BayCEER Workshop 2015

An orchid´s exquisite taste for truffles – partial mycoheterotrophy in the genus Epipactis

Introduction: The orchid genus Epipactis contains 49 species distributed throughout the Northern hemisphere occurring in habitats from closed forests to open wetlands. Roots of Epipactis species form orchid mycorrhiza either with rhizoctonias or ectomycorrhizal (ECM) fungi. Information about fungal partners and trophic strategies such as autotrophy, partial or full mycoheterotrophy remains scarce.

Material and Methods: We sampled leaves and roots of eight Epipactis species and accompanying autotrophic plant species as reference for site conditions at 11 locations in NE Bavaria and on the coastline of the Netherlands. Fruiting bodies of ECM fungi were collected at some sites consecutively. C and N stable isotope natural abundance analyses of dried leaf and sporocarp material were conducted using EA-IRMS coupling. Identity of mycorrhizal fungi present in the Epipactis roots were determined using ITS sequencing.

Results: Stable isotope abundances showed that all Epipactis species in this survey were significantly enriched in ¹³C and ¹⁵N towards autotrophic references. The Epipactis species lie on a gradient in ¹⁵N enrichment between 3 and 25 ‰. Analyses of fungal partners separated Epipactis species into three clusters: Species mostly associated with ECM ascomycetes, ones that form mycorrhiza with obligate ECM basidiomycetes and rhizoctonial E. gigantea and E. palustris.

Conclusions: All Epipactis species are partially mycoheterotrophic while the degree of isotopic enrichment is driven by type of fungal partner. The stable isotope profiles in ¹⁵N of sporocarps of ECM ascomycetes and ECM basidiomycetes corresponds to the ¹⁵N enrichment of the Epipactis species mycorrhizal with ECM ascomycetes and ECM basidiomycetes, respectively. This allows drawing conclusions to food-web interactions: ectomycorrhizal Epipactis species obviously obtain carbon simultaneously from photosynthesis and the mycorrhizal fungi while the complete nitrogen gain seems to be from the fungal source.