Kinetics of nitrate and ammonium absorption and accompanying H+ fluxes in roots of Lolium perenne L. and N2-fixing Trifolium repens L.

Henning Høgh Jensen, B. Wollenweber, J.K. Schjørring

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Abstract

Kinetic parameters for NH4+ and NO3 uptake were measured in intact roots of Lolium perenne and actively N2-fixing Trifolium repens. Simultaneously, net H+ fluxes between the roots and the root medium were recorded, as were the net photosynthetic rate and transpiration of the leaves. A Michaelis–Menten-type high-affinity system operated in the concentration range up to about 500 mmol m−3 NO3 or NH4+. In L. perenne, the Vmax of this system was 9–11 and 13–14 μmol g−1 root FW h−1 for NO3 and NH4+, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g−1 root FW h−1. The Km for NH4+ uptake was much lower in L. perenne than in T. repens (c. 40 compared with 170 mmol m−3), while Km values for NO3 absorption were roughly similar (around 130 mmol m−3) in the two species. There were no indications of a significant efflux component in the net uptake of the two ions. The translocation rate to the shoots of nitrogen derived from absorbed NO3-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH4+. Trifolium repens had higher rates of transpiration and net photosynthesis than L. perenne. Measurements of net H+ fluxes between roots and nutrient solution showed that L. perenne absorbing NO3 had a net uptake of H+, while L. perenne with access to NH4+ and T. repens, with access to NO3 or NH4+, in all cases acidified the nutrient solution. Within the individual combinations of plant species and inorganic N form, the net H+ fluxes varied only a little with external N concentration and, hence, with the absorption rate of inorganic N. Based on assessment of the net H+ fluxes in T. repens, nitrogen absorption rate via N2 fixation was similar to that of inorganic N and was not down-regulated by exposure to inorganic N for 2 h. It is concluded that L. perenne will have a competitive advantage over T. repens with respect to inorganic N acquisition.

Original languageEnglish
JournalPlant, Cell and Environment
Volume20
Issue number9
Pages (from-to)1184-1192
ISSN0140-7791
DOIs
Publication statusPublished - 1997
Externally publishedYes

Keywords

  • Lolium perenne
  • Trifolium repens
  • Ammonium uptake
  • Kinetic parameters
  • N-2-fixation
  • Nitrate uptake
  • pH stat
  • Proton fluxes
  • Ryegrass
  • Translocation
  • White clover

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