Partial mycoheterotrophy in the leafless orchid Cymbidium macrorhizon

Premise of the Study: The evolution of full mycoheterotrophy is one of the most interesting topics within plant evolution. The leafless orchid Cymbidium macrorhizon is often assumed to be fully mycoheterotrophic even though it has a green stem and fruit capsule. Here, we assessed the trophic status of this species by analyzing the chlorophyll content and the natural ¹³C and ¹⁵N abundance in the sprouting and the fruiting season. Methods: The chlorophyll content was measured in five sprouting and five fruiting individuals of C. macrorhizon that were co-occurring. In addition, their ¹³C and ¹⁵N isotopic signatures were compared with those of neighboring autotrophic and partially mycoheterotrophic reference plants. Key Results: Fruiting individuals of C. macrorhizon were found to contain a remarkable amount of chlorophyll compared to their sprouting counterparts. In addition, the natural abundance of ¹³C in the tissues of the fruiting plants was slightly depleted relative to the sprouting ones. Linear two-source mixing model analysis revealed that fruiting C. macrorhizon plants obtained approximately 73.7 ± 2.0% of their total carbon from their mycorrhizal fungi when the sprouting individuals were used as the 100% carbon gain standard. Conclusions: Our results indicated that despite its leafless status, fruiting plants of C. macrorhizon were capable of fixing significant quantities of carbon. Considering the autotrophic carbon gain increases during the fruiting season, its photosynthetic ability may contribute to fruit and seed production. These results indicate that C. macrorhizon should, therefore, be considered a partially mycoheterotrophic species rather than fully mycoheterotrophic, at least during the fruiting stage.