Orchids rarely live alone. Below the ground they partner with mycorrhizal fungi. These are fungi that provide nutrients in exchange for sugars from a plant. These fungi can provide support for seedlings as they germinate, and even as adult plants the fungi increase the ability of orchids to extract nutrients from the soil. In return, the orchids return sugars to the fungi. But what happens when one of the partners in this arrangement goes missing?
Attempts to germinate new seeds ran into a problem. No one knew which mycorrhiza was helping the seeds germinate. Botanists took fungi associated with the adult plants, but they were unsuccessful. It seems that orchids vary their partners as they grow, so a different, unknown, fungus would be needed to germinate the seeds.
In fact, the orchids have been propagated in the lab to preserve them, but this is only a temporary measure. If they are to survive in the wild, they will need to re-form their relationships with their former mycorrhizal partners.
Michael Fay and colleagues have been investigating some orchids that have been planted in the wild to see if these partnerships re-emerge. They have found that some have been growing more vigorously than others. This difference might indicate that the supporting fungi live on at some sites, but how do you show it’s fungi that make the difference?
Fay and colleagues tested samples of the orchids looking for isotopes, variations of chemical elements, in the plants. For example, all plants pick up nitrogen, but not all nitrogen is equal. Some nitrogen has an extra neutron, making it fractionally heavier than the usual nitrogen. In some orchids with mycorrhizal partners, this heavier nitrogen accumulates in the tissues of the plant, in comparison with other plants around it. Fay’s team looked for carbon, nitrogen and hydrogen to see what the orchids were picking up.
What they found is that the orchids were more likely to have heavier forms of hydrogen and nitrogen – but not carbon. The results for hydrogen and nitrogen confirmed that the orchids were working with mycorrhizal fungi. However, the lack of heavy carbon was also important. This tell-tale similarity between orchids and other plants in the carbon result gave away what type of fungus it was.
The fungus was a rhizoctonia-type fungus – the same sort as the wild orchids were seen using. It seems that the introduced orchids are effectively integrating back into the ecosystem, and acting like wild orchids. However, because this is happening at all the locations, it means that success isn’t simply about whether a site has the right fungus or not.
As for the future, that’s still uncertain. The transplanted plants were adult plants and orchids can switch partner fungi as they develop. Fay and colleagues note: “[T]he existence of fungi capable of establishing mycorrhizal associations with adult plants of C. calceolus does not necessarily indicate that the same fungus would be capable of inducing germination and supporting early seedling development” So while the orchid has returned, more research is needed to see if this will become a permanent feature in the landscape.