Effect of amino acids on nitrate reductase activity in maize roots

Nitrate reductase activity in roots of maize was examined after amino acids treatment. Two different concentrations of glutamine, asparagine or isoleu cine (1 and 10 mM) were applied individually into incubation medium contain ing three different levels of KNO3 (0.1 mM - low; 1.0 mM - medium, and 10.0 mM - high). Generally, the low concentration (1 mM) of the amino acids use d in this study showed an inhibitory effect on NR activity, regardless of t he nitrate level in root medium. Only very weak (at 1 mM of Gln) or no effe ct at all (at 10 mM) on NR activity was found in the roots at high level of nitrate (10 mM KNO3). Surprisingly pretreatment of roots with high concent rations of amino acids (mainly Ile) caused the stimulation of nitrate reduc tase activity in maize roots kept at low nitrate level (0.1 mM KNO3). This stimulation of NR activity (by Gin and Ile in roots kept at low nitrate lev el) does not fit with the proposed mechanism of the amino acid effects on N R activity. It seems that amino acids are not the dominant regulatory facto rs in nitrate assimilation in maize roots and that some other dominant regu latory factors do exist.