Hearing the pitter-patter of rain, seeds sprout faster, according to a new study published in the journal Scientific Reports. Researchers from the Massachusetts Institute of Technology found rice seeds exposed to the patter of rain germinated faster than those that were not exposed to it. They also found that five centimetres is the sweet spot for germination. That is because if they are too far down, the seeds cannot drink in the moisture of the rain and their sprouts cannot come up to the surface.

The lead author of the study, Professor Nicholas Makris, relates how it all started. About three years ago, his co-author, Cadine Navarro, a former graduate student in MIT's Department of Urban Studies and Planning, came to him asking about sounds seeds might make. After some discussion and investigation, “we decided it would be more interesting to look at how seeds may respond to sound.”

When they dug into the literature, they found studies on how plants sense gravity. That proved to be a key factor in the work they would eventually do. In the literature, they found papers from 20 years ago from Japan. Japanese scientists were investigating how seeds responded to vibrations of the kind you'd find in agricultural machinery. They were looking at large vibrations. They attached seeds to moving platforms and found there was a response—seeds sprouted faster.

Then another group in Japan investigated further by testing seeds that were not able to sense gravity—the mutant strains—the way normal seeds would do. And they found that they didn't get that response from the mutant seeds.

“So that gave us a clue that it seemed to be something related to gravity sensing,” Nicholas Makris tells Hot Rock. The next step was investigating the properties by which plants sense gravity. They deduced that particularly strong environmental sounds might be able to elicit a response and cause the gravity sensing cells in a plant to respond to the sound of rain, which is quite intense in the near surface, near the drops, within a couple to a few centimetres.

They then theorised as to how this would happen. Inside the plant sensing cells, there are little granules that are heavier than the fluid in the cell cytoplasm and they sink to the bottom of the cell.

Makris says you can think of them like particles of salt in a salt shaker and as you move the salt shaker around, as you reorient it, the salt tends to move to the bottom of the shaker.  That's how these gravity sensing cells work.

These little granules are calcium carbonate crystals called statoliths. Housed in the root cells of plants, they guide the plants on which way is down so that they can send their roots in that direction.

In humans, too, there are calcium carbonate crystals, called otoliths, that act like gravity sensors. Situated within the vestibular system in the ear, they help us maintain balance and spatial orientation.

Coming back to the plants, as the plant moves around the little particles at the bottom will find the bottom of the cell. “It basically says if your root cell grows down, your stem cells grow up.” Makris says.

Another clue from the past literature fine-tuned their investigation. It was found that even if the statoliths, that rest on the bottom of the gravity sensing cells, are displaced, you get a response of growth.

“So the motion and the contact location of these little particles on the cell membrane is one of the key things,” says Makris.

The next step for the scientists was to test it. They investigated what kind of sound in the environment would vibrate the plant so much that the little particles on the bottom of the gravity sensing cells would be dislodged, just like shaking a salt shaker. They found that they needed “pretty strong environmental sounds”.

That is when the sound of rain came up. Makris says it is because he knew colleagues back in the 1980s who had studied the sound of rain.  They'd studied more of the bubbles and splashes, but this team needed the impact itself.

For you, the sound of rain may be a plink. If you’re a seed, the sound could be entirely different. Makris says: “Water is denser than air, so the same drop makes larger pressure waves underwater. If you’re a seed that’s within a few centimetres 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 metres of a jet engine in the air.”

They designed experiments that dropped raindrops on pools of water with rice seeds submerged below and measured their effect versus seeds in similar water but not with the rain dropping on top. They found that the germination rate of seeds exposed to the rain increased by the order 30-40 per cent.

The impact of rain on the seeds was visible only when the seeds were within a few centimetres of the surface—whether the water surface or the air or soil surface. If the seeds were buried too deep, more than about five centimetres, they wouldn't be able to sense the sound of the rain.

Makris says that is an advantage because it turns out for the types of seeds that they investigated—rice seeds and seeds of similar nature—have similar gravity sensing properties and similar size. They tend to favour depths of no more than about five centimetres for optimal germination.

If they go too deep, it's difficult for them to reach the surface when the shoots start to come up. So, this ended up being something quite beneficial for the plants, for the seeds. Not only were they getting moisture from the rain they were alerted to, but they're also at a good depth for optimal germination.

“It makes sense that something like this would happen. Just as human hearing is adapted to the needs of humans, the ability of a plant to sense sound should be adapted to what the plant needs,” Makris says.