Do Plants Actually Respond To Sound?

Advances in sensors and communication systems, such as those pinpointing the amount of water and fertilizers needed in just one corner of a plot of land, allow farmers to use technology to produce more food. Now there's a surprising new tool to add to the precision agriculture arsenal: sound.

In this episode of Gastropod, we investigate how scientists and farmers are listening in to the sounds of the farm, from James Bond-inspired spy devices that can capture the wing-beats of hungry insects from dozens of feet away, to microphone-equipped drones patrolling henhouses in search of sick chickens.

Can sound help us keep Florida's citrus groves alive, make Brussels sprouts even healthier, and put livestock in charge of their own welfare? Listen now!​

Mozart For Plants

The idea that plants can hear and respond to music has a long and checkered history. Charles Darwin made his son, Francis, play the bassoon in front of a herb while he watched to see whether its leaves twitched (the plant was unmoved); Barbra Streisand caused a veritable explosion of color when singing to her tulips in the musical On a Clear Day You Can See Forever; and, as recently as the 1970s, UNC Greensboro physicist Dr. Gaylord Hageseth claimed that his experimental "pink" noise could make turnips sprout much faster.

While the claims that playing Mozart in a cornfield will lead to a dramatic increase in yield have proved impossible to replicate, scientists are sure that plants do respond to sounds in their environment, with small changes in gene expression, for example, or slightly different germination rates. But, as Heidi Appel, senior research scientist at the University of Missouri, told Gastropod, "we never understood why plants would have that ability."

Pest Sounds

Intrigued, Appel teamed up with her colleague Reginald Cocroft, a behavioral ecologist, to focus on a sound that, they thought, might be particularly useful to plants: the vibrations caused by insect feeding. "These are one of the earliest and most quickly transmitted signals plants have that they're being attacked," Appel told Gastropod. And while plants can't hear insects the same way we do — they don't have ears, after all — they can sense vibrations, much like club-goers feel the thump of bass or worshippers hear an organ reverberate through a church. "In that case, your body is a substrate," picking up the sound vibrations, Appel explained. "That's much more like what plants experience."

To test their theory, Appel and Cocroft used lasers to measure the minute leaf tremors, about 1/10,000th of an inch, that caterpillars make when they munch on Arabidopsis (rockcress), a spindly relative of cabbage and broccoli that is commonly used in plant research. Next, they played those sounds back to one set of plants, and left the control group in peace. Finally, they let the caterpillars loose on both plant populations. Astonishingly, they found that the plants that had undergone audio training actually responded to the attack by producing much higher levels of mustard oil, their innate pesticide — which made them much less appetizing to the hungry caterpillars.

"That was very exciting and we were very happy," Appel said. "But, at one level, we thought, 'So what?' Plants might respond to everything." So they tested the plants again, this time using recordings of wind and treehoppers, a bug that looks like a thorn and sings with a high-pitched whine but does not like to dine on Arabidopsis. In response to these vibrations, however, the plants produced no increase in mustard oil. With this elegant experiment, Appel and Cocroft had solved a basic question of plant evolutionary biology: plants evolved the ability to respond to sound vibrations in order to recognize and ward off attackers.

Musical Mustard

In doing so, Appel and Cocroft may have also hit upon a potent environmentally-friendly pesticide. Perhaps a field full of speakers blasting the sounds of crunching caterpillars might help terrified crops prime themselves to ward off a real attack, removing the need to apply chemical pesticides. This summer, Appel and Cocroft are testing commercially useful Arabidopsis relatives in the brassica family, such as kale and Brussels sprouts, to see if they demonstrate the same response.

But, as Appel pointed out to Gastropod, the use of sound might have an even more direct impact on our health. While plants evolved these chemical responses to deter pests, for humans, they often provide both flavor and health benefits. In fact, the sulfurous compounds produced by Arabidopsis and its fellow brassicas form the basis of America's favorite hot dog condiment, mustard. And those same chemicals are actively being studied by cancer researchers for their potent health benefits. Maybe, by playing predator sounds in the field, farmers could actually grow more healthful plants.

Appel is testing this hypothesis with an African plant that is currently harvested for medicinal use, to determine whether playing predator sounds will increases the plants' production of beneficial chemicals. "When we look at a plant as a source of flavor or medicine, what we are looking at is the product of millions of years of evolution of the plant interacting with its own pests — and those are largely insects," said Appel. Insects that, it turns out, plants can hear.

This is the first of a two-part series exploring the relationship between sound and food. Listen to this episode of Gastropod for much more on the experimental history and emerging science of acoustic agriculture, from the perfect bovine playlist to the lost rhythms of Southern farming. And, if you like what you hear, subscribe to make sure you don't miss out on hearing the difference between hot and cold tea, learning how the sound of tiny bubbles in soda changes its taste, and discovering the science behind pairing wine with music.

DVD Extras: Field Recordings Playlist

We've uploaded all the odd sounds we played during the
episode, so that you too can scare your houseplants with the sounds of
caterpillars munching on leaves! From Heidi Appel, we have the infamous
caterpillars themselves, the treehopper,
and the sound
of the wind. Richard Mankin's entire audio library is online,
but check out the sounds of a German
cockroach scurrying, an old
house borer, a rice
weevil, and its larvae
at 17 and 18
days old. Eamonn Keogh shared a
recording of several fruit flies passing the sensor, as well as a female
mosquito. And Wayne Daley shared the squawks
of sick chickens and healthy
ones, for comparison. Mary Caton Lingold's Sonic Dictionary contains
several agricultural sounds: we played a field holler, the
sound of sugarcane
harvesting, and the sound of a diesel tractor
starting up.