NITRATE REDUCTASE, PHOSPHOENOLPYRUVATE CARBOXYLASE, AND GLUTAMINE-SYNTHETASE IN ANNUAL RYEGRASS AS AFFECTED BY SALINITY AND NITROGEN

The concentration of organic acids, organic nitrogen (N), nitrate (NO3 ), and total cations increased in annual ryegrass (Lolium multiflorum Lam.) with salinity and N concentration in the growth medium. Increasi ng salinity and N in the growth medium induced changes in the level of key enzymes of N assimilation and organic acids: nitrate reductase (N R, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31), a nd glutamine synthetase (GS, EC 6.3.1.2). Plants grown in pots filled with sand were irrigated with nutrient solutions with an electroconduc tivity of 2 or 11.2 dS m(-1) and N applied as ammonium nitrate (NH4NO3 ), sodium nitrate (NaNO3), or ammonium applied as ammonium nitrate (NH 4NO3), sodium nitrate (NaNO3), or ammonium (NH4) as ammonium sulfate [ (NH4)(2)SO4] at concentrations of 0.5, 4.5 or 9.0 mM. Nitrate reductas e, PEPc, and GS increased with salinity and N level. Shoot NR was high est in the presence of NH4NO3 irrespective of salinity level, while ro ot NR activity responded best to NO3. Enhancement of PEPc activity in both shoots and roots was highest with NH4NO3 and lowest with NH4. Nit rogen source had no significant effect on GS activity in shoots or roo ts of ryegrass. Shoot NR activity increased with NO3 concentration in the tissue, as calculated from repression coefficients. The PEPc activ ity correlated positively with total cations and NO3 concentrations in the plants, irrespective of the salinity level, suggesting that the i ncrease in total cations and NO3 induced by salinity may have triggere d the changes in enzyme activities. The concentration of organic acids in both shoots and roots correlated positively with PEPc activity irr espectively of the salinity level. The PEPc activity was higher in roo ts than in shoots, while organic acid concentration was higher in shoo ts. These results suggest that a significant part of the organic acids produced in the roots were used as carbon skeleton for transamination reactions. The increased activity of NR, PEPc, and GS in roots may co nstitute part of an adaptation strategy of the plant to increasing sal inity in the medium. These enzymes have an important role in the metab olism of amino acids and the synthesis of organic N in annual ryegrass irrigated with saline water, and boosting them with suitable N fertil izers could increase the nutritional value and protein content of the crop.