Photosynthetic Mediterranean meadow orchids feature partial mycoheterotrophy and specific mycorrhizal associations

Premise of the study: We investigated whether four widespread, photosynthetic Mediterranean meadow orchids (Ophrys fuciflora, Anacamptis laxiflora, Orchis purpurea, and Serapias vomeracea) had either nutritional dependency on mycobionts or mycorrhizal fungal specificity. Nonphotosynthetic orchids generally engage in highly specific interactions with fungal symbionts that provide them with organic carbon. By contrast, fully photosynthetic orchids in sunny, meadow habitats have been considered to lack mycorrhizal specificity. • Methods: We performed both culture-dependent and culture-independent ITS sequence analysis to identify fungi from orchid roots. By analyzing stable isotope (13C and 15N) natural abundances, we also determined the degree of autotrophy and mycoheterotrophy in the four orchid species. • Key results: Phylogenetic and multivariate comparisons indicated that Or. purpurea and Oph. fuciflora featured lower fungal diversity and more specific mycobiont spectra than A. laxiflora and S. vomeracea. All orchid species were significantly enriched in 15N compared with neighboring non-orchid plants. Orchis purpurea had the most pronounced N gain from fungi and differed from the other orchids in also obtaining C from fungi. • Conclusions: These results indicated that even in sunny Mediterranean meadows, orchids may be mycoheterotrophic, with correlated mycorrhizal fungal specificity.