Résumé
1. Some forest understorey plants recover carbon (C) not only from their own photosynthesis, but also from mycorrhizal fungi colonizing their roots. How these mixotrophic plants use the resources obtained from mycorrhizal and photosynthetic sources remains unknown. 2. We investigated C sources and allocation in mixotrophic perennial orchids from the genus Epipactis. Based on the assumption that fungal biomass has high delta C-13 and N content, while photosynthetic biomass has lower delta C-13 and N content, we indirectly estimated the respective contributions of these two resources to various organs, at various times over the growth season. Fully heterotrophic and fully autotrophic plants from the same sites were used as references for delta C-13 and N content of biomass purely issuing from fungi and photosynthesis, respectively. 3. In four investigated populations, the biomass shifted from fully heterotrophic in young spring shoots to 80-100% autotrophic in leaves and fruits at fruiting time, suggesting that photosynthesis supported mostly fruiting costs. In addition, fungal colonization decreased in roots over this period. 4. Based on delta C-13 and N content, below-ground organs and young spring shoots from green (mixotrophic) individuals and spontaneous achlorophyllous variants (fully heterotrophic) displayed similar fungal C contributions. Similar fungal contributions were also found in shoots of individuals that were either sprouting (and thus partially photosynthetic) or dormant (and thus fully heterotrophic) in the previous years. Therefore, fungal C supported mostly young spring shoots and below-ground organs. 5. Although experimentally shaded plants had decreased contributions of photosynthetic C in shoots, experimentally defoliated plants showed no increase in fungal C contribution as compared with non-defoliated controls. Strikingly, these defoliated plants maintained the same seed production: they likely compensated defoliation by increasing stem and fruit photosynthesis. 6. Synthesis. We propose a falsifiable model of C resource allocation in mixotrophic orchids, where mycorrhizal fungi mostly support below-ground organs and survival, while photosynthesis mostly supports above-ground sexual reproduction, but not below-ground reserves. We discuss how this allocation pattern, where seed production depends on photosynthesis, complicates the evolutionary route to full heterotrophy in mixotrophic orchids.