Impact of Fungal Symbionts of Co-occurring Orchids on the Seed Germination of Serapias orientalis and Spiranthes spiralis.

Interactions with mycorrhizal fungi are increasingly recognized as crucial ecological factors influencing orchids' distribution and local abundance. While some orchid species interact with multiple fungal partners, others show selectivity in their mycorrhizal associations. Additionally, orchids that share the same habitat often form relationships with different fungal partners, possibly to reduce competition and ensure stable coexistence. However, the direct impact of variations in mycorrhizal partners on seed germination remains largely unknown. We examined how fungal associates' specific identity and origin affect seed germination in Spiranthes spiralis and Serapias orientalis through in situ symbiotic germination experiments. A total of four fungal isolates, Tulasnellaceae and Ceratobasidiaceae were successfully isolated and cultured from S. spiralis and S. orientalis and two additional orchid species  found in the same habitat: Neotinea tridentata and Orchis provincialis. While all fungal strains facilitated the swelling of seed embryos, only the fungal associate, a member of the Ceratobasidiaceae family isolated from N. tridentata, (NT2) was capable of inducing protocorm formation and subsequent seedling growth of S. spiralis seeds. Another fungal associate belonging to the Tulasnellaceae family and isolated from O. provincialis (OP3) supported seed germination up to the seedling stage of S. orientalis seeds. However, the remaining two fungal strains did not support seed germination. We conclude that fungal associates of co-occurring orchids can promote seed germination and seedling growth in S. spiralis and S. orientalis.