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

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 V_max 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 K_m 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.