The study published in Cell found that those sounds vary depending on the plant emitting them as well as the kind and gravity of the threat that prompts them.
The findings dispel the myth that plants exist in the background, acting as a silent support for the animal life around them.
Instead, they demonstrate that those plants may be able to send out signals that nearby animals can pick up on and hear, which they could then use to possibly alter their behavior.
The study’s co-author, mathematician Lilach Hadany of Tel Aviv University, told The Hill that tomatoes left without water start to make noise “on the second day — even while the tomato still looks good.”
After five days of water stress, the sounds—which sounded somewhat like popcorn popping—reached their peak, and as the plant dried out, they started to diminish.
The sounds occur at approximately the volume of human speech but outside of our hearing range, according to the study.
They also differed depending on the plant making them and the type of injury, with cut and dehydrated plants making different noises.
The study raises the tantalizing possibility that, for organisms capable of hearing these pitches, a plant landscape is also a soundscape of information, revealing vital information about both plants and the larger environment.
Hadany directs a lab at Tel Aviv University in Israel that uses machine learning to study plant evolution, including the emerging field of plant acoustics and, in particular, how plants use sound.
The word “use” in the sentence is a landmine in scientific circles: It can imply a level of intent that scientists have traditionally been reluctant to ascribe to plants.
However, the hard consensus on the subject has gradually weakened in recent years.
A growing body of evidence suggests that plants emit cues that other plants and pollinators pick up on, most notably by releasing floating airborne chemicals, as discovered by Richard Karban of the University of California, Davis.
Hadany told The Hill in a video interview that it made sense for plants to use sound because it requires little energy to produce and travels a long distance.
When Hadany first considered investigating whether plants could hear sounds in their environment – and even send audio signals of their own – colleagues advised her to wait for fear of jeopardizing her career.
The colleague said, “‘Do not work on it before you have tenure,’” Hadany said, noting that the whole topic had a slightly disreputable flavor in academia.
“But now that I am a full professor, it is good,” she said, smiling over the Zoom video.
Previous research from her lab discovered that some plants can hear and change their behavior in response to what they hear.
According to a 2019 study her team published in Ecology Letters, when her team played the sound of buzzing bees near primrose bushes, their flowers began to release sweeter nectar within a few minutes — something they did not do when exposed to other frequencies.
The primrose bushes “heard” the bees through the flowers, which may explain why bees hover and buzz near flowers, according to Hadany.
According to Hadany, the 2019 findings raised larger questions. The existence of one such channel of useful, audible information — which allows pollinators to signal their presence to plants and plants to woo them — suggested the type of two-way relationship that evolution frequently works to strengthen, she added.
“Once you have this interaction, there is selection on both sides to improve hearing and emissions of sounds,” she said.
Hearing and interpreting such information is especially important for plants, which have a greater need than animals “to interact with their environment, to respond to the environment — because it cannot go to a different environment,” according to Hadany.
Plants use environmental information to trigger the production of new chemicals or physiological responses, such as insecticide production or water retention, as well as turning to follow the sun.
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