Plants Make Sounds Especially When Stressed, Study Reveals

VIVA – Researchers from Israel have found that stressed plants, specifically tomato and tobacco plants, emit ultrasonic sounds that make the sound of bubble wrap being opened, accirding to a study published in the journal Cell on March 30.

These sounds, which are comparable in volume to normal human conversation, are beyond the range of human hearing but can likely be detected by insects, other mammals, and perhaps even other plants.

"Even in quiet places, there are actually sounds that we don't hear, and they carry information," said senior author Lilach Hadany as a biologist and evolutionary theorist at Tel Aviv University, Israel on Friday.

"There are animals that can hear these sounds, so there are likely a lot of acoustic interactions going on," Hadany added.

While ultrasonic vibrations have previously been recorded in plants, this study provides the first evidence that these sounds are airborne, making them more relevant to other organisms in the environment.

"Plants interact with insects and other animals all the time, and many of these organisms use sound to communicate, so it would be suboptimal if plants didn't use sound at all," Hadany said. 

To conduct the study, the researchers recorded healthy and stressed tomato and tobacco plants using microphones. They first placed the plants in a soundproof acoustic chamber before moving them to a noisier greenhouse environment.

The plants were stressed through two methods: stem cutting and dehydration by withholding water for several days. After obtaining the recordings, the researchers used machine learning algorithms to distinguish between unstressed, thirsty, and cut plants.

The results showed that stressed plants emit significantly more sound than non-stressed plants.

The sounds produced by these plants resembled popping or clicking sounds, with one stressed plant emitting about 30-50 clicks per hour at seemingly random intervals. In contrast, non-stressed plants produced far fewer sounds.

"When tomatoes are not stressed at all, they are very quiet," Hadany informed. 

Water-deprived plants made sounds before showing signs of dehydration, and the frequency of these sounds peaked after five days without water before decreasing when the plants were completely dry.

In addition, the type of sound produced varied depending on the stressor. The researchers used a machine learning algorithm that could accurately distinguish between dehydration stress and stress caused by cutting, as well as tell whether the sound was coming from tomato or tobacco plants.

While the study focused primarily on tomato and tobacco plants due to ease of cultivation and standardization in the lab, the research team also recorded sounds from a variety of other plant species.

"We found that many plants – corn, wheat, grapes, and cactus plants, for example, make sounds when stressed," Hadany stated.