Differential NO₃⁻ dependent patterns of NO₃⁻ uptake in <i>Pinus pinaster, Rhizopogon roseolus</i> and their ectomycorrhizal association

Abstract

• 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 NO₃ ^- 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.