Like popcorn or bubbles in packing plastic, this is how plants sound when they are under stress. In a series of experiments, a group of Israeli scientists has verified that they emit sounds. The most striking thing is that their sound profile is different depending on whether they lack water, have a stem cut or are infected by a virus. They were also able to differentiate between sound waves coming from a tomato plant, tobacco or a cactus. Does this mean that plants speak or have communicative intent? No, the explanation is much simpler and is from basic physics.
Plants respond to changes in the environment, especially if it causes them stress: they lose their greenness, their leaves droop, and they become withered. Recent research has also shown that they can communicate chemically, releasing volatile substances when attacked by insects. In 2019, a team led by biologist and mathematician Lilach Hadany, from Tel Aviv University (Israel), also showed that the flowers raised the concentration of sugars in their nectar within minutes of hearing the buzz of pollinators. Now, Hadany and his colleagues have shown that they also make sounds and that these contain information.
The series of experiments began in a soundproof room. In it they placed dozens of tomato and tobacco plants, both common in laboratories. Since they had never been heard before, they assumed that if they made sounds, they must be in parts of the spectrum that humans cannot hear. So they were placed near ultrasound microphones, capable of recording sound waves with a frequency between 20 and 250 kilohertz (kHz), when an adult human rarely hears beyond 16 kHz. Some of them were left without water for five days, while others had some of the stems cut off. For comparison, they left some alone.
“Any system of pipes that transports a fluid generates sounds and that does not mean that a water pipe is trying to communicate with anyone”
Carlos Vicient, Agricultural Genomics Research Center
The results, published in the scientific journal Cell, show that the cared-for plants emitted on average one sound per hour. However, those stressed by drought increased their noise to 35 per hour for tomato plants, and 11 per hour for tobacco. In the case of the cuts, the sounds were 25 and 15 respectively. A striking detail is that the tomato plants increased the number of sounds as the days without water passed. But, at a given moment, when dehydration threatened their lives, they slowed down until they stopped emitting them. The researchers designed an algorithm with these first data and repeated the experiments in a more realistic environment, a greenhouse. This artificial intelligence was able to detect which bushes were making sounds, how many, and what type of stressor was causing them.
“In this study we resolve a very old scientific controversy: we prove that plants emit sounds,” says Hadany. “Our findings suggest that the world around us is full of plant sounds and that they contain information, for example, about water scarcity or about injuries. We assume that the sounds emitted in nature are detected by nearby creatures, such as bats, rodents, various insects and possibly other plants as well,” adds the Israeli biologist.
The phenomenon is not limited to tomato and tobacco. The researchers repeated their experiments with corn, wheat, nettles or cacti. In all cases, they made sounds. In addition, they found that the plants also emitted them when they were infected with the tobacco mosaic virus, a pest of Solanaceae. Some of the ultrasound waves were picked up by microphones up to five meters away. What the Israeli scientists also saw is that in the most woody plants, such as the almond tree or the vine, the sound production was almost negligible.
The study authors aren’t sure why the latter is the case. “There is a possibility that the cortex is interfering or that they are emitting sounds at other frequencies. We need to study it more,” Hadany says in an email. They are also not entirely clear on how these sounds are produced. Discarded the consciousness of plants that some defend, since in the plant kingdom there is nothing similar to a brain or a nervous system that controls these communications, why do plants produce ultrasound? The researchers suggest that it could be due to the formation and bursting of air bubbles in the vascular system of the plant, in a phenomenon known as cavitation.
“Our findings suggest that the world around us is full of plant sounds and that these sounds contain information.”
Lilach Hadany, Tel Aviv University
The researcher at the Agricultural Genomics Research Center Carlos Vicient, who has been able to review Hadany’s study, summarizes what cavitation is: “It is a dysfunction that occurs in the water-conducting vessels of plants (or any other type of conduit ) when they are subjected to some type of water stress. The water columns break and air bubbles are generated. This process can generate sounds.” But he immediately adds: “The fact that a plant emits sounds does not mean that it is communicating with its congeners. Any pipe system that transports a fluid generates sounds and that does not mean that a water pipe is trying to communicate with anyone.
The professor of genetics at the Polytechnic University of Valencia, José Blanca, who knows the history and genetics of tomatoes very well, agrees with Vicient. “The sounds seem to be due to something very mundane, fluids moving through tubes. In other words, no plant intelligence or deliberate communication, ”he highlights. The authors mention a series of possible ecological implications, as Blanca suggests: “That there are species, for example mice, that take advantage of these sounds in some way, but this is completely speculative, they have not carried out any experiments or observations in this regard.”
This is where Hadany qualifies: “We have no idea about the intention, and we don’t even know if the sounds are used for communication. But now we are studying the responses of animals and plants, which could give us clues about this.” And he adds: “It must be taken into account that even if there is no intention and the sounds are emitted passively, they contain information, so natural selection could be acting on the organisms for which plant stress is relevant, favoring listening and response to sound. But this is entirely theoretical for now.”
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