Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Inhibition of Nitrate Uptake by Ammonium in Barley. Analysis of Component Fluxes1

Herbert J. Kronzucker*, Anthony D.M. Glass, and M. Yaeesh Siddiqi

Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7 (H.J.K.); and Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (A.D.M.G., M.Y.S.)

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 N 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.

1   This work was supported by a National Science and Engineering Research Council grant to A.D.M.G. and by a University of Western Ontario grant to H.J.K.
*   Corresponding author; e-mail kronzuck{at}julian.uwo.ca; fax 1-519-661-3935.

Plant Physiol. (1999) 120: 283-292
Copyright Clearance Center:   0032-0889/99/120//10
© 1999 American Society of Plant Physiologists




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