File photo by Ksenia Pixelesse
Seeds may come alive to the soothing sound of rainfall, sensing the drops while they are still beneath the surface, suggests a new study.
A series of experiments at the Massachusetts Institute of Technology demonstrated that rice seeds sprouted faster to the sound of rainfall.
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 findings, published yesterday in the journal Scientific Reports, are the first direct evidence that plant seeds and seedlings can sense sounds in nature.
Rice seeds can germinate in either soil or water, and these experiments involved seeds submerged in shallow water.
The researchers suspect that many similar seed types may also respond to the sound of rain.
The MIT team 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.
The vibrations can be strong enough to dislodge a seed’s “statoliths”, tiny gravity-sensing organelles within certain cells of a seed. When the statoliths are jostled, their movement is a signal for seeds and seedlings to grow and sprout.
“The energy of the rain sound is enough to accelerate a seed’s growth,” said study author Professor Nicholas Makris.
Credit: Cadine Navarro / MIT (CC BY-NC-ND 3.0)
“What this study is saying is that seeds can sense sound in ways that can help them survive.”
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Plants have evolved to sense and respond to many stimuli in their surroundings to help them survive: some plants snap shut when touched, some curl inward when exposed to toxic smells, and most respond to light, reaching toward the sun to help them grow.
Prof. Makris says 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, which are denser than a cell’s cytoplasm and can drift and sink through the cell, like 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.
Makris became curious when Cadine Navarro, an MIT graduate, asked him about seeds and sound. They wondered if sound could be enough to jostle the statoliths and stimulate a seed to grow.
“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.”
The researchers suspected that such rain-induced sound waves might be enough to jostle statoliths and subsequently stimulate a seed’s growth—and they were right.
Growing 30-40% faster
They submerged around 8,000 individual seeds of rice in shallow tubs of water and exposed sections of them to dripping water.
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The researchers varied the size and height of each water droplet to mimic raindrops during light, moderate, and heavy storms. They also used a hydrophone to measure the acoustic vibrations created underwater by the water droplets.
The team compared the measurements to recordings they took in the field, such as in puddles, ponds, wetlands, and soils during rainstorms. Comparisons confirmed that their water droplets in the lab were generating rain-induced acoustic vibrations like those found in nature.
The groups of seeds that were exposed to the sound of water drops were able to germinate 30% to 40% faster than the seed groups that were not exposed to rain sounds but were otherwise in identical conditions.
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The team 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.
The researchers believe that there may be a biological advantage to seeds that can sense rain. If seeds 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. They found that the experiments they performed on rice seeds were consistent with their calculations: 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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The team plan to investigate other natural vibrations and sounds, like wind, that plants may perceive.
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