Plants Make Ultrasonic Sounds When They Are Stressed

Roald Dahl may, it appears, have been right about one thing: when a plant is harmed, it screams.

At least, in a sense. Plants do not scream as people do; instead, they produce ultrasonic popping or clicking sounds beyond human hearing, and these become more frequent when a plant is under stress.

A study published in 2023 suggests that this may be among the ways 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.

"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

Stressed plants are less passive than they may seem. They can undergo striking changes, including releasing strong scents - perhaps the most readily detectable change for humans. Their colour and form can change, too.

Such shifts may 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.

Yet the question of whether plants send other signals, including sounds, had not been fully investigated. Several years ago, Hadany and her team established that plants can detect 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 began with unstressed plants to establish a baseline, before recording dehydrated plants and others whose stems had been cut. The work was carried out first in a soundproof acoustic chamber and later in an ordinary greenhouse.

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

The noises produced by plants resemble pops or clicks at pitches far too high for people to hear, although they can be detected from more than 1 metre away. Unstressed plants are almost silent, simply getting on with being plants.

Plants experiencing stress, however, are considerably louder, making an average of up to roughly 40 clicks an hour depending on the species. Water-deprived plants, in particular, produce a distinct sound pattern. Their clicking intensifies before they display visible dehydration, increases as they become drier, then diminishes as the plant wilts.

The algorithm could differentiate these sounds and identify the plant species that made them. Tomato and tobacco are not alone, either. The researchers examined a range of species and found that making sound seems to be a fairly widespread plant activity. Wheat, maize, grape, cactus and henbit were all recorded producing noise.

What causes plants to click?

Several questions remain unanswered. For one, researchers do not yet know exactly how plants make these sounds. Earlier studies found that dehydrated plants undergo cavitation, in which air bubbles form, expand and collapse within the stem. In human knuckle-cracking, this creates an audible pop, and a comparable process may occur in plants.

Nor is it known whether other forms of distress can trigger sound. Pathogens, attacks, UV exposure, extreme temperatures and other unfavourable conditions might also make plants pop like bubble wrap.

It is similarly uncertain whether producing sounds is an adaptive trait in plants or simply an incidental occurrence. The team's findings did show that an algorithm can be trained to recognise and distinguish plant sounds, however. Other organisms may well have acquired the same ability.

Those organisms might also have learnt 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 calls of thirsty plants and water them before dehydration becomes a problem.

Whether other plants can detect and respond to these sounds remains unknown. Previous research has found that plants can improve their drought tolerance in response to sound, making the possibility entirely plausible. This is the direction the team intends to pursue in the next phase of its research.

"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.