Plasticity and constraint in growth and protease production of ectomycorrhizal fungi in a nitrogen enrichment scenario.
EATON, G.K.* and M.P.AYRES
Dartmouth College, Hanover, NH 03755-3576 U.S.A. 1
Ectomycorrhizal fungi allow their host plants access to organic forms of nitrogen through enzymatic mineralization of the substrate and enhanced absorption of amino acids and mineral nitrogen. The cost to the plant for this benefit is carbohydrates supplied to support fungal growth and metabolism. Enrichment of soils with mineral nitrogen, as through atmospheric deposition, may affect the growth and function of these fungi by direct effects of increased nitrogen on fungi and indirect effects through reduced plant carbon allocation to roots. We tested the potential of nitrogen enrichment and altered carbohydrate supply to affect the growth and ectoprotease enzyme activity of 9 ectomycorrhizal fungi in sterile liquid media. Nitrogen treatments consisted of organic N only (pure protein Bovine Serum Albumin) vs. organic plus mineral N (the same protein with an additional 40% total N as NH4NO3). Carbon treatments consisted of 5 g vs. 1 g per liter glucose. Fungi differed widely in their growth and protease responses to these variables. Eight of 9 fungi had at least 20% reduced growth with reduced carbohydrates. Only 2 of 9 grew at least 20% more with increased nitrogen. Carbohydrates affected growth more in a purely organic nitrogen environment suggesting an energy limitation to mineralization. Reduction in carbohydrates depressed protease activity in most fungi, while increased nitrogen increased this activity. Carbohydrates had a stronger effect than nitrogen on fungal growth suggesting important indirect effects of N enrichment mediated through altered plant C allocation patterns. Fungi differed widely in the magnitude of their growth response to C supply. Protease activity was more strongly affected by nitrogen availability directly. These results indicate a strong potential for nitrogen enrichment to alter the composition and function of the ectomycorrhizal fungus community. Mechanisms producing these changes are likely to be interactions between physiological responses of fungi and those of host plants to a changing resource environment.
Keywords: mycorrhizae, nitrogen deposition, mineralization, ectoenzymes
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