Differential NO3- dependent patterns of NO3- uptake in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association

A different NO3- dependent pattern of NO3- uptake at low [NO3-] (0-0.1 mM) is shown in Pinus pinaster and in the ectomycorrhizal fungus Rhizopogon roseolus. In ectomycorrhizal symbiosis, the fungal pattern is pre-eminent. Net NO3- uptake rates were deduced in plant and fungus from solution depletion measurements. Net NO3- fluxes were estimated at the surface of mycorrhizal and nonmycorrhizal short roots, using NO3- selective microelectrodes. In NO3 - starved seedlings, maximum NO3- uptake rates were reached after 3 d of incubation in 0.05 mM NO3-. In R. roseolus mycelia, NO3- uptake rates did not change after withdrawing NO,- for up to 7 d, or after adding NO3- for 3 d. Net NO3- fluxes in nonmycorrhizal short roots were increased twofold by a 3-d exposure to NO3- whereas in ectomycorrhiza they were similar irrespective of the NO3- pretreatment, but always higher than the fluxes measured in nonmycorrhizal roots. Ectomycorrhiza have a greater capacity to use NO3- than nonmycorrhizal short roots, whatever the NO3- concentration in the solution. This may give mycorrhizal plants a greater ability to use fluctuating concentrations of NO3- in the soil solution.