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Breakthrough: Plants emit sounds when they are stressed

In a groundbreaking study published in the journal Cell on March 30, researchers from Israel have discovered that stressed plants, specifically tomato and tobacco plants, emit ultrasonic noises resembling the sound of bubble-wrap being popped. 

These sounds, which are comparable in volume to normal human conversation, are beyond the range of human hearing but are likely detectable by insects, other mammals, and possibly even other plants. The research sheds new light on the largely unexplored world of acoustic communication among plants and other organisms.

“Even in a quiet field, there are actually sounds that we don’t hear, and those sounds carry information,” explained senior author Lilach Hadany, an evolutionary biologist and theoretician at Tel Aviv University. “There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring.”

While ultrasonic vibrations have previously been recorded in plants, this study provides the first evidence that these sounds are airborne, making them more relevant to other organisms within the environment. “Plants interact with insects and other animals all the time, and many of these organisms use sound for communication, so it would be very suboptimal for plants to not use sound at all,” said Hadany.

To conduct the study, the researchers recorded both healthy and stressed tomato and tobacco plants using microphones. They first placed the plants in a soundproofed acoustic chamber before moving them to a noisier greenhouse environment.

The plants were subjected to stress through two methods: stem severing and dehydration by withholding water for several days. After obtaining the recordings, the researchers used a machine-learning algorithm to differentiate between unstressed, thirsty, and cut plants.

The study’s results revealed that stressed plants emit significantly more sounds than their unstressed counterparts. The sounds produced by these plants resemble pops or clicks, with a single stressed plant emitting approximately 30–50 clicks per hour at seemingly random intervals. By contrast, unstressed plants produce far fewer sounds. “When tomatoes are not stressed at all, they are very quiet,” said Hadany.

Water-stressed plants emit noises before they show visible signs of dehydration, and that the frequency of these sounds peaks after five days without water before decreasing as the plants dry up completely. Furthermore, the types of sounds produced vary depending on the cause of stress. The researchers employed a machine-learning algorithm that was able to accurately differentiate between dehydration stress and stress caused by cutting, as well as discern whether the sounds originated from tomato or tobacco plants.

While the study focused primarily on tomato and tobacco plants due to their ease of cultivation and standardization in a laboratory setting, the research team also recorded sounds from a variety of other plant species. “We found that many plants – corn, wheat, grape, and cactus plants, for example – emit sounds when they are stressed,” said study senior author Lilach Hadany.

The exact mechanism behind the production of these noises remains unclear, but the researchers propose that it may be related to the formation and bursting of air bubbles within the plant’s vascular system, a process known as cavitation.

The question of whether plants produce these sounds with the intent to communicate with other organisms is still up for debate, but the mere existence of these sounds holds significant ecological and evolutionary implications. 

“It’s possible that other organisms could have evolved to hear and respond to these sounds. For example, a moth that intends to lay eggs on a plant or an animal that intends to eat a plant could use the sounds to help guide their decision,” said Hadany.

It is possible that other plants are also “listening in” and benefiting from these sounds. Prior research has demonstrated that plants can respond to sounds and vibrations. Hadany and several other members of the research team had previously shown that plants increase the sugar concentration in their nectar when they “hear” sounds made by pollinators. 

Moreover, other studies have revealed that plants alter their gene expression in response to sounds. “If other plants have information about stress before it actually occurs, they could prepare,” explained Hadany.

The sound recordings of plants have potential applications in agricultural irrigation systems. The authors suggest that these recordings could be used to monitor crop hydration status and, in turn, help distribute water more efficiently across the fields. By understanding the acoustic cues related to plant stress, farmers may be able to optimize water use and maintain healthier crops.

“We know that there’s a lot of ultrasound out there – every time you use a microphone, you find that a lot of stuff produces sounds that we humans cannot hear – but the fact that plants are making these sounds opens a whole new avenue of opportunities for communication, eavesdropping, and exploitation of these sounds,” said co-senior author Yossi Yovel, a neuro-ecologist at Tel Aviv University.

“So now that we know that plants do emit sounds, the next question is – ‘who might be listening?’” said Hadany. “We are currently investigating the responses of other organisms, both animals and plants, to these sounds, and we’re also exploring our ability to identify and interpret the sounds in completely natural environments.”

This groundbreaking research opens the door to further investigation into plant communication and its potential impact on the interactions between plants and other organisms in their surrounding ecosystems. Understanding the role of these sounds in the plant world could eventually lead to new insights in agriculture, pest management, and the broader understanding of the complex relationships between plants and their environments.

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