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

In lab tests, three of the wasps responded strongly to whiffs of their own fig species but not significantly to the others. Yet there was a problem with the wasps that the scientists had thought would pollinate the fourth fig. Those insects turned up their antennae at all four fig aromas. So the researchers began to rethink the taxonomy. They suspect that the trees they had considered varieties of the fourth species were actually members of two other species. The wasps seem to be more discerning than human taxonomists.

There's more to attractive scent than a pretty flower though. Raguso, Levin, and Lucinda McDade of the Academy of Natural Sciences in Philadelphia, remind floral biologists to stop and sniff the foliage. Raguso has found that hawkmoths searching for alluring odors respond to leaf fragrances as well as to those of flowers. Levin's recent survey of four o'clocks revealed that four species in the genera Acleisanthes and Selinocarpus emit up to 80 percent of their total volatile emissions not from the flowers but from the leaves. The leafy allure even includes hefty proportions of sesquiterpenoids, which are among the ingredients that have been proposed as hawkmoth attractants.

Evolutionary biologists are also waking up and smelling the flowers. Because plants use scents to flirt with their pollinators, the odors strongly influence the complex dynamic that determines species.

Kim Steiner of the California Academy of Sciences in San Francisco has been investigating South African orchids and relatives of snapdragons that attract bees with the unusual lure of insect baby food. The bees don't eat the oil themselves but instead mix it into a lump of provisions for their larvae.

In March, Steiner told a conference in Ventura, Calif., that he had overlaid a family tree of these unusual species with information about their scents. For example, he found two Pterygodium orchid species growing in the same area in southwestern Africa and another species, with a different pollinator bee, far to the east. Yet all three species had a distinctive scent that Steiner calls "soapy and pungent." Chemist Roman Kaiser of the fragrance company Givaudan in Dubendorf, Switzerland, has identified eight compounds in their scents. The most abundant, with a benzene ring, has never before been isolated from a living thing.

On the family tree, the three orchid species are one another's closest relatives, so the scent may have persisted as the species diverged. In other eases, plant lineages appear to have switched their scent chemistry, taking advantage of different pollinating species.

DOLLARS AND SCENTS Shifts in pollinators may have economic, even culinary, consequences. In the early 1990s, Allen Young of the Milwaukee Public Museum and David Severson of the University of Notre Dame compared floral fragrances from nine varieties of the cacao plant that aren't closely related. In an old cacao plantation in Costa Rica, the researchers set out traps scented with the oils from the different varieties. Then, they watched to see which insects responded.