Nitrate uptake rates were determined for nonmycorrhizal and mycorrhizal maritime pine (Pinus pinaster) associated with the ectomycorrhizal basidiomycete Hebeloma cylindrosporum and for this fungal species grown in vitro. Different [NO3−] (0–1.3 and 10 mM) were supplied to nonmycorrhizal or mycorrhizal P. pinaster plants grown axenically in test tubes for 21 weeks. Uptake of NO3− by the plants was determined by measuring NO3− depletion from the medium that was added to the culture tubes without root pertubation. For all applied [NO3−] the uptake rates for NO3− in mycorrhizal plants were increased about twofold compared with nonmycorrhizal plants. The same range of [NO3−] was supplied to the fungus in pure culture, and uptake rates were calculated either from the measurement of NO3− depletion from the medium or by 15N accumulation in the mycelium. For the same external nitrate concentrations the results obtained with these two methods were not significantly different. The amount of fungal biomass in mycorrhizal root systems was determined with the chitin assay. The contribution of each partner to nitrate uptake in the mycorrhizal plant was calculated from the kinetic parameters of nitrate uptake by the fungus and host plant alone given by the Lineweaver–Burk plots. For external [NO3−] higher than 0.1 mM the measured uptake rates for NO3− in mycorrhizal plants were greater than the calculated sum of the individual symbiotic partners. The significance of this excess nitrate uptake is discussed in relation to the role of ectomycorrhizae in nitrogen accumulation.
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Plassard et al. (1994) studied this question.