Rain may be doing more than watering seeds. A new MIT study suggests rice seeds can sense the sound of falling droplets before they sprout, and that sound helps wake them from dormancy.

The findings, published in Scientific Reports, are described as the first direct evidence that plant seeds and seedlings can sense sounds in nature.

In a series of experiments, researchers found rice seeds sprouted faster when exposed to the sound of rainfall. The sound vibrations from falling droplets shook the seeds out of a dormant state and sped up germination compared with seeds kept in the same conditions without those rain sounds.

“The energy of the rain sound is enough to accelerate a seed’s growth,” said study author Professor Nicholas Makris. “What this study is saying is that seeds can sense sound in ways that can help them survive.”

Rice seeds can germinate in soil or water. In these experiments, the team used seeds submerged in shallow water.

The researchers said that when a raindrop hits the surface of a puddle or the ground, it generates a sound wave that makes the surroundings vibrate, including shallowly submerged seeds. They found those vibrations can be strong enough to dislodge a seed’s statoliths, tiny gravity-sensing organelles inside certain cells.

When statoliths are jostled, their movement acts as a signal for seeds and seedlings to grow and sprout.

Makris said plants already are known to sense and respond to many things in their surroundings, including touch, toxic smells, light and gravity. Roots grow down while shoots grow up against gravity’s pull.

One way plants sense gravity is through statoliths, which are denser than a cell’s cytoplasm and can drift and sink through the cell. When a statolith settles at the bottom, its resting place on the cell membrane reflects gravity’s direction and signals where a seed’s root or shoot should grow.

Makris said the idea for the study began when MIT graduate Cadine Navarro asked him about seeds and sound. They wanted to know if sound could jostle statoliths enough to stimulate growth.

“I went back to look at work done by colleagues in the 1980s, who measured the sound of rain underwater,” explained Makris. “If you check, you’ll see it’s much greater than in the air.

“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.”

To test the idea, researchers submerged about 8,000 rice seeds in shallow tubs of water and exposed sections of them to dripping water. They varied the droplet size and height to mimic light, moderate and heavy rainstorms.

The team also used a hydrophone to measure the underwater acoustic vibrations created by the droplets, then compared those readings with recordings from puddles, ponds, wetlands and soils during rainstorms in the field.

Those comparisons showed the lab droplets were producing rain-induced acoustic vibrations like those found in nature.

Seeds exposed to the sound of water drops germinated 30 percent to 40 percent faster than seeds that were not exposed to rain sounds but were otherwise kept in identical conditions.

The team also found seeds closer to the surface sensed the droplet sounds better and grew faster than seeds that were deeper or farther away.

The researchers said that may point to a biological advantage. Seeds close enough to the surface to respond to rain sound are also likely at a good depth to absorb moisture and grow safely to the surface.

They then calculated whether the droplets’ physical vibrations would be enough to jostle the seeds’ microscopic statoliths. They found the rice seed experiments matched those calculations, and that the sound of rain can dislodge and jostle a seed’s statoliths.

“Brilliant research has been done around the world to reveal the mechanisms behind the ability of plants to sense gravity,” said Prof. Makris. “It gives new meaning to the fourth Japanese micro-season, entitled ‘Falling rain awakens the soil.’”

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