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; Ian A Newman

doi:10.1071/FP03087

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

Funct Plant Biol. 2003 Jan;30(11):1165-1176. doi: 10.1071/FP03087.

Authors

Trevor P Garnett¹, Sergey N Shabala², Philip J Smethurst³, Ian A Newman⁴

Affiliations

¹ Cooperative Research Centre for Sustainable Production Forestry, Private Bag 12, Hobart, Tas. 7001, Australia. School of Plant Science, Private Bag 54, Hobart, Tas. 7001, Australia. Current address: South Australian Research and Development Institute, GPO Box 397, Adelaide, SA 5001, Australia. Corresponding author; email: garnett.trevor@saugov.sa.gov.au.
² School of Agricultural Science, University of Tasmania, Private Bag 37, Hobart, Tas. 7001, Australia.
³ Cooperative Research Centre for Sustainable Production Forestry, and Private Bag 12, Hobart, Tas. 7001, Australia. CSIRO Forestry and Forest Products, Private Bag 12, Hobart, Tas. 7001, Australia.
⁴ School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, Tas. 7001, Australia.

PMID: 32689098
DOI: 10.1071/FP03087

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.