Light and ethylenediamine tetraacetic acid mediated differential effects of ammonium on nitrate reductase activity in maize seedlings

Abstract Effect of ammonium on in vivo activity of nitrate reductase in roots, shoots and leaves of maize (Zea mays L.) seedlings was studied in relation to light/dark conditions and EDTA supply. Supply of 5 mM (NH₄)₂SO₄ increased the steady state level of enzyme only in leaves and in light, while it had no effect in roots and shoots and in the dark. The substrate induction of enzyme was also little affected by 1 to 10 mM (NH₄)₂SO₄ in roots and shoots. In the leaves the activity in the dark was either inhibited (minus EDTA) or stimulated (plus EDTA) by 5 to 10 mM (NH₄)₂SO₄. The activity was stimulated in the light also in the presence of EDTA at higher concentrations of ammonium. When different concentrations of ammonium were supplied without any exogenous nitrate in the light, the enzyme activity increased at low concentration and was either inhibited or unaffected at higher concentrations depending upon the tissue used. Supply of EDTA with ammonium modified its effect to some extent. It is suggested that the effect of ammonium on nitrate reductase activity depends upon the tissue used and the effective concentration of the ammonium.