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See Inside Scientific American Volume 306, Issue 4

Orchids Are as Finicky as the Fungi That Nourish Them

Fungi that orchids need to grow are just as finicky as the exotic flowers themselves



Larry West/Minden Pictures

In The Orchid Thief, writer Susan Orlean describes the cultlike devotion that these exotic-looking flowers inspire among plant collectors. One reason, in addition to their beauty, that orchids are so prized is that they are fragile: although they grow in every U.S. state and on every con­tinent except Antarctica, many are endangered, and the flowers are exceed­ingly sensitive to environ­mental changes. Native orchids’ dustlike seeds will grow only if nourished by certain groups of root fungi, known as mycorrhizal fungi.

Little is known about these organisms—so little that many have not been named. They grow into the roots of orchids, which digest the fungi to obtain needed nutrients. Recently a four-year study has shed new light on where my­corrhizal fungi grow and under what conditions they stimulate orchids to germinate. The results, published online Jan­uary 24 in Molecular Ecology, will help ecol­ogists preserve rare or­chid varieties.

The team of researchers, led by ecologist Melissa McCormick of the Smithsonian Environ­mental Research Center in Edgewater, Md., planted and tracked three U.S. orchid species—all present in the East and endangered somewhere in the country—in six study sites: three in younger forests, which were 50 to 70 years old, and three in older forests, which were 120 to 150 years old. Investigators covered each plot with leaf litter, decomposing wood or nothing and provided half the plots with the specific fungi known to promote growth in each orchid.

The researchers also identified the existing fungi in each forest. Because the organisms have no fruiting structures, they can be tough to detect, so the team pio­neered the use of testing for DNA in the soil to identify where and how much fungus was present. Older forests, McCormick and her colleagues found, had about five to 12 times more orchid-friendly fungi than younger forests, and the fungi in older forests were more diverse.

Each orchid had different requirements to grow. For Goodyera pubescens (a stalk of its small white flowers is pictured), only older forests held enough fungus for it to flourish. Adding the fun­gus to younger forests alone or in combination with decom­posing wood did not make Goodyera seeds germinate. The host fungus of Tipularia dis­color, which has many small mauve-purple flowers with yellowish centers, was wide­spread in young and old forests alike but could support germi­nation only on decom­posing wood. The host fungus for Liparis lilii­folia wasn’t common in the wild, but the orchid would germinate if the fungus was added.

Orchid conservation plans gen­erally do not account for orchid fungi abundance or require-ments, simply because the techniques and knowledge to identify the fungi haven’t been in place. Says McCormick, “We’re hoping others can apply these tech­niques to figure out what environmental conditions affect the fungi.”

This article was published in print as "Picky Eaters Club."

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