Differential effects of exogenous L-glutamine, L-glutamic acid, sodium glutamate, and casamino acids on glutamine synthetase and nitrate reductase in isolated pea roots cultured with or without sucrose

Exogenous L-glutamine, the sodium salt of L-glutamic acid, and casamino acids do not decrease glutamine synthetase (GS) level in isolated pea (Pisum sativum L. cv. Jupiter and cv. Proteus) roots cultured with 20 g∙L−1 sucrose while L-glutamic acid does decrease it. The effect of L-glutamic acid is stronger in solutions lacking nitrate. By contrast, only the exogenous sodium salt of L-glutamic acid does not enhance the decrease in GS level caused by sugar starvation in isolated roots cultured without any sugar while, the other compounds tested do enhance this decrease. These facts confirm our earlier conclusion that sugar availability and the concentration of H+ ions are more important for GS level regulation in pea roots than nitrogen substrate availability and the presence of the end products. Nitrate reductase (NR) level is depressed by exogenous L-glutamine, the sodium salt of L-glutamic acid, casamino acids, and a low (0.2 mM) concentration of L-glutamic acid whereas it is increased by higher (0.8 and 1.0 mM) concentrations of L-glutamic acid, by α-ketoglutaric acid (0.4 to 0.6 mM), and by nitric acid (0.2 to 0.4 mM) added to saturating concentration (10 mM) of nitrate present in the form of potassium and calcium salts. The negative effect of L-glutamine, sodium glutamate, and casamino acids can be reversed by L-glutamic acid. This suggests that more mechanisms may be involved in NR regulation by these compounds and that the mechanism controlled by increased concentration of H+ ions is of great importance.