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In a new and definitely pathbreaking research, biologists at Tel Aviv University recorded ultrasonic sounds emitted by tomato and tobacco plants inside an acoustic chamber, and in a greenhouse, while monitoring the plants’ physiological parameters. They developed machine learning models that succeeded in identifying the condition of the plants, including dehydration level and injury, based solely on the emitted sounds. The frequency of these sounds is too high for human ears to detect, but they can probably be detectable by other organisms such as insects, mammals, and possibly other plants. The scientists may not be sure of exactly what is causing the emitted sounds and say it could simply be bubbles forming and popping within the plants’ water-carrying tissues. But, it has led to an interesting revelation anyway.
To investigate plants’ airborne sound emissions, the scientists constructed a reliable recording system, where each plant was recorded simultaneously by two microphones; first, they recorded plants within an acoustic box and developed machine learning algorithms to classify the recorded sounds; then they tested the system in a greenhouse, while monitoring physiological parameters of the recorded plants. Before placing the plants in the acoustic box, the researchers subjected them to various treatments: some plants had not been watered for five days, in some the stem had been cut, and some were untouched. They intended to test whether the plants emit sounds, and whether these sounds are affected in any way by the plant's condition: "Our recordings indicated that the plants in our experiment emitted sounds at frequencies of 40-80 kilohertz. Unstressed plants emitted less than one sound per hour, on average, while the stressed plants – both dehydrated and injured – emitted dozens of sounds every hour. Even in a quiet field, there are actually sounds that we don’t hear, and those sounds carry information,” said Tel Aviv University’s Professor Lilach Hadany, senior author of the study.
Plants show significant changes in their phenotypes in response to stress. They differ visually- with respect to both colour and shape- from unstressed plants. They also emit volatile organic compounds, e.g. when exposed to drought or herbivores. These compounds can also affect neighboring plants, resulting in increased resistance in these plants. Altogether, plants have been demonstrated to produce visual, chemical, and tactile cues, which other organisms can respond to. Nevertheless, the ability of plants to emit airborne sounds — that could potentially be heard by other organisms — has not been sufficiently explored. She added: “There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring. Although ultrasonic vibrations have been recorded from plants before, this is the first evidence that they are airborne, a fact that makes them more relevant for other organisms in the environment.”
Hadany’s team has tried something new. She and her colleagues at Tel Aviv University set up ultrasonic microphones next to, but not touching, living plants. The team wanted to find out if the plants could generate airborne sounds — vibrations that travel through the air. The researchers first detected the horticultural hiccups coming from plants set up on tables in the lab. But the team couldn’t be sure that something else wasn’t making the noises. So, the researchers ordered sound-dampening acoustic boxes and tucked them in the basement away from the lab’s hustle and bustle. Inside the hushed boxes, thirsty tomato plants emitted about 35 ultrasonic clicks per hour, the team found. Tomato plants cut at the stem were slightly less noisy, and tobacco plants clicked even less. Plants not water-stressed or chopped kept mostly quiet. The plants’ short sounds were about as loud as a typical conversation, but too high-pitched for humans to hear (though dogs’ ears might perk up). And each plant species had a recognizable “voice”. A machine learning algorithm the team created could tell the difference between clicks from tomato plants and tobacco plants. It could also pick out thirsty and hydrated plants. The algorithm could even differentiate between plants when they sat in a noisy greenhouse, filled with the sounds of people talking and building renovations next door.
In their research, Professor Hadany and colleagues used microphones to record healthy and stressed tomato and tobacco plants, first in a soundproofed acoustic chamber and then in a noisier greenhouse environment. They stressed the plants via two methods: by not watering them for several days and by cutting their stems. After recording the plants, the researchers trained a machine-learning algorithm to differentiate between unstressed plants, thirsty plants, and cut plants. They found that stressed plants emit more sounds than unstressed plants. The plant sounds resembled pops or clicks, and a single stressed plant emitted around 30-50 of these clicks per hour at frequencies of 40-80 kHz and seemingly random intervals, but unstressed plants emitted far fewer sounds. “When tomatoes are not stressed at all, they are very quiet,” Professor Hadany said.
Water-stressed plants began emitting noises before they were visibly dehydrated, and the frequency of sounds peaked after five days with no water before decreasing again as the plants dried up completely. The types of sound emitted differed with the cause of stress. The machine-learning algorithm was able to accurately differentiate between dehydration and stress from cutting and could also discern whether the sounds came from a tomato or tobacco plant. Although the study focused on tomato and tobacco plants because of their ease to grow and standardize in the laboratory, the researchers also recorded 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,” Professor Hadany said.
The exact mechanism behind these noises is unclear, but the authors suggest that it might be due to the formation and bursting of air bubbles in the plant’s vascular system, a process called cavitation. Whether or not the plants are producing these sounds in order to communicate with other organisms is also unclear, but the fact that these sounds exist has big ecological and evolutionary implications. “It is possible that other organisms could have evolved to hear and respond to these sounds,” Professor Hadany said. “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 its decision.” The study was published in the journal Cell.
The sounds might be akin to “someone’s creaking joints,” says Tom Bennett, a plant biologist at the University of Leeds in England who was not involved with the research. “It doesn’t mean that they’re crying for help.” Still, it is possible that other organisms eavesdrop on the noises, he says, something Hadany’s team is currently investigating. She is curious whether other plants or insects like moths, some of which can hear in the ultrasonic range, are tuning in. It’s possible moths, as well as mice and other mammals, could detect the noises as far as five meters away, the team suggests.
And tomato and tobacco weren’t the only plants that prattled. Similar sounds came from wheat, corn, Cabernet Sauvignon grapevines, and pincushion cactus. “It is happening in so many different plants that grow in so many different environments,” says Ravishankar Palanivelu, a plant developmental biologist at the University of Arizona in Tucson who did not work on the study. “It seems like this is not a random thing.” He doesn’t know if the sounds have any evolutionary significance, but, Palanivelu says, he thinks the study’s results will certainly generate some ‘noise’.
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