Cortical Cell Fluxes of Ammonium and Nitrate in Excised Root Segments ofAllium cepaL; Studies using15N

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Abstract
From compartmental analysis of 15N elution measurements, concentrations and fluxes of NH+4 and NO3− were estimated for cortical cells in excised root segments, when bathed in a complete nutrient solution, in which 20 mol m−3 NH4+ or NO3− were the single N sources. The results were compared with those for nutrient solution containing 20 mol m−3 NH4NO3. No nitrate reductase activity was detected in the roots but rapid assimilation of NH4+ occurred, due to glutamine synthetase activity. The efflux curves for NH4+, on a 'μg 15N remaining against time' basis, deviated from the criteria determining conformity to first order kinetics, since the slowest rate constant was an order of magnitude lower than that exhibited by the curve for efflux versus time. The data were transformed to conform to the appropriate criteria, revealing a large slowly exchanging pool equated with assimilated NH4+. The presence of NO3− had little effect on NH4+ uptake and exchange, but NH4+ suppressed NOj 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 total 15NH4 influx at the plasmalemma and that at the tonoplast, after assimilation. The only active NO3− transfer was influx at the plasmalemma. The results were compared with those of others using13N and 36C1O3 analogues to measure NH4+ and NO3− fluxes in cereal roots.