Relative effectiveness of Tulasnella fungal strains in orchid mycorrhizal symbioses between germination and subsequent seedling growth

Orchids rely strictly on mycorrhizal symbioses for nutrition, including carbon, from seed germination to protocorm growth. However, whether orchids establish symbiotic compatibility selectively with specific mycorrhizal fungi during this early growth period remains to be determined for most species. We examined pair-wise symbiotic compatibility between three terrestrial orchid species, Bletilla striata, Pecteilis radiata, and Spiranthes australis, and five closely related strains of Tulasnella, using a procedure we recently developed for detailed and quantitative evaluation of mycorrhizal symbiosis during post-germination growth. Our analyses revealed significant differences among colonization effects of the Tulasnella strains on the hosts, both at seed germination and during post-germination growth. The effects of the mycorrhizal associations tended to differ by stage, suggesting a variety of determinants for symbiotic compatibility. One Tulasnella strain exhibited high compatibility across the three distantly related host species, implying that inherent properties of mycorrhizal fungi can influence their symbiotic compatibility with a wide range of orchid species.