Atmospheric Aspirin

Atmospheric Aspirin

Trees release an aspirin-like compound under stress.


Aspirin in the atmosphere. I'm Bob Hirshon and this is Science Update.

When people feel stressed, their hearts start beating fast, they breathe harder, and their stomach ties in knots. When trees get stressed out, they spew chemicals into the air. Thomas Karl and colleagues at the National Center for Atmospheric Research in Boulder, Colorado, measured the myriad compounds given off by a walnut tree plantation. Karl says one chemical was methyl salicylate—a compound related to aspirin—previously only detected in the lab.


And what was also surprising to us, what we could see from our measurements, is that not just one tree responded to it, but the whole forest triggered the formation of that compound.

Karl says the aspirin-like compound is a stress signal plants use to communicate with each other. And monitoring it might give farmers an early warning for when their crops are under attack. I’m Bob Hirshon for AAAS, the science society.

Making Sense of the Research

Plants can't see, hear, or speak, but they do communicate in a number of ways. Many plants release distinctive chemicals when bugs munch on their leaves. These chemicals may kill or repel the bugs directly, attract predators that eat the bugs, and/or warn other plants to ramp up their natural defenses. Plants can sense one another through their roots, too. When the roots of two plants come into contact, they often release signals that inhibit one another's growth, so that no one plant hogs all the soil. Recent studies suggest that some plants can even distinguish the roots of genetic relatives from those of non-relatives, and treat the relatives more generously.

Originally, these researchers were actually studying other compounds released by walnut trees, which can affect climate and bind with air pollutants. However, they also detected airborne, aspirin-like chemicals floating around the walnut grove, which at the time was suffering from a local drought. The chemicals in the air spiked even higher after additional stresses like dramatic temperature changes.

The chemical has been shown to boost the walnut tree's natural defenses, which resemble our own immune system. This helps the tree recover more quickly from diseases and resist new ones. Evidence suggests that chemicals from one tree also can cause another tree to build up its resistance. Although it hasn't yet been proven that this can happen through the air, it would make good evolutionary sense for trees to be able to respond in this way.

In any event, the chemical may soon alert farmers that their trees might be in trouble. Mechanical detectors could notice rising levels of the chemical in the atmosphere long before more obvious signs of damage show up. As a result, farmers could intervene and save their crops before it's too late.

Now try and answer these questions:

  1. What is the main function of methyl salicylate in walnut trees?
  2. What's significant about the fact that it was detectable in the air after stressful events?
  3. Why are chemical signals like these useful from a human perspective?
  4. Propose an experiment that would test whether walnut trees can communicate through the air with this chemical. What factors would you have to control in designing your experiment? 

You may want to check out the October 3, 2008, Science Update Podcast to hear further information about this Science Update and the other recent programs. This podcast's topics include: THE SCIENCE OF TREES: Electricity from trees, building a tree from the ground up, a forest of aspirin, and trading rainforest conservation for healthcare.


Going Further

For Educators

The National Geographic News article Plants on the Warpath focuses on antagonistic plant relationships, including ones resembling conspiracy and murder.

The Plant Physiology Information Website, created by Eastern Connecticut State University biology professor Ross Koning, presents a wealth of information about plant biology.

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