Are carbon and nitrogen exchange between fungi and the orchid Goodyera repens affected by irradiance?

BACKGROUND AND AIMS: The green orchid Goodyera repens has been shown to transfer carbon to its mycorrhizal partner, and this flux may therefore be affected by light availability. This study aimed to test whether the C and N exchange between plant and fungus is dependent on light availability, and in addition addressed the question of whether flowering and/or fruiting individuals of G. repens compensate for changes in leaf chlorophyll concentration with changes in C and N flows from fungus to plant. METHODS: The natural abundances of stable isotopes of plant C and N were used to infer changes in fluxes between orchid and fungus across natural gradients of irradiance at five sites. Mycorrhizal fungi in the roots of G. repens were identified by molecular analyses. Chlorophyll concentrations in the leaves of the orchid and of reference plants were measured directly in the field. KEY RESULTS: Leaf δ(13)C values of G. repens responded to changes in light availability in a similar manner to autotrophic reference plants, and different mycorrhizal fungal associations also did not affect the isotope abundance patterns of the orchid. Flowering/fruiting individuals had lower leaf total N and chlorophyll concentrations, which is most probably explained by N investments to form flowers, seeds and shoot. CONCLUSIONS: The results indicate that mycorrhizal physiology is relatively fixed in G. repens, and changes in the amount and direction of C flow between plant and fungus were not observed to depend on light availability. The orchid may instead react to low-light sites through increased clonal growth. The orchid does not compensate for low leaf total N and chlorophyll concentrations by using a (13)C- and (15)N-enriched fungal source.

C and N stable isotope signatures reveal constraints to nutritional modes in orchids from the Mediterranean and Macaronesia.

We compared the nutritional modes and habitats of orchids (e.g., autotrophic, partially or fully mycoheterotrophic) of the Mediterranean region and adjacent islands of Macaronesia. We hypothesized that ecological factors (e.g., relative light availability, surrounding vegetation) determine the nutritional modes of orchids and thus impose restrictions upon orchid distribution. Covering habitats from dark forests to open sites, orchid samples of 35 species from 14 genera were collected from 20 locations in the Mediterranean and Macaronesia to test for mycoheterotrophy. Mycorrhizal fungi were identified via molecular analyses, and stable isotope analyses were applied to test whether organic nutrients are gained from the fungal associates. Our results show that orchids with partial or full mycoheterotrophy among the investigated species are found exclusively in Neottieae thriving in light-limited forests. Neottioid orchids are missing in Macaronesia, possibly because mycoheterotrophy is constrained by the lack of suitable ectomycorrhizal fungi. Furthermore, most adult orchids of open habitats in the Mediterranean and Macaronesia show weak or no N gains from fungi and no C gain through mycoheterotrophy. Instead isotope signatures of some of these species indicate net plant-to-fungus C transfer.