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.