Stressed plants that have been damaged or dehydrated do not go quietly—some emit high-pitched sounds, according to a study published last week in the journal Cell.
In an experiment, researchers denied water to some tomato and tobacco plants and cut the stems of others, then they placed microphones nearby. The devices picked up on noises coming from the plants that were outside the range humans can hear. However, researchers say other animals could theoretically detect the sounds from about 10 to 15 feet away, according to the paper.
The researchers found no evidence that the plants were making the sounds on purpose—the noises might be the plant equivalent of a person’s joints inadvertently creaking, Tom Bennett, a plant biologist at the University of Leeds in England who did not contribute to the study, tells Science News’ Meghan Rosen. “It doesn’t mean that they’re crying for help.”
But the sounds the plants made did hint at the specific types of stresses they were experiencing. A machine learning algorithm was able to distinguish between the sounds of a cut plant and the sounds of a dehydrated plant 70 percent of the time.
“That the plants are making different noises that have some information seems like the main contribution of this study,” Richard Karban, an ecologist at the University of California, Davis, who did not contribute to the study, tells the New York Times’ Darren Incorvaia. “I think it will move the field forward.”
Previous studies have shown that stressed plants send out other signals—they release chemicals called volatile organic compounds and vibrate when experiencing drought. Past experiments searching for vibrations and sounds had used sensors attached to plants, but this team of researchers wanted to see if they could detect plants’ sounds in the air.
The scientists placed microphones about four inches away from the plants and found that the stressed crops made noises more frequently than their unstressed counterparts. Dehydrated tomato and tobacco plants emitted an average of about 35 and 11 sounds per hour, respectively. Cut tomato and tobacco plants made 25 and 15 noises per hour. Healthy plants, on the other hand, emitted far fewer sounds, at less than one per hour.
Humans can’t hear these noises, though—we can pick up frequencies as high as 20 kilohertz (kHz), but the plants made sounds mostly between 40 and 80 kHz, Lilach Hadany, a co-author of the study and an evolutionary biologist at Tel Aviv University, tells Business Insider’s Marianne Guenot. When the researchers lowered the frequency of the sounds into the audible range for humans and removed the silence in between them, they sounded like popping.
“It is a bit like popcorn—very short clicks,” Hadany tells Nature News’ Emma Marris. “It is not singing.”
The researchers theorize the sounds are caused by a process called cavitation, where air bubbles form and pop in the plants’ xylem, the tissue that carries water from the roots to the leaves. It’s the same process that causes the vibrations recorded in previous studies with the sensors attached to the plants.
Stressed plants’ popping sounds appear to be reliable—the machine learning algorithm could even tell the thirsty plants from the injured ones amid other noise in a greenhouse, such as conversations and construction activity, writes Science News. “We were particularly happy that the sounds turned out to be informative—containing information on the type of the plant and the type of the stress,” Hadany told Vice’s Becky Ferreira via email.
In theory, recording these sounds could help inform farmers about which of their crops are most in need of water, allowing for more precise irrigation, per the paper. “When more and more areas are exposed to drought due to climate change, efficient water use becomes even more critical, for both food security and ecology,” the authors write. In addition to the tomatoes and tobacco, the team recorded sounds from other plants, including wheat, corn and Cabernet Sauvignon grapevine.
Though we can’t detect them, the sounds are within the range of frequencies that some animals, such as mice and moths, can hear. “Natural selection may be acting on other organisms (animals and plants) to whom the sounds are relevant, to be able to hear the sounds and interpret them,” Hadany told Vice.
Graham Pyke, a retired biologist at Macquarie University in Australia who did not contribute to the research, tells Nature News he thinks the sounds would be too faint for animals to pick up on. “It is unlikely that these animals are really able to hear the sound at such distances,” he says to the publication.
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