Looks like Roald Dahl might be onto something: If I were you Hurt the plant, it screams.
Well sort of. Not in the same way that you or I might scream. Instead, they make popping or clicking sounds at ultrasonic frequencies outside the range of human hearing that increase when the plant becomes stressed.
And this, according to a study published this year, could be one of the ways plants communicate their suffering to the world around them.
“Even in the quiet domain, there are already sounds that we don’t hear, and those sounds carry information. There are animals that can hear these sounds, so there is potential for a lot of vocal interaction.” said evolutionary biologist Lilash Hadani From Tel Aviv University in Israel.
“Plants interact with insects and other animals all the time, and many of these organisms use sound to communicate, so it would be counterproductive for plants not to use sound at all.”
Plants under stress are not as passive as you might think. They undergo some pretty interesting changes, one of the most obvious (to us humans, at least) is the release of some very strong odors. They can also change their color and shape.
These changes can signal danger to other plants nearby, causing their defenses to be strengthened; or Attract animals to deal with pests which may harm the plant.
However, whether plants emit other types of signals – such as sounds – has not been fully explored. A few years ago, Hadani and her colleagues found that plants can detect sound. The next logical question to ask is whether they can produce it too.
To find out, they recorded tomato and tobacco plants under a number of conditions. First, they recorded unstressed plants to obtain a baseline. Then they recorded the plants that had been dried, and the plants whose stems had been cut off. These recordings were made first in a soundproofed acoustic chamber, and then in an ordinary greenhouse environment.
Next, they trained a machine learning algorithm to distinguish between the sound produced by unstressed plants, cut plants, and dried plants.
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The sounds made by plants are similar to popping or clicking with a frequency too high-pitched for humans to make, and detectable within a radius of more than a meter (3.3 feet). Unstressed plants don’t make much noise at all; They’re just hanging out and quietly doing their vegetarian thing.
By contrast, stressed plants are noisier, making an average of about 40 clicks per hour depending on the species. And plants deprived of water have a noticeable acoustic appearance. They begin to tap more often before they show obvious signs of drying, escalating as the plant grows drier, before subsiding as the plant wilts.
The algorithm was able to distinguish between these sounds, as well as the types of plants that made them. And it’s not just tomato and tobacco plants. The team tested a variety of plants, and found that healthy production seemed to be a very popular vegetative activity. Wheat, corn, grapes, cacti, and honeybees have all been recorded making sounds.
But there are still a few unknowns. For example, it is not clear how sounds are produced. In previous research, dried plants were found to experience cavitation, a process in which air bubbles in the shape of the stem expand and collapse. This results, in the cracking of the knuckles, into an audible snap; Something similar can happen with plants.
We don’t know yet whether other distresses can produce sound as well. Pathogens, attack, exposure to ultraviolet light, temperature extremes, and other adverse conditions can trigger plants to start looking like bubble wrap.
Nor is it clear whether proper production is an adaptive evolution in plants, or if it is just something that happens. However, the team showed that the algorithm can learn to recognize and differentiate between plant sounds. It is certainly possible that other organisms would have done the same.
In addition, it is possible that these organisms have learned to respond to the noise of distressed plants in different ways.
“For example, a butterfly intending to lay its eggs on a plant or an animal intending to eat a plant can use sounds to help guide its decision.” Hadani said.
For us humans, the implications are very clear; We can listen to the distress calls of thirsty plants and water them before they become a problem.
But whether other plants sense and respond is not known. Previous research work has shown that plants can increase Drought tolerance in response to sound, so it is certainly reasonable. And this is where the team marks the next stage of their research.
Now that we know that plants make sounds, the next question is, “Who is listening?” Hadani said.
“We are currently investigating the responses of other organisms, both animals and plants, to these sounds, and we are also exploring our ability to identify and interpret sounds in completely natural environments.”
The research was published in cell.
A previous version of this article was published in March 2023.