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 NH 4 + concentrations, and NO3 - and H+ fluxes were measured simultaneously at a range of NO 3 - 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 Km, was 16 μM for NH4 + and 18 μM for NO3 -. Values of Vmax 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 NO 3 -: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 NH 4 + 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.