STRUCTURE AND FUNCTION OF THE ECTOMYCORRHIZAL ASSOCIATION BETWEEN PAXILLUS-INVOLUTUS (BATSCH) FR AND BETULA-PENDULA ROTH .1. DYNAMICS OF MYCORRHIZA FORMATION

Detailed examination of the structural and functional integration taki ng place during mycorrhiza formation necessitates rapid and aseptic sy nthesis of uniform mycorrhizal structures at well-defined stages of de velopment. A system is described for formation of ectomycorrhizas betw een Betula pendula Roth. and Paxillus involutus (Batsch) Fr. Small ste rile seedlings were placed directly on fungal colonies growing on cell ophane covered agar plates. Microscopic examination during the course of development indicated an initial rapid colonization of the roots an d formation of a fully developed mantle within 2-4 d. Hartig net forma tion was evident after 8 d of contact and involved a transition from a paraepidermal organization to a mature periepidermal Hartig net after 15 d of contact. Using a standard fungal growth medium, formation and development of mycorrhizas were not greatly affected by changes in th e overall concentrations of nitrogen and phosphorus but changes in the ratio of nitrogen to phosphorus had strong effects on development and mycorrhiza formation was completely suppressed when the ratio of nitr ogen to phosphorus was increased. These differences were not related t o the effects of nitrogen or phosphorus on linear growth of the fungus .