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ASSIMILATION OF 15 NH4 + BY BEECH (FAGUS SYLVATICA L.) ECTOMYCORRHIZAS.

Ammonia assimilation has been followed in ectomycorrhizal roots of Fagus sylvatica. The absorption of ammonium ions was associated with a rapid synthesis of free amino acids in mycorrhizal tissues, glutamine being the most prominent. In the presence of [15 N]ammonium, glutamate, glutamine and alanine became the most strongly labelled metabolites of ectomycor-rhizas. Nitrogen-15 nuclear magnetic resonance spectroscopy demonstrated that the glutamine amide-N was the most highly enriched component of the extracts. Methionine sulphoximine and albizine, inhibitors of glutamine synthetase and glutamate synthase, almost completely blocked the incorporation of 15 N label into amino acids and induced an accumulation of NH4 + . These observations suggest that in the ammonia-fed beech ectomycorrhizas, ammonia assimilation occurs mainly via the glutamine synthetase/glutamate synthase pathway, and that glutamate dehydrogenase plays little, if any, part in this process. Alternative models for the nitrogen assimilation pathways in fungal and host tissues are presented.