Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes.

Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH and NO fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane & Maiden) Maiden. In addition, NH and H fluxes were measured simultaneously at a range of NH concentrations, and NO and H fluxes were measured simultaneously at a range of NO concentrations. Nitrogen concentrations ranged from 10-250 μM, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH and NO fluxes exhibited saturating Michaelis-Menten-style kinetics. The K was 16 μM for NH and 18 μM for NO. Values of V were 53 nmol m s for NH and 37 nmol m s for NO. Proton fluxes were highly correlated with NH and NO fluxes, but the relationships were different. Proton efflux increased with increasing NH concentration and mirrored the changing NH fluxes. The ratio between NH and H fluxes was 1 : -1.6. Proton influx was evident with initial exposure to NO, with the flux stoichiometry for NO : H being 1 : 1.4. Subsequent increases in NO concentration caused a gradual increase in H efflux such that the flux stoichiometry for NO : H became 1 : -0.8. The presence of 100 μM NH greatly reduced NO fluxes and caused a large and constant H efflux. These results are evidence that E. nitens has a preference for NH as a source of N, and that the fluxes of NH and NO are quantitatively linked to H flux.