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

Abstract

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 the13NO3− 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 mmNO2− (to generate plant roots induced for NO3− uptake). Exposure to 1 mmNH4+ 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.