Plants make a popping sound – a bit like bubble wrap – when they are stressed.
Even though the volume is comparable to people speaking, humans can’t hear plant noises because of their high frequency.
Researchers from Tel Aviv University in Israel used two microphones to record sounds emitted by individual tomato and tobacco plants. They recorded noises under normal conditions, and when the plants were stressed from dehydration or having their stems severed.
The research is published in the journal Cell.
“Even in a quiet field, there are actually sounds that we don’t hear, and those sounds carry information,” says professor Lilach Hadany, an evolutionary biologist and theoretician at Tel Aviv University.
Humans might not be able to hear them, but sounds emitted by stressed plants might be audible to insects, other mammals and even other plants, she says.
Hadany says, it’s possible other organisms might hear and respond to plant noises, for instance helping a moth decide where to lay its eggs or which plants to eat.
The researchers’ microphones recorded healthy and stressed tomato and tobacco plants, first in a soundproofed acoustic chamber and then in a noisier greenhouse environment.
They deliberately stressed the plants by not watering them for several days and by cutting their stems.
The researchers then trained a machine-learning algorithm to differentiate between unstressed plants, thirsty plants, and cut plants. The algorithm could also tell the difference between sounds from a tobacco or tomato plant.
“When tomatoes are not stressed at all, they are very quiet,” says Hadany.
Under stressful situations plants emitted many more sounds, some 30 – 50 pops or clicks per hour, at seemingly random intervals.
Thirsty plants began emitting noises before they were visibly dehydrated. After five days with no water, the frequency of sounds peaked before decreasing again as the plants dried up completely.
Although the study focused on tomato and tobacco plants because of their ease to grow and standardise in the laboratory, the research team also recorded a variety of other plant species.
“We found that many plants—corn, wheat, grape, and cactus plants, for example—emit sounds when they are stressed,” says Hadany.
While the exact mechanism behind the plant noises is unclear, the researchers suggest the sounds might be due to the formation and bursting of air bubbles in the plant’s vascular system, a process called cavitation.
Sound recordings of plants could potentially be useful in agricultural irrigation systems to monitor crop hydration status and help distribute water more efficiently, the authors say.