WHEN STRESSED PLANTS MAKE SOUNDS TO COMMUNICATE

For the first time in the world, researchers from Tel Aviv University recorded and analysed sounds clearly emitted by plants. The clicks, 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 sounds of plants are informative: they are emitted mainly when the plant is under stress, and they contain information about its state.

The researchers recorded mainly tomato and tobacco plants, but also wheat, corn and cacti. This means that an idyllic field of flowers can be quite a noisy place, even if we cannot hear the sounds.

The sounds that plants make when they are under stress vary, and each plant and each type of 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 insects.

The study was conducted and led by researchers at Tel Aviv University. The work was published in the prestigious scientific journal Cell. It was known from previous studies that vibrations were recorded when vibrometers were attached to plants, but until now it had not been verified whether these vibrations were converted into airborne sound waves, that is, into sounds that can be recorded at a distance. This is a question that researchers have debated for many years.

In the first phase of the study, the researchers placed the plants in a loudspeaker box in a quiet, isolated basement with no background noise. Ultrasonic microphones were placed at a distance of about 10 cm from each plant, recording sounds at frequencies of 20-250 kilohertz (the maximum frequency detected by an adult human is about 16 kilohertz).

Before placing the plants in the acoustic box, they were subjected to various treatments to stress them: some plants had not been watered for five days, in some the stem had been cut, while others were intact. The intention was to test if the the state of the plant. The recordings indicated that the plants made sounds at frequencies of 40-80 kilohertz. Unstressed plants made less than one sound per hour, on average, while stressed, dehydrated or injured plants made dozens of sounds every hour.

The recordings thus collected were analysed using machine learning (AI) algorithms specially developed for the experi- 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 in a greenhouse with a lot of background noise. In the greenhouse, the researchers followed the dehydrated plants over time and found that the amount of noise they made increased to a certain peak, and then decreased.

The researchers believe that humans can use this information if they have the right tools, such as sensors that tell growers when to water plants.

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