Stressed Plants Make Ultrasonic Sounds, Study Finds

It appears Roald Dahl may have been right in one respect: when a plant is harmed, it screams.

Well, in a manner of speaking. Plants do not scream as people do; instead, they produce ultrasonic popping or clicking sounds beyond the limits of human hearing. These noises become more frequent when a plant is under stress.

A 2023 study suggests this may be one method by which plants communicate distress to the surrounding world.

"Even in a quiet field, there are actually sounds that we don't hear, and those sounds carry information. There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring," said evolutionary biologist Lilach Hadany of Tel Aviv University in Israel.

"Plants interact with insects and other animals all the time, and many of these organisms use sound for communication, so it would be very suboptimal for plants to not use sound at all."

How stressed plants communicate

Plants experiencing stress are not as inactive as they may seem. They undergo striking changes, including the release of strong scents - one of the effects most readily noticed by humans. Their colour and form can change too.

Such changes may warn nearby plants of danger, prompting them to strengthen their own defences. They can also draw in animals that deal with pests damaging the plant.

Yet it has not been fully investigated whether plants send out other signals, including sounds. Several years ago, Hadany and her colleagues discovered that plants can perceive sound. The obvious follow-up was to determine whether they could make sounds as well.

Recording ultrasonic sounds from plants

To investigate, the researchers recorded tomato and tobacco plants under several conditions. They first captured sounds from unstressed plants to establish a baseline, before recording dehydrated plants and plants whose stems had been cut. The work was carried out initially in a soundproof acoustic chamber and later in an ordinary greenhouse.

They then taught a machine-learning algorithm to tell apart sounds made by unstressed, cut and dehydrated plants.

The sounds produced by plants resemble pops or clicks at frequencies far too high for people to hear, although they can be detected from more than a metre away. Unstressed plants make very little sound; they simply go about their quiet plant business.

Stressed plants, however, are considerably louder, producing an average of up to around 40 clicks per hour, depending on the species. Plants lacking water also display a distinctive sound pattern. Their clicking increases before any visible symptoms of dehydration appear, becoming more intense as they dry out and then declining as the plant wilts.

The algorithm could differentiate these sounds and identify the plant species responsible for them. Nor is the phenomenon limited to tomato and tobacco. The team examined a range of plants and found that making sounds seems to be a widespread plant activity. Wheat, maize, grape, cactus and henbit were all recorded producing noise.

Why plants make clicking noises

Several questions remain unanswered. For instance, the way these sounds are generated is unclear. Earlier research found that dehydrated plants can undergo cavitation, in which air bubbles form, grow and collapse in the stem. In humans, this process creates the audible pop of cracking knuckles; a similar mechanism may be at work in plants.

It is also not yet known whether other stressful circumstances trigger sounds. Pathogens, attacks, UV exposure, extreme temperatures and other harmful conditions could potentially make plants pop like bubble wrap.

Nor is it clear whether sound production is an adaptive trait in plants or merely an incidental effect. The researchers did demonstrate, though, that an algorithm can be trained to recognise and distinguish plant sounds. Other organisms may well have learned to do this too.

Those organisms might also have developed different responses to the noises made by stressed plants.

"For example, a moth that intends to lay eggs on a plant or an animal that intends to eat a plant could use the sounds to help guide their decision," Hadany said.

For humans, the possible application is straightforward: we could listen for the distress calls of thirsty plants and water them before the problem develops.

Whether other plants detect and react to these sounds remains unknown. Previous studies have shown that plants can improve their drought tolerance in response to sound, making this a plausible possibility. This is the direction in which the team intends to take the next phase of its work.

"Now that we know that plants do emit sounds, the next question is – 'who might be listening?'" Hadany said.

"We are currently investigating the responses of other organisms, both animals and plants, to these sounds, and we're also exploring our ability to identify and interpret the sounds in completely natural environments."

The research was published in Cell.

An earlier version of this article was published in March 2023.