by April Thompson
While flowers are known to lean toward light, a growing body of research is demonstrating plants also respond to sounds and scents—and then herald the news to their neighbors. Far from being passive life forms, members of the plant kingdom are adept at interacting with their environments and with each other.
“Plants don’t have specialized sense organs, but like animals, plants are very capable of sensing their environment. They perceive cues, weigh different alternatives and allocate resources in very sophisticated ways,” says Richard Karban, professor of entomology at the University of California at Davis and the author of Plant Sensing and Communication.
Better Living Through Chemistry
Early evidence of plant communication was discovered by accident, according to Jack Schultz, senior executive director of research development at the University of Toledo, in Ohio. “In the 1970s, researchers began to notice plants under attack respond by increasing defensive chemistry—things that make a plant distasteful or toxic to predators,” he says. Researchers noticed that control plants also seemed to respond to their neighbors being attacked.
Since then, Schultz, Karban and other investigators have discovered that plants emit complex profiles of odors in the form of volatile compounds that can be picked up by other plants, as well as insects. Studying sagebrush in the Sierra Nevada mountains, Karban found that plants under duress emit chemical cues that trigger nearby plants to increase their defenses.
These odors vary with the type of threat and time, working to attract pollinators during the day and fending off enemies at night, Schultz says. A plant being eaten by an insect may release a chemical that attracts predatory insects looking for herbivore prey. “There is a clear adaptive advantage in attracting the ‘enemy of your enemy’, who can act as a bodyguard for the plant being attacked.”
Smells are just part of a plant’s multisensory life, says Heidi Appel, a professor in the Department of Environmental Sciences at the University of Toledo and one of Schultz’s collaborators. Appel’s research with collaborator Rex Cocroft, at the University of Missouri, demonstrates they’re listening for threats, too.
Her lab exposed plants from the mustard family to the sound of a caterpillar feeding, with control plants in silence or “listening” to a recording of the wind or other insects, and found that those vibrations didn’t effect the same defensive-priming response as that of the plant-munching caterpillar. “Plants have no special sense organs, so their sophisticated sense of hearing is very surprising,” says Appel.
Karban’s lab isolated plants to determine that their chemical signals were transmitted by air rather than soil or root systems. Yet researcher Suzanne Simard, a professor of forest ecology at the University of British Columbia, in Vancouver, is digging into the underground connections, finding that trees are interacting with one another below the ground in complex ways.
Trees have a symbiotic relationship with fungi that’s built on a mutually beneficial exchange of nutrients, says Simard. This underground network links root systems of trees together, enabling them to exchange carbon, water and other nutrients in a kind of natural balance sheet. Simard discovered these networks had hubs—typically older “mother trees”—that can connect to hundreds of saplings and send them excess carbon that can quadruple their survival rates.
Simard also found that trees engage in “defense signaling” similar to plants, increasing their natural defenses in response to damage inflicted on their neighbors, but only if the mycorrhizal networks of fungi that aid in sending such messages are intact. Simard’s research seeks to understand how environmental threats like climate change and logging may further disrupt these communication networks.
Recognizing all of the communication that exists between plants, we might wonder if human words of encouragement can help them grow. Perhaps, but not for the reasons one might hope, says Appel. “Whenever we feel a sense of connection to another life form, we are more likely to take better care of it,” says the researcher. “We underestimate what plants can do because their communication is invisible to us. Yet we also have to be careful about overestimating their abilities. We need an understanding to be driven by science, and not wishful thinking.”
April Thompson is a freelance writer in Washington, D.C.