Floral foragersHoneybees pick up pollen on a visit to a poppy.Stephen L. Buchmann/Univ. of Arizona

Take time to stop and smell the flowers, goes a familiar saying. But that might be harder to do today than it used to be. Scientists recently reported that air pollution quickly destroys some of the sweet-smelling perfumes flowers produce. It's a problem that could have long-lasting effects on plant reproduction and diversity, and might help explain why populations of bees and other pollinators are declining.

Many flowering plants produce fragrant compounds that attract pollinators, such as bats, bees and moths. These plants rely on pollinators to move a powdery material called pollen from one flower to another. This movement of pollen must take place for flowering plants to reproduce.

Some pollinators rely more on scent than sight to find flowers, so scent is especially important to these pollinators. Like fairy tales' Hansel and Gretel, who followed a trail of pebbles to their home in the woods, pollinators home in on flowers by following the scent trail that drifts out from a flowering plant. Anything that interferes with pollinators' ability to find flowers could make it difficult for flowering plants to produce new generations, and for pollinators to find important sources of food.

Several years ago, however, researchers learned that flower perfumes react with the air pollutants that are produced when we burn fossil fuels, such as gasoline and coal. While scientists knew these chemicals could change or destroy a flower's scent, until now, nobody knew just how quickly that happened.

To find out, scientists at the University of Virginia in Charlottesville wrote a computer program that modeled the chemical reactions between three common floral scent compounds and three harmful components of air pollution: ozone, nitrate and a class of compounds called hydroxyl radicals. To build their computer model, the scientists recorded the weather conditions at a flower farm, including factors such as air temperature and wind speed.

Specifically, the model helped the scientists understand what happened to a flower's scent compounds as those perfumes were released from the plant and drifted across the landscape on the wind. The computer model showed that when no air pollution was present, a flower's scent trail, or the path over which its scent drifts, extended beyond a kilometer. But when any of the components of air pollution were added, both the concentration of scent compounds and the distance of the scent trail decreased. In fact, when substantial air pollution was present, half of a flower's scent trail could be lost within 200 meters of the plant, the researchers calculated.

The results suggest that air pollution may affect the signals that nature depends on, says Laurie Adams of the Pollinator Partnership in San Francisco. However, bees rely less on scent than certain other pollinators — such as moths and bats, other researchers point out. So bees probably will be less affected by the loss of flowers’ perfumes than might these other animals, which are still very important pollinators.

Because the new study only considered computer models of the chemical reactions between scent and air pollution, the research team needs to run tests under natural conditions to confirm what can be learned from the models. The researchers will have more answers soon: They plan to begin new tests in the field this summer.

Power words:

fossil fuel A source of energy that was formed millions of years ago from the remains of plants and animals. Oil, natural gas and coal are fossil fuels. Gasoline, derived from oil, is also considered a fossil fuel.

ozone A poisonous form of oxygen that is produced naturally in the atmosphere by sunlight or lightning. Ozone is often present in a form of air pollution known as smog that forms from car and truck exhaust. It is also made in factories for use as bleach and to kill germs in drinking water.

pollen Powdery grains that contain the male sex cells of most plants.

pollinator An animal that carries pollen from one flower to another.

pollination The process that allows pollen to move from a male plant part to fertilize the sex cells in a female plant part. In most flowering plants, pollination takes place when wind, insects or birds carry pollen from a stamen on one flower to a pistil on another.

smog Form of air pollution that is produced when sunlight reacts with certain gases emitted by factories and automobiles that burn fossil fuels.

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