Even on the quietest days, the world is full of sounds: birds chirping, wind rustling through trees, and insects humming about their business. The ears of both predator and prey are attuned to one another’s presence.
Sound is so elemental to life and survival that it prompted Tel Aviv University researcher Lilach Hadany to ask: What if it wasn’t just animals that could sense sound—what if plants could, too? The first experiments to test this hypothesis, published recently on the pre-print server bioRxiv, suggest that in at least one case, plants can hear, and it confers a real evolutionary advantage.
The sweetest sound
As an evolutionary theoretician, Hadany says her question was prompted by the realization that sounds are a ubiquitous natural resource—one that plants would be wasting if they didn’t take advantage of it as animals do. If plants had a way of hearing and responding to sound, she figured, it could help them survive and pass on their genetic legacy.
Since pollination is key to plant reproduction, her team started by investigating flowers. Evening primrose, which grows wild on the beaches and in parks around Tel Aviv, emerged as a good candidate, since it has a long bloom time and produces measurable quantities of nectar.
To test the primroses in the lab, Hadany’s team exposed plants to five sound treatments: silence, recordings of a honeybee from four inches away, and computer-generated sounds in low, intermediate, and high frequencies. Plants given the silent treatment—placed under vibration-blocking glass jars—had no significant increase in nectar sugar concentration. The same went for plants exposed to high-frequency (158 to 160 kilohertz) and intermediate-frequency (34 to 35 kilohertz) sounds.
But for plants exposed to playbacks of bee sounds (0.2 to 0.5 kilohertz) and similarly low-frequency sounds (0.05 to 1 kilohertz), the final analysis revealed an unmistakable response. Within three minutes of exposure to these recordings, sugar concentration in the plants increased by an astounding 20 percent.
A sweeter treat for pollinators, their theory goes, may draw in more insects, potentially increasing the chances of successful cross-pollination. Indeed, in field observations, researchers found that pollinators were more than nine times more common around plants another pollinator had visited within the previous six minutes.
“We were quite surprised when we found out that it actually worked,” Hadany says. “But after repeating it in other situations, in different seasons, and with plants grown both indoors and outdoors, we feel very confident in the result.”
Flowers for ears
As the team thought about how sound works, via the transmission and interpretation of vibrations, the role of the flowers became even more intriguing. Though blossoms vary widely in shape and size, a good many are concave or bowl-shaped. This makes them perfect for receiving and amplifying sound waves, much like a satellite dish.
To test the vibrational effects of each sound frequency test group, Hadany and her co-author Marine Veits, then a graduate student in Hadany’s lab, put the evening primrose flowers under a machine called a laser vibrometer, which measures minute movements. The team then compared the flowers’ vibrations with those from each of the sound treatments.
“This specific flower is bowl- shaped, so acoustically speaking, it makes sense that this kind of structure would vibrate and increase the vibration within itself,” Veits says.