When stressed, plants emit sounds at a frequency that humans cannot hear, but are detectable at more than one meter. The sound resembles the bursting of a bubble wrap and its volume is similar to that of a normal conversation.
A group of researchers from Tel Aviv University has studied these sounds emitted by stressed tomato and tobacco plants, either due to lack of irrigation or because a stem has been cut
. The study has been published in the journal Cell. Although human ears can’t pick up these sounds, there are animals and plants that “can hear those sounds, so there’s a chance there’s a lot of acoustic interaction going on,” said study coordinator Lilach Hadany.
Although ultrasonic vibrations had already been recorded in plants, this is the first proof that they are transmitted through the air, a fact that makes them more relevant to other organisms in the environment, explains the publication. Plants interact with insects and other animals, many of which use sound to communicate, “so it would be very suboptimal for plants not to use sound at all,” Hadany said.
The researchers used microphones to record healthy, stressed tomato and tobacco plants, first in a soundproof acoustic chamber and then in a noisier greenhouse.
The scientists recorded the plants’ sounds with microphones.
The scientists recorded the plants’ sounds with microphones.
“Non-stressed tomato plants are quieter”
After recording the plants, they trained a machine-learning algorithm to differentiate between stressed and unstressed plants, as well as between thirsty and cut plants.
The team found that stressed plants make more sounds than those that are not, and that these resemble “pops or clicks.” A single stressed plant emits between 30 and 50 of those clicks per hour at what seem like random intervals, while healthy ones emit fewer sounds. “When the tomato plants are not stressed, they are very, very quiet,” Hadany said.
The types of sound emitted differed depending on the cause of the stress and the algorithm was able to differentiate between dehydration and cut-off stress, as well as discern whether the sounds came from a tomato or tobacco plant.
Although the study focused on tomato and tobacco plants, because they are easy to grow in a standard lab, the team also recorded a variety of plants from other species. “We found that many plants — corn, wheat, grapes and cacti, for example — make sounds when they’re stressed,” Hadany said.
Are we surrounded by communication?
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 is also not known whether they produce these sounds to communicate with other organisms, but the fact that they exist has great ecological and evolutionary implications.
“It is possible that other organisms have evolved to hear and respond to these sounds,” for example, explained the researcher, “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.” The team is now studying the responses of other organisms, both animal and plant, to these sounds, and the researchers’ ability to identify and interpret sounds in completely natural environments.
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