Recent developments in the study of orchid mycorrhiza

Orchids are mycoheterotrophic during their seedling stage and in many species the dependency on fungi as a carbohydrate source is prolonged into adulthood. The mycobionts in orchid mycorrhiza belong in at least 5 major taxonomic groups of basidiomycetes. Traditional records have mainly focused on saprotrophic mycobionts but the participation of both ectomycorrhizal and parasitic fungi in orchid mycorrhiza has been corroborated. There is an increasing evidence of specific relationships between orchids and fungi, though usually not on a species-to-species level. Physiological compatibility demonstrated under artificial conditions, as in vitro, may be much broader, however. Recent development of field sowing techniques has improved the possibilities of evaluating orchid-fungal relations in an ecological context. Although the general nutrient flow in orchid mycorrhiza is well known, some questions remain regarding breakdown processes of fungi within orchid tissues, especially the ptyophagic syndrome that has recently been illustrated at the ultrastructural level for the first time.