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

Abstract

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; th..