Symbiotic germination and development of myco-heterotrophic plants in nature: ontogeny of Corallorhiza trifida and characterization of its mycorrhizal fungi

The processes of symbiotic germination and seedling development were analys ed in the myco-heterotrophic orchid Corallorhiza trifida, seeds of which we re buried in 'packets' either adjacent to or at varying distances from adul t plants in defined communities of ectomycorrhizal tree species. Germinatio n occurred within eight months of burial under Betula-Alnus and within seve n months under Salix repens. It was always associated with penetration of t he suspensor by a clamp-forming mycorrhizal fungus. Four distinct developme ntal stages were defined and the rates of transition through these stages w ere plotted. There was no evidence of a relationship between extent of germ ination or rate of development and the presence of naturally distributed pl ants of C. trifida at the spatial scale of 1 m. The best germination and th e most rapid rate of development of C. trifida seedlings occurred in a Sali x repens community located at a considerable distance from any extant C. tr ifida population. Determination of internal transcribed spacer (ITS) RFLPs and of gene sequences of the fungi involved in symbiotic germination and gr owth of C. trifida, revealed them to belong exclusively to the Thelephora-T omentella complex of the Thelephoraceae. These fungi are known also to be e ctomycorrhizal associates of trees. It is hypothesized that the rate of gro wth of the C. trifida seedlings is determined by the ability of the fungal symbionts to transfer carbon from their ectomycorrhizal co-associates.