Influence of amino acids and ammonium on nitrate reduction in corn seedlings

Abstract

Corn seedlings were transferred 48 h after imbibition to a medium containing 1/10 Hoagland''s salts and 5 mM nitrate (K14NO3 or K15NO3). Three treatments were used during the ensuing 6-h induction period: no further additions; corn mixture of amino acids plus 10 mM glutamine and 10 mM asparagine; and 10 mM (NH4)2SO4. The shoots, mature root sections (25-35 mm from the tip), and root tip sections (0-10 mm) were then examined for nitrate reductase activity (NR) and the ability to reduce 15NO3 in vivo. Both amino acid and ammonium additions resulted in less NR in the shoots. In the roots, the development of NR was inhibited slightly by the amino acids and was enhanced by ammonium additions. Treatment with either corn amino acids or (NH4)2SO4 had no influence on the incorporation of 15N into the ammonium and amino acid fraction. Although the potential end products of NR have slight effects on measured in vitro levels of NR, they have no effect on the reduction of NO3- in the intact corn seedling.