• Ben Derico

Plant Acoustics: or how the plant kingdom got vibey

Honey bees flap their wings 250 times per second—imperceptible to humans—when in flight. It turns out that plants, particularly pollinating flowers, may actually be able to respond to the faint vibrations produced by the bee’s buzzing wings. Researchers discovered in 2019 that plants increase pollen production within moments of detecting the specific vibration pattern of a pollinator.

“Because plants don't have specialized organs like we do,” says Dr. Heidi Appel, principal investigator at the University of Toledo’s Appel-Schultz Chemical Ecology Lab, “we underestimate their sensory capabilities.” Plant life may not see, hear, or feel the same way we do, but their sensory system is tuned, Appel says, to stimuli on an entirely different bandwidth.

And if we can figure out how to get onto that bandwidth, we may be able to find ways to grow stronger, healthier plants or even protect them from disease without harmful chemicals.

By most accounts, humans began farming around 10,000 years ago. Ever since then, they’ve tinkered with the soil, water, and light to grow more robust, dependable crops. But their experience playing with sound and vibrations to affect change in cultivated plants is a bit more recent.

In 2014, a few years before the previously mentioned bee study, Appel, and colleague Dr. Rex Cocroft, professor of biological sciences at the University of Missouri, set out to see if they could influence plant behavior with specific sounds from nature.

Using a tiny laser microphone connected directly to the leaf of an Arabidopsis Thaliana, a type of small watercress, they recorded the munching of a hungry caterpillar climbing on its leaves. Next they played that chewing sound to one set of plants and left another to sit quietly alone as a control group. When they released eager caterpillars onto both sets of plants, the watercress that had already been exposed to the chomping audio released increased levels of a mustard oil compound used in the plant's internal defense system. The sound of a predator led the plant to release a chemical to entice the caterpillar to stop eating. Previous exposure primed the plants to produce more oil, and, reduce its chance of being eaten.

The breakthrough, published in a 2014 article in Oecologia, was the first evidence that plants could respond to an ecologically relevant sound in their environment and earned Appel and Cocroft a National Science Foundation grant to continue their work.

For Appel, whose interest in plants dates back to the 1970s when she was a teenager, this experiment represented a potential end to an era of ridicule for scientists interested in the effects of sound on plants. In 1973, The Secret Life of Plants, a now widely discredited book claiming to prove plant sentience, feelings, and emotions, was published and pushed plant life to the forefront of American popular culture. Featuring page-turning passages of former CIA agents conducting polygraphs on frightened dracaena, the book captivated popular culture and increased interest into the “minds” of the plant kingdom. But the book also pushed unrepeatable, shoddy science regarding plants and sound into the spotlight. The book, and its proximity to new-age culture, left a long-lasting chill on those studying how plants respond to auditory stimuli.

Forty years later, that chill is lifting. With Appel’s 2014 paper leading the way, the burgeoning field of plant acoustics has been growing steadily. A 2015 paper published from GD Goenka University near Delhi, India, for example, found notable results on growth and germination of chickpea plants exposed to “light meditation music” compared to their counterparts that were exposed only to noise. In a separate 2010 Chinese study working with strawberries cultivated in greenhouses, researchers found that the fruit exposed to stimulation from a single audible tone “grew stronger than in the control” and that “resistance of the strawberry against disease and insect pests were enhanced.” With more and more studies bringing back scientifically-sound reports from the lab, Appel says, “It's clear that plants are responding to vibrations...under lab conditions. We don't know what happens in the field, but that's where I'm going next.”

Eyeing a move out of the sterile, refrigerator-like environment of her lab’s reach-in chambers, Appel plans to head into the field next summer to focus on native North American plants in their natural habitat. Out in fields, plains, and deserts across different stretches of the country, Evening Primroses and Partridge Peas will undergo the same exposure to herbivore vibrations as previous studies did in the lab, while also encountering the whole host of pathogens and pests the real world has to throw at them.

If the results are the same as they were in the lab, it could show that targeted sonic exposure could prove to be a new tool in the 21st century agrarian arsenal.

Appel’s forthcoming study involves exposing plants to the same caterpillar vibrations and will also look at the effects of vibrations from buzz pollination. Understanding how these vibrations affect or influence plant life can help scientists understand how plants naturally defend themselves or even cooperate with partners from the animal kingdom.

One example that’s already drawing interest - buzz pollination seems to lead to increased nectar production, which potentially acts as a reward. Sweeter nectar can attract hungry ants, coaxing them to climb up a plant’s stalks and pick off threatening herbivores and their eggs. It’s an indirect, non-chemical defense that, if triggered by sound waves, could be used by farmers as a non-chemical pesticide.

It’s those kinds of potential outcomes that, six years following the paper’s publication, keep Appel inspired to continue. In the British journal The Biologist, Appel explained after the study’s results were released, “The field of plant acoustics has only begun to explore the acoustic interactions between plants and their environment.”

Despite the growing promise of this rigorous research, there remains in the background a 21st century breed of pop culture pseudoscience and renewed interest in the “minds'' of plants, much like the wave in the 1970s. Works by authors like Monica Gagliano, the University of Sydney-based researcher whose more “spiritual” work with plants has been featured in the New York Times style section, are gaining popularity and making the conversation on what exactly plants perceive complicated.

“A lot of scientists get very frustrated with that,” Appel says, but she’s not one of them. “People want to understand how sophisticated plants are, because that’s the reality,” she continued. Increased attention, in any context—including in terms of human thought, feelings, cognition—will just increase focus on the study of plant acoustics.