Stoichiometry of carbon, nitrogen and phosphorus is closely linked to trophic modes in orchids.

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.

A fine-scale spatial analysis of fungal communities on tropical tree bark unveils the epiphytic rhizosphere in orchids.

Approximately 10% of vascular plants are epiphytes and, even though this has long been ignored in past research, are able to interact with a variety of fungi, including mycorrhizal taxa. However, the structure of fungal communities on bark, as well as their relationship with epiphytic plants, is largely unknown. To fill this gap, we conducted environmental metabarcoding of the ITS-2 region to understand the spatial structure of fungal communities of the bark of tropical trees, with a focus on epiphytic orchid mycorrhizal fungi, and tested the influence of root proximity. For all guilds, including orchid mycorrhizal fungi, fungal communities were more similar when spatially close on bark (i.e. they displayed positive spatial autocorrelation). They also showed distance decay of similarity with respect to epiphytic roots, meaning that their composition on bark increasingly differed, compared to roots, with distance from roots. We first showed that all of the investigated fungal guilds exhibited spatial structure at very small scales. This spatial structure was influenced by the roots of epiphytic plants, suggesting the existence of an epiphytic rhizosphere. Finally, we showed that orchid mycorrhizal fungi were aggregated around them, possibly as a result of reciprocal influence between the mycorrhizal partners.