Plants signal stress with a toluene burst.

Toluene, a volatile organic compound often used as an indicator of human-made pollution, also emanates from plants, a new study finds. These emissions could confuse efforts to trace pollution from automobiles, factories, and other nonbiological sources.

Scientists at the Research Center Julich in Germany found that sunflowers and Scotch pine trees give off small amounts of toluene under normal conditions and expel larger bursts under stress. Juergen Wildt and his colleagues report their surprise findings in the May 1 GEOPHYSICAL RESEARCH LETTERS.

Volatile organic compounds (VOCs) in the atmosphere react with other chemicals to form smog. Globally, plants emit about 90 percent of the VOC concentration in the air, says Alex Guenther of the National Center for Atmospheric Research in Boulder, Colo. In urban areas, however, human-made sources predominate.

"Emission of other VOCs from [botanical] plants is so large, having another one wouldn't be a big deal," says Guenther. However, the more interesting impact would be an undermining of measurements that use toluene as a tracer for air pollution generated by people.

"If toluene indeed turns out to be a significant emission from plant under stress," says Joost A. de Gouw of the University of Utrecht in the Netherlands, it will "have some implications for the interpretation of measurements of aromatic compounds in the atmosphere."

The Julich researchers monitored toluene emission from a sunflower by enclosing the plant in a plastic bag and periodically taking samples of the gases inside. The scientists found that an empty bag didn't show measurable toluene, but the sunflower gave off the compound at a rate equal to a few percent of its emission of alpha-pinene, the VOC that gives pine needles their scent.

Putting stress on the plant turned up its toluene production. When the researchers deprived the sunflower of nutrients, it released more VOCs overall and doubled its discharge of toluene. When the researchers cut off a leaf, the injured plant responded with a VOC surge, in which toluene emission increased to 10 times its original rate.

In tests on Scotch pines, toluene emissions provided an early sign that the plants had been attacked by some unknown pathogen. "The plants were not wounded, did not suffer from insect attack, and were not exposed to high ozone concentrations," says Julich's Michael Komenda. Yet their emissions of VOCs, including toluene, increased to 10 times the amount emitted by a healthy pine. After a few days, the green needles turned yellow from the mysterious ailment.

No one understands how the plants synthesize toluene, but the process is likely to be some kind of defense mechanism, says Guenther. VOCs are responsible for the familiar smells of certain plants, such as the scent of fresh-cut grass (SN: 4/3/99, p. 223).

The idea of toluene-emitting plants remains "slightly controversial," says de Gouw, but "the experiments seem to have been done carefully." Guenther agrees. Only further study, they say, can show whether plant toluene has an impact an

atmospheric chemistry.