Researchers at MIT have suggested that rice seeds can hear the sound of rain, according to a new study. MIT calls it "the first direct evidence that plant seeds and seedlings can sense sounds in nature". Perhaps surprisingly, the effects reported in this new study are not as radical as they may appear.
Playing music to your plants may sound eccentric, but a few previous studies have found it has some effect. For example, a 2024 study found bok choi grew better to classical music but less well to rock and roll. Sound can have a range of effects on plant behaviour. Some flowers use the pitch of an insect's buzz to determine whether they will release their pollen. Both Arabidopsis (thale cress) and tobacco plants produce higher levels of toxins in response to the sound of caterpillars chewing on neighbouring plants.
In contrast to previous experiments using electronic tones from a speaker, the MIT researchers instead tested the effect of a natural sound upon rice germination: the fall of rain. Rice can grow in soil or under water, and the researchers started by measuring the sound made by raindrops falling onto shallow puddles similar to the paddies they sowed seed in. The volume of sound waves created by drops landing on water was incredibly loud, equivalent to someone shouting straight into your ear, but mostly at frequencies too low or too high for a human to hear.
They then poured simulated rain on some of the pools containing rice and compared their rate of sprouting with seeds in still water. They found that although water droplets imitating light rain had little effect, heavier rain increased germination, and the heaviest by more than 30%.
They also picked up on an important clue from a previous study about how the rice might be detecting the sound. A 2002 study found that mutant Arabidopsis plants, which can't make starch, didn't respond to vibration in the same way that normal Arabidopsis do. Sound waves are just vibrating energy traveling through a gas, liquid, or solid that makes objects, such as the eardrum membranes we use to hear, shake as they pass. The MIT researchers theorised that perhaps plants needed to be able to make starch to detect sound.
This drew their attention to structures called statoliths, from the Greek for "standing stone". Plant cells that can detect gravity each contain several statoliths filled with highly dense starch, which sink through the cell. As they fall, the statoliths brush against other structures in the cell and come to rest pressing on its bottom, telling the plant which way is down.
To test their theory, the researchers modelled the effect of the recorded sound upon statoliths in the rice seeds. They found that the rain sounds could make the statoliths bounce up from the bottom of the cell like beads on a drum. Light rain would have little effect, but as the rain sound got heavier, the statoliths jumped higher and faster, matching the stimulation of germination. It also seemed that the layer of statoliths at the bottom of the cell would behave almost like a liquid, and that the sound energy would stir this "liquid" and help spread chemical messages to the rest of the plant. This suggests that statoliths may be one way that plants "hear".
Although there is now little doubt among scientists that plants can detect and respond to sounds, is this really hearing, or is a mind needed to perceive the signal? Plants don't have a nervous system and a centralised brain like humans and most other animals. There has, however, been a lively debate amongst scientists about whether plants demonstrate some type of intelligence or not. Observations of plant behaviour that appear intelligent include a 2017 study in which pea roots seemed to follow the sound of water through a simple maze.
Scientists have observed electrical signals in plants of a similar type to those in our nerves. For example, electrical signals trigger Venus flytraps to close on their prey. They are also used in Mimosa pudica, also known as the shyplant, which rapidly closes its leaves when touched.
Hearing may also require an organism that is aware of sound. Scientists Lynn Margulis and Dorian Sagan have argued that, at its most fundamental, consciousness is simply an awareness of the world outside the organism. If so, this is surely something that all species must possess if they are to respond to their environment and survive, even if it varies in complexity and nature. Maybe the world of a rice seedling is too different from ours for us to understand, but it may not be too much of a stretch to say that they hear the sound of rain.