Mycelial uptake, translocation and assimilation of nitrogen from N-15-labelled ammonium by Pinus sylvestris plants infected with 4 different ectomycorrhizal fungi

Abstract:
The uptake and assimilation of 15N-labelled ammonium was followed in Pinus sylvestris L. plants infected with four different ectomycorrhizal fungi, Rhizopogon roseolus Fr. Suillus bovinus (Fr.) O. Kuntze, Pisolithus tinctorius (Fr.) Fr. and Paxillus involutus (Mich. ex Pers.) Cohen & Couch. Plants were grown in flat perspex observation chambers or in Petri dishes containing non-sterile peat; in each case 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 was fed to the mycelium, and the shoot, root and mycelial tissues analysed for total and 15N-labelled free amino acid contents after a feeding period of 72 h. High proportions of 15N-labelled glutamate/glutamine, aspartate/asparagine, and alanine were found in the fungal mycelia of all species except Pax. involutus where labelled aspartate/asparagine was not found. Lower proportions of labelled serine, threonine, tyrosine, lysine, ornithine and arginine were also found in the mycelium. The degree of 15N enrichment declined throughout the transport pathway but between 5 and 50% of the amino acids were 15N-labelled in the plant shoots. In total, at least 2-3% of the nitrogen supplied was assimilated as labelled amino acid during the 3 day feeding period, the largest amounts of labelling occurred in glutamic acid/glutamine and aspartic acid/asparagine.