Stressed Plants Make Ultrasonic Clicking Sounds, Study Finds

Roald Dahl may, it seems, have been right about something: damage a plant and it screams.

Not quite in the way a person would scream, of course. Instead, stressed plants release ultrasonic popping or clicking sounds that lie beyond human hearing.

How plants communicate stress through sound

A 2023 study suggests these noises could be one means by which plants signal distress to the environment around them.

"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," explained evolutionary biologist Lilach Hadany of Tel Aviv University in Israel when the research was published.

"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."

Plants facing difficult conditions are not as inactive as they may appear. They can undergo striking changes, including the release of strong scents – among the alterations most readily noticed by humans. Their colour and form can change as well.

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

Whether plants sent other signals, including sounds, had not been thoroughly investigated. Several years earlier, Hadany and her team established that plants can detect sound. The obvious follow-up was to determine whether plants could make sounds themselves.

Recording ultrasonic sounds from tomato and tobacco plants

To investigate, the researchers recorded tomato and tobacco plants in several states. They began with unstressed plants to establish a baseline, then recorded dehydrated plants and plants whose stems had been cut. The work was conducted first in a soundproof acoustic chamber and later in an ordinary greenhouse.

The team then trained a machine-learning algorithm to tell apart sounds from unstressed, cut and dehydrated plants.

Plant noises resemble pops or clicks at pitches too high for people to hear, and can be detected from more than a metre away. Plants without stress make very little sound, simply continuing their quiet plant lives.

Stressed specimens, however, are considerably louder, producing an average of up to around 40 clicks an hour, depending on the species. Water-starved plants also have a distinct sound pattern: they begin clicking more frequently before visible dehydration develops, intensify the clicking as they become drier, and then become quieter as they wither.

The algorithm could identify both these different sounds and the plant species that produced them. Nor is the phenomenon limited to tomatoes and tobacco. After examining a range of plants, the researchers found that sound production seems to be a fairly widespread plant behaviour. Wheat, maize, grape, cactus and henbit were all recorded producing noises.

What remains unknown about plant distress calls

Several questions remain unanswered. For instance, the mechanism that creates the sounds is unclear. Earlier research found that dehydrated plants can undergo cavitation, in which air bubbles form, expand and collapse within the stem. In human knuckle cracking, this process creates an audible pop; plants could be experiencing something comparable.

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

Nor is it certain whether plant sound production is an adaptive trait or merely an incidental occurrence. However, the team demonstrated that an algorithm can learn to recognise and distinguish plant sounds. Other organisms may well have developed the same ability.

Those organisms might also have learned to react in different ways to the sounds made by distressed 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 potential use 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 sounds remains uncertain. Previous studies have found that plants may improve their drought tolerance in response to sound, making the possibility plausible. This is the direction in which the researchers intend to take the next stage of their 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.