「植物看得見你」公開課筆記／3.6 Plant Communication: Volatile Signaling

In the late 1970s, early 1980s, primarily by two scientists at the University of Washington, David Rhodes and Gordon Orians, they noticed if you look at trees that have been attacked by caterpillars, the trees that were growing next to them, were often resistant to the caterpillars, even though there were no physical connection between the trees. The branches and the roots weren't touching.

It was resistant to the caterpillars is because those leaves were making chemicals, which are called phenolic compounds / tannin compounds, which are actually poisonous to the bugs.

Their hypothesis was that the attack trees were releasing some type of airborne signal, which was then signalling to the neighbour to watch out and start making these chemicals to protect themselves.

This hypothesis was picked up and studied by two scientist at Dartmouth College, Ian Baldwin, who is then a graduate student, and his mentor Jack Schultz.

And their results were so ground breaking that they were published in the very prestigious Journal of Science in an article that was called Rapid Changes in Tree Leaf Chemistry Induced by Damage, Evidence for Communication Between Plants.

They studied both poplar and sugar maple seedlings. They built airtight plexiglass cages, and in these cages they put about 15 of the seedlings. They used two different cages for two different conditions. In the first cage they put 15 seedlings but to two of the seedlings they ripped their leaves.

In the second cage, you had the same number of tree seedlings, but their leaves were not ripped at all, they were just left there as a control.

So we have three different types of seedlings in one cage. Two trees with ripped leaves, the rest without ripped leaves. And in the other cage, nothing had been done to the seedlings.

After two days, they came back, then measured the chemicals in the leaves. Now when they measured the chemicals in the leaves from the trees that had had their own leaves ripped, they found that these leaves made those same anti-bug chemicals.

But not only did those same seedlings that had had their leaves ripped make those chemicals, but the other trees in those same boxes also made those chemicals.

Baldwin and Schultz proposed that the damaged leaves whether by tearing as in their experiment, or by the caterpillars foraging on them as in the earlier experiment from the University of Washington, cause the leaves to release a volatile signal. A gas up into the air which was then picked up by the neighboring leaves, this gas is telling them to make the anti-bug chemicals.

Now this result was incredibly controversial. Many scientists believed the results were completely over-interpreted. In 1985, scientists from the University of York, even published an article where they believed that much of the evidence in this general field is unsatisfactory.

What's wonderful about the process of science is that, in the long run, the science that's done with proper controls, in the end tends to become accepted. Whereas pseudoscience that's done with improper controls, is often relegated to what I call the wastebasket of scientific history. Volatile signaling between plants is now an accepted paradigm.