© Getty Images/iStockphoto
“Apparently, an idyllic field of flowers can be a rather noisy place. It’s just that we can’t hear the sounds,” study coauthor Lilach Hadany quipped.
Mum's the word.

Thirsty or stressed plants emit popping sounds undetectable to the human ear, according to research published Thursday in the journal Cell.

Tel Aviv University scientists used special microphones to record ultrasonic sounds produced by tomato and tobacco plants inside a sound-protected box and a greenhouse.

The researchers say they developed machine learning models to identify the condition of the plants, including if they were dehydrated or diseased, based on the detected sounds.

"From previous studies we know that vibrometers attached to plants record vibrations. But do these vibrations also become airborne soundwaves — namely sounds that can be recorded from a distance? Our study addressed this question, which researchers have been debating for many years," study coauthor Lilach Hadany said in a statement.

Researchers say they put the plants in a box in a basement with no background noise, placing ultrasonic microphones about 10 centimeters away from each specimen.

Tomato and tobacco plants were the focus, but wheat, corn, cactus and henbit were studied as well.

"Before placing the plants in the acoustic box we 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," Hadany said.

"Our intention was 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."

The maximum frequency detected by a human adult is about 16 kilohertz.

Researchers found unstressed plants typically emitted less than one sound per hour, while plants that were dehydrated and injured produced dozens of sounds every hour.
plant sounds 1
© Cell
Plants were tested in an “acoustically isolated box” in a basement without background noise.
The team noted the clicks could be detected even when the plants were placed in a noisy greenhouse.

"We assume that in nature the sounds emitted by plants are detected by creatures nearby, such as bats, rodents, various insects, and possibly also other plants — that can hear the high frequencies and derive relevant information," Hadany said.

"We believe that humans can also utilize this information, given the right tools — such as sensors that tell growers when plants need watering."

Hadany quipped: "Apparently, an idyllic field of flowers can be a rather noisy place. It's just that we can't hear the sounds."

plant sounds 2
© Cell
The plants, mostly tomato and tobacco, were recorded when cut and dehydrated.
Scientists not involved in the new research caution there is no evidence sounds produced by plants are a form of communication.

"This result adds to what we know about plant responses to stress. It is a useful contribution to the field and to our general appreciation that plants are responsive organisms capable of sophisticated behaviors," Richard Karban, a professor who studies interactions between herbivores and their host plants, told CNN.

"However, it should not be interpreted as showing that plants are actively communicating by making sounds," Karban added.