Cédric Gonneau

Centre d'Ecologie Fonctionnelle et Evolutive, Montpelhièr, Languedoc-Roussillon, France

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Publications (4)26.07 Total impact

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    ABSTRACT: 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 δ13C and N content, while photosynthetic biomass has lower δ13C 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 δ13C 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 δ13C 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.This article is protected by copyright. All rights reserved.
    Journal of Ecology 05/2014; · 5.43 Impact Factor
  • Ecological Monographs 01/2013; 83(1):95-117. · 8.09 Impact Factor
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    ABSTRACT: Characterizing the architecture of bipartite networks is increasingly used as a framework to study biotic interactions within their ecological context and to assess the extent to which evolutionary constraint shape them. Orchid mycorrhizal symbioses are particularly interesting as they are viewed as more beneficial for plants than for fungi, a situation expected to result in an asymmetry of biological constraint. This study addressed the architecture and phylogenetic constraint in these associations in tropical context. We identified a bipartite network including 73 orchid species and 95 taxonomic units of mycorrhizal fungi across the natural habitats of Reunion Island. Unlike some recent evidence for nestedness in mycorrhizal symbioses, we found a highly modular architecture that largely reflected an ecological barrier between epiphytic and terrestrial subnetworks. By testing for phylogenetic signal, the overall signal was stronger for both partners in the epiphytic subnetwork. Moreover, in the subnetwork of epiphytic angraecoid orchids, the signal in orchid phylogeny was stronger than the signal in fungal phylogeny. Epiphytic associations are therefore more conservative and may co-evolve more than terrestrial ones. We suggest that such tighter phylogenetic specialization may have been driven by stressful life conditions in the epiphytic niches. In addition to paralleling recent insights into mycorrhizal networks, this study furthermore provides support for epiphytism as a major factor affecting ecological assemblage and evolutionary constraint in tropical mycorrhizal symbioses.
    Molecular Ecology 07/2012; 21(20):5098-5109. · 6.28 Impact Factor
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    Molecular Ecology 04/2012; · 6.28 Impact Factor