Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes

Abstract

An investigation to determine the magnitude of the back reactions which occur during net ammonium uptake by roots and during net ammonium assimilation within roots was undertaken with maize (Zea mays L.). Ten-day-old seedlings, which had been grown on 250 mmol m~3 ammonium at pH 4 or 6, were pretreated for 3 h in the absence or presence of 500 mmol m3 MSX (methionine-DL-sulphoximine), an inhibitor of the glutamine synthetase-catalysed pathway of ammonium assimilation. They were then exposed for 2 h to 99 A% 15N-ammonium + MSX. Substantial ammonium cycling occurred during net ammonium uptake. Efflux was enhanced by MSX treatment, reflecting a 2- to 3-fold accumulation of ammonium in the root tissue. Influx of ammonium was also increased by treatment with MSX, indicating that influx was enhanced when products of ammonium assimilation were dissipated. The decline in root 14N-ammonium accounted for only a small fraction of the 14N-ammonium recovered in the ambient 15N-ammonium solution, revealing a substantial generation of endogenous 14N-ammonium during the 2 h exposure. The net quantity of ammonium generated was increased appreciably when assimilation of ammonium was restricted by MSX and it was estimated to occur at least 50% faster than net ammonium uptake. Presence of MSX severely decreased translocation of 15N to shoots but had a smaller influence on incorporation of 15N into macromolecules of the root tissue. The various ammonium flux rates were not greatly affected by growth at pH 4 0, implying a considerable resistance of ammonium assimilation processes in these maize roots to the high ambient acidity commonly induced by exposure to ammonium.