Israeli researchers have found that tomato and tobacco plants stressed, due to dehydration or because their stems have dried up, emit sounds of a volume comparable to that of a normal human conversation. The frequency of these noises is too high for our ears to detect, but they can probably be heard by insects, other mammals and, possibly, other plants, according to the journal 'Cell'.

"Even in a quiet field there are sounds that we don't hear and that contain information," says Lilach Hadany, an evolutionary biologist and theorist at Tel Aviv University--. There are animals that can hear those sounds, so there's a chance of a lot of acoustic interaction."

Although ultrasonic vibrations had already been recorded in plants, this is the first evidence that they are transmitted through the air, a fact that makes 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 very suboptimal for plants not to use sound at all," explains Hadany.

The researchers used microphones to record healthy, stressed tomato and tobacco plants, first in a soundproof acoustic chamber and then in a noisier greenhouse. They subjected the plants to stress by two methods: not watering them for several days and cutting off the stems.

After recording the plants, the researchers trained a machine-learning algorithm to differentiate between stress-free plants, thirsty plants and cut plants.

The team found that stressed plants make more sounds than non-stressed ones. A single stressed plant emits between 30 and 50 of these clicks per hour at seemingly random intervals, while non-stressed plants emit far fewer sounds. "When tomatoes aren't stressed, they're very quiet," Hadany says.

Water-stressed plants began to make noises before they were visibly dehydrated, and the frequency of sounds peaked after 5 days without water before decreasing again as the plants dried up completely.

The types of sound emitted differed depending on the cause of the stress. The machine-learning algorithm was able to accurately differentiate between dehydration and cut-off stress and was also able to discern whether the sounds came from a tomato or tobacco plant.

Although the study focused on tomato and tobacco plants for their ease of cultivation and standardization in the laboratory, the research team also recorded other plant species. "We found that many plants—corn, wheat, grapes, and cacti, for example—make sounds when they're stressed," Hadany says.

The exact mechanism of these noises is unclear, but the researchers suggest it could be due to the formation and rupture of air bubbles in the plant's vascular system, a process called cavitation.

It's also unclear whether plants produce these sounds to communicate with other organisms, but the fact that they exist has major ecological and evolutionary implications. "It's possible that other organisms evolved to hear and respond to these sounds," Hadany said--. For example, a moth that intends to lay eggs in a plant or an animal that intends to eat a plant could use sounds to guide its decision."

Gene expression

Other plants may also be hearing and benefiting from sounds. It is known from previous research that plants can respond to sounds and vibrations. Hadany and several other team members previously showed that plants increase the concentration of sugar in their nectar when they "hear" sounds emitted by pollinators, and other studies have shown that plants change their gene expression in response to sounds.

"If other plants have information about stress before it actually occurs, they could prepare," Hadany reasons.

According to the authors, sound recordings of plants could be used in agricultural irrigation systems to monitor the hydration status of crops and help distribute water more efficiently.

"We know there's a lot of ultrasound out there — every time you use a microphone, you find that a lot of things produce sounds that humans can't hear--, but the fact that plants produce these sounds opens up a whole new avenue of opportunities for communication, listening, and exploitation of these sounds," says co-senior author Yossi Yovel. neuroecologist from Tel Aviv University.

"Now that we know that plants make sounds, the next question is: who can be listening? "He wonders--. We are currently investigating the responses of other organisms, both animal and plant, to these sounds, and we are also exploring our ability to identify and interpret sounds in completely natural environments."

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