COMPARATIVE INDUCTION OF NITRATE AND NITRITE UPTAKE AND REDUCTION SYSTEMS BY AMBIENT NITRATE AND NITRITE IN INTACT ROOTS OF BARLEY (HORDEUM-VULGARE L) SEEDLINGS

The induction by ambient NO3- and NO2- of the NO3- and NO2- uptake and reduction systems in roots of 8-d-old intact barley (Hordeum vulgare L.) seedlings was studied. Seedlings were induced with concentrations of NaNO3 or NaNO2 ranging from 0.25 to 1000 muM. Uptake was determined by measuring the depletion of either NO3- or NO2- from uptake solutio ns. Enzyme activities were assayed in vitro using cell-free extracts. Uptake and reduction systems for both NO3- and NO2- were induced by ei ther ion. The K(m) values for NO3- and NO2- uptake induced by NO2- wer e similar to those for uptake induced by NO3-. Induction of both the u ptake and reduction systems was detected well before any NO3- or NO2- was found in the roots. At lower substrate concentrations of both NO3- and NO2- (5-10 muM), the durations of the lag periods preceding induc tion were similar. Induction of uptake, as a function of concentration , proceeded linearly and similarly for both ions up to about 10 muM. T hen, while induction by NO3- continued to increase more slowly, induct ion by NO2- sharply decreased between 10 and 1000 muM, apparently due to NO2- toxicity. In contrast, induction of NO3- reductase (NR) and NO 2- reductase (NiR) by NO2- did not decrease above 10 muM but rather co ntinued to increase up to a substrate concentration of 1000 muM. NO3- was a more effective inducer of NR than was NO2-; however, both ions e qually induced NiR. Cycloheximide inhibited the induction of both upta ke systems as well as NR and NiR activities whether induced by NO3- or NO2-. The results indicate that in situ NO3- and NO2- induce both upt ake and reduction systems, and the accumulation of the substrates per se is not obligatory.