Stable isotope signatures of pelotons and rhizomorphs reveal nutritional plasticity in Cremastra appendiculata (Orchidaceae).

Many photosynthetic orchids combine photosynthesis with fungal carbon acquisition, yet the isotopic relationships between orchids and their fungal partners remain poorly resolved. Direct stable isotope analysis of pelotons and extraradical fungal tissues can refine fungal baselines and clarify nutritional dynamics in orchid mycorrhizas. We investigated Cremastra appendiculata, a photosynthetic orchid with contrasting mycorrhizal states, by comparing fungal communities between individuals with and without coralloid rhizomes and analyzing the δ13C and δ15N values of leaves, protocorms, coralloid rhizomes, pelotons, Psathyrellaceae rhizomorphs, dead wood, and autotrophic reference plants. Psathyrellaceae-associated individuals were strongly enriched in 13C relative to autotrophic reference plants, indicating substantial acquisition of fungal carbon, whereas rhizoctonia-associated individuals exhibited little 13C enrichment and appeared predominantly autotrophic. Leaf δ15N values exceeded those of the associated fungal tissues in both states, suggesting plant-mediated 15N enrichment during nitrogen assimilation and allocation to leaves. Coralloid rhizome pelotons did not differ detectably in δ13C or δ15N between digestion stages. Pelotons resembled extraradical Psathyrellaceae rhizomorphs in δ15N but had higher δ13C values, indicating tissue-specific carbon isotope baselines. Psathyrellaceae tissues were less 13C enriched than fungal tissues reported from fully mycoheterotrophic orchids associated with wood-decaying fungi. These results support contrasting nutritional states in C. appendiculata: a predominantly autotrophic, rhizoctonia-associated state and a more fungus-dependent, Psathyrellaceae-associated state. The fungal isotope pattern raises the possibility that bidirectional carbon transfer persists even in the latter state. Determining whether the orchid transfers carbon to its fungal partners requires controlled culture or tracer experiments.