Researchers from Tel Aviv University (TAU) have recorded high-pitched airborne noises emitted by water-stressed plants. The noises could be interpreted as an indication that plants cry out for help (please water me!), much like a distressed animal would.

In their study, the researchers suggest that whenever a plant is cut, suffers from an infection, or is under stress from water deprivation, it repeatedly emits a click sound. Humans are not able to hear these clicks because their frequency is above the audible range (40 to 80 KHz, whereas humans can hear sounds only up to 20 KHz).

However, bats, insects, mice, and some other animals can detect a plant’s audio clicks from a distance of up to three to five meters. Researchers have yet to study how these sounds affect them, though.  

“If plants emit airborne sounds, these sounds can potentially trigger a rapid response in nearby organisms, including both animals and plants. Organisms that are capable of hearing these sounds could use them for their own benefit,” the researchers note. 

Is there a way for us to hear a plant’s ‘scream’?

Researchers recorded the clicks produced by stressed plants in an acoustic chamber and in a greenhouse setting with the help of ultrasonic microphones. The study authors recorded sounds emitted by wheat, corn, cactus, tomato, and tobacco plants under different conditions — both when they were stressed and healthy.

In addition, they also used some machine learning models to identify the different physiological states of the plants from the sounds they emitted. 

“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 researchers said. 

They noticed that when plants were healthy, they clicked rarely (~ once an hour) but in stressful conditions, they clicked multiple times an hour. Moreover, the researchers also discovered that different plants made different types of clicks under different conditions.

“We can separate between sounds emitted by tomato and tobacco, between tomato and cacti, and also between cut tomato and dry tomato a little bit dry tomato and very dry tomato,” Lilach Hadany, one of the study authors and a professor at TAU, told Insider.

This kind of acoustic technique could be beneficial to farmers as it could help them identify stressed plants in their fields, and then take action accordingly. 

However, the researchers are still not sure how exactly plants make the click sounds. They suggest that the clicks are possibly a result of the popping of air bubbles in the plant’s vascular tissues that transport water and minerals.

“Plants exposed to drought stress have been shown to experience cavitation – a process where air bubbles form, expand, and collapse in the xylem, causing vibrations,” the researchers note.

However, further research is required to confirm this theory.

Plants can’t have emotions — or can they?

When animals feel stress or pain, they whine and mewl. But when it comes to plants, whether or not they feel pain and stress is itself a subject of debate. Most biologists will tell you that it’s impossible for plants to feel pain because they lack pain receptors or a nervous system, while others argue that they respond to pain stimuli in their own way.

The current study suggests that plants make certain sounds when they’re water-deprived, but Professor Hadany and her team cannot say with certainty that this means plants can ‘feel’ stress.

“We cannot say the plant feels stress and therefore makes sounds. It might be that the sounds are made completely passively, like a physical process,” Hadany told Insider.

The study is published in the journal Cell.