Partial mycoheterotrophy in orchids

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In the mycorrhizal symbiosis, plants exchange carbohydrates from photosynthesis for mineral nutrients acquired by fungi from the soil. This mutualistic arrangement has been subverted by a few hundreds of mycorrhizal plant species that lack the ability to photosynthesize. The most numerous examples of this mycoheterotrophic nutrition are found among the orchid family. Although chlorophyll free orchid species are known since long to be mycoheterotrophic, green orchids have been thought until recently to be fully autotrophic. Recent data of C and N stable isotope abundances together with molecular ecological identification of fungi in orchid roots, however, provide evidence that at least some of the green orchids (i) gain C and N partially at the cost of fungi and (ii) switch the fungal partner towards mycorrhizas with ectomycorrhizal fungi of forest trees. This project aims to deepen our knowledge (i) on the distribution of this partially mycoheterotrophic nutrition among terrestrial orchids by including a broader spectrum of orchid habitats and taxonomic groups and (ii) on the environmental conditions (light climate, nitrogen availability as limiting factors) under which this hitherto unknown plant nutritional mode occurs.