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Effect of Exogenous and Endogenous Nitrate Concentration on Nitrate Utilization by Dwarf Bean ¹

Research Institute ITAL, P. O. Box 48, 6700 AA Wageningen, The Netherlands, Department of Microbiology and Plant Physiology, University of Bergen, N-5000 Bergen, Norway

The effect of the exogenous and endogenous NO₃^– concentration on net uptake, influx, and efflux of NO₃^– and on nitrate reductase activity (NRA) in roots was studied in Phaseolus vulgaris L. cv. Witte Krombek. After exposure to NO₃^–, an apparent induction period of about 6 hours occurred regardless of the exogenous NO₃^– level. A double reciprocal plot of the net uptake rate of induced plants versus exogenous NO₃^– concentration yielded four distinct phases, each with simple Michaelis-Menten kinetics, and separated by sharp breaks at about 45, 80, and 480 micromoles per cubic decimeter.

Influx was estimated as the accumulation of ¹⁵N after 1 hour exposure to ¹⁵NO₃^–. The isotherms for influx and net uptake were similar and corresponded to those for alkali cations and Cl^–. Efflux of NO₃^– was a constant proportion of net uptake during initial NO₃^– supply and increased with exogenous NO₃^– concentration. No efflux occurred to a NO₃^–-free medium.

The net uptake rate was negatively correlated with the NO₃^– content of roots. Nitrate efflux, but not influx, was influenced by endogenous NO₃^–. Variations between experiments, e.g. in NO₃^– status, affected the values of K_m and V_max in the various concentration phases. The concentrations at which phase transitions occurred, however, were constant both for influx and net uptake. The findings corroborate the contention that separate sites are responsible for uptake and transitions between phases.

Beyond 100 micromoles per cubic decimeter, root NRA was not affected by exogenous NO₃^– indicating that NO₃^– uptake was not coupled to root NRA, at least not at high concentrations.

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