The C-13 content of the orchid Epipactis palustris (L.) Crantz responds to light as in autotrophic plants

Most temperate green orchids form mycorrhizae with rhizoctonias fungi and are considered autotrophic. Some orchids, however, associate with fungi that also form ectomycorrhizae with surrounding trees and derive part of their carbon from these fungi. This evolutionarily derived condition, called mixotrophy, is characterized by natural C-13 enrichment and high N content. Some orchid genera display both syndromes depending on species. Here, we further document the isotopic features of a rhizoctonias-associated species, Epipactis palustris, in a genus otherwise encompassing many mixotrophic orchids. First, E. palustris in two wetland sites from Northern Europe displayed similar to slightly-depleted C-13 abundance and identical N content as compared with surrounding autotrophic plants, confirming published results on this species. Second, some individuals growing in darker conditions (63% reduced irradiation) revealed a decrease of C-13 abundance as expected (and observed at the same place) for autotrophic plants. The latter result expands previous observations on other rhizoctonias-associated orchid species. We discuss our results in relationship with the current views of the two nutritional syndromes of green orchids (especially, with the doubts on autotrophy of rhizoctonias-associated species) and in the framework of the phylogeny of Epipactis that evolved secondarily mixotrophy. The latter observation entails that the tribe Neottieae repeatedly evolved mixotrophy.