Plants have long been known to react to changes in their environment, and may respond to light, temperature, and touch.
But are they listening too?
For the Arabidopsis plant, the answer is a loud and clear “yes.”
The distinct, high-amplitude vibrations produced by a cabbage butterfly caterpillar munching on a leaf of this flowering mustard plant, commonly called mousear cress, throws its defenses into high gear, according to a study published in Oecologia this month by two researchers at the University of Missouri.
The study, which combined audio and chemical analysis, is the first to find evidence that plants respond to an ecologically relevant sound in the environment, said Heidi Appel, a senior research scientist in the Division of Plant Sciences at Missouri.
Appel, along with Rex Cocroft, a professor in the Division of Biological Sciences, used a laser and a small piece of reflective material to record the caterpillar’s chewing vibrations, which moves an Arabidopsis leaf up and down about 1/10,000 of an inch. They then played two-hour recordings of the vibrations to one set of plants and left another set in silence.
The plants that heard the recording of chewing vibrations created an increased amount of mustard oil, a defense meant to deter an insect attacker.
“[The vibrations] trigger them to be better prepared for subsequent attacks,” said Appel. “So they make more defenses, faster, when they’ve been primed by these feeding vibrations.”
The plants were also selective about what type of vibrations they responded to. Shortly after the first experiment, Appel and Cocroft exposed the plants to other vibratory sounds, including those from the wind and nonthreatening insects. The other sounds did not trigger any response.
“We don’t think it’s anything as simple as some frequencies or pitches of sound are better than others,” Cocroft said. “They responded to the chewing vibrations and not to the insect song, even though they had the same frequencies in them. It suggests that the plants’ acoustic perception is more complicated than simply looking for a particular pitch of sound.”