Not only were stressed plants noisier, but the researchers claim that they gave off different sounds depending on what was happening. Angel House Studio/Shutterstock

Some plants emit “ultrasonic clicks” inaudible to the human ear when under life-threatening stress, a new study has found. Recordings reveal each sound contains information about a plant’s current state of being.

“These findings can alter the way we think about the Plant Kingdom, which has been considered to be almost silent until now,” write the study authors in a paper available to read on preprint server bioRxiv. The findings have not been published in a journal nor have they been certified by peer review, but the researchers say that their findings add to the scientific understanding of plant evolution and ecology around the world.

Previous studies have found that stress caused by changes in temperatures and light, as well as “herbivore attack,” can alter a plant’s phenotype, resulting in changes in color, smell, and shape. Stressed plants have also been found to give off volatile organic compounds. To examine whether plants make audible sounds as well, researchers at Tel Aviv University recorded tomato and tobacco plants that were deprived of water, having their stems cut, or were otherwise comfortable. Microphones focused on an ultrasonic sound range between 20 and 150 kHz.

Stressed plants were found to emit significantly more sounds than plants from the comfortable control group. Not only were they noisier, but the researchers claim that they gave off different sounds depending on what was happening; sounds that carried information about the physiological state of the plant. A specially designed machine learning model was reportedly capable of distinguishing between plant sounds and general background noise, going so far as to identify whether a plant was water-deprived or being cut.

Plant subjects were first placed in an acoustic box and recorded using two microphones. The process was later repeated in a greenhouse. bioRxiv

Without vocal cords, how might a plant give off sounds? The researchers say that it may be the result of an internal process known as cavitation, whereby air bubbles form and explode in the xylem. Previous research has found that cavitation can produce vibrations but has not been linked to the transmission of sound.

The sounds were also capable of being heard up to 5 meters (16 feet) away, a distance that researchers believe implies that other sensitive-hearing organisms, such as mice and moths, may also be able to pick up on and interpret such sounds.

“Our results suggest that animals, humans, and possibly even other plants, could use sounds emitted by a plant to gain information about the plant’s condition,” write the authors, adding that their work may help open new avenues to understanding plants and how they interact with their environment – particularly in an agricultural setting. Listening to plants may help to monitor whether or not a plant needs water, potentially saving usage and increasing yields in the future.