Symbiotic germination and development of myco-heterotrophic plants in nature: transfer of carbon from ectomycorrhizal Salix repens and Betula pendulato the orchid Corallorhiza trifida through shared hyphal connections

Seedlings of the myco-heterotrophic orchid Corallorhiza trifida which had b een germinated in the field in mesh bags developed hyphal links and mycorrh izas with Betula pendula and Salix repens, but not with Pinus sylvestris, w hen transplanted into soil microcosms. The fungus connecting the myco-heter otroph to Betula and Salix formed endomycorrhiza in the orchid with typical pelotons, but formed ectomycorrhizas with the autotrophs. The orchid plant s, when linked to Betula and Salix by fungal hyphae, gained 6-14% in weight over 25-28 wk. In microcosms supporting P. sylvestris, and in control micr ocosms which lacked autotrophs, the Corallorhiza plants lost 13% of their w eight over the same period. In the course of the 28-wk experimental period new Corallorhiza seedlings, in addition to those added as part of the exper iment, appeared in the microcosms containing Salix and Betula but not in th e Pinus microcosms. Shoots of Betula and Salter plants grown in association with Corallorhiza were fed with (CO2)-C-14, and the movement of the isotop e was subsequently traced by a combination of digital autoradiography and t issue oxidation. Direct transfer of C from both autotrophs to the myco-hete rotroph occurred in all cases where the associates had become connected by a shared fungal symbiont. Orchid seedlings lacking these hyphal connections , introduced to the microcosms as controls immediately before isotope feedi ng, failed to assimilate significant amounts of C. The results provide the first experimental confirmation that growth of Corallorhiza trifida can be sustained by supply of C received directly from an autotrophic partner thro ugh linked fungal mycelia.