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Flower Chemicals Attract Pollinating Insects, Repel Predators
REBECCA RAWLS
To humans, St. John's wort has a lovely all-yellow flower (left). But that's not the way it looks to pollinating insects, such as honeybees (right). Because these insects can see near-ultraviolet light, UV-absorbing compounds concentrated in the center of the flower help insects zero in on this pollen-containing region.
In Hypericum calycinum, a member of the St. John's wort family, these UV-absorbing pigments are dearomatized isoprenylated phloroglucinols (DIPs), according to chemical biologist Thomas Eisner, chemist Jerrold Meinwald, and their colleagues at Cornell University [Proc. Natl. Acad. Sci. USA, 98,13745 (2001)]. These are the same compounds that give the female flowers of the hop plant the distinctive bitter flavor that beer makers use to flavor their beverage.
In H. calycinum, however, the compounds also serve another function: They deter caterpillars from feasting on the seed-producing parts of the flowers. DIPs are particularly highly concentrated in the plant's ovary walls, where they make up as much as one-fifth of the tissue's dry weight, the researchers found. That led them to suspect that the compounds might have a protective role for this vital organ. They proved it by isolating the compounds and offering them to caterpillars. Most caterpillars refused to eat paper discs soaked in the compounds; the few who did were killed by them.
"With the same chemical," Eisner says, "the plant is saying to pollinating insects that it needs to attract, 'this bud's for you,' and to herbivores that pose a threat, 'bug off.'"
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