DIFFERENTIAL EFFECT OF AMMONIUM ON THE INDUCTION OF NITRATE AND NITRITE REDUCTASE ACTIVITIES IN ROOTS OF BARLEY (HORDEUM-VULGARE) SEEDLINGS

The effect of exogenous NH4+ on the induction of nitrate reductase act ivity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7 .1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3- in the presence of 0, 1 or 10 mM NH4+. Exogenous NH4+ partially inhib ited the induction of NRA when roots were exposed to 0.1 mM, but not t o or 10 mM NO3-. In contrast, the induction of NiRA was inhibited by N H4+ at all NO3- levels, Maximum inhibition of the enzyme activities oc curred at 1.0 mM NH4+. Pretreatment with NH4+ had no effect on the sub sequent induction of NRA in the absence of additional NH4+; whereas th e induction of NiRA in NH4+-pretreated roots was inhibited in the abse nce of NH4+. At 10 mM NO3- L-methionine sulfoximine stimulated the ind uction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irresp ective of NH4+ supply. During the postinduction phase, exogenous NH4decreased NRA in roots supplied with 0.1 mM but not with 1 mM NO3-; wh ereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NR A in roots by limiting the availability of NO3-. Conversely, it has a direct effect, independent of the availability of NO3-, on the inducti on of NiRA. The lack of an NH4+ effect on NiRA during the postinductio n phase is apparently due to a slower turnover rate of that enzyme.