Your tomato plant may be crying out for you to water it, but although other animals and plants might hear it, your human ears are deaf to the sound of its laments. That’s the finding of new research that captured the pops and clicks of stressed plants, which have been tuned down to the human hearing range.
The novel experiment revealed that plants exposed to harmful pressures, such as injuries and dehydration, produce ultrasonic noises that may be audible to animals and plants as far as 16 feet away. In this way, plants may use sound to communicate with their wider ecosystems, a finding that sheds light on their mysterious inner worlds and could help mitigate agricultural challenges presented by climate change.
Plants have devised many ways to interact with their fellow creatures that humans can appreciate, such as complex fragrances, visual splendor, spiky surfaces, and tasty treats like fruits and nuts. However, they are not especially famous for being noisy. Though scientists have previously recorded botanical sounds by placing devices directly on plants, it has remained unclear whether these acoustic signals carried over a significant distance, thereby allowing other species in an environment to listen in to the plant world.
Now, researchers led by Itzhak Khait, a plant scientist at Tel Aviv University, have recorded tobacco and tomato plants making high-frequency noises that could be detectable at distances of five meters, or 16 feet. The sound of the stressed plants was so specific that a machine learning tool was able to tell whether a plant was thirsty, suffering from cuts, or part of a control group based only on the pattern of its acoustic vibrations.
The results “can alter the way we think about the plant kingdom, which has been considered to be almost silent until now,” according to a study published on Thursday in the journal Cell.
“We started this project from the evolutionary question: why are plants mute?” said Lilach Hadany, an evolutionary biologist at Tel Aviv University and the senior author of the study, in an email to Motherboard. “It appears that plants could have a lot to benefit from acoustic communication.”
Hadany added that her team had predicted that plants would make airborne sounds, and yet they were still surprised by the clear results. “We were particularly happy that the sounds turned out to be informative—containing information on the type of the plant and the type of the stress,” she noted.
Previous research has suggested that plants can emit acoustic vibrations by forming and collapsing air bubbles in their vascular system as part of a process called cavitation. To test out the range of these sounds, Hadany and her colleagues deliberately stressed out a bunch of tomato and tobacco plants by cutting their stems and depriving them of water.
The results show that a stressed plant can produce up to 50 sounds a minute—far more than their unstressed counterparts—and that the patterns of their clicks are linked to specific complaints. For instance, a machine learning tool that listened to the plants in a silent acoustic chamber was then able to identify the ailments of plants in a later version of the test conducted in a noisier greenhouse environment, with a high degree of accuracy.
“Here we show that plants indeed emit airborne sounds, which can be detected from several meters away, both in acoustic chambers and in greenhouses,” the team said in the study. “Moreover, we show that the emitted sounds carry information about the physiological state of the plant. By training machine learning models, we were able to distinguish between drought-stressed, cut, and control plants, based only on the sounds they emit.”
The study focused on tobacco and tomato plants because these species are so well-studied, but the researchers were also able to record sounds from a variety of other plants, such as wheat, corn, cactus, and the grapevine that makes Cabernet Sauvignon. Hadany said that it’s not yet known whether the noises are a specific mode of botanical communication, or they are just an incidental acoustic byproduct of cavitation, but it does seem to be a common response across a diversity of species.
“What we do know is that there are sounds in the air, and they contain information,” she explained. “Thus, natural selection may be acting on other organisms (animals and plants) to whom the sounds are relevant, to be able to hear the sounds and interpret them. That includes animals that can hear the sounds and can use the information to choose a food source or a laying site, or potentially plants that can prepare for the stress.”
“That may or may not lead to natural selection acting on the plants emitting the sounds (to emit more or less) but this is a complete speculation at the moment,” Hadany continued. “I think future studies would reveal who is listening to the sounds, and that may give us a hint on their evolution as a mode of communication.”
To that end, future studies might unravel the ultrasonic language of plants, allowing us to listen in on their conversations (though it seems they get most chatty when they’re upset). In addition to helping scientists understand how plants interact with wild environments, these efforts might give farmers a new tool for evaluating the health of their crops.
“Plant sound emissions could offer a way for monitoring crops water and possibly disease states—questions of crucial importance in agriculture,” the team said. “In times when more and more areas are exposed to drought due to climate change, efficient water use becomes even more critical, for both food security and ecology.”
“Plants were already shown to react to sounds, and specifically to increase their drought tolerance in response to sounds,” the team concluded. “Could plants potentially respond adaptively to the sounds of their drought-stressed or injured neighbors? We suggest that more investigation in the plant bioacoustics field, and particularly in the ability of plants to emit and react to sounds under different conditions and environments, may reveal a pathway of signaling between plants and their environment.”
