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© 1989 Oxford University Press

RESEARCH-ARTICLE

Reciprocal Ammonium Transport Into and Out of Plant Roots: Modifications by Plant Nitrogen Status and Elevated Root Ammonium Concentration1

M. A MORGAN1,1 and W A. JACKSON2,1

1Department of Agricultural Chemistry and Soil Science, University College Dublin Belfield, Dublin 4, Ireland
2Department of Soil Science, North Carolina State University Raleigh, NC, 27695-7619, U S A

1To whom correspondence should be addressed.

Wheat and oat were grown for 20 d on a nitrate-containing solution (nitrogen-replete plants) or for the last 6 d of this period on a nitrate-free solution (nitrogen-depleted plants). Exposure of the nitrogen-depleted plants on day 20 to nitrate-free solutions containing 500 mmol m–3 ammonium (96 A% 15N) resulted in a cumulative net influx of 15N-ammonium over an 8 h period that was appreciably greater than that of the nitrogen-replete plants. Both the initial rate and the more restricted rate after the first hour were enhanced by nitrogen deprivation. In the nitrogen-replete plants, cumulative net efflux of endogenous 14N-ammonium was approximately equivalent to net ammonium uptake during the first hour, and was essentially complete after 1–2 h. Pretreating nitrogen-depleted plants for 5 h in 500 mmol m–3 15N-ammonium (99 A% 15N) resulted in root ammonium concentrations of 12.7±1.1 and 16.0±0.4 µmol for wheat and oat, respectively. Subsequent net efflux of 15N-ammonium to 500 mmol m–3 exogenous 14N-ammonium exceeded these initial amounts within 2 h. Increasing ambient 14N-ammonium to 5000 mmol m–3 increased net 15N-ammonium efflux such that net loss of the maximal original amount in the root tissue was exceeded within 0.75 h. The data for both species indicate substantial reciprocal transfers of ammonium into and out of roots of ammonium-treated plants and a significant degradation of recently synthesized products of ammonium assimilation concurrent with ammonium assimilation.

Key words: Accumulation, ammonium, efflux, oat, root, uptake, wheat


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