Influence of some growth regulators and divalent cations on the inhibition of nitrate reductase activity by lead in maize leaves

Abstract

Substrate induction of in vivo nitrate reductase (NR) activity in excised maize leaf segments from seedlings grown with nutrient containing no nitrogen, was substantially inhibited by 0.5 mM Pb acetate, during a 24 h incubation of leaf segments. Inclusion of IAA, GA 3, kinetin and salicylic acid in the incubation medium increased NR activity in the absence of Pb, but only GA and salicylic acid did so in the presence of Pb. Salicylic acid negated the inhibitory effect of the Pb, which was related to its concentration. The phenolic acid had little effect on in vitro Pb effects on NR activity. Among CaCl 2, MgCl 2, MnCl 2, ZnCl 2 and CuSO 4, only MnCl 2 to some extent, negated the inhibitory effect of Pb on in vivo NR activity in the leaf segments, although the chlorides of Ca 2+, Mg 2+ and Mn 2+ caused some increase in enzyme activity in the absence of Pb. The effect of Mn 2+ caused some increase in enzyme activity in the absence of Pb. The effect of Mn 2+ was concentration dependent. But in vitro supply of MnCl 2 to NR preparation prevented the inhibitory effect of Pb on its activity at each concentration, 5, 10, and 20 mM. The experiments demonstrate that the inhibitory effect of Pb on enzyme activity could be antagonised by salicylic acid and Mn.