Plants May Let Out Ultrasonic Squeals When Stressed

Human ears can’t hear them, but other plants or animals might

Microphones picked up ultrasonic sounds produced when tomato or tobacco plants were cut or dried out. joffi / Pixabay by CC0

If a drought-parched plant lets out a scream, but it’s at a frequency too high to hear, does it count as a cry of distress? According to a study posted on the preprint server bioRxiv last week, the answer could very well be yes. (And we’re not talking about folklorish mandrakes.)

For the first time, researchers appear to have evidence that, like animals, plants can audibly vocalize their agony when deprived of water or forced to endure bodily harm. The study, which has yet to be published in a peer-reviewed scientific journal, adds another dimension to scientists’ growing understanding of how plants detect and interact with their surroundings—despite lacking many of the sensory organs their faunal counterparts deploy.

In recent years, it’s become abundantly clear that plants are far more sensitive than researchers once gave them credit for. They respond when touched by insects, turn toward sources of light, and some even sniff out other plants. Others are even sensitive to anesthetics, suggesting that they’re capable of experiencing something akin to “pain.”

“Plants are not just robotic, stimulus-response devices,” Frantisek Baluska of the University of Bonn in Germany told Joanna Klein at the New York Times last year. “They’re living organisms which have their own problems.”

Actually making that anguish audible, however, is another matter entirely. To test that possibility, a team led by Itzhak Khait, a plant scientist at Tel Aviv University in Israel, placed microphones capable of detecting ultrasonic frequencies four inches from tomato and tobacco plants, then either stopped watering them or snipped their stems.

Measuring in the range of 20 to 150 kilohertz, the researchers found that even happy, healthy plants made the occasional noise. But when cut, tobacco plants emitted an average of 15 sounds within an hour of being cut, while tomato plants produced 25 sounds. Stress from drought—brought on by up to ten days without water—elicited about 11 squeals per hour from the tobacco plants, and about 35 from the tomato plants.

The shrieks were also surprisingly informative. When the team fed the recordings into a machine learning model, it was able to use the sounds’ intensity and frequency distinguish whether they were related to dryness or physical harm, or were just regular, day-to-day chatter. One odd pattern? Thirsty tobacco makes a bigger ruckus than tobacco that’s been snipped, reports Adam Vaughan at New Scientist.

Researchers aren't yet sure how plants produce these sounds, but Khait and his colleagues propose one possibility in their paper. As water travels through the plants’ xylem tubes, which help keep them hydrated, air bubbles will form and explode, generating small vibrations. Previous studies have picked up these waves, but only through devices attached directly to plants. Still, the process, called cavitation, could explain longer-range sound production as well, as Edward Farmer, a plant biologist at the University of Lausanne in Switzerland who wasn’t involved in the study, tells Vaughan. But Farmer also remains cautious about the recordings, which may have picked up ambient noise as well,. Even drying soil can produce faint sounds, reports Nicolette Lanese for Live Science.

All this stress-induced “screaming” wasn’t in a range detectable by human ears. But organisms that can hear ultrasonic frequencies—like mice, bats or perhaps other plants—could hear the plants' cries from as far as 15 feet away.

It’s not yet clear how ubiquitous stressed squeals are among plants, though the researchers have started to listen in on some other species. Plants also experience many kinds of stress, such as those brought on by extreme temperatures or salinity, and may not always react in the same way, Anne Visscher, a plant biologist at the Royal Botanic Gardens in the United Kingdom who wasn’t involved in the study, told Vaughan. And any ideas on what purpose the sounds might serve—from warning other plants to passing information onto animals—remains speculative, she adds.

For now, it’s useful to simply know what plants are truly capable of. Something to chew on, perhaps, the next time you’re pruning your tomato plants.

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