How plants communicate has long been a question up for debate.
In fact, it was initially very controversial: Books like “The Secret Life of Plants” (1973) seemingly undermined the credibility of some of the first studies in the field by suggesting that plants thrive if you, say, sing or play classical music.
While many of those statements were later debunked, researchers remain adamant that there is some sort of communication going on between plants — and between plants and animals.
Can Plants Communicate With Each Other?
Plant communication is a perplexing topic. Perhaps because scientists are still learning about the process. What we do know is that plants have a wide range of mechanisms for communicating with each other and their surroundings.
“It's quite clear that plants are not just unresponsive victims of their herbivores, but that they are very aware of all kinds of things in their environment. And they respond to reliable information,” says Richard Karban, professor of entomology at the University of California, Davis.
“Yet it's not clear how important these various sources of information might be,” he continues.
Luckily, experts are tackling the problem from different points of view.
Read More: Can Plants Feel Pain?
The Wood-Wide Web
A pillar of the plant communication debate takes its name from the Internet with the “wood-wide web”. This concept first came about in the 1990s, when scientists began suggesting that forests were connected underground via intricate webs of elongated fungi known as mycorrhizas.
A 1997 paper published in Nature served as the cornerstone of this theory, suggesting that trees in forests don’t compete with each other; they cooperate. In other words, trees use these fungal links to share information, water, nutrients and more.
Read More: Why Some Plants Close Their Leaves at Night
How Do Plants Communicate?
The fact that the scientific jury is out on the existence of the wood-wide web doesn’t rule out all communication, though. To get to the root of the issue, perhaps the question should be how plants communicate rather than whether they communicate at all.
Plants Use Chemicals
When you get a delicious whiff of cut grass, those blades of grass are actually crying for help with a warning of danger. A growing body of research shows that when plants are damaged by things that want to eat them, like insects, they emit airborne chemicals.
Other sensitive leaves on that same plant, as well as the leaves of their neighbors, perceive those chemical signals and subsequently increase their own defenses, according to Karban.
His own research, for instance, shows that sagebrush plants sound the alarm when they’re attacked by pests so that other sagebrush plants respond by growing faster and stronger. Even other species, like tobacco, can sense and react to the alarm.
“These volatile chemical cues are one potential source of reliable information for plants,” Karban says. “What's still unclear is how general this is, how universal it is among plants and just how important it is.”
Plants Emit Sounds
Lilach Hadany, a professor at Tel Aviv University’s School of Plant Sciences and Food Security, wanted to better understand whether plants use the acoustic realm of communication, too.
In a study published this year, her team put tomato and tobacco plants in an acoustically isolated box and then recorded any ultrasonic sounds produced between 20 and 150 kilohertz. They experimented with cutting stems or leaving them without water as if to simulate drought.
The researchers found that the plants emitted popping and clicking sounds at around 60 decibels, approximately as loud as human chatter. These sounds were at an ultrasonic frequency that humans cannot naturally hear, however.
Hadany’s team even used artificial intelligence to match different sounds to the plants’ environment. Whether they were being cut or starved of water, each type of stress could be matched with a specific, identifiable sound.
How the sound is produced, what the biological costs of producing sound are and whether they have any use is still unknown. In fact, researchers still aren’t sure whether the sounds are used for communication purposes at all.
“We don’t know if they're using the sound, or if the sounds are emitted in a completely passive way due to physiological changes,” Hadany says. “But we do know they are in the air, and they contain information.”
Plant Communication Through Soil
Like the top part of plants, roots also produce volatile chemicals, hormones and other signaling molecules that can be transmitted to and detected by other roots through soil.
And, most importantly, there’s a large swath of microbes living underground. These microbes also react to whatever the plants are emitting.
“We focus on the mycorrhizal fungal network [wood-wide web], I think, because we can see it, we can visualize it in our head,” Field says. But, she continues, the role that other microbes play is underappreciated and misunderstood.
If, for instance, a root secretes sugars or other carbohydrates, it could change the makeup of the microbiome. The microbial communities that live in the soil may then exert an influence over what happens to another plant.
“So, there’s also this indirect communication, [and] we don't know how it works or what it does,” Field says.
The Role of Fungi in Plant Communication
But while it’s true that most plant roots are colonized by fungi — fungi that give plants water and nutrients from the soil in exchange for sugars and carbon — proof that these networks are used to communicate between plants is still scarce, according to research published in Nature this past February.
Experiments showed that most of the molecules “transferred” between trees stay in the fungi. Additionally, there’s very little research into the other elements that could be at play in these processes, and the scholars questioned why so little research has been done into what the fungi must gain from the process.
“Fungus has got its own agenda. It's doing its own thing. And we don't really know what it's doing. We don't know how it's doing it. And we don't know why it's doing it,” says Katie Field, a professor of plant-soil processes at the University of Sheffield in England. “I think we need to better understand that before we start ascribing roles in communication.”
Read More: If Fungi Could Talk
Why Plants Communicate
A key takeaway from all this research is that plants might not be actively communicating. Rather, according to Karban, they may be actively eavesdropping.
“If there's information in the environment that's reliable and that can increase their fitness, it's reasonable to put forward an argument that plants will be responsive to that kind of information,” Karban says.
That could be why plants are sensitive to the quality and quantity of light — something that's important to them — and why they’re unresponsive to their caretakers talking to them or playing music for them. These things just don’t increase their fitness.
“We think that as long as these cues are sort of reliable, plants will pay attention to them,” Karban says. “But that's very different than the idea that there's been selection to actively emit these cues.”
Read More: 20 Things You Didn’t Know About Trees