(The Hill) — Tomato and tobacco plants make distinctive sounds when cut or dehydrated, a new study has found.

Those sounds change depending on the plant emitting them and the type and severity of the threat that prompts them, according to the study in Cell.

The findings shatter the common perception of plants as silent, passive background players to the animal life in their environments.

Instead, they show those plants could send out signals that animals in their environment can hear and pick up on — and potentially use to change their behavior.

Tomatoes left without water begin making noise “on the second day — even while the tomato still looks good,” Lilach Hadany, a Tel Aviv University mathematician who co-authored the study, told The Hill.

The sounds, which somewhat resembled the noise of popcorn popping, peaked after five days of water stress, and then began to decline as the plant dried out. 

The sounds happen at the approximate volume of human speech but outside the range of our hearing, the study found.

They also differed depending on the plant making them and the form of injury, with cut and dehydrated plants making different noises.

The research opens the tantalizing possibility that for organisms able to hear these pitches, a landscape of plants is also a soundscape of information — revealing essential information about both plants and the wider environment.

Hadany runs a lab at Tel Aviv University in Israel that uses machine learning to study plant evolution, including the emerging field of plant acoustics and, in particular, how plants use sound.

The word “use” in the sentence is a landmine in scientific circles: It can imply a level of intent that scientists have traditionally been reluctant to ascribe to plants.

Hard consensus on the matter has slowly softened in recent years, however. A considerable body of evidence now suggests that plants emit cues that other plants and pollinators pick up on — in particular by releasing floating airborne chemicals, as Richard Karban of the University of California, Davis has found.

It made sense that plants would also use sound, Hadany told The Hill in a video interview — sound takes little energy to produce and carries a long way. 

But when Hadany began considering earlier in her career whether to investigate whether plants might hear sounds in their environment — and even send audio signals of their own — colleagues warned her to wait for fear of damaging her career.

The colleague said, “‘Do not work on it before you have tenure,’” Hadany said, noting that the whole topic had a slightly disreputable flavor in academia.

“But now that I am a full professor, it is good,” she said, smiling over the Zoom video. 

Previous research out of her lab found that some plants can hear — and change their behaviors based on what they are hearing.

When her team played the sound of buzzing bees near primrose bushes, their flowers began within a few minutes to release sweeter nectar — something they did not do when exposed to other frequencies, according to a 2019 study her team published in Ecology Letters.

The primrose bushes “heard” the bees through the flowers themselves, which perhaps helps explain why bees hover and buzz near flowers, Hadany said.

Those 2019 findings opened the door to larger questions, Hadany said. The existence of one such channel of helpful, audible information — which gives pollinators a way to signal their presence to plants, and plants an opportunity to woo them — suggested the kind of two-way relationship that evolution often works to strengthen, she added. 

“Once you have this interaction, there is selection on both sides to improve hearing and emissions of sounds,” she said.

Hearing and interpreting such information can be particularly important to plants, which have an even greater need than animals “to interact with their environment, to respond to the environment —  because it cannot go to a different environment,” Hadany said.

Plants use environmental information to trigger the production of new chemicals or physiological responses, from generating insecticides or retaining water to turning to follow the sun.

Discovering that plants could hear hovering bees led Hadany’s team to ask in Thursday’s paper if they could also transmit news of drought or distress. The team found that they could, through means that are still poorly understood but may involve the release of bubbles through the plants’ stalks.

Further study will be needed to determine if other plants — or animals — can hear the sounds of crying tomatoes or tobacco.  

If they do, such knowledge could drive other nearby plants to take protective action, like closing up their hatches against water loss or bacterial threat, Hadany said.

It could also provide a definite advantage to animals who can hear it. For example, moths that lay eggs on tomatoes can hear frequencies in the range that tomatoes are transmitting. 

Does that mean they are more or less likely to lay eggs on water-stressed plants? Hadany’s group is working on this question. 

But she noted that Thursday’s findings showed the benefits of scientists arriving “at a project from a completely evolutionarily open question,” rather than assuming they understood how systems worked.