This is what a plant sounds like when it's stressed

New research challenges assumptions that the plant kingdom is silent. Turns out, plants make a lot of noise when they're stressed.

Other plants and animals might even be able to interpret those sounds. And the ability to listen in could even help humans get smarter about the way we grow our crops, especially in a world where climate change is increasingly stressing us all out.

"Even in a quiet field, there are actually sounds that we don't hear, and those sounds carry information," Lilach Hadany, senior author of the paper and an evolutionary biologist and theoretician at Tel Aviv University, says in a press release.

"Even in a quiet field, there are actually sounds that we don't hear, and those sounds carry information."

Tomato and tobacco plants in particular make clicking sounds when they're dehydrated or being cut, according to the paper Hadany and her colleagues published in the journal Cell yesterday.

Humans haven't been able to hear those sounds coming from tomato and tobacco plants because they're at a frequency too high for us to detect. That is until researchers at Tel Aviv University set up microphones to listen to the plants in a greenhouse and in a soundproofed acoustic chamber. They listened to healthy plants, dehydrated plants, and plants after their stems were cut.

They discovered that the plants regularly make noises and get louder when they're struggling. Healthy plants in the control groups made an average of less than one sound an hour. Cut tomato and tobacco plants made around 25 and 15 sounds an hour, respectively.

Distress sounds were more prolonged for plants deprived of water. They made increasingly more noise in the first few days without water, reaching a crescendo before quieting as they dried out. Because of that, it was possible to hear the difference between plants that were only a little dry versus those that were very dehydrated. Researchers were also able to train a machine learning algorithm to differentiate between dry, cut, and healthy plants.

What's still a mystery is how exactly the plants make these noises. One hypothesis is that the sounds come from air bubbles that form and burst within the vascular system of a plant. It's a phenomenon called cavitation that's known to take place in plants experiencing drought.

You can listen to a recording of the plant sounds that researchers shared online. They lowered the frequency so that humans can hear it. The recording does kind of sound like Bubble Wrap popping.

The study focuses on tomato and tobacco plants because they're easy to grow and control in a lab. But the team also recorded sounds from a handful of other plants. Corn, wheat, and cactus plants also make sounds when they're stressed. So do Cabernet Sauvignon grapes famous in winemaking. Wine grapes whine! And if all these species emit sound, it's likely that others do, too, the paper suggests. There just needs to be more research on it.

This is the first time scientists have documented airborne sounds coming from plants, which means the sound travels and could potentially be heard by other living things up to several meters away. Since scientists don't know how or why the plants make the noise, we can't say whether they're intentionally trying to communicate. Nevertheless, animals and other plants might be able to glean helpful information from it.

Mice and moths are able to hear the high pitch of the tomato sound, for example. And since some moths lay larvae on tomato plants, they might be able to listen in to decipher which particular plant could give their young the best odds of survival. Other kinds of plants have also been found to react to sound and might start to prepare themselves for drought, for instance.

Farmers could set up listening sensors to gauge whether their crops are healthy or need extra attention. And since they could potentially hear how dry the plants are -- whether they're just starting to get dehydrated or nearing the point of no return -- this kind of application could help farmers water crops more efficiently.

Climate change is intensifying droughts in many places around the world, and crop yields are expected to suffer as a result. Precision irrigation could conserve up to 50 percent of the water farms use, the paper notes, while still producing more bountiful harvests. To be sure, a lot more research -- literally in the fields instead of in the lab -- needs to be done before that can be accomplished through sound.

The paper published yesterday opens up a lot more questions to answer. "We're also exploring our ability to identify and interpret the sounds in completely natural environments," Hadany says in the press release. And her team is still curious about how other critters might respond to the din. "So now that we know that plants do emit sounds, the next question is--'who might be listening?'"