Nitrate reductase, phosphoenolpyruvate carboxylase, and glutamine synthetase in annual ryegrass as affected by salinity and nitrogen

Abstract

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. Increasing salinity and N in the growth medium induced changes in the level of key enzymes of N assimilation and organic acids: nitrate reductase (NR, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31), and glutamine synthetase (GS, EC 6.3.1.2). Plants grown in pots filled with sand were irrigated with nutrient solutions with an electroconductivity of 2 or 11.2 dS m^‐1 and N applied as ammonium nitrate (NH₄NO₃), sodium nitrate (NaNO₃), or ammonium applied as ammonium nitrate (NH₄NO₃), sodium nitrate (NaNO₃), or ammonium (NH₄) as ammonium sulfate [(NH₄)₂SO₄] at concentrations of 0.5, 4.5 or 9.0 mM. Nitrate reducíase, PEPc, and GS increased with salinity and N level. Shoot NR was highest in the presence of NH₄NO₃ irrespective of salinity level, while root NR activity responded best to NO₃. Enhancement of PEPc activity in both shoots and roots was highest with NH₄NO₃ and lowest with NH₄. Nitrogen source had no significant effect on GS activity in shoots or roots of ryegrass. Shoot NR activity increased with NO₃ concentration in the tissue, as calculated from regression coefficients. The PEPc activity correlated positively with total cations and NO₃ concentrations in the plants, irrespective of the salinity level, suggesting that the increase in total cations and NO₃ induced by salinity may have triggered the changes in enzyme activities. The concentration of organic acids in both shoots and roots correlated positively with PEPc activity irrespectively of the salinity level. The PEPc activity was higher in roots than in shoots, while organic acid concentration was higher in shoots. 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 constitute part of an adaptation strategy of the plant to increasing salinity in the medium. These enzymes have an important role in the metabolism of amino acids and the synthesis of organic N in annual ryegrass irrigated with saline water, and boosting them with suitable N fertilizers could increase the nutritional value and protein content of the crop.