When you have a headache, you take a couple aspirin, but when plants get stressed out, they just make their own.

Scientists had known that plants in laboratories produce a chemical called methyl salicylate — a form of the painkiller aspirin — when stressed out, but they had never detected it in plants out in nature.

A team of scientists from the National Center for Atmospheric Research (NCAR) in Boulder, Colo., discovered by accident plants in the wild emitting methyl salicylate. They set up instruments last year in a walnut grove near Davis, Calif., to monitor plant emissions of certain volatile organic compounds (or VOCs). VOCs emitted by plants can actually combine with industrial emissions and contribute to smog.

To their surprise, the NCAR scientists found that the emissions of VOCs their instruments recorded in the atmosphere included methyl salicylate.

They noticed that the methyl salicylate emissions increased dramatically when the plants, already stressed by a local drought, experienced unseasonably cool nighttime temperatures followed by large temperature increases during the day.

Scientists think that the methyl salicylate has two functions: stimulating a process similar to the immune response in animals that helps plants resist and recover from disease, and acting as a form of chemical communication to warn neighbors of threats.

"These findings show tangible proof that plant-to-plant communication occurs on the ecosystem level," said study team member Alex Guenther. "It appears that plants have the ability to communicate through the atmosphere."

The research, funded by the National Science Foundation and detailed in the Sept. 8 issue of the journal Biogeosciences, could give farmers and forest managers an early warning signal that all is not right with their plants, either because of disease, insect infestation, or other types of stress.

"The earlier you can detect that something's going on, the more you can benefit in terms of using fewer pesticides and managing crops better," said study leader Thomas Karl.

Methyl salicylate and other plant hormones emitted into the atmosphere could also account for some of the fraction of VOCs scientists had suspected were in the atmosphere but had escaped detection until now.