Background and Aims: Partially mycoheterotrophic plants are enriched in 13C and 15N compared to autotrophic plants. Here, it is hypothesized that the type of mycorrhizal fungi found in orchid roots is responsible for variation in 15N enrichment of leaf tissue in partially mycoheterotrophic orchids.
Methods: The genus Epipactis was used as a case study and carbon and nitrogen isotope abundances of eight Epipactis species, fungal sporocarps of four Tuber species and autotrophic references were measured. Mycorrhizal fungi were identified using molecular methods. Stable isotope data of six additional Epipactis taxa and ectomycorrhizal and saprotrophic basidiomycetes were compiled from the literature.
Key Results: The 15N enrichment of Epipactis species varied between 3·2 ± 0·8 ‰ (E. gigantea; rhizoctonia-associated) and 24·6 ± 1·6 ‰ (E. neglecta; associated with ectomycorrhizal ascomycetes). Sporocarps of ectomycorrhizal ascomycetes (10·7 ± 2·2 ‰) were significantly more enriched in 15N than ectomycorrhizal (5·2 ± 4·0 ‰) and saprotrophic basidiomycetes (3·3 ± 2·1 ‰).
Conclusions: As hypothesized, it is suggested that the observed gradient in 15N enrichment of Epipactis species is strongly driven by 15N abundance of their mycorrhizal fungi; i.e. ɛ15N in Epipactis spp. associated with rhizoctonias < ɛ15N in Epipactis spp. with ectomycorrhizal basidiomycetes < ɛ15N in Epipactis spp. with ectomycorrhizal ascomycetes and basidiomycetes < ɛ15N in Epipactis spp. with ectomycorrhizal ascomycetes.