Plants Make Aspirin When Under the Weather

Sept. 19, 2008 -- Plants make aspirin when they need an immune boost, according to new research, sending a form of the compound airborne to signal a health problem to the rest of the tree or to other trees.

The finding may help growers more readily identify plants under stress by monitoring for the airborne distress signal.

Researchers at the National Center for Atmospheric Research in Boulder, Colo. made the finding over a walnut tree grove in California. They used sensitive instruments to detect the organic compounds in the air over the grove at different locations, including different heights.

The researchers found levels of an unidentified compound that swamped the levels of the compounds they set out to look for.

"It was kind of a surprise to us because we weren't really looking for this product," said study lead author Thomas Karl. "We weren't sure what we were seeing to begin with."

The compound turned out to be methyl salicylate, a volatile form of salicylic acid. Salicylic acid was originally derived from willow bark and shown to have the pain- and fever-relieving effects known for aspirin. A modified form of salicylic acid, acetosalicylic acid, is now sold as the active ingredient in aspirin.

Salicylic acid is produced by plants when they are under stress like drought or attack by a fungus or insect. It travels through the plant's vascular system and activates the plant's version of an immune response.

Until the new study, nobody had detected the high values of the airborne version of salicylic acid, which the researchers believe is a way to send the stress signal farther and faster. Leaves on the stressed tree or on nearby trees can detect the methyl salicylate signal and convert it into the immune-response-triggering salicylic acid.

"It's faster to send the volatile form to the other leaves, rather than sending through the plant," Karl pointed out. "It might be a more effective way for the same tree to signal that's what's going on."

The researchers observed spikes in methyl salicylate over the walnut grove after nighttime temperatures dipped low, suggesting the plants were reacting to cold stress. The peaks were higher during a dry period, pointing to combined stress of cold nighttime temperatures and mild drought.

The researchers published the work in Biogeosciences.