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

Abstract

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.