MIT engineers studying the effect of sound waves on plant reproduction have shown that elements within rice seeds evolved to detect gravity can “hear” the sound of raindrops impacting the water’s surface above them.
Unlike humans, who often describe raindrops as calming, the seeds used in the experiments appeared to be stimulated by the sound waves, shaking them out of a dormant state and accelerating germination compared to seeds not exposed to the sounds of falling raindrops.
The team behind the unusual discovery said their work is the first direct evidence that plant seeds and seedlings can sense and react to sounds in nature. A follow-up effort will investigate the effects of other natural vibrations on seed germination, with a focus on sounds seeds could realistically perceive, like strong winds and thunderstorms.
A Plant Seed’s Ability to Sense Gravity also Lets it “Hear” the Sound of Rain
Science has repeatedly shown that plants are extremely perceptive of their surroundings. Besides their obvious reaction to the sun, plants have evolved the ability to sense water in soil, respond to touch by closing (as in a Venus Flytrap), or curling their leaves to protect against toxic smells or extreme heat. Still, most may not know that plants and their seeds can sense gravity.
For example, a plant can use gravity’s pull to direct its roots downward, increasing access to water and nutrients. In plant seeds, this sensing and reaction to gravity is driven by small, physical features called statoliths. The MIT researchers described statoliths as denser than a cell’s cytoplasm, allowing them to drift and sink over time “like a bit of sand in a jar of water.”
After some time, gravity will cause the statolith to reach the seed’s bottom.’ Once settled against the seed cell’s membrane, the plant will direct roots to grow in that direction. According to the research team’s statement, previous studies have also shown that a dislodged statolith can trigger additional seed growth.
This idea prompted Nicholas Makris, an MIT professor of mechanical engineering whose work spans several disciplines, to reach out to eventual study co-author Cadine Navarro, a former graduate student in MIT’s Department of Urban Studies and Planning.
Together, the researchers asked, “Could sound be enough to jostle the statoliths and stimulate a seed to grow? And if so, what sounds in nature could be strong enough to have such an effect?”
Sound Pressures Equal to ‘Within a Few Meters of a Jet Engine in the Air’
The team’s first step involved tracing back the scientific literature to a study measuring the sound of rain underwater. Markis noted that the search revealed the underwater sound was much louder than the sound of rain falling above ground.
“If you check, you’ll see it’s much greater than in the air,” Makris said.
According to the MIT professor, the main difference lies in water’s higher density compared to air. This allows more of the raindrop’s energy to travel through the water, resulting in “larger pressure waves” beneath the water compared to above it.
“So, if you’re a seed that’s within a few centimeters of a raindrop’s impact, the kind of sound pressures that you would experience in water or in the ground are equivalent to what you’d be subject to within a few meters of a jet engine in the air,” Makris explained.
To test if this level of vibrations would be sufficient to stimulate plant growth, the team conducted a series of experiments using rice seeds. The MIT researchers said they chose rice seeds because they can germinate in soil or in shallow watery fields.
Seedlings Exposed to Rain Sounds Germinated 30 to 40 Percent Faster
Throughout a series of experiments, the team submerged approximately 8,000 individual rice seeds in shallow tubs of water. Some of the tubs were exposed to dripping water simulating rain, while others were not. To best approximate natural conditions, the team said they varied the size and height of each water droplet “to mimic raindrops during light, moderate, and heavy rainstorms.”
While exposing some of the seed tubs to simulated rain, the team monitored the underwater acoustic vibrations. When comparing these vibrations with recordings they’d previously recorded in puddles, ponds, wetlands, and soil during real rainstorms, the MIT team confirmed that their simulated raindrops were generating the same “rain-induced acoustic vibrations” as a real rainstorm causes in nature.
As expected, the rice seeds exposed to simulated raindrops germinated 30-40% faster than the unexposed seeds. Critically, the conditions for both seed groups were identical, save for the rain sounds. Even more telling, the seeds closer to the surface, where they could “hear” the louder rain sounds, germinated faster than the more deeply submerged seeds, which were also exposed to simulated raindrops.
To confirm that the added vibrations from the rain could rattle the seeds enough to dislodge their internal statoliths and stimulate growth, the MIT research team performed calculations that factored in a raindrop’s terminal velocity and the resulting vibrations its collision with soil or water surface could realistically deliver to the submerged or buried seeds.
After running the numbers, the researchers found that their experiments, which spurred accelerated germination, were “consistent: with the calculations. The sound of falling raindrops is sufficient to dislodge and “jostle” a seed’s statolith enough to spur the observed growth.
“This mechanism is likely at the root of a plant’s ability to ‘sense’ the sound of rain and grow in response,” they explained.
“Falling Rain Awakens the Soil”
When discussing the implications of their current findings, Professor Markis said their results say unequivocally that “seeds can sense sound in ways that can help them survive,” adding that the energy from the sound of falling rain “is enough to accelerate a seed’s growth.”
“Brilliant research has been done around the world to reveal the mechanisms behind the ability of plants to sense gravity,” Makris said of their findings. “Our study has shown that these same mechanisms seem to be providing plant seeds a means of perceiving submergence depths in the soil or water that are beneficial to their survival by sensing the sound of rain.”
Although further research is needed to confirm the evolutionary reasons for the observed behavior, the team suggested a potential biological advantage to seeds that can hear rain. For example, they said that seeds closer to the surface can respond more quickly to a new rainfall, placing them at an “optimal depth to soak up moisture and safely grow to the surface.”
Next, the team is planning a future study to determine whether other natural sounds, such as wind or thunder, also affect seed germination. For now, Makris noted that their study offered him a new perspective on a much older tome.
“It gives new meaning to the fourth Japanese microseason, entitled ‘Falling rain awakens the soil.’” The professor said.
The study “Seeds accelerate germination at beneficial planting depths by sensing the sound of rain” was published in Scientific Reports.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.