Plants don’t have ears. But they can still detect sound

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In 1986, when he was a mere prince, King Charles, Britain’s eco-minded monarch, told a television interviewer that it was important to talk to one’s plants. He was widely mocked. But that piece of princely wisdom seems to have been ahead of its time, for there is now plenty of evidence that plants can detect sound, react to it, and even, perhaps, produce it.

Scientists have been experimenting with playing sounds to plants since at least the 1960s, during which time they have been exposed to everything from Beethoven to Michael Jackson. Over the years, evidence that this sort of thing can have an effect has been growing. One paper, published in 2018, claimed that an Asian shrub known as the telegraph plant grew substantially larger leaves when exposed to 56 days of Buddhist chants—but not if it was exposed to Western pop music, or silence. Another, published last year, found that marigolds and sage plants exposed to the noise of traffic from a busy motorway suffered stunted growth, and produced a range of stress compounds.

Other research, much of it done in China, reports that certain frequencies, played in acoustically controlled environments like greenhouses, can affect seed germination and even boost crop yields. And plants can make noises, too, albeit not deliberately. Earlier this year a group of researchers at Tel Aviv University published an article in Cell Press, reporting that several species of plants emitted different noises in response to different stresses—although not at the sorts of frequencies that humans can hear.

If all that sounds strange, perhaps it should not. After all, sound carries useful information about an organism’s environment. From an evolutionary point of view, there is no reason to expect that information to be exploited only by animals.

I’m picking up bad vibrations

Plants have been evolving alongside the insects that pollinate them and eat them for hundreds of millions of years. With that in mind, Heidi Appel, a botanist now at the University of Houston, and Reginald Cocroft, an entomologist at the University of Missouri, wondered if plants might be sensitive to the sounds made by the animals with which they most often interact. The researchers recorded the vibrations made by certain species of caterpillar as they chewed on leaves. These vibrations are not powerful enough to produce sound waves in the air. But they are able to travel across leaves and branches, and even to neighbouring plants if their foliage touches.

The researchers then exposed Thale cress—the plant biologist’s version of the laboratory mouse—to the recorded vibrations while no caterpillars were actually present. Later, they put real caterpillars on the plants to see if exposure had led them to prepare for an insect attack. The results were striking. Leaves that had been exposed had significantly higher levels of defensive chemicals like glucosinolates and anthocyanins, making them much harder for the caterpillars to eat. Leaves on control plants that had not been exposed to vibrations showed no such response. Other sorts of vibration—caused by the wind, for instance, or other insects that do not eat leaves—had no effect.

Dr Appel and Dr Cocroft published their findings in 2014. They have since been replicated many times in both cress and the tobacco plant, another common lab organism, and with a variety of caterpillars. While the vibrations created by different insects chewing on different leaves vary, the plants in question are consistently able to recognise them as a threat and defend themselves accordingly.

In 2019 the researchers took a closer look at what exactly was going on, biochemically speaking, with the plants exposed to the chewing sounds. Many of the chemicals released to cope with insect attacks turned out to be the same as those that are produced to better endure cold weather. Drs Appel and Cocroft propose that both situations activate similar signalling pathways associated with stress.

The research may have practical consequences, too. “Drones armed with speakers and the right audio files could warn crops to act when pests are detected but not yet widespread,” says Dr Cocroft. Unlike chemical pesticides, sound waves leave no toxic residue. With the help of weather forecasts, the system could even be used to prepare crops for cold snaps.

If it makes evolutionary sense that some plants are able to eavesdrop on insects, then it would make just as much sense if others were able to “hear” a noisy resource that all plants rely upon: water. In 2017 Monica Gagliano, an ecologist at the University of Western Australia, published evidence suggesting that they can.

Dr Gagliano knew from previous work that plants’ roots are sensitive to even minute amounts of water in soil, and will aggressively follow moisture gradients. But seeds seemed to be able to send roots to nearby water even when the soil in the immediate vicinity was dry. Dr Gagliano hypothesised that the roots might be detecting groundwater by sound, and tested her theory on pea plants.

The plants were grown in pots with forked bases, which could be filled with either dry or damp soil. As expected, roots proved very sensitive to the presence of moisture, and readily grew towards it. Then Dr Gagliano added a twist. Some forks were surrounded by plastic pipes full of water, creating watery noises but remaining inaccessible to the roots. When the alternative was a tube full of dry soil, those noises proved just as enticing to the growing roots as water itself.

image: Caroline Péron

Fascinated, Dr Gagliano and her colleagues placed small speakers at the bases of some tubes and played either a recorded sound of water, white noise, or nothing. Intriguingly, the plants seemed able to tell that they were being duped. Even when the alternative was parched soil, almost all chose to grow away from the speaker. They could only be persuaded to grow towards a speaker when forced to choose between two, in which case they chose the one playing watery noises. Dr Gagliano suspects—but cannot yet prove—that the small magnets found in the speakers are responsible for such discerning behaviour. A few older papers have suggested that plants can detect magnetic fields.

Still, the findings suggest that, in the absence of soil moisture, pea plants can detect the sound of water in pipes and follow it to its source. That too could prove to be valuable information. Plant roots are a big cause of damage to sewer systems all over the world. In Germany, the annual cost of root removal and associated pipe repair is around €28m ($30m). The assumption had been that it was leaks that attracted the roots. Dr Gagliano’s results suggest that even watertight pipes might still come under attack. The solution, she says, might be to invest in pipes that are silent as water runs through them.

A cry for help

And while plants are able to detect sounds, some also produce them, albeit unintentionally. This was demonstrated in April by the team at Tel Aviv University. Lilach Hadany, the team’s leader, knew that plants could sometimes be made to vibrate. This can happen when they do not have enough water. That causes air bubbles to form in the xylem, a specialised tissue that transports water from a plant’s roots to its leaves. When those bubbles collapse, they transmit small shock waves into the surrounding tissues. Previous work had shown that those vibrations could be measured with devices stuck to the plants themselves. Dr Hadany wondered whether they might be audible from farther away.

So the researchers put tomato and tobacco plants inside a microphone-lined box. Half had been watered, while half had been left parched. The researchers repeated the experiment with another set of plants, half of which had their stems cut, and half of which were left undamaged.

The microphones picked up very little sound from healthy plants. But those lacking water, or which had been cut, made a fair bit of noise, albeit at frequencies too high for humans to hear. Different stresses produced different kinds of sound. When the recordings were fed to a machine-learning algorithm, it was able to tell the sounds emitted from thirsty plants from those from the damaged ones.

When the experiment was repeated in a noisy greenhouse, Dr Hadany found that microphones could still detect the sounds from 10cm away. Experiments on cacti, corn, grapevines and wheat produced similar results, as did tomato plants suffering from an infection of mosaic virus, a common pathogen that can damage yields.

Farmers monitor the health of their crops by eye. (Mosaic virus, for instance, is so named because of the mottled pattern produced on the leaves of suffering plants.) That can be hard to do properly over an entire field. But if plants are broadcasting auditory indicators of distress, then wiring a field with microphones might help farmers keep an ear out for trouble.

That plants live in a world full of sound is no longer in doubt. But plenty of questions remain. One is the effect of human civilisation. It is well known that the din of city life makes bird calls harder to hear, forcing the animals to sing more loudly. Since trickling water, hungry caterpillars and suffering plants are all very quiet, it seems worth investigating whether plants face similar problems. Researchers might even apply to King Charles for funding.

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