How Plants Secretly Talk With Each Other
“Oh the horror!”
Do you enjoy the smell of freshly cut grass in the summer? Remind you of those days as a youth, when you made great memories playing soccer? You monster. That’s the smell of untold numbers of blades of grass announcing their grisly wounds to their neighbors. Love the aroma of a bouquet of flowers? That scent you’re enjoying is the plant frantically trying to attract some insects to help it pollinate, the only means for these plants to have any children. Not likely to attract any bees sitting in a vase on your kitchen counter. There, now that I’ve properly set the stage for this essay with a hostile accusatory rant, let’s talk about Plant Communication. The common theme here is that we’re unknowingly walking around in the result of countless plants attempting to communicate — with themselves, with their neighbors, and with animals. We tend to think of plants as inert photosynthesizers, greening up our yards or salad-ing up our dinners. But plants are constantly sending messages, react and process information from their environment, maybe even have a semblance of intelligence to them. Let’s learn more! (You monster)
Chemical Communication In Plants
Think about it this way — if you’re a plant, you don’t have the benefit of a nervous system to send rapid signals around your body. (Though more on that later.) Something chewing on the leaves on your upper left branch? It’d be nice to somehow let the lower right branch know. But how? You don’t have any internal mechanism to send a signal down one branch and up the other — your insides only have slow-moving fluid and nutrient transport tubes. What else you got? Well, you could release a scent — if the leaves on one branch released some pungent smell, the volatile chemicals could possibly waft over to the other branches. While the sense of smell isn’t usually known for speed, you really don’t have any better options.
An experienced gardener providing volatile fertilizer for his plants
And it turns out that plants are using this mode of communication all the time. For example, this paper discusses how plants release volatile organic compounds (VOCs, for those in the biz) for just the purposes I discussed, to send long-distance signals between distant parts of the plant to mount a defense against predators. And those signals aren’t just specific to that one plant — neighboring plants can relay the stress signal to each other — a stressed plant will induce a stress response in its neighbor, who in turn induces a stress response in its neighbor. And those neighbors will increase their defenses, making the whole local area less affected by parasite damage. It might not be intentional communication on the damaged plant’s part (after all, why should it waste energy warning its neighbors?), but even eavesdropping counts as communication. And it might not only be through the air, from leaf to leaf — there’s evidence that plants also communicate through chemical signals in their roots.
Pretty cool, eh? Yeah, pretty cool, I guess… but it is sort of common knowledge by this point. 20 or 30 years ago, this was a controversial topic, but by this point chemical communication among plants is a pretty much settled topic. Not really what we go for here at Timeblimp. Howbout something weirder?
Non-chemical communication among plants
It turns out that chemical communication among plants is the least of the strangeness going on here. Some plants can also visually detect each other. They don’t have eyes, of course, but they can judge by how much light they themselves receive whether they must have plant neighbors nearby. Why would they need to know about neighbors? Competition, of course! By looking for evidence of neighbors in infrared light, they can decide whether they need to be worried about competitors for nutrients and sunlight. And plants can possibly listen in to the sound of herbivores muching on them — Heidi Appel showed that plants can react to the sound of a predator and mount a defense accordingly. When she played a recording of the sound of a caterpillar eating a leaf, her plants reacted by mounting a chemical defense. And they responded specifically to a caterpillar chewing with its mouth open, not other nature-borne but irrelevant sounds like wind or singing insects. These examples are both very cool, in that they show evidence of plants monitoring some sensory modality other than scent, but they’re not quite communication. Is there any evidence of non-chemical communication in the plant world?
Probably the most vocal champion of research in this field is Dr. Monica Gagliano, who has garnered a lot of press lately for applying serious scientific analysis to the question of plant communication. Sometimes you get to watch a particular topic make its first tentative steps into proper scientific study, after decades of being dismissed as not a proper topic for scientists. Non-chemical plant communication is making those first steps now.
Gagliano in her own lab has shown compelling (albeit indirect) evidence for communication among plants by showing they can influence the growth of neighbors even when known communication channels are blocked. By letting plants grow in close proximity but isolating them chemically and visually, she has shown that having neighbors nearby increases the plants’ growth rates. Pretty cool, but it begs the question of how the plants are communicating. In her own work she hasn’t located the source yet, but offers some interesting speculation that it might be acoustic or magnetic. Acoustic would make sense — we’ve known for a while that plants react positively to Van Halen (and other sound). Hell, even Mythbusters tackled the old legend that talking to your plants helps them grow better, and found it plausible. But magnetic signals? There’s apparently evidence that plants can sense magnetic fields (which is a surprise to me), but no clear evidence yet that plants can create them. But Gagliano does suggest some ideas, including appealing to quantum effects, which goes to confirm my old college prof’s belief that when a scientific concept isn’t understood, someone will invoke quantum mechanics or consciousness to explain it. (More on plant consciousness later.) Gagliano points to another paper on coherent quantum effects in algae photosynthesis, which sounds like good bathroom reading for later.
“Seymour, stop mowing the lawn, you monster!
So is there any evidence that plants can intentionally emit sounds? Well, it is known that they unintentionally emit sounds sometimes, for example during extremely dry conditions. Plants suck up water from the ground, transporting it to the leaves via internal tubes that run up through the stem. If its really dry, these tubes can sometimes pick up a trapped air bubble or two. An air bubble in the vascular system is just as healthy for the plant as it is for a human, so the plant will do the best it can to pop those bubbles, and you can hear the popping if you listen with the right microphones. But this really doesn’t count as an intentional act on the plant’s part, so as an example of communication, it’s a little disappointing.
More tantalizing is some work she cites by another researcher, Mario Pagano of the University of Firenze, who showed that 1) corn plant roots are attracted to (and grow toward) a ~200 Hz sound source, and 2) the plant roots themselves emit little clicking sounds, at about the same frequency! This is (as far as I can tell) the first experimental evidence of plant roots emitting sound. Sound communication sounds preposterous at first, but it has some logic to it — aside from the solid evidence that plants react to sound (Pagano wasn’t the first), As Gagliano points out, having body parts embedded in the ground could be an advantage for sensing sound, as sound waves travel much more efficiently in more dense material. Could plants be talking to each other, underground?
So we might be on the cusp of a revolution here. Still, we should take this with a grain of salt, as this exciting screaming-root data couldn’t be more buried away in her papers than if it were written in lemon juice on the back of the page. She brings up Pagano’s root-listening data in a review paper in 2012, but really only goes into detail about it down in a figure caption (which by law must be written in 3-point impossible-to-read font). Dr. Pagano hasn’t published his data himself — it’s apparently in his as-yet-unpublished PhD thesis. And results from her own lab, published later, only have the indirect proof — proof by removing known communication channels, not by sensing the actual new channel itself. My guess? Maybe Dr. Gagliano had trouble reproducing the root clicking data. It wouldn’t be the first time a really cool result turned out to be a false alarm.
So it has the air of “extremely preliminary” about it — it’s possible the barking corn roots aren’t actually barking. But showing that plants emit sounds for the purpose of communication would be astounding. And I hate to be a party pooper for such a cool idea — let’s let other people do that.
Further Reading
A great Wired article, ‘The Secret Language of Plants”
Dr. Gagliano’s review paper, which mentions the preliminary data on roots making clicking noises
Dr. Gagliano’s paper on how plants can still communicate, even if you cut off visual and chemical access
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