NITROGEN-METABOLISM OF EXTERNAL HYPHAE OF THE ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS INTRARADICES

The arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck and Smith was grown in symbiosis with Cucumis sativus L. cv. Aminex (F1 h ybrid) in mesh bags surrounded by a sand-filled hyphal compartment (HC ), allowing only the fungal hyphae to protrude into the HC. The hyphae in the HC were supplied with N-15-labelled NH4+ or NO3- after 60 d (e xpt 1). Following a 48 h labelling period, the sand was removed from t he HC and the hyphae extracted. In another experiment (expt 2), the hy phae were extracted from the sand before being incubated in vitro in a nutrient solution containing N-15-labelled NH4+ for 15 h. The hyphal material was incubated in a 0 or 2.5 mM solution of the GOGAT-inhibito r albizzine prior to labelling. In both experiments the hyphal content of free amino acids and fatty acids were measured as well as the amin o acid N-15 enrichment. Asparagine was the hyphal amino acid measured in highest concentration followed by glutamine, glutamate, aspartate a nd alanine. Ornithine, serine and glycine were detected in lower conce ntrations, but gamma-aminobutyric acid and citrulline were not detecta ble. This pattern was the same in both experiments and was unaffected by the type of N applied to the hyphae or by preincubation with albizz ine, although the amino-acid concentration decreased considerably in e xpt 2 compared with expt 1. Both NH4+-N and NO3--N were assimilated in to amino acids but the levels of N-15 enrichment following application of NO3- were much lower than those following application of NH4+ indi cating that the latter was more readily assimilated. Albizzine decreas ed the hyphal amino acid concentration by c. 30% (without affecting th e N-15 enrichment of the individual amino acids) indicating that the A M-fungal hyphae might possess a GS-GOGAT enzyme system for assimilatio n of inorganic N. The fatty-acid profiles (especially phospholipid fat ty acids 16:1 omega 5 and 20:5) obtained from the hyphae of G. intrara dices showed that contamination of the samples by fungi other than C. intraradices and bacteria was insignificant, and confirmed the usefuln ess of specific fatty-acid measurement to estimate soil AM-fungal cont ent.