It seems like Roald Dahl may have been onto something after all: if you hurt a plant, it emits a sound. 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 a study published this year, 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,” said 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 Respond Actively to Stress
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.
Recording the Sounds of Plants
To find out, Hadany and her team recorded tomato and tobacco plants in a variety of conditions. First, they recorded unstressed plants to establish a baseline. Then they recorded plants that were dehydrated and plants that had their stems cut. These recordings were done in both a soundproofed acoustic chamber and a normal greenhouse environment.
By analyzing the recordings, they trained a machine learning algorithm to differentiate between the sound produced by unstressed plants, cut plants and dehydrated plants.
The Sounds of Plants under Stress
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 of around 40 clicks per hour, depending on the species. 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 emissions, falling in the range of 20 to 100 kilohertz, can’t be heard by humans but are within the frequency range that some animals, like bats and mice, can detect.
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.
Unanswered Questions
The driving force behind these emissions remains an area of active scientific investigation. Some researchers posit that they might be the result of air pockets forming and collapsing inside the plant’s xylem tubes, which are responsible for water transport. When plants face drought, they might produce more of these air pockets, leading to these ultrasonic emissions.
It’s also not known if other distress conditions can induce sound. Pathogens, attacks, UV exposure, temperature extremes, and other adverse conditions could also prompt plants to start producing sounds.
Furthermore, it’s still unclear whether sound production is an adaptive development in plants or if it is just something that happens. However, the team showed that an algorithm can learn to identify and distinguish between plant sounds, raising the possibility that other organisms could have done the same.
The Implications and Future Research
Beyond mere scientific curiosity, understanding these “screams” has practical applications. These ultrasonic emissions can serve as early warning signals for farmers, indicating when crops are under duress and need intervention, whether it’s more water, pest control, or other care. By tuning into the silent signals of plants, we could optimize agricultural practices and ensure healthier, more resilient crops. Previous research has shown that plants can increase their drought tolerance in response to sound, so it’s certainly plausible.
The next stage of research for Hadany and her team is to investigate the responses of other organisms, both animals and plants, to the sounds emitted by distressed plants. They are also exploring their ability to identify and interpret these sounds in completely natural environments.
“Now that we know that plants do emit sounds, the next question is – ‘who might be listening?'” Hadany said. “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 study, titled “Plants actively communicate distress using ultrasonic sounds,” was published in the scientific journal Cell.
What Else Can’t We Hear?
Humans possess a fascinating and complex auditory system, allowing us to perceive a wide range of sounds from the softest whisper to the roaring thunder. Typically, the human ear can detect sounds ranging from 20 hertz (Hz) to 20,000 Hz. This range, however, can vary among individuals and tends to decrease with age, especially at the higher frequencies.
While this auditory range serves us well in daily life, enabling communication, music appreciation, and awareness of our surroundings, there are many sounds in the living kingdom that remain beyond our grasp. These inaudible sounds fall into two categories: infrasound, which is below our range of hearing, and ultrasound, which is above it.
- Infrasound (below 20 Hz):
- Many large animals, such as elephants and whales, use infrasound to communicate over vast distances. An elephant’s low-frequency rumble, for instance, can travel for miles, providing a means to convey messages about threats, mating, or food sources. These sounds are too low-pitched for human ears to detect but hold a wealth of information and communication among these majestic creatures.
- Ultrasound (above 20,000 Hz):
- Bats are perhaps the most famous users of ultrasound. They emit high-frequency sounds and then listen to the echoes to locate and catch prey, a process known as echolocation. Similarly, dolphins use ultrasonic clicks for communication and hunting. On the other end, certain rodents emit ultrasonic vocalizations, often associated with mating or distress calls. Even plants, as previously discussed, can emit ultrasonic “screams” under stress. However, all these remain inaudible to the human ear.
Beyond the realm of biology, various technologies harness these out-of-range sounds. Ultrasound, for instance, is used in medical imaging, while infrasound can detect volcanic eruptions or nuclear explosions.
In the grand tapestry of life, every species has evolved to suit its environment and needs. While our auditory range captures a vast array of sounds, it’s humbling to recognize the vast symphony of communication and interaction in the living kingdom that we might never directly hear.