Plants emit ultrasonic screams when stressed: Study

Scientists have long known that plants have the capacity to exhibit changes in their phenotypes in response to stress. Some plants can, for instance, wilt, change color, or release volatile organic compounds. Apparently they can cry, too.

They found that under all circumstances, the tobacco and tomato plants made sounds, but were much more sonically active when stressed.

Tomato and tobacco plants subjected to drought conditions made 35 sounds and 11 sounds an hour, respectively. However, unstressed plants made fewer than one sound per hour on average.

The researchers noted that in the pot control, where a pot with soil but no plant was recorded, their system did not record any sound ">500 h of recordings."

The researchers successfully trained machine learning models to classify different plant conditions and species on the basis of the sounds they made. In light of the accuracy of the interpretation, the researchers concluded, "plant sounds carry information that can be interpreted and used for classification of plant type and condition."

This emission and receipt of informative sound was possible outside the acoustic chamber as well.

"In order to distinguish between the sounds generated by plants and background greenhouse noises, we first constructed a library of greenhouse noises, by recording in an empty greenhouse," wrote the researchers. "We then trained a convolution neural network (CNN) model to distinguish between these empty greenhouse noises and the sounds of dry tomatoes recorded in the acoustic box."

Again, they found that the tomato plant was noisily indicating its condition, such that the researchers could distinguish between the drought-stressed plants and the control plants with roughly 84% accuracy.

Edward Farmer at the University of Lausanne told New Scientist that cavitation is most likely the cause behind the sounds stressed plants make.

The study underscored, "These findings can alter the way we think about the plant kingdom, which has been considered to be almost silent until now."

Khait and his colleagues only provided evidence of tomato and tobacco plants' thoughtless cries, but are confident other plants — such as the vegetarian's coveted kale, avocado plant, and spinach — might similarly make sounds.

Although human beings may have been deaf to plant chatter up until now, the researchers indicated that other organisms may have evolved to pick up and comprehend the meaning of these sounds.

"For instance, many moths—some of them using tomato and tobacco as hosts for their larvae—can hear and react to ultrasound in the frequencies and intensities that we recorded. Nearby plants may also respond to the sounds emitted by plants. Plants were already shown to react to sounds and specifically to increase their drought tolerance in response to sounds," said the study.

With the help of machine learning algorithms, the researchers indicated humans, farmers in particular, can exploit this new understanding: "Plant sound emissions could offer a way for monitoring crops water and possibly disease states—questions of crucial importance in agriculture. More precise irrigation can save up to 50% of the water expenditure and increase the yield, with dramatic economic implications."

Anne Visscher, a research fellow at the Royal Botanic Gardens, cast doubt on the agricultural application of this research, telling New Scientist, "The suggestion that the 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."

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