INWARD AND OUTWARD TRANSPORT OF AMMONIUM IN ROOTS OF MAIZE AND SORGHUM - CONTRASTING EFFECTS OF METHIONINE SULFOXIMINE

To examine the adaptive responses of ammonium efflux of nitrate-grown plants during first exposure to ammonium nutrition, isotopic flows of ammonium in and out of root tissue of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) were delineated. Appreciable efflux of N-14-ammonium occurred during the first 2 min of exposure, suggesting that ammonium efflux is characteristic of plants grown with nitrate as the sole source of nitrogen. Treatment with methionine sulphoximine (MSX) enhanced the initial N-14-ammonium efflux 5-fold in maize but less than 2-fold in sorghum. Increasing ambient acidity from pH 6 to pH 4 had relatively little effect on N-14-ammonium efflux in either species. Initial influx of exogenous N-15-ammonium was stimulated by MSX in maize (slightly less at pH 4 than at pH 6), but either was not affected (at pH 6) or was inhibited (at pH 4) in sorghum. Assuming that the rate of ammonium influx remained constant during the 2 h adaptation period, and that efflux of N-15-ammonium was negligible during the 0-2 min period, permitted estimates to be made of the unidirectional inward and outward flow of the two isotopic ammonium species. The decline in efflux of endogenous N-14-ammonium after the 2 min exposure was more than compensated for by an increase in efflux of the exogenously-supplied N-15-ammonium. As a consequence, total ammonium efflux increased significantly during the adaptation period. By this analysis about 60% of the total efflux in maize was as N-15-ammonium within the 2-5 min period whereas in sorghum it was 20% to 40%. The data indicate that appreciable ammonium efflux occurs in nitrate-grown seedlings in spite of low root ammonium concentrations, and that substantial futile cycling of ammonium occurs in both maize and sorghum during net transport of ammonium into their root tissue.