BACKGROUND: Mycorrhiza is a ubiquitous form of symbiosis based on the mutual, beneficial exchange of resources between roots of autotrophic (AT) plants and heterotrophic soil fungi throughout a complex network of fungal mycelium. Mycoheterotrophic (MH) and mixotrophic (MX) plants can parasitise this system, gaining all or some (respectively) required nutrients without known reciprocity to the fungus. We applied, for the first time, an ecological stoichiometry framework to test whether trophic mode of plants influences their elemental carbon (C), nitrogen (N), and phosphorus (P) composition and may provide clues about their biology and evolution within the framework of mycorrhizal network functioning. RESULTS: We analysed C:N:P stoichiometry of 24 temperate orchid species and P concentration of 135 species from 45 plant families sampled throughout temperate and intertropical zones representing the three trophic modes (AT, MX and MH). Welch's one-way ANOVA and PERMANOVA were used to compare mean nutrient values and their proportions among trophic modes, phylogeny, and climate zones. Nutrient concentration and stoichiometry significantly differentiate trophic modes in orchids. Mean foliar C:N:P stoichiometry showed a gradual increase of N and P concentration and a decrease of C: nutrients ratio along the trophic gradient AT < MX < MH, with surprisingly high P requirements of MH orchids. Although P concentration in orchids showed the trophy-dependent pattern regardless of climatic zone, P concentration was not a universal indicator of trophic modes, as shown by ericaceous MH and MX plants. CONCLUSION: The results imply that there are different evolutionary pathways of adaptation to mycoheterotrophic nutrient acquisition, and that the high nutrient requirements of MH orchids compared to MH plants from other families may represent a higher cost to the fungal partner and consequently lead to the high fungal specificity observed in MH orchids.

Department of Experimental Plant Biology Faculty of Science Charles University Viničná 5 Prague 12844 Czech Republic

Department of Microbiology Viçosa Federal University P H Rolfs Street Viçosa Minas Gerais CEP 36570 900 Brazil

Department of Mycorrhizal Symbioses Institute of Botany Czech Academy of Sciences Lesní 322 Průhonice Czech Republic

Evolution Biodiversité Institut de Systématique Muséum national d'Histoire naturelle CNRS Sorbonne Université EPHE 57 rue Cuvier Paris CP 39 F 75005 France

Faculty of Biology Department of Plant Taxonomy and Nature Conservation University of Gdańsk ul Wita Stwosza 59 Gdańsk 80 308 Poland

Faculty of Biology Department of Vertebrate Ecology and Zoology University of Gdańsk ul Wita Stwosza 59 Gdańsk 80 308 Poland

Faculty of Science University of South Bohemia Branišovská České Budějovice 1760 37005 Czech Republic

Institute of Microbiology ASCR Vídeňská Praha 1083 142 20 Czech Republic

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