It seems like Roald Dahl may have been onto something after all: if you hurt a plant, it screams.
Well, sort of. Not in the same way you or I might scream. Rather, they emit popping or clicking noises in ultrasonic frequencies outside the range of human hearing that increase when the plant becomes stressed. This, according to scientists, could be one of the ways in which plants communicate their distress to the world around them.
"Even in a quiet field, there are actually sounds that we don't hear, and those sounds carry information. There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring," explains evolutionary biologist Lilach Hadany of Tel Aviv University in Israel.
"Plants interact with insects and other animals all the time, and many of these organisms use sound for communication, so it would be very suboptimal for plants to not use sound at all."
Plants under stress aren't as passive as you might think. They undergo some pretty dramatic changes, one of the most detectable of which (to us humans, at least) is the release of some pretty powerful aromas. They can also alter their color and shape.
These changes can signal danger to other plants nearby, which in response boost their own defenses; or attract animals to deal with the pests that may be harming the plant.
However, whether plants emit other kinds of signals – such as sounds – has not been fully explored. A few years ago, Hadany and her colleagues found that plants can detect sound. The logical next question to ask was whether they can produce it, too.
To find out, they recorded tomato and tobacco plants in a number of conditions. First, they recorded unstressed plants, to get a baseline. Then they recorded plants that were dehydrated, and plants that had had their stems cut. These recordings took place first in a soundproofed acoustic chamber, then in a normal greenhouse environment.
Then, they trained a machine learning algorithm to differentiate between the sound produced by unstressed plants, cut plants, and dehydrated plants.
The sounds plants emit are like popping or clicking noises in a frequency far too high-pitched for humans to make out, detectable within a radius of over a meter (3.3 feet). Unstressed plants don't make much noise at all; they just hang out, quietly doing their plant thing.
By contrast, stressed plants are much noisier, emitting an average up to around 40 clicks per hour depending on the species. And plants deprived of water have a noticeable sound profile. They start clicking more before they show visible signs of dehydrating, escalating as the plant grows more parched, before subsiding as the plant withers away.
The algorithm was able to distinguish between these sounds, as well as the species of plant that emitted them. And it's not just tomato and tobacco plants. The team tested a variety of plants, and found that sound production appears to be a pretty common plant activity. Wheat, corn, grape, cactus, and henbit were all recorded making noise.
But there are still a few unknowns. For example, it's not clear how the sounds are being produced. In previous research, dehydrated plants have been found to experience cavitation, a process whereby air bubbles in the stem form, expand and collapse. This, in human knuckle-cracking, produces an audible pop; something similar could be going on with plants.
We don't know yet if other distress conditions can induce sound, either. Pathogens, attack, UV exposure, temperature extremes, and other adverse conditions could also induce the plants to start popping away like bubble wrap.
It's also not clear whether sound production is an adaptive development in plants, or if it is just something that happens. The team showed, however, that an algorithm can learn to identify and distinguish between plant sounds. It's certainly possible that other organisms could have done the same.
In addition, these organisms could have learned to respond to the noise of distressed plants in various ways. "For example, a moth that intends to lay eggs on a plant or an animal that intends to eat a plant could use the sounds to help guide their decision," Hadany says. For us humans, the implications are pretty clear; we could tune into the distress calls of thirsty plants and water them before it becomes an issue.
But whether or not other plants are sensing and responding is unknown. Previous research works have shown that plants can increase their drought tolerance in response to sound, so it's certainly plausible. And this is where the team is pointing the next stage of their research.
"Now that we know that plants do emit sounds, the next question is – 'who might be listening?'" Hadany says. "We are currently investigating the responses of other organisms, both animals and plants, to these sounds, and we're also exploring our ability to identify and interpret the sounds in completely natural environments."
The research has been published in Cell.