Making scents of flowers: it's time for science to close its eyes and sniff - Brief Article

Science News, July 27, 2002 by Susan Milius

Take a deep, sweet lungful of jasmine fragrance--and admire its courage. Its swaggering machismo, even. For a strongly scented flower takes terrible risks, says scent biologist Robert Raguso of the University of South Carolina. The same perfume that draws invited guests to the flower can also tip off pests, thieves, and killers. "It's like a peacock's tail," he says. Raguso has had to turn to the science of visual signals for a classic example of inconvenient exuberance. There's barely been a science of biological displays of scent, he laments. In textbooks or seminal papers, "odor is either ignored or treated as insolubly complex," says Raguso.

"This is a human bias," he argues. We're such visual animals that, of course, the rainbow of floral colors and flowers' myriad shapes fixed scientists' attention first. Also, the techniques for studying scents haven't allowed much precision--until recently.

Now, Raguso says, advances in collecting and identifying volatile compounds are opening a new world. At last, evolutionary biologists can examine the same issues--such as attraction powers, evolutionary history, and exposure to predators--for olfactory displays as for visual ones. Harnessing modern genetics to the new scent chemistry is revealing the genes and enzymes behind the scenes. There's even talk about boosting scents in crops and commercial flowers for bigger yields, more sniff appeal, and perhaps swaggering courage, too.

SCENT MATTERS Nature uses plenty of perfume. By the early 1990s, scent biologist Jette Knudsen of Goteborg University in Sweden had tallied some 700 volatile compounds wafting from 441 kinds of plants. While nobody has kept an exact count, newly discovered substances join the list every year.

The complexity of the chemistry depends on the species. Snapdragons and petunias release relatively simple blends of 7 to 10 compounds. Some orchids, however, secrete scents with around 100 ingredients.

Scent alone can do the vital job of attracting customers to a flower. In 1876, Charles Darwin described wrapping flowers in muslin so insects couldn't see the blooms. Pollinators showed up, anyway, so Darwin concluded that odor was the lure.

In a 1986 refinement on the protocol, Olle Pellmyr, now of the University of Idaho, led an experiment covering the yellow, tightly packed flowers of the western U.S. skunk cabbage, Lysichiton americanum, with glass so insects could see them but not catch the scent. On other skunk cabbages, the researchers blocked the view but not the smell. Beetles ignored the glass-covered plants but swarmed toward the scent.

An even more vivid demonstration focused on the snakeroot, Cimicifuga simplex, which grows in three forms, each with its own habitats, pollinators, and scents. In 1990, Pellmyr and his colleagues reported that fritillary butterflies seek the scent of only one form, probably because of its irresistible methyl anthranilate and isoeugenol. The researchers spiffed up flowers of the other forms by adding dashes of those compounds, and the butterflies started visiting them, too.

That's not to say that looks don't matter. Some pollinators, if forced to choose, will respond to visual cues rather than scented ones. And in the real world, scents and sights interact. Pellmyr's group found that the number of beetles alighting on a skunk cabbage flower more than doubled when the researchers permitted insects to see the flower as well as smell it.

Raguso says that intense whiffs of the perfume of the night-blooming datura he studies drive hawkmoths to start probing at patches of white. Lucky moths quickly find the white trumpets of the flowers, but "often a moth will poke at my socks," says Raguso. If he walks far enough away from the strongest odors, though, the moths lose interest in his ankles.

There's a dark side to this power of alluring scent. Some Australian orchids release scents that trick male wasps into ridiculous positions and, according to a new study, threaten the social lives of female wasps. The orchid Chiloglottis trapeziformis belongs to a group of about 300 species that lure pollinators by mimicking a female insect. Males grappling flower parts in attempts to mate pick up pollen and then rub it onto another orchid during the next delusional encounter.

The C. trapeziformis flowers don't look much like female wasps beyond their dark green-to-brown color range. However, scent seems to be the most important advertisement, say Bob Wong of Australian National University in Canberra and Florian Schiestl of ETH Zurich in Switzerland. When they hid a real female of the wasp Neozeleboria cryptoides and an orchid in identical opaque chambers in the woods, allowing only the scents to waft out, males showed no sign of being able to distinguish the odors.

The males do learn from bad experiences, though. The researchers set out orchid flowers at 2-minute intervals to mimic a colony blooming in the woods. The first blooms attracted visits and mating attempts from male wasps, but within 20 minutes, the males largely ignored them.