Comparative induction of nitrate and nitrite uptake and reduction systems by ambient nitrate and nitrite in intact roots of barley (Hordeum vulgare L.) seedlings

Abstract

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 μM. Uptake was determined by measuring the depletion of either NO3- or NO2- from uptake solutions. Enzyme activities were assayed in vitro using cell-free extracts. Uptake and reduction systems for both NO3- and NO2- were induced by either ion. The Km values for NO3- and NO2- uptake induced by NO2- were similar to those for uptake induced by NO3-. Induction of both the uptake 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 μM), the durations of the lag periods preceding induction were similar. Induction of uptake, as a function of concentration, proceeded linearly and similarly for both ions up to about 10 μM. Then, while induction by NO3- continued to increase more slowly, induction by NO2- sharply decreased between 10 and 1000 μM, apparently due to NO2- toxicity. In contrast, induction of NO3- reductase (NR) and NO2- reductase (NiR) by NO2- did not decrease above 10 μM but rather continued to increase up to a substrate concentration of 1000 μM. NO3- was a more effective inducer of NR than was NO2-; however, both ions equally induced NiR. Cycloheximide inhibited the induction of both uptake 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 uptake and reduction systems, and the accumulation of the substrates per se is not obligatory.