Kinetics of nitrate and ammonium absorption and accompanying H+ fluxes in roots of Lolium perenne L. and N-2-fixing Trifolium repens L.

Kinetic parameters for NH4+ and NO3- uptake were measured in intact roots of Lolium perenne and actively N-2-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 mu mol g(-1) root FW h(-1) for NO3- and NH4+, respectively. The corresponding values in T. repens were 5-7 and 2 mu mol 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 K-m 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 than 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 absorption rate of inorganic N. Based on assessment of the net H+ fluxes in T. repens, nitrogen absorption rate via N-2 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.