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

Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH₄⁺ and NO₃^- fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane & Maiden) Maiden. In addition, NH₄⁺ and H⁺ fluxes were measured simultaneously at a range of NH₄⁺ concentrations, and NO₃^- and H⁺ fluxes were measured simultaneously at a range of NO₃^- concentrations. Nitrogen concentrations ranged from 10-250 μM, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH₄⁺ and NO₃^- fluxes exhibited saturating Michaelis-Menten-style kinetics. The K_m was 16 μM for NH₄⁺ and 18 μM for NO₃^-. Values of V_max were 53 nmol m^-2 s^-1 for NH₄⁺ and 37 nmol m^-2 s^-1 for NO₃^-. Proton fluxes were highly correlated with NH₄⁺ and NO₃^- fluxes, but the relationships were different. Proton efflux increased with increasing NH₄⁺ concentration and mirrored the changing NH₄⁺ fluxes. The ratio between NH₄⁺ and H⁺ fluxes was 1 : -1.6. Proton influx was evident with initial exposure to NO₃^-, with the flux stoichiometry for NO₃^- : H⁺ being 1 : 1.4. Subsequent increases in NO₃^- concentration caused a gradual increase in H⁺ efflux such that the flux stoichiometry for NO₃^- : H⁺ became 1 : -0.8. The presence of 100 μM NH₄⁺ greatly reduced NO₃^- fluxes and caused a large and constant H⁺ efflux. These results are evidence that E. nitens has a preference for NH₄⁺ as a source of N, and that the fluxes of NH₄⁺ and NO₃^- are quantitatively linked to H⁺ flux.