Cephalanthera longifolia (Neottieae, Orchidaceae) is mixotrophic: a comparative study between green and nonphotosynthetic individuals

We investigated an Estonian population of the orchid Cephalanthera longifolia (L.) Fritsch. (Neottieae tribe), which harbours green and achlorophyllous individuals (= albinos), to understand albino survival and compare mycorrhizal associates, development, and nutrition of the two phenotypes. Albinos never changed phenotype over 14 years and had development similar to green individuals; their chlorophyll content was reduced by 99.4%, making them heterotrophic. Molecular typing by polymerase chain reaction amplification of fungal intergenic transcribed spacer and microscopic analyses showed that Thelephoraceae (Basidiomycetes, usually forming ectomycorrhizae with trees) were mycorrhizal on both phenotypes. Molecular typing also demonstrated that additional fungi were present on roots, including many endophytes (such as Helotiales) and various ectomycorrhizal taxa, whose role and pattern of colonization remained unclear. Mycorrhizal colonization was increased in albinos by about twofold, but no obvious difference in fungal partners compared with green individuals was demonstrated. Analysis of stable isotope composition (N and C) showed that albinos were dependent on their fungi for carbon (mycoheterotrophy), while green individuals recovered 33% of their carbon from fungi (mixotrophy). Surrounding trees, which formed ectomycorrhizae with at least one Thelephoraceae found in orchids, were likely the ultimate carbon source. These data are discussed in the framework of evolution of mycoheterotrophy in orchids, especially in Neottieae.