Mutualistic Mycorrhiza in Orchids: Evidence from Plant-Fungus Carbon and Nitrogen Transfers in the Green-Leaved Terrestrial Orchid Goodyera repens

$\bullet$ The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (N) to adult orchids are poorly understood. Here, we employed isotopically labelled sources of C and N to investigate these processes in the green forest orchid, Goodyera repens. $\bullet$ Fungus-to-orchid transfers of C and N were measured using mass spectrometry after supplying extraradical mycelial systems with double-labelled $\lbrack^{13}C-^{15}N\rbrack glycine$. Orchid-to-fungus C transfer was revealed and quantified by radioisotope imaging and liquid scintillation counting of extraradical mycelium following 14CO2 fixation by shoots. $\bullet$ Both 13C and 15N were assimilated by the fungus and transferred to the roots and shoots of the orchid. Contrary to previous reports, considerable quantities (2.6% over 72 h) of fixed C were shown to be allocated to the extraradical mycelium of the fungus. $\bullet$ This study demonstrates, for the first time, mutualism in orchid mycorrhiza, bidirectional transfer of C between a green orchid and its fungal symbiont, and a fungus-dependent pathway for organic N acquisition by an orchid.