Symbiotic germination and development of myco‐heterotrophic plants in nature: transfer of carbon from ectomycorrhizal <i>Salix repens</i> and <i>Betula pendula</i> to the orchid <i>Corallorhiza trifida</i> through shared hyphal connections

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<jats:p>Seedlings of the myco‐heterotrophic orchid <jats:italic>Corallorhiza trifida</jats:italic> which had been germinated in the field in mesh bags developed hyphal links and mycorrhizas with <jats:italic>Betula pendula</jats:italic> and <jats:italic>Salix repens</jats:italic>, but not with <jats:italic>Pinus sylvestris</jats:italic>, when transplanted into soil microcosms. The fungus connecting the myco‐heterotroph to <jats:italic>Betula</jats:italic> and <jats:italic>Salix</jats:italic> formed endomycorrhiza in the orchid with typical pelotons, but formed ectomycorrhizas with the autotrophs. The orchid plants, when linked to <jats:italic>Betula</jats:italic> and <jats:italic>Salix</jats:italic> by fungal hyphae, gained 6–14% in weight over 25–28 wk. In microcosms supporting <jats:italic>P. sylvestris</jats:italic>, and in control microcosms which lacked autotrophs, the <jats:italic>Corallorhiza</jats:italic> plants lost 13% of their weight over the same period. In the course of the 28‐wk experimental period new <jats:italic>Corallorhiza</jats:italic> seedlings, in addition to those added as part of the experiment, appeared in the microcosms containing <jats:italic>Salix</jats:italic> and <jats:italic>Betula</jats:italic> but not in the <jats:italic>Pinus</jats:italic> microcosms. Shoots of <jats:italic>Betula</jats:italic> and <jats:italic>Salix</jats:italic> plants grown in association with <jats:italic>Corallorhiza</jats:italic> were fed with <jats:sup>14</jats:sup>CO<jats:sub>2</jats:sub>, and the movement of the isotope was subsequently traced by a combination of digital autoradiography and tissue oxidation. Direct transfer of C from both autotrophs to the myco‐heterotroph 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 feeding, failed to assimilate significant amounts of C. The results provide the first experimental confirmation that growth of <jats:italic>Corallorhiza trifida</jats:italic> can be sustained by supply of C received directly from an autotrophic partner through linked fungal mycelia.</jats:p>

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