REGULATION OF THE ACCUMULATION AND REDUCTION OF NITRATE BY NITROGEN AND CARBON METABOLITES IN MAIZE SEEDLINGS

The accumulation and reduction of nitrate in the presence of the nitro gen metabolites asparagine (Asn) and glutamine (Gln) and the carbon me tabolite sucrose (Suc) were examined in maize (Zea mays L.) seedlings in an attempt to separate their effects on the nitrate uptake system a nd the nitrate reduction system. After 8 h of exposure to nitrate in t he presence of 1 mM Asn, tissue nitrate accumulation was reduced at 25 0 mu M external nitrate, but not at 5 mM Asn. The induction of nitrate reductase (NR) activity was reduced at both external nitrate concentr ations. In the presence of 1 mM Gln or 1% Suc, tissue nitrate concentr ation was not significantly altered, but the induction of root NR acti vity was reduced or enhanced, respectively. The induction of root nitr ite reductase (NiR) activity was also reduced in the presence of Asn o r Gln and enhanced in the presence of Suc. Transcript levels of NR and NiR in roots were reduced in the presence of the amides and enhanced in the presence of Suc. When Suc was present in combination with eithe r amide, there was complete relief from the inhibition of NiR transcri ption observed in the presence of amide alone. In the case of NR, howe ver, this relief from inhibition was negligible. The inhibition of the induction of NR and NiR activities in the presence of Gln and Asn is a direct effect and is not the result of altered nitrate uptake in the presence of these metabolites.