Martin Heil and his team at the Center for Research and Advanced Studies in Irapuato, Mexico, have been studying wild lima beans (Phaseolus lunatus) for the past several years to further explore this question. Heil knew that scientists had observed that when a lima bean plant is eaten by beetles, it responds in two ways. The leaves that are being eaten by the insects release a mixture of volatile chemicals into the air, and the flowers (though not directly attacked by the beetles) produce a nectar that attracts beetle-eating arthropods. Early in his career at the turn of the millennium, Heil had worked at the Max Planck Institute for Chemical Ecology in Jena, Germany, the same institute where Baldwin was (and still is) a director, and like Baldwin before him Heil wondered why it was that lima beans emitted these chemicals.
Heil and his colleagues placed lima bean plants that had been attacked by beetles next to plants that had been isolated from the beetles and monitored the air around different leaves. They chose a total of four leaves from three different plants: from a single plant that had been attacked with beetles they chose two leaves, one leaf that had been eaten and another that was not; a leaf from a neighboring but healthy “uninfested” plant; and a leaf from a plant that had been kept isolated from any contact with beetles or infested plants. They identified the volatile chemical in the air surrounding each leaf using an advanced technique known as gas chromatography/mass spectrometry (often featured on the show CSI and employed by perfume companies when they are developing a new fragrance).
Heil found that the air emitted from the foraged and the healthy leaves on the same plant contained essentially identical volatiles, whereas the air around the control leaf was clear of these gases. In addition, the air around the healthy leaves from the lima beans that neighbored beetle-infested plants also contained the same volatile chemicals as those detected from the foraged plants. The healthy plants were also less likely to be eaten by beetles.
But Heil was not convinced that damaged plants “talk” to other plants to warn them against impending attack. Rather he proposed that the neighboring plant must be practicing a form of olfactory eavesdropping on an internal signal actually intended for other leaves on the same plant.
Heil modified his experimental setup in a simple, albeit ingenious, way to test his hypothesis. He kept the two plants next to each other but enclosed the attacked leaves in plastic bags for 24 hours. When he checked the same four types of leaves as in the first experiment, the results were different. While the attacked leaf continued to emit the same chemical as it did before, the other leaves on the same vine and neighboring vines now resembled the control plant; the air around the leaves was clear.
Heil and his team opened the bag around the attacked leaf, and with the help of a small ventilator usually used on tiny microchips to help cool computers, they blew the air in one of two directions: either toward the neighboring leaves farther up the vine or away from the vine and into the open. They checked the gases coming out of the leaves higher up the stem and measured how much nectar they produced. The leaves blown with air coming from the attacked leaf started to emit the same gases themselves, and they also produced nectar. The leaves that were not exposed to the air from the attacked leaf remained the same.
The results were significant because they revealed that the gases emitted from an attacked leaf are necessary for the same plant to protect its other leaves from future attacks. In other words, when a leaf is attacked by an insect or by bacteria, it releases odors that warn its brother leaves to protect themselves against imminent attack, similar to guard towers on the Great Wall of China lighting fires to warn of an oncoming assault.