Differential effects of mineral and organic N sources, and of ectomycorrhizal infection by Hebeloma cylindrosporum, on growth and N utilization in Pinus pinaster

Abstract

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH4+ or NO3-) or organic N (L-glutamate) and at 5 mol m-3. Plants Were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non-mycorrhizal plants grew better with NH4+ than with NO3- and grew very slowly when supplied with L-glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L-glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change 15NH4+ uptake rate by roots, suggesting that the growth decrease of the host-plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH4+ acquisition. In contrast, in NO3--grown plants, in addition to draining carbon for NO3- reduction the fungus competed with the root for NO3- uptake. With NH4+ or NO3- feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L-glutamate-grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.