Exchange processes in the mycorhizosphere are essential for the nutrition of Orchidaceae. Orchids exhibit different trophic strategies ranging from autotrophy to mycoheterotrophy, with the latter being recognized as a partial or entire carbon gain by exploiting diverse associating fungi. Particularly in their below-ground development stage as protocorms orchids depend on fungal nutrition.
We examined how a change in fungal community and subterranean morphology accompanies a nutrition mode alteration during the ontogenesis of the chlorophyllous, terrestrial orchid Cremastra appendiculata from Taiwan. Trophic strategies were revealed by comparing different development stages of C. appendiculata to surrounding autotrophic reference plants based on δ13C, δ15N, δ2H and δ18O stable isotope natural abundance and total N concentration analyses. Mycorrhizal fungi of C. appendiculata were determined using next generation DNA sequencing.
We identified saprotrophic non-rhizoctonia Psathyrellaceae as dominant fungi in protocorm and seedling coralloid rhizomes, while roots of seedlings and mature C. appendiculata were mainly colonizes with rhizoctonia fungi. Mature C. appendiculata did not differ in isotopic signature from autotrophic reference plants suggesting a fully autotrophic nutrition. Characteristic of mycoheterotrophic orchid specimens, C. appendiculata protocorms were enriched in 15N, 13C and 2H compared to reference plants. Seedlings showed a dispersive, intermediate isotope signature, underpinning their transitional nutrition mode and the differences in fungal community depending on their subterranean morphology.
Considering recent literature we suggest a high within-species variability in nutrition and fungal association depending on subterranean morphology and development stage of C. appendiculata. To conclude, C. appendiculata is a key species covering the entire spectrum on the continuous gradient from autotrophic to mycoheterotrophic nutrition.