Inhibition of Nitrate Uptake by Ammonium in Barley. Analysis of Component Fluxes1

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.