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PLANT PHYSIOLOGY , Vol 102, Issue 3 811-819, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Comparative Induction of Nitrate and Nitrite Uptake and Reduction Systems by Ambient Nitrate and Nitrite in Intact Roots of Barley (Hordeum vulgare L.) Seedlings
M. Aslam, R. L. Travis and R. C. Huffaker
Department of Agronomy and Range Science, University of California, Davis, California 95616
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 [mu]M. 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
[mu]M), 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 [mu]M. Then, while induction by NO3-
continued to increase more slowly, induction by NO2- sharply decreased
between 10 and 1000 [mu]M, apparently due to NO2- toxicity. In contrast,
induction of NO3- reductase (NR) and NO2- reductase (NiR) by NO2- did not
decrease above 10 [mu]M but rather continued to increase up to a substrate
concentration of 1000 [mu]M. 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.
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