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. (See "Plants 'Listen' to the Good Vibes of Other Plants.")
"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."
Little Plants Have Big Ears
Appel and Cocroft predict that although their research focused on one plant and one predator, their results suggest a widespread response throughout the plant kingdom. The next phase of research will explore the phenomenon with other types of plants and herbivores. It is too early, they say, to speculate on what applications their finding may have for commercial agriculture.
Appel and Cocroft's results join "a growing list of really interesting pieces of information that we really never appreciated that is useful for plants," said John Orrock, associate professor of zoology at the University of Wisconsin-Madison.
Orrock, who studies "the ecology of fear" between plants and their predators and has researched the effect snail slime has on a plant's defense system, said he believes the Missouri findings "open up a rich array of new and interesting questions" about the field of plant behavior.
"There are so many other forms of information that we know plants respond to," he said. "We know they pay attention to light; we know they pay attention to a vast array of chemicals both in the soil and in the air around them ... Plants are balancing a lot of information at any given moment in time. And now we know that acoustic information is something they're also balancing.
"It suggests that plants, in some ways, are listening."
Comment: More bad news for vegetarians in that science of their beloved faceless plantbased food source, no longer can be considered without functions of consciousness, but aware beings with ability to feel some kind of pain and reciprocate in more complex ways than formerly thought (purely electric and chemical communications).