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Cortical Cell Fluxes of Ammonium and Nitrate in Excised Root Segments ofAllium cepa L; Studies using15N
Authors:MACKLON, A. E. S.   RON, M. M.   SIM, A.
Abstract:From compartmental analysis of 15N elution measurements, concentrationsand fluxes of NH+4 and NO3– were estimated for corticalcells in excised root segments, when bathed in a complete nutrientsolution, in which 20 mol m–3 NH4+ or NO3– werethe single N sources. The results were compared with those fornutrient solution containing 20 mol m–3 NH4NO3. No nitratereductase activity was detected in the roots but rapid assimilationof NH4+ occurred, due to glutamine synthetase activity. Theefflux curves for NH4+, on a 'µg 15N remaining againsttime' basis, deviated from the criteria determining conformityto first order kinetics, since the slowest rate constant wasan order of magnitude lower than that exhibited by the curvefor efflux versus time. The data were transformed to conformto the appropriate criteria, revealing a large slowly exchangingpool equated with assimilated NH4+. The presence of NO3–had little effect on NH4+ uptake and exchange, but NH4+ suppressedNOj uptake and reduced exchange across plasmalemma and tonoplast.It was established that NH4+ absorption was an active process.However, NH4+ entering and leaving the vacuole was overestimated,since the flux equation used did not differentiate between total15NH4 influx at the plasmalemma and that at the tonoplast, afterassimilation. The only active NO3– transfer was influxat the plasmalemma. The results were compared with those ofothers using13N and 36C1O3 analogues to measure NH4+ and NO3–fluxes in cereal roots. Key words: Ammonium, nitrate, compartmental analysis, 15N, active transport
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