Inhibition of nitrate uptake by ammonium in barley. Analysis of component fluxes

NO3- uptake by plant roots is rapidly inhibited by exposure to NH4+. The rapidity of the effect has led to the presumption that the inhibition results from the direct effects of NH4+ at the plasma membrane. The mechanism of this inhibition, however, has been in contention. In the present study we us...

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Bibliographic Details
Published in:Plant physiology (Bethesda) 1999-05, Vol.120 (1), p.283-291
Main Authors: Kronzucker, H.J, Glass, A.D.M, Siddiqi, M.Y
Format: Article
Language:English
Subjects:
Ammonium
ammonium compounds
application rate
Barley
efflux
Elution
Half lives
Hordeum vulgare
inhibition
isotope labeling
Kinetics
Nitrates
Nitrites
nitrogen
nitrogen metabolism
nutrient sources
nutrient transport
nutrient uptake
Plant roots
Plants
plasma membrane
Quaternary ammonium compounds
roots
Seedlings
stable isotopes
Whole-Plant Processes
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Summary:NO3- uptake by plant roots is rapidly inhibited by exposure to NH4+. The rapidity of the effect has led to the presumption that the inhibition results from the direct effects of NH4+ at the plasma membrane. The mechanism of this inhibition, however, has been in contention. In the present study we used the radiotracer 13N to determine the relative effects of short-term exposures to NH4+ on the 13NO3- influx, efflux, and partitioning of absorbed 13N in barley (Hordeum vulgare) roots. Plants were grown without NO3- or NO2- (uninduced for NO3- uptake), or with 0.1, 1.0, 10 mM NO3-, or 0.1 mM NO2- (to generate plant roots induced for NO3- uptake). Exposure to 1 mM NH4+ strongly reduced influx; the effect was most pronounced in plants induced for NO3- uptake when NO3- absorption was measured at low external NO3-. At higher [NO3-] and in uninduced plants the inhibitory effect was much diminished, indicating that NH4+ inhibition of influx was mediated via effects on the inducible high-affinity transport system rather than on the constitutive high-affinity transport system or the low-affinity transport system. Exposure to NH4+ also caused increased NO3- efflux; the largest effect was at low external [NO3-] in uninduced plants. In absolute terms, the reduction of influx made the dominant contribution to the observed reduction of net uptake of NO3-. Differences in response between plants induced with NO3- and those induced with NO2- indicate that NO2- may not be an appropriate analog for NO3- under all conditions.
ISSN:0032-0889
1532-2548
DOI:10.1104/pp.120.1.283