Mycoheterotrophic growth of Cephalanthera falcata (Orchidaceae) in tripartite symbioses with Thelephoraceae fungi and Quercus serrata (Fagaceae) in pot culture condition.

Mixotrophy, obtaining carbon by mycoheterotrophy and photosynthesis, has been suggested in Cephalanthera species (Orchidaceae) by analyses on stable isotopes of carbon. In this study, we examined the growth of Cephalanthera falcata in pot cultured tripartite symbioses with Thelephoraceae fungi and Quercus serrata. Mycorrhizal fungi were isolated from roots of C. falcata in natural habitats. Two fungal isolates identified as Thelephoraceae were cultured and inoculated to fine roots of non-mycorrhizal seedlings of Q. serrata (Fagaceae). After the ectomycorrhizal formation, non-mycorrhizal seedlings of C. falcata were co-planted. The pots with tripartite symbioses were cultured in greenhouse for 30 months, and growth of C. falcata seedlings was examined. Fresh weight of C. falcata seedlings was significantly increased by the tripartite symbioses even in those with no shoot, thus providing further evidence for the mycoheterotrophic nature of this orchid. The achievement of seedling culture in tripartite symbioses would be valuable for conserving many forest orchids and for conducting experiments to understand their physiology and ecology.