Resolving the physiological ecology of symbiotic germination of epiphytic and terrestrial orchids

In the early life-stages, both epiphytic and terrestrial orchid seeds obtain carbon and nitrogen from mycorrhizal fungi for germination and seedling development. Epiphytic orchids are exposed to comparatively high light levels from an early stage of development and often produce green seeds therefore their responses to light and requirements for mycorrhiza-supplied C is likely to be different from terrestrial orchids. The effect of exogenous C, light and fungal partner on germination and establishment were studied in vitro for three terrestrial orchid and one epiphyte orchid. I found that terrestrial orchids require their fungal partner for development and that light inhibits this process. In epiphytic species, light only inhibited germination and early development in the absence of the mycorrhizal symbiont. The findings in this study shows for the first time the potential ecological importance of mycorrhizal fungi in overcoming light inhibition of seed germination and growth in both terrestrial and epiphytic orchid. I also investigate the impact of light, C and N budgets of terrestrial and epiphytes (using 13C and 15N tracers) and the amount of C and N the plant obtains from fungus to the adult orchid plants. The biomass and transfer of C and N from fungus to plant was increased upon exposure to light but only after germination had been already begun. The most cost benefit of mycorrhizal symbioses was examined as other studies have ignored C flow from plant to fungus. All three species gave C back to the fungus but in different quantities. Encyclia phoenicea provided significantly more C to its mycorrhizal fungi; twice the amount of G. repens and 2.4 times greater than D. fuchsii by mass. This confirms the mutualistic nature of G. repens-mycorrhiza interactions and reveals that this same trophic strategy operates in two additional orchid species for the first time.