2023 study suggests plants “scream” with ultrasonic clicks when stressed

It turns out Roald Dahl might not have been entirely wrong: when you damage a plant, it can “scream”.

Not as you or I would, of course. Instead, stressed plants give off popping and clicking noises at ultrasonic frequencies beyond human hearing, and those emissions rise as the plant comes under strain.

A 2023 study suggests this could be one route by which plants broadcast distress to their surroundings.

"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."

From Roald Dahl to plant stress signals

Plants experiencing stress are not nearly as inert as they may seem. They can go through striking shifts, including-at least to human senses-the release of potent aromas. They may also change their colour and their form.

Such responses can act as warnings to neighbouring plants, prompting them to ramp up their own defences, or they can draw in animals that deal with the pests harming the plant.

Even so, the question of whether plants send out other signals-sound, for instance-has not been examined in depth. A few years before this work, Hadany and colleagues showed that plants are able to sense sound. The natural follow-up was whether plants also generate sound themselves.

How the team recorded tomato and tobacco plants

To investigate, the researchers made recordings of tomato and tobacco plants under several conditions. They first captured audio from plants that were not stressed, to establish a baseline. After that, they recorded plants that had been deprived of water, as well as plants whose stems had been cut.

Recordings were carried out in two settings: initially inside a soundproof acoustic chamber, and then again in a standard greenhouse environment.

Next, the team trained a machine learning algorithm to tell apart the sounds associated with unstressed plants, cut plants, and dehydrated plants.

What the ultrasonic clicks revealed

The noises produced by plants resemble pops or clicks at frequencies far too high for humans to perceive. They can be picked up from more than 1 metre away (3.3 feet). Plants that are not stressed make very little sound; they simply sit there, quietly getting on with being plants.

Stressed plants, however, are markedly louder. Depending on the species, they produced an average of up to roughly 40 clicks per hour. Plants lacking water also displayed a distinctive pattern: they began clicking more before obvious visual signs of dehydration appeared, intensified as the plant became drier, and then eased off as the plant ultimately withered.

The algorithm successfully separated these sound patterns and could also identify which plant species had made them. And the phenomenon was not confined to tomatoes and tobacco. The researchers tested other species and found that sound production seems to be widespread. Wheat, corn, grape, cactus, and henbit were all recorded producing noise.

Open questions: cavitation, other stresses, and who is listening?

Plenty remains uncertain. One key mystery is the mechanism behind the sounds. Earlier studies have shown that dehydrated plants can undergo cavitation, in which air bubbles form within the stem, grow, and then collapse. In humans, cracking knuckles can create an audible pop through a similar process; something along these lines could be occurring in plants.

It is also unknown whether different kinds of distress might trigger sound. Infection, physical attack, UV exposure, temperature extremes, and other unfavourable conditions could potentially set plants off, popping like bubble wrap.

Another open issue is whether plant sound production is an adaptation shaped by evolution, or simply a by-product of other processes. What the team did demonstrate is that an algorithm can be trained to recognise and discriminate between plant sounds, which means it is entirely plausible that other organisms could have learned to do something similar.

Those organisms might also have evolved responses to the sounds of plants in distress.

"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 takeaway is straightforward: we might be able to monitor the distress calls of thirsty plants and water them before the problem becomes severe.

Whether other plants can hear and react is still unclear. Past research has shown that plants can improve their drought tolerance in response to sound, making the idea plausible. This is the direction the team says it will pursue next.

"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.