For the first time in the world, Tel Aviv University (TAU) researchers have recorded and analyzed sounds distinctly emitted by plants. They found that the sounds contain information about the plant’s condition, and they are emitted mostly when it is under stress.
The click-like sounds, similar to the popping of popcorn, are emitted at a volume similar to human speech, but at high frequencies beyond the hearing range of the human ear.
“We found that plants usually emit sounds when they are under stress and that each plant and each type of stress is associated with a specific identifiable sound,” the researchers wrote in a research paper that was published in the prestigious scientific journal Cell under the title “Sounds emitted by plants under stress are airborne and informative.”
“Stressed plants show altered phenotypes, including changes in color, smell and shape. Yet, airborne sounds emitted by stressed plants have not been investigated before. While imperceptible to the human ear, the sounds emitted by plants can probably be heard by various animals, such as bats, mice, and insects.”
“Apparently, an idyllic field of flowers can be a rather noisy place,” the researchers wrote. “It’s just that we can’t hear the sounds!”
The study was led by Prof. Lilach Hadany from TAU’s School of Plant Sciences and Food Security at the Wise Faculty of Life Sciences, together with Prof. Yossi Yovel, head of the Sagol School of Neuroscience and a faculty member at the School of Zoology and the Steinhardt Museum of Natural History. He was assisted by research students Itzhak Khait and Ohad Lewin-Epstein, in collaboration with researchers from TAU’s Raymond and Beverly Sackler School of Mathematical Sciences at the Institute for Cereal Crops Research and the Sagol School of Neuroscience.
“From previous studies, we know that vibrometers attached to plants record vibrations,” Hadany said. “But do these vibrations also become airborne sound waves – namely, sounds that can be recorded from a distance? Our study addressed this question, which researchers have been debating for many years.”
How did the researchers come to their conclusions?
In the first stage of the study, the researchers put plants in an acoustic box in a quiet, isolated basement with no background noise. Ultrasonic microphones recording sounds at frequencies of 20 to 250 kilohertz (the maximum frequency detected by a human adult is about 16 kilohertz) were set up at a distance of about 10 centimeters from each plant. The study focused mainly on tomato and tobacco plants, but wheat, corn, cactus and henbit also were recorded.
Plants differ visually, with respect to both color and shape, from unstressed plants. They also emit volatile organic compounds (VOCs) when exposed to drought or herbivores. VOCs can also affect neighboring plants, resulting in increased resistance in these plants. It has been demonstrated that organisms can respond to visual, chemical and tactile cues produced by plants. Nevertheless, the ability of plants to emit airborne sounds that could potentially be heard by other organisms has not been sufficiently explored, the team said.
“Before placing the plants in the acoustic box, we subjected them to various treatments – some plants had not been watered for five days; in some, the stem had been cut, and some were untouched,” Hadany said. “Our intention was to test whether the plants emit sounds and if these sounds are affected in any way by the plant’s condition. Our recordings indicated that the plants in our experiment emitted sounds at frequencies of 40 to 80 kilohertz. Unstressed plants emitted less than one sound per hour on average, while the stressed plants – both dehydrated and injured – emitted dozens of sounds every hour.”
The recordings they collected were analyzed by specially developed machine learning (AI) algorithms that learned how to distinguish among different plants and different types of sounds and were ultimately able to identify the plant and determine the type and level of stress from the recordings.
In addition, the algorithms identified and classified plant sounds even when the plants were put in a greenhouse with a great deal of background noise. In the greenhouse, the researchers monitored plants subjected to a process of dehydration over time and found that the number of sounds they emitted increased up to a certain peak and then diminished.
“In this study, we resolved a very old scientific controversy: We proved that plants do emit sounds! Our 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. “We assume that in nature, the sounds emitted by plants are detected by creatures nearby, such as bats, rodents, various insects and possibly also other plants that can hear the high frequencies and derive relevant information. We believe that humans can also use this information, given the right tools – such as sensors that tell growers when plants need watering.”
In future studies, the researchers said they would continue to explore a range of intriguing questions, including: What is the mechanism behind plant sounds? How do moths detect and react to sounds emitted by plants? Do other plants also hear these sounds?