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“They do interact with their environment,” Hadany said. “So it would be somewhat of a waste if they were completely deaf and mute.”
After six years of research, Hadany and her colleagues discovered that plants — when distressed — make ultrasonic noises, which are inaudible to humans. According to a study released last week, plants emit popping sounds when they’re cut or when they become dehydrated or infected — noises that researchers say might be their version of a call for help.
“We always thought that plants are silent,” researcher Yossi Yovel said. “And now we realize that they actually make those sounds quite often, and they are meaningful to some extent.”
The researchers placed plants in soundproof boxes in a quiet room and sat two ultrasonic microphones nearby. They studied tomato, tobacco, cactus, corn, wheat and other plants in varying conditions — some had cut stems, some had not been watered for days and others were untouched.
The result: The microphone picked up sounds at frequencies between 40 and 80 kilohertz — far above what the human ear can detect. The noises sound similar to popcorn kernels popping, the researchers found.
Distressed plants generated dozens of sounds every hour and sometimes roughly one every minute, the team found. Undamaged plants made less than one sound every hour.
“We were happy and surprised,” Hadany said. “It took us some time to really believe there were sounds.”
Researchers said the noises are the sound of popping air bubbles, which plants produce in their xylem, a tissue that moves water. Plants often lose air bubbles as their water intake decreases, Yovel said.
The rhythms produced depend on the plant and its condition. For example, a dehydrated wheat plant made more frequent and louder noises than a grapevine that had been cut. Researchers found that dehydrated plants generated more sounds as their conditions worsened.
Using artificial intelligence, researchers said, they can identify the type of plant and its condition based on the volume, frequency and tempo of its sounds. But researchers don’t know whether other creatures pay attention to the noises.
Previous studies have found that plants interact with their environments. In 2019, Hadany and Yovel discovered that flowers produced nectar when they detected bees and other pollinators nearby. A study in May in the Plant Cell found that plants communicate with electrical signals from their leaves.
Hadany and Yovel said they will continue their research in hopes of learning whether damaged plants communicate with animals and other plants. That’s a question that has stumped other researchers, including a team from Michigan that, in 1988, uncovered that plants made noises when they were dehydrated. Robert Haack, one of the researchers, wrote in an email that the team couldn’t determine whether insects heard the sounds.
“We didn’t have the right equipment back then,” Haack wrote.
Hadany and Yovel have the benefit of more technology to try to determine the answer. For now, they hope their findings will help farmers learn when their plants need water or are infected.
“The big question is whether animals, or perhaps some other organism, evolved to use these sounds,” Yovel said. “There’s a huge new world of possibilities.”
