Plants Emit Ultrasonic Sounds When They Are Stressed

Roald Dahl may, it seems, have been right about something: when a plant is injured, it screams.

Well, in a sense. It does not scream as a person would. Instead, plants give off popping or clicking sounds at ultrasonic frequencies beyond human hearing, and these increase when a plant is under stress.

According to research published in 2023, this may be one method through which plants convey their 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 facing stress are less passive than they may appear. They can undergo striking changes, including releasing particularly potent scents - one of the clearest signs to humans. Their colour and form may change too.

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

Whether plants send out other forms of signals, including sound, had not been examined in full. Several years ago, Hadany and her team discovered that plants can perceive sound. The obvious follow-up was to investigate whether they could make sounds as well.

Recording tomato and tobacco plant sounds

To test this, the researchers recorded tomato and tobacco plants in several situations. They began with unstressed plants to establish a baseline, before recording dehydrated specimens and others whose stems had been cut. The sessions were conducted first in a soundproof acoustic chamber and then in an ordinary greenhouse.

Next, they 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, but they can be detected from more than a metre away. Unstressed plants make very little sound; they simply remain there, quietly getting on with 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. Plants lacking water show a distinctive sound pattern: they begin clicking more often before visible dehydration appears, with the rate rising as they become drier, then falling away as the plant wilts.

The algorithm could differentiate these sounds and identify the plant species that produced them. Nor is the phenomenon limited to tomato and tobacco. The researchers trialled a range of plants and concluded that producing sound seems to be a fairly widespread plant activity. They recorded wheat, maize, grape, cactus and henbit all making noises.

What causes plants to click?

Several questions remain unanswered. For instance, researchers do not yet know exactly how plants create these sounds. Earlier work found that dehydrated plants can undergo cavitation, in which air bubbles form, expand and collapse inside the stem. In human knuckle-cracking, this creates an audible pop; plants could be experiencing something similar.

It is also unknown whether other stressful circumstances trigger sound. Pathogens, attacks, UV exposure, extreme temperatures and other harmful conditions might likewise lead plants to pop like bubble wrap.

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

Those organisms may 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 might listen for the distress calls of thirsty plants and water them before dehydration becomes a problem.

Whether other plants detect and react to these noises remains unknown. Previous studies have found that plants can improve their drought tolerance in response to sound, making the possibility plausible. This is the direction in which the researchers are taking the next phase 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.