
%0 Journal Article
%A Garnett, Trevor P. 
%A Shabala, Sergey N. 
%A Smethurst, Philip J. 
%A Newman, Ian A. 
%T Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes
%J Functional Plant Biology
%D 2003/11/25
%V 30
%N 11
%P 1165-1176
%K ammonium, eucalyptus, ion fluxes, microelectrode, nitrate, proton.
%R https://doi.org/10.1071/FP03087
%U https://www.publish.csiro.au/paper/FP03087 
%X Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH4+ and NO3&#8211; fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane &#38; Maiden) Maiden. In addition, NH4+ and H+ fluxes were measured simultaneously at a range of NH4+ concentrations, and NO3&#8211; and H+ fluxes were measured simultaneously at a range of NO3&#8211; concentrations. Nitrogen concentrations ranged from 10&#8211;250 &#956;M, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH4+ and NO3&#8211; fluxes exhibited saturating Michaelis&#8211;Menten-style kinetics. The Km was 16 &#956;M for NH4+ and 18 &#956;M for NO3&#8211;. Values of Vmax were 53 nmol m&#8211;2 s&#8211;1 for NH4+ and 37 nmol m&#8211;2 s&#8211;1 for NO3&#8211;. Proton fluxes were highly correlated with NH4+ and NO3&#8211; 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 : &#8211;1.6. Proton influx was evident with initial exposure to NO3&#8211;, with the flux stoichiometry for NO3&#8211; : H+ being 1 : 1.4. Subsequent increases in NO3&#8211; concentration caused a gradual increase in H+ efflux such that the flux stoichiometry for NO3&#8211; : H+ became 1 : &#8211;0.8. The presence of 100 &#956;M NH4+ greatly reduced NO3&#8211; 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&#8211; are quantitatively linked to H+ flux. 
