Effect of exogenous and endogenous nitrate concentration on nitrate utilization by dwarf bean.

The effect of the exogenous and endogenous NO(3) (-) concentration on net uptake, influx, and efflux of NO(3) (-) and on nitrate reductase activity (NRA) in roots was studied in Phaseolus vulgaris L. cv. Witte Krombek. After exposure to NO(3) (-), an apparent induction period of about 6 hours occurred regardless of the exogenous NO(3) (-) level. A double reciprocal plot of the net uptake rate of induced plants versus exogenous NO(3) (-) 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 (15)N after 1 hour exposure to (15)NO(3) (-). The isotherms for influx and net uptake were similar and corresponded to those for alkali cations and Cl(-). Efflux of NO(3) (-) was a constant proportion of net uptake during initial NO(3) (-) supply and increased with exogenous NO(3) (-) concentration. No efflux occurred to a NO(3) (-)-free medium.The net uptake rate was negatively correlated with the NO(3) (-) content of roots. Nitrate efflux, but not influx, was influenced by endogenous NO(3) (-). Variations between experiments, e.g. in NO(3) (-) 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 NO(3) (-) indicating that NO(3) (-) uptake was not coupled to root NRA, at least not at high concentrations.