The next time you find yourself lulled by the patter of rain outside your window, think how that same sprinkle might sound if you were a tiny seed planted directly below a free-falling droplet. Would you still be similarly soothed?
In fact, MIT engineers have found the opposite to be the case: Some seeds may come alive to the sound of rain. In experiments with rice seeds, the team found that the sound of falling droplets effectively shook the seeds out of a dormant state, stimulating them to germinate at a faster rate compared with seeds that were not exposed to the same sound vibrations.
The team's findings, which are published today in the journal Scientific Reports, are the first direct evidence that plant seeds and seedlings can sense sounds in nature. Their experiments involved rice seeds that they submerged in shallow water. Rice can germinate in both soil and shallow water. The researchers suspect that many similar seed types may also respond to the sound of rain.
The team worked out a hypothesis to explain how the seeds might be doing this. They found that when a raindrop hits the surface of a puddle or the ground, it generates a sound wave that makes the surroundings vibrate, including any shallowly submerged seeds. These vibrations can be strong enough to dislodge a seed's "statoliths," which are tiny gravity-sensing organelles within certain cells of a seed. When these statoliths are jostled, their movement is a signal for seeds and seedlings to grow and sprout.
"What this study is saying is that seeds can sense sound in ways that can help them survive," says study author Nicholas Makris, a professor of mechanical engineering at MIT. "The energy of the rain sound is enough to accelerate a seed's growth."
Makris and his co-author, Cadine Navarro, a former graduate student in MIT's Department of Urban Studies and Planning, suspect that the sound of rain is similar to the vibrations generated by other natural phenomena such as wind. They plan to follow up this work to investigate other natural vibrations and sounds plants may perceive.
Sound vibration
Plants are surprisingly perceptive. To help them survive, plants have evolved to sense and respond to stimuli in their surroundings. Some plants snap shut when touched, while others curl inward when exposed to toxic smells. And of course, most plants respond to light, reaching toward the sun to help them grow.
Plants can also sense gravity. A plant's roots grow down, while its shoots push up against gravity's pull. One way that plants sense and respond to gravity is through their statoliths. Statoliths are denser than a cell's cytoplasm and can drift and sink through the cell, like a bit of sand in a jar of water. When a statolith finally settles to the bottom, its resting place on the cell's membrane is a reflection of gravity's direction and a signal for where a seed's root or shoot should grow. If the statolith is dislodged, scientists have found that this can also trigger the seed to grow more.
Makris, whose work focuses on acoustics across a range of disciplines, became curious when Navarro asked him questions about seeds and sound. They wondered: 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?
"I went back to look at work done by colleagues in the 1980s, who measured the sound of rain underwater. If you check, you'll see it's much greater than in the air," Makris says. "It has to do with the fact that water is denser than air, so the same drop makes larger pressure waves underwater. 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."
Such rain-induced soundwaves, Makris and Navarro suspected, might be enough to jostle statoliths and subsequently stimulate a seed's growth.
Connecting a droplet's dots
To test this idea, the researchers carried out experiments with rice seeds, which naturally grow in shallow watery fields. Over a large number of repeated experiments, the team submerged roughly 8,000 individual seeds of rice in shallow tubs of water and exposed sections of them to dripping water. The seeds were placed sufficiently far away from the falling droplets that only sound waves would reach them. The team varied the size and height of each water droplet to mimic raindrops during light, moderate, and heavy rainstorms.
The sound of rain, recorded by MIT researchers from underwater, within a rain puddle in Massachusetts during a moderate to heavy rainstorm.
Credit: Courtesy of the researchers
They also used a hydrophone to measure the acoustic vibrations created underwater by the water droplets. They compared these measurements to recordings they took in the field, such as in puddles, ponds, wetlands, and soils during rainstorms. The comparisons confirmed that their water droplets in the lab were generating rain-induced acoustic vibrations as in nature.
As they observed the rice seeds, the researchers found that the groups of seeds that were exposed to the sound of water were able to germinate 30 to 40 percent faster than the seed groups that were not exposed to rain sounds but were otherwise in identical conditions. They also found that seeds that were closer to the surface could better sense the droplets' sounds and grow faster, compared to more submerged or more distant seeds.
These experiments showed that there is a connection between the sound of a water droplet and a seed's ability to grow. The researchers propose that there may be a biological advantage to seeds that can sense rain: If they are close enough to the surface to respond to the sound of rain, they are likely at an optimal depth to soak up moisture and safely grow to the surface.
The team then worked out calculations to see whether the physical vibrations of the droplets would be enough to jostle the seeds' microscopic statoliths. If so, this would point to the mechanism by which sound can directly stimulate a plant's growth.
In their calculations, the researchers factored in a rain droplet's size and terminal velocity (the constant speed that a falling object eventually reaches), and worked out the amplitude of sound vibration the droplet would generate. From this, they determined to what degree these vibrations in water or soil would displace, or shake a submerged or buried seed, and how a shaking seed would affect microscopic statoliths within individual cells.
Makris and Navarro found that the experiments they performed on rice seeds were consistent with their calculations: The sound of rain can indeed dislodge and jostle a seed's statoliths. This mechanism is likely at the root of a plant's ability to "sense" the sound of rain and grow in response.
"Brilliant research has been done around the world to reveal the mechanisms behind the ability of plants to sense gravity," Makris notes. "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. It gives new meaning to the fourth Japanese microseason, entitled 'Falling rain awakens the soil.'"
This work was supported, in part, by the MIT Bose Fellowship and the MIT Koch Chair.
