Plants Make Sounds When They're Stressed Out, Recordings Reveal

Researchers have detected, for the first time, plants emitting airborne sounds when they are stressed

According to a study published in the pre-print website bioRxiv, a team of Israeli scientists recorded tomato and tobacco plants producing sound frequencies which humans cannot hear in stressful situations—such as when they experienced a lack of water or their stems were cut.

The team identified the sounds with microphones placed around 10 centimeters (around four inches) away from the plants, although the scientists say the noises could potentially be heard several feet away by some mammals and insects, such as mice and moths.

Previous research has shown that plants respond to stress by producing several visual, chemical and tactile clues. For example, stressed plants may differ in color and shape compared to unstressed plants. Meanwhile, some are also known to emit substances known as "volatile organic compounds" in response to drought or being eaten.

Furthermore, plants exposed to drought stress have been shown to experience cavitation—a process where air bubbles form, expand and explode inside tissue that transports water. These explosions produce sound, but they have only ever been recorded using devices directly connected to the plant.

The latest study, meanwhile, is the first to identify plants making sounds which can be detected over a distance. And the researchers say that cavitation could potentially be the source of these sounds.

"These findings can alter the way we think about the Plant Kingdom, which has been considered to be almost silent until now," the authors wrote in the study.

The team detected the tomato plants making 35 sounds an hour on average when they were exposed to drought conditions, while the tobacco plants produced 11. When the stems of the plants were cut, the tomato plants made 25 sounds an hour on average and the tobacco plants produced 15. As a comparison, unstressed plants made less than one sound per hour on average, according to the study.

The team say that while they only tested tomato and tobacco, it's possible that other plants could also produce sounds, adding that the latest findings could have implications for agriculture.

"Plant sound emissions could offer a novel way for monitoring crops water state—a question of crucial importance in agriculture," the authors wrote in the study. "More precise irrigation can save up to 50 percent of the water expenditure and increase the yield, with dramatic economic implications.

"In times when more and more areas are exposed to drought due to climate change, while human population and consumption keep increasing, efficient water use becomes even more critical, for both food security and ecology," they said. "Our results, demonstrating the ability to distinguish between drought-stressed and control plants based on plant sounds, open a new direction in the field of precision agriculture."

According to Anne Visscher from the Royal Botanic Gardens, Kew, in the U.K., the idea that the sounds could be used in precision agriculture is "feasible" although she urges caution regarding the Israeli team's suggestion that other animals could hear the sounds at a distance, New Scientist reported.

Meanwhile, Edward Farmer from the University of Lausanne, Switzerland, told the magazine that although cavitation is the most likely way a plant could make sounds when stressed, he said more research was needed to validate the team's conclusions.