It is not only human who can react to stress, but a new study found that plants that are stressed by drought or physical damage may emit ultrasonic squeals.
A report by Live Science revealed that unlike human screams, plant sounds are too high-frequency for humans to hear them. The report mentioned a research, which was posted Dec. 2 on the bioRxiv database, whose authors placed microphones near stressed tomato and tobacco plants, the instruments picked up the crops' ultrasonic squeals from about 4 inches (10 centimeters) away. The noises fell within a range of 20 to 100 kilohertz, a volume that could feasibly "be detected by some organisms from up to several meters away."
According to the research, animals and plants might listen and react to the silent screams of plants, and perhaps humans could too, with the right tools in hand.
Anne Visscher, a fellow in the Department of Comparative Plant and Fungal Biology at the Royal Botanic Gardens in the U.K., told New Scientist that the idea that "sounds that drought-stressed plants make could be used in precision agriculture seems feasible if it is not too costly to set up the recording in a field situation."
Visscher explained that, like animals, the green living organisms respond to stress in a variety of ways; including releasing smelly chemical compounds or change their color and shape in response to drought and bites from hungry herbivores. Animals seem to recognize and respond to these botanical stress signals, and even other plants appear to pick up on the airborne scents wafting from their tense neighbors. Some previous research had suggested that plants react to sound, too, but questions remained about whether plants themselves emit detectable noises.
Te report noted that in previous studies, researchers affixed recording devices directly to listen for secret sounds inside stems. In plants stressed by drought, air bubbles formed, popped and triggered vibrations within the tissue that normally carries water up the plants' stems. The process, known as cavitation, was picked up by the attached recording devices, but this time the researchers wanted to know if any plants sounds could travel through the air.
So the team set up microphones near stressed-out tomato and tobacco placed in either a soundproof box or an open greenhouse space. The researchers subjected one set of crops to drought conditions and another to physical damage (a snipped stem). A third untouched group served as a point of comparison. The recordings revealed that the different species made distinct sounds at varying rates, depending on their stressor.
In specific, drought-stressed tomato plants emitted about 35 ultrasonic squeals per hour, on average, while those with cut stems made about 25. Drought-stressed tobacco plants let out about 11 screams per hour, and cut crops made about 15 sounds in the same time. In comparison, the average number of sounds emitted by untouched plants fell below one per hour.
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