Scientifically Speaking | Crying plants: When science fiction predicts science

The recent discovery that plants emit inaudible sounds under stress has caused a stir in the scientific community and reminded me of a story from decades ago.

It’s not that often that I read a research paper in a scientific journal, and I’m reminded of a science fiction story that I read as an impressionable child. But a remarkable discovery that is making waves this week had precisely this effect on me.

Here’s what the researchers found when they placed microphones next to plants in soundproof boxes. They make sounds when they’re stressed. (Shutterstock)

There are a few notable examples of predictions made in science fiction stories and movies that have come true. Mobile phones, space stations, and credit cards are instances where imagination preceded real-life innovations.

To this list, we can now add the inaudible crying of plants, a provocative idea presented in a science fiction story written by Satyajit Ray. Ray is well-known across the world as a filmmaker. In West Bengal and Bangladesh, he is also a beloved writer of science-fiction and detective novellas. Many of these stories originally written in Bengali — including those detailing the inventions of the fictional scientist, Professor Trilokeshwar Shonku — have found new audiences in English translations and cinematic adaptations.

In Professor Shonku O Golok Rahasya (translated into English as The Mysterious Sphere) written by Ray for Sandesh magazine in 1965, our scientist invents a machine called the microsonograph which is capable of picking up and amplifying soundwaves that humans can’t hear. In the story, Professor Shonku plucks a rose from a potted plant on his verandah. He then notices that the wounded plant cries out in frequencies that can be picked up with his fictitious microsonograph. The Professor then uses this machine to ascertain the origins of a mysterious sphere. I will not spoil the rest of the story for anyone who has yet to read it, but suffice it to say that it stuck with me for over three decades.

Imagine my bemusement last week when I found that researchers based in Israel reported that “plants emit ultrasonic airborne sounds when stressed.” The research was published in the prestigious scientific journal Cell on March 30.

Here’s what the researchers found when they placed microphones next to plants in soundproof boxes. They make sounds when they’re stressed.

Plants that need water or have had stems cut emit around 35 sounds in an hour. In comparison, plants that are well-watered and uninjured plants are much quieter. They make around one popping sound an hour. Truth is indeed stranger than fiction.

By training machine learning models, the researchers were also able to distinguish between the kinds of anguished “cries” made by plants. Not only could they differentiate between stressed and non-stressed plants, but quite amazingly they could distinguish between the stressors. Using this approach, they could separate drought-stricken and cut plants based on their sounds with 70% accuracy.

The researchers could also distinguish between plants at high and low levels of dehydration with over 80% accuracy. It’s worth stepping back and thinking about this for a minute. Extending our earlier metaphor, not only can we hear plants crying, but also in at least one case, we can tell how much they’re distressed.

Well, if plants are crying when distressed then who is listening? Certainly not us. The sounds made by the injured plants are in the range of 20 to 100 kilohertz rendering them inaudible to most humans. But the scientists opine that these cries are not falling on deaf ears.

The researchers write that “these sounds can potentially trigger a rapid response in nearby organisms, including both animals and plants. Even if the emission of the sounds is merely a result of the plant’s physiological condition, nearby organisms that are capable of hearing these sounds could use them for their own benefit.” Or in other words, even if the crying is unintentional, other animals might be listening in. And according to the authors, eavesdropping animals might include moths, mice, and bats.

Then there’s the question of how these sounds are actually made. After all, plants don’t have vocal cords. And indeed, here is where fact diverges from fiction. In Satyajit Ray’s story, the wounded rose plant let out a moaning lament. Here, the “crying” has been compared to a high-pitched popping sound.

How these sounds are made is still unknown, but the researchers hypothesize that there could be physical processes like bubbles forming and popping inside the vascular system of plants which lead to these distressed audible signals.

Not all scientists are convinced that plant sounds are actually used for anything by animals or other plants (and this would rule out the role of these sounds as a means of meaningful communication). But that’s the beauty of cutting-edge science. These findings will be corroborated and built on, or we will find other (possibly prosaic) explanations.

Scientific papers are usually written in a dry and understated style. The pronouncements are conservative. But it is difficult not to be swept away by the statement the authors make that “these findings can alter the way we think about the plant kingdom, which has been considered to be almost silent until now.”

The thoughts that pop into my head are both humbling and exhilarating. If plants do indeed cry in response to stress, then why are we only finding out just now? And, looking at the world around us, what else is waiting to be heard?

Anirban Mahapatra is a scientist by training and the author of a book on COVID-19

The views expressed are personal