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

Trevor P. Garnett, Sergey N. Shabala, Philip J. Smethurst and Ian A. Newman

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.

Keywords: ammonium, eucalyptus, ion fluxes, microelectrode, nitrate, proton.

Functional Plant Biology 30(11) 1165 - 1176  doi:10.1071/FP03087