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 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 muM, 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 muM for NH4+ and 18 muM 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 muM 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.