Comparative growth of ectomycorrhizal basidiomycetes (Hebeloma spp.) on organic and inorganic nitrogen

In the largely organic soils in which ectomycorrhizas are commonly found, a preference for absorbing organic nitrogen over mineral forms is likely to be an advantage, especially where mineralisation rates are low. To determin e rates of both independent and preferential growth of ectomycorrhizal basi diomycetes on organic and inorganic nitrogen, strains of Hebeloma were grow n on nutrient agar media containing either NH4+ or glutamic acid as the sol e source of nitrogen, on both single medium and split plate Petri dishes. G rowth rates on the split plate Petri dishes, where thr fungi had access to both nitrogen soul-ces, were generally greater than on the single medium di shes. Growth on glutamic acid was at least equal to, and usually greater th an, that on NH4+. In some cases growth on NH; alone appeared severely inhib ited, a condition th;lt was partially alleviated by access to glutamic acid on the split plates Petri dishes. This highlights a potential pitfall of s ingle nitrogen source growth studies. The greater growth of most strains on glutamic acid suggests an adaptation to organic nitrogen utilisation in th ese strains. If this is so in soils with low mineralisation rates, direct u ptake of amino acids by ectomycorrhizal plants could by-pass the bottle nec k that requires mineral nitrogen to be made available for plant uptake.