Uptake, translocation and assimilation of nitrogen from 15N-labeled ammonium and nitrate sources by intact ectomycorrhizal systems of Fagus sylvatica infected with Paxillus involutus

The uptake and assimilation of nitrogen from 15N-labelled ammonium and nitrate sources was followed in intact ectomycorrhizal systems containing Fagus sylvatica L. plants infected with the fungus Paxillus involutus (Mich. ex Pers.) Cohen & Couch. Plants were grown in flat perspex observation chambers containing non-sterile peat; the fungal mycelium growing from a host plant was allowed to cross a barrier and to colonize an area of peat from which roots had been excluded. Labelled ammonium chloride or sodium nitrate was fed to the mycelium, and the shoot, root and mycelial tissues analysed for total and 15N-labelled amino acid contents after a feeding period of 72 h. Both free and protein-incorporated amino acids were analysed. Labelled nitrogen was incorporated into a range of free amino acids, the principal sinks for assimilation being alanine, aspartate/asparagine and glutamate/glutamine. The spectrum of labelling in protein-incorporated amino acids was wider with significant incorporation into the above compounds but additional assimilation of nitrogen as glycine, valine, serine, leucine, isoleucine and arginine. In total 78% of the nitrogen assimilated was incorporated into proteinaceous material. Label was incorporated from both nitrogen sources but the levels of enrichment in most free and protein-bound amino acids were usually higher in systems supplied with ammonium than in those supplied with nitrate. The mean amount of nitrogen assimilated from nitrate was only 62% of that assimilated from ammonium