Mycorhizomes and ectomycorrhizal associations facilitate a mycoheterotrophic nutritional mode in the green orchid Cymbidium kanran.

Nutritional modes in mature orchids range from complete autotrophy to full mycoheterotrophy. Within the genus Cymbidium, interspecific variation in the degree of mycoheterotrophy has been documented, yet intraspecific variation remains poorly understood. Intriguingly, some green Cymbidium species often possess mycorhizomes (coralloid rhizomes), structures commonly found in fully mycoheterotrophic orchids, and morphologically similar to fully mycoheterotrophic seedlings (protocorms). This study examines Cymbidium kanran individuals with and without mycorhizomes to assess whether these specialized subterranean structures enhance fungal carbon acquisition through the use of stable isotope analyses (δ¹³C and δ¹⁵N) and high-throughput metabarcoding techniques. Cymbidium kanran with mycorhizomes exhibit significantly elevated δ¹³C and δ¹⁵N compared to autotrophic reference species and congeneric individuals lacking mycorhizomes. The mycorhizome-bearing plants are partially mycoheterotrophic, obtaining nearly half of their carbon from fungi. The degree of mycoheterotrophy is similar to that of other partially mycoheterotrophic Cymbidium species, such as C. lancifolium and C. goeringii. Fungal community profiling revealed that mycorhizome-bearing C. kanran individuals mostly associate with ectomycorrhizal fungi, such as Sebacinaceae, whereas individuals lacking mycorhizomes primarily recruit non-ectomycorrhizal rhizoctonia fungi. Notably, this morphological and nutritional plasticity parallels patterns observed in other orchid lineages, such as Cremastra and Oreorchis, which associate with wood-decay fungi. The findings strongly suggest that persistent protocorm-like mycorhizomes enhance fungal carbon uptake in partially mycoheterotrophic orchids associated not only with wood-decaying fungi but also with ectomycorrhizal fungi. These insights expand understanding of orchid nutritional strategies and highlight how intraspecific morphological plasticity may contribute to the evolution of mycoheterotrophy.