The symbiosis between members of the Orchidaceae and mycorrhizal fungi differs from most other mycorrhizal associations in a number of important ways, including the effects of early fungal colonization, the fungi involved, the direction of carbon flow, the structures formed by the association, and the apparent lack of benefit to the fungal symbiont. There may also be considerable variation in the interactions between orchids and fungi within orchid mycorrhizal symbioses. Non-photosynthesizing orchids (eg. Corallorhiza spp.) are unusual among the Orchidaceae for their association with the mycorrhizal symbionts of ectomycorrhizal trees. Recent research into the mycorrhizal symbiosis of Cypripedium reginae in vitro suggested that the carbon contribution of a mycorrhizal fungus to the seedlings was minimal, although phosphorous uptake was enhanced by the symbiosis. There is also some evidence for fungal 'cheaters' in the symbiosis. Orchid mycorrhizas are therefore not only inherently interesting in their own right; they are important ‘exceptions’ or contrast tools to examine the structure, function, ecology and other features of mycorrhizal symbioses.

Key words: orchid mycorrhizas, Orchidaceae