“When tomatoes aren’t stressed, they are very quiet,” says one of the study’s authors.
Plants make sounds similar to “pops” that the human ear cannot detect, according to recordings made in New study And making more sounds when thirsty or under stress.
The research changes what most botanists thought they knew about the plant kingdom, which was largely considered silent, and suggests that the world around us is a cacophony of plant sounds, says study co-author Lilac Hadani.
For a long time, she says, she was skeptical that plants are completely silent.
“There are many organisms that respond to sound, and I thought there was no good reason why plants should be deaf and dumb,” says Hadani, a professor in the College of Plant Sciences and Food Safety and program director in the College of Life Sciences. • George S. Wise of Tel Aviv University.
The first plant Hadani recorded, using an ultrasonic microphone, was a cactus in her lab six years ago, but at the time she couldn’t rule out that the sound she detected was caused by another element in the environment. Previous studies had shown that plants vibrated, but it was not known if these vibrations were converted into sound waves.
To see if plants actually make sounds, Hadani and her team got soundproofing speakers.
The researchers placed tobacco and tomato plants in boxes fitted with ultrasonic microphones that register frequencies between 20 and 250 kilohertz. (The maximum frequency that an adult ear can detect is about 16 kilohertz.) Some plants had their stems cut off or had not been watered for five days, and some had been left untouched.
The team found that plants make sounds with a frequency of 40 to 80 kilohertz, and when amplified and translated into a frequency humans can hear, the sounds were somewhat similar to the popping of popcorn or the bursting of plastic bubbles.
One stressed plant made about 30 to 50 of these clicks, or clicks, per hour at seemingly random intervals, but plants that were not stressed made much fewer clicks—about one per hour.
“When the tomato isn’t under stress, it’s very calm,” Hadani says.
The team conducted the study inside a greenhouse and placed the plants in soundproof boxes. Photo courtesy of Tel Aviv University
Do plants communicate? It’s not that simple
Researchers don’t know exactly how the sounds are made, but they think the noise comes from cavitation—a process in which an air bubble in a plant’s water column gives way to some kind of pressure, creating a crackling or popping sound.
But rest assured, the bouquet of cut flowers in your vase is not screaming in pain. There is no evidence that the noises produced by the plants are intentional or a form of communication.
“These data complement what we know about plant stress responses. They are a useful contribution to the field and to our general knowledge that plants are delicate organisms capable of complex behaviors,” says Richard Karpan, UCLA eminent professor of entomology. Davis, who studies the interactions between herbivores and host plants. He did not participate in this investigation.
“However, this should not be interpreted as evidence that plants actively communicate by making sounds,” adds Karpan.
Sensory ecologist Daniel Robert, professor of biology at the University of Bristol’s School of Biological Sciences in the UK, says that while plant sounds are a passive phenomenon, other organisms may be able to use plant sound signals for their own benefit. He also did not participate in the investigation.
For example, he says, the sounds can send a message to a female butterfly that a particular tomato plant is stressed and not suitable for laying eggs or feeding on it.
“Many sounds are created in the world that are not ‘intentional’ signals, but that can nonetheless be heard and used by other organisms for their own benefit. So the concept of communication is really challenging… does it have to be two-way to work and be considered as such?”, he asks in an email.
Yossi Yovel, left, and Lilac Hadani, lead authors of the study. Both are professors at Tel Aviv University. Photo courtesy of Tel Aviv University
Who is listening?
The team repeated the experiment with tobacco plants and tomato plants in a noisier greenhouse environment. After recording the plants, the researchers designed a machine-learning algorithm that could distinguish between stress-free plants, thirsty plants, and cut plants.
“Discovering that it is possible to obtain information from acoustic emissions using neural network classification is exciting because it is a rapid method and can identify data structures that are inaccessible to the human eye or ear,” notes Robert.
The researchers used tobacco and tomato plants because they are easy to grow in a standard way, but they also recorded sounds from a variety of other plant species, such as wheat, corn, cacti, and vines, and also found that they made more sounds when stressed.
In addition to insects or mammals that may detect and use plant sounds, Hadani says other plants may also listen and take advantage of sounds. Previous work by Hadani and other team members showed that plants increase the concentration of sugar in their nectar when they “listen” for sounds made by pollinators.
Hadani says she looks at plants and flowers differently now. “There are a lot of songs we don’t hear.”
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