Stressed Plants Make Ultrasonic Clicking Sounds, Study Finds

It appears Roald Dahl may have been right in one respect: damage a plant and it screams.

At least, in a fashion. Plants do not scream as people do. Instead, when stressed, they produce popping or clicking sounds at ultrasonic frequencies beyond human hearing.

A 2023 study suggests these noises may be one means by which plants convey 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 when the study was released.

"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 signal danger

Plants experiencing stress are less passive than they may seem. They can undergo striking changes, including the release of strong scents, which is among the effects humans can most readily detect. Their colour and form may change too.

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

Whether plants give off other forms of signals, including sounds, had not been comprehensively investigated. Several years ago, Hadany and her team established that plants can perceive sound. The obvious follow-up was to determine whether plants could generate sound themselves.

Tomato and tobacco plants recorded under stress

To investigate, the researchers recorded tomato and tobacco plants in several situations. They first captured sounds from unstressed specimens to establish a baseline, then from dehydrated plants and from plants with cut stems. The work was carried out initially in a soundproof acoustic chamber and subsequently in an ordinary greenhouse.

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

The sounds emitted by plants resemble pops or clicks at frequencies too high for people to hear, and can be detected from more than a metre away. Unstressed plants make very little sound; they simply carry on quietly being plants.

Stressed plants, on the other hand, are considerably louder, producing an average of up to roughly 40 clicks an hour, depending on the species. Water-deprived plants also display a distinct sound pattern. Their clicking increases before visible dehydration appears, becomes more intense as they dry out, and then declines as the plant wilts.

The algorithm could differentiate these sounds and identify the plant species producing them. Nor is this limited to tomato and tobacco plants: after examining a range of species, the team concluded that making sounds seems to be a fairly widespread plant activity. Wheat, maize, grapevines, cacti and henbit were all recorded producing noise.

What creates plants' popping sounds?

Several questions remain unanswered. For instance, the mechanism behind these sounds is uncertain. Earlier research found that dehydrated plants can undergo cavitation, in which air bubbles form, enlarge and collapse in the stem. In the case of cracking human knuckles, this creates an audible pop; plants may be undergoing something similar.

It is also not yet known whether other forms of distress trigger sounds. Pathogens, attacks, UV exposure, temperature extremes and other unfavourable circumstances might likewise make plants pop like bubble wrap.

Nor is it clear whether producing sound is an adaptive trait in plants or merely an incidental occurrence. The team did show, however, that an algorithm can be trained to recognise and distinguish plant sounds. Other organisms may well have acquired the same ability.

Those organisms might also have learned to react to the sounds of distressed plants in different ways.

"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 potential application is straightforward: we could listen for the distress signals of thirsty plants and water them before dehydration becomes a problem.

Whether other plants can detect and respond to these signals remains unknown. Previous studies have shown that plants can improve their drought tolerance in response to sound, making the possibility plausible. This is the direction the team plans to pursue in 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.