For the first time, it has been possible to record and analyze sounds emitted by plants. The sounds, similar to the popping of popcorn, are emitted at a volume similar to that of human speech, but at high frequencies, beyond the range of hearing of the human ear.
The authors of the finding have verified that plants, at least those of the classes examined, tend to make these sounds when they are subjected to physiological stress, and that each plant and each type of physiological stress is associated with a specific identifiable sound. Although imperceptible to the human ear, the sounds emitted by plants can probably be heard by various animals, such as bats, mice, and many insects.
The discovery was made by a team made up of, among others, Lilach Hadany, Yossi Yovel, Itzhak Khait and Ohad Lewin-Epstein, from Tel Aviv University in Israel.
From previous studies, it was known that vibrometers attached to plants record vibrations. But do these vibrations also become airborne sound waves, that is, sounds that can be recorded at a distance? The new study has answered this question.
In the first phase of the study, the researchers placed plants in a speaker box in a quiet, isolated basement with no background noise. At a distance of about 10 centimeters from each plant, ultrasonic microphones were placed that recorded sounds at frequencies between 20 and 250 kilohertz (the maximum frequency detected by an adult human is usually about 16 kilohertz). The study focused primarily on tomato and tobacco plants, but tests were also done on a few other plants, including cacti and wheat and corn plants.
Before placing the plants in the acoustic box, the researchers subjected them to various situations capable of generating physiological stress: some plants had not been watered for five days, others had had a cut in the stem and some were intact. The intention was to test if the plants made sounds, and if these sounds were somehow affected by the state of the plant.
A cactus surrounded by microphones. (Photo: Tel Aviv University. CC BY)
The recordings indicated that the plants in the experiment made sounds at frequencies between 40 and 80 kilohertz. Unstressed plants made less than one sound per hour, on average, while stressed plants, both dehydrated and injured, made dozens of sounds every hour.
The recordings thus collected were analyzed using specially developed machine learning algorithms (a form of artificial intelligence). The algorithms learned to distinguish between 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 the sounds of the plants even when they were placed in a greenhouse with a lot of background noise.
The findings in this study suggest that the world around us is full of plant sounds and that these sounds contain information about what is happening to plants. Presumably in nature, the sounds emitted by plants are detected by nearby animals, such as bats, rodents and various insects, which can hear ultrasound, and which can probably extract relevant information. Perhaps even those sounds are also heard by other plants.
“Apparently, an idyllic field of flowers can be quite a noisy place, we just can’t hear that noise,” Hadany comments wryly.
The study is entitled “Sounds emitted by plants under stress are airborne and informative”. And it has been published in the academic journal Cell.
In future studies within this line of research, Hadany and his colleagues will continue to search for answers to several intriguing questions: What is the underlying mechanism of plant sound generation? How do insects with ultrasonic hearing react to sounds made by plants? Do plants pick up the sounds emitted by their neighbors? And many others. (Fountain: NCYT de Amazings)