So much goes on in nature and in our Universe that we humans don’t perceive. We think we’ve got our world all figured out, and then something like this comes along.
Researchers at Tel Aviv University have discovered that plants make audible, airborne sounds in response to different conditions, especially stressful ones. These sounds are at frequencies higher than humans can hear, but they are within the audible range of many insects and other living things. Not only that, but the sounds plants make under different kinds of stress (such as wounding, infection, and lack of water) can be distinguished from one another, giving the listener information about the type and level of stress the plant is facing.
This research is described in the March 30 edition of the journal Cell as the Featured Article (with open access!)
It’s extremely tempting to say that insects and other herbivores can and do listen to plants to gain information about them, and that they have been doing this for millions of years, and it has influenced evolution in a way we’ve never been aware of.
On the more practical side, we know for sure now that we can pick up these sounds with microphones and even interpret them, and that could be very useful in a greenhouse or field setting to keep an eye (or an ear, I guess) on the stresses our crop might be facing. Just listen to the plants!
So, what do they sound like? Click HERE or HERE to listen to a stressed plant (a 36-second audio clip), where the frequency of the sound has been lowered so that you can perceive it. Hey, you’ll be one of the first people on Earth ever to do so! (Of course, if you are a tree cricket, you may find my enthusiasm a bit passé).
As far as animals go, we humans aren’t exactly virtuoso sound perceivers. Our hearing ranges from about 20 hertz (vibrations per second, abbreviated Hz) up to about 20,000 hertz, or 20 kilohertz (kHz).
Here, take the test! I admit I zoinked out around 16 kHz, and speaking as a … shall we say non-teenager, that is actually about what’s supposed to happen. But um, maybe it’s just my cheap headphones. Yeah, yeah, definitely the headphones. I haven’t aged a bit!
The frequencies typically emitted by plants — and in this study, those included tomato, tobacco, wheat, corn, grape, and even cactus — are in the neighborhood of 50-60 kHz. That easily leaves all of us out, but it’s within the range of many animals, including my cat, who can hear up to 64 kHz. Dogs can get up around there as well, and their hearing is a lot more sensitive than humans’ to boot. Your pet can hear plants!
One reason flying insects would want to hear high-frequency sounds is because bats use echolocation to hunt prey at frequencies up to about 200 kHz. But the record frequency for insect hearing is held by the greater wax moth, which can hear at up to 300 kHz! The greater wax moth, and lots of other insects, are of course vitally concerned with information about plant health as well:
We also know that plants can respond to various sounds and vibrations by altering their gene expression and metabolism, so is it possible that plants can “hear” a stressed plant near them, interpret the sound, and prepare for a similar stress? More study will be needed to find out, but at least now we have the basis to do it.
By now you must be wondering how a plant could make sounds just by sitting there in the ground.
It’s been known for some time that air bubbles can start to form within the liquid in a plant’s xylem, the part that conducts water upward through the stem to hydrate the rest of the plant. They’re especially susceptible to this when water levels are getting low. It’s not great for the plant because air bubbles in the xylem make a narrower channel for water to get through. The formation of bubbles under these kinds of conditions is called “cavitation”.
We’ve also been aware for awhile that this can cause measurable vibrations in a plant. In 1966, researchers at the University of Aberdeen directly hooked up a wire to freshly cut leaves and detected vibrations that they concluded were caused by cavitation. Here’s the groovy gizmo they used to do that:
They had been inspired by a 1914 experiment by Henry H. Dixon of Trinity College, Dublin that was actually pretty cool. He filled a glass tube with water and introduced a small air bubble, then sealed up the tube. He found that when he heated the tube, the water would expand a little bit faster than the glass, and the bubble would gradually disappear. He found he couldn’t go too far with the heat, or the water would keep expanding and break the tube. So if he heated the tube just enough to make the bubble disappear, when he let the tube cool back down, the water would contract a little faster than the glass, until the point where the force of the water sticking to the glass wasn’t enough to prevent a bubble from forming. And indeed a bubble would suddenly materialize — cavitation — accompanied by an audible “click”.
And it appears to be this phenomenon that causes audible sounds to be made by plants. But the cavitation in plant capillaries like the xylem and stems and leaf veins happens on a teenier scale than in Dixon’s glass tube, so it produces higher-frequency sounds, out of the range of human hearing.
Somehow, even though we knew all of this, until Lilach Hadany’s group in Tel Aviv finally checked it out in the current study, nobody had ever tested to see whether the sounds could travel through the air and be detected remotely as sound.
A surprising aspect of this is that Hadany’s group was able to train a machine-learning algorithm to distinguish different kinds of stress simply by listening to the plant. In tobacco and tomato, they could discern stress vs. non-stress correctly 98% of the time, and they could distinguish between cutting stress and drought stress correctly 70% of the time. And that’s with a pretty small training set, so there’s a lot of room for improvement.
So if I’m an insect that likes to lay its eggs on leaves, and I can hear that this leaf I’m sitting on is stressed and may not be able to support my offspring because it’s in danger of wilting or getting whacked by a pathogen, I’m moving on and laying my eggs somewhere else.
And if I’m a farmer, and you tell me I can monitor different sectors of my land for drought stress, for example, by just walking around with a sound-gathering device, or by placing a few of these devices throughout my field, that means I can distribute water where and when it’s needed, do better on my yields, and even be less wasteful.
It seems plants have been trying to tell us a lot, and we’re finally alert enough to start listening. Nature always, always, always continues to amaze.
But I didn’t mean to interrupt the plants, so let’s get back to the show…..