Plants Sound-Off When Under Stress: Study

Lilach Hadany, professor at the School of Plant Sciences and Food Security at the Wise Faculty of Life Sciences, Tel Aviv University in an undated photo. Courtesy of Tel Aviv University

Plants under stress emit ultrasonic sounds that are specific to each species and to the type of stress they experience, according to a recent study.

The click-like sounds emitted by plants—similar to the popping of popcorn—that were recorded and analyzed for the first time, can probably be heard by various animals, such as bats, mice, and insects, yet are beyond the hearing range of the human ear.

The study mainly analyzed tomato and tobacco plants, yet wheat, corn, cactus, and henbit were recorded as well.

The Israeli study, led by researchers at Tel Aviv University, was published on March 30 in the scientific journal Cell.

The study was led by Lilach Hadany, a professor at the School of Plant Sciences and Food Security at the Wise Faculty of Life Sciences, and professor Yossi Yovel, head of the Sagol School of Neuroscience and faculty member at the School of Zoology and the Steinhardt Museum of Natural History, in collaboration with researchers from the Raymond and Beverly Sackler School of Mathematical Sciences, the Institute for Cereal Crops Research, and the Sagol School of Neuroscience.

Cactus plant with ultrasonic microphones in Israel, in an undated photo. Courtesy of Tel Aviv University

Airborne Soundwaves

Previous studies showed that vibrometers attached to plants recorded vibrations, said Hadany in a press statement. The current study addressed the question of whether these vibrations also become airborne soundwaves—sounds that can be recorded from a distance, she said.

“Our study addressed this question, which researchers have been debating for many years,” she said.

In the first stage of this study, plants were placed in an acoustic box in a quiet, isolated basement with no background noise.

Ultrasonic microphones recording sounds at frequencies of 20–250 kilohertz were placed about 4 inches from each plant. The maximum frequency a human adult can detect is about 16 kilohertz.

Before placing the plants in the acoustic box they were subjected to different treatments: some had not been watered for five days, some had the stem cut, and some were left untouched.

The recordings indicated that the plants emitted sounds at frequencies of 40–80 kilohertz.

Plants that were not under stress emitted on average less than one sound per hour. The stressed plants, which were either dehydrated or injured, produced dozens of sounds every hour. Drought-stressed plants emitted on average 35.4 ± 6.1 and 11.0 ± 1.4 sounds per hour for tomato and tobacco, respectively, and cut tomato and tobacco plants emitted 25.2 ± 3.2 and 15.2 ± 2.6 sounds per hour, respectively.

The recordings of the plants were analyzed by specially developed machine learning (AI) algorithms. The algorithms were trained to distinguish different plants and different types of sounds and were able to identify the plant and determine the type and level of stress from the recordings.

The algorithms identified and classified plant sounds even when the plants were placed in a greenhouse with a lot of background noise.

The researchers succeeded in differentiating between the dry and cut plants according to the sounds they emitted with an accuracy of 70 percent. The accuracy of the distinction between drought-stressed and control plants in a greenhouse based on their sounds was 84 percent.

The researchers also monitored the distribution of the emitted sounds with respect to days of dehydration, time of the day, soil moisture, and transpiration rate.

They found that the hourly pattern of sound emission correlated with the plant’s transpiration rate. The number of sounds emitted by plants that were subjected to dehydration over time in the greenhouse increased during the first days up to a certain peak and then declined as the plant dried up.

The sounds emitted by plants that were at high and low levels of dehydration were found to be different, and the researchers were able to classify them with an accuracy of 81 percent.

One possible mechanism that may be responsible for the emission of at least part of the sounds “is cavitation in the stem” the researchers wrote. Cavitation is a process where “air bubbles form, expand and collapse in the xylem”—the tissue that transports water and nutrients from the root to the stem and the leaves—causing vibrations.

Eavesdropping on a cut plant in Israel, in an undated photo. Courtesy of Tel Aviv University

Monitoring Moisture or Disease

The findings suggest that the world around us is full of plant sounds and that these sounds contain information—for example about water scarcity or injury, Hadany said.

The researchers assume that these emitted sounds can be detected by creatures nearby, such as bats, rodents, various insects, and possibly other plants—that are able to hear the high frequencies and derive relevant information, according to Hadany.

“We believe that humans can also utilize this information, given the right tools—such as sensors that tell growers when plants need watering,” she said.

A potential application of the findings could be for monitoring plants in the field or greenhouse, according to the study. Sounds emitted by plants could allow the monitoring of crops for moisture and disease. More accurate irrigation “can save up to 50% of the water expenditure and increase the yield,” the researchers wrote.

In future studies, the scientists plan to examine additional questions such as what mechanism is behind the plants’ sounds and whether other plants can hear them as well.