Abstract
Orchids are highly dependent on mycorrhizal fungi for seed germination and subsequent growth to a seedling as they provide essential carbon, water, and mineral nutrients to developing seeds. Although there is mounting evidence that orchid seeds are often colonized by multiple fungi simultaneously, most in vitro germination experiments focus on mycorrhizal monocultures and little is known about how mycorrhizal assemblages affect seed germination and growth of seedlings. In this study, we compared the effects of mycorrhizal monocultures and co-cultures on seed germination and seedling growth of the epiphytic orchid Dendrobium nobile. In situ baiting was used to isolate mycorrhizal fungi from protocorms for germination experiments. Germination experiments were conducted under two light regimes for 90 days. In total, five fungal strains were isolated from protocorms of D. nobile, indicating that the species was not highly specific to its fungal partners. Four strains (JC-01, JC-02, JC-04, and JC-05) belonged to the Serendipitaceae and one (JC-03) to the Tulasnellaceae. In vitro germination experiments showed that germination percentages were higher under light-dark conditions than under complete dark conditions, supporting previous findings that light facilitates germination in epiphytic orchids. While all strains were able to induce protocorm formation and growth into the seedling stage, large differences between fungal strains were observed. Co-cultures did not result in significantly higher seed germination percentages and seedling development than monocultures. Taken together, these results demonstrate that effects of fungal assemblages are not predictable from those of component species, and that more work is needed to better understand the role of fungal assemblages determining seed germination and subsequent growth under natural conditions.
Highlights
As most other plant species, orchids rely on multiple interactions with other organisms to complete their life cycle
Fungus CY (KM226996.1), which was obtained from the related D. devonianum, was located in the sister clade of JC-03 fungus clade, but with low support values (Figure 2B)
We investigated the effect of mycorrhizal assemblages on seed germination and seedling formation in the epiphytic orchid D. nobile
Summary
As most other plant species, orchids rely on multiple interactions with other organisms to complete their life cycle. Because most orchid seeds are very small (microspermy) and lack sufficient resources, orchid seeds critically depend on mycorrhizal fungi for essential carbon, water, and mineral nutrients (Meng et al, 2019b; Yeh et al, 2019). Before they appear aboveground, orchids are completely reliant on fungi, a condition called initial mycoheterotrophy (Merckx et al, 2013; Jacquemyn and Merckx, 2019). Orchids are completely reliant on fungi, a condition called initial mycoheterotrophy (Merckx et al, 2013; Jacquemyn and Merckx, 2019) Depending on their dependency on fungi, orchids can remain mycoheterotrophic throughout their entire life or become autotrophic. A substantial number of orchids most likely combines both modes of carbon supply and are partially mycoheterotrophic (Gebauer et al, 2016; Jacquemyn et al, 2017; Schiebold et al, 2017, 2018; Jacquemyn and Merckx, 2019)
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