissolved inorganic carbon (DIC) in the growth medium of salinity‐stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity‐stressed tomato plants, enriched CO2 in the rhizosphere increases NO−3uptake. In the present study, wild‐type and nitrate reductase‐deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO2 on NO−3 uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH4+and aerated with ambient air, were transferred to either NO3− or NH4+ solutions and aerated with air containing between 0 and 6 500 μmol mol−1 CO2. Nitrogen uptake and tissue concentrations of NO3− and NH4+ were measured as well as activities of NR and PEPc. The uptake of NO−3 by the wild‐type was increased by increasing CO2. This was associated with increased in vitro NR activity, but increased uptake of NO3− was found also in the NR‐deficient genotype when exposed to high CO2 concentrations; so that the influence of CO2 on NO3− uptake was independent of the reduction of NO3− and assimilation into amino acids. The increase in uptake of NO3− in wild‐type plants with enriched CO2 was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO3− or CO2 in the medium did not influence NO3− uptake. Uptake of NH4+ was decreased by enriched CO2 in a pH (5 or 7) independent fashion. Thus NO3− and NH+4 uptakes are influenced by the CO2 component of DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO2 may directly affect the uptake of NO3− and NH4+ in ways unrelated to the NR activity in the tissue. [ABSTRACT FROM AUTHOR]