Unlocking germination: the role of mycorrhizal strain and seed provenance in driving seed germination of a widespread terrestrial orchid.

Orchids represent an important component of biodiversity in many ecosystems worldwide, notwithstanding their seed germination and distribution may to a large extent be determined and influenced by mycorrhizal fungi. While it is commonly assumed that widespread orchids are mycorrhizal generalists, the degree to which mycorrhizal diversity supports seed germination remains relatively underexplored. In this study, we investigated the role of a variety of Ceratobasidium fungi in supporting seed germination of the widespread terrestrial orchid Gymnadenia conopsea across China. Twelve Ceratobasidium strains isolated from G. conopsea and other orchids were examined for their ability to support germination of G. conopsea seeds collected from twelve sites across China. Of the twelve tested strains, six were able to support seed germination, while the remaining six strains showed no activity. Compatible strains showed a broad phylogenetic breadth, indicating the G. conopsea is capable of initiating associations with a diverse array of Ceratobasidium fungi. However, the six compatible strains differed in their ability to support protocorm formation. Moreover, germination success of seeds collected from different sites differed among Ceratobasidium strains. Seeds from northern China had a significantly higher number of compatible strains (average 5.6) than seeds from southwestern China (average 3.5). Our results suggest that G. conopsea is not only a mycorrhizal generalist in the adult stage but also in the seed germination stage, at least towards Ceratobasidium fungi. However, the significant strain-provenance interactions indicate regional differences in orchid-fungus interactions. These findings are important for improving local population restoration programs and germplasm conservation of this widespread and endangered orchid species.