New research from MIT suggests that rice seeds may be able to "hear" the sound of rainfall and regulate their germination process accordingly. This work, called "the first direct demonstration that plant seeds and seedlings can sense sounds in nature," provides new clues about how plants use sound signals to adapt to their environment.
Long before this study, the scientific community had repeatedly noted the effects of sound on plants. Some studies have found that playing classical music to Shanghai Qing can promote growth, while rock music may inhibit its growth, indicating that different sound sources can have differential effects on plants. Other experiments also show that sound can widely affect plant behavior: some flowers use the specific pitch of an insect's wings to decide whether to release pollen; Arabidopsis and tobacco will increase the content of toxic substances such as nicotine in their bodies to enhance their defense capabilities after "hearing" the sound of neighboring plants being chewed by caterpillars. The tones emitted by the synthesizer have also been reported to promote seed germination and seedling growth of mung beans, cucumbers, and rice.
Different from the previous use of speakers to play electronic sounds, this time the MIT team chose a sound source that is closer to a natural situation - rainfall. They first measured the sound produced when raindrops fall into the water in a shallow water environment similar to a rice field. The results show that the intensity of this sound wave is equivalent to someone shouting loudly in your ear, but the frequency range mostly falls in the high and low frequency bands that are difficult for human ears to hear. The researchers then poured simulated rainfall into shallow pools of rice seeds and compared their germination rates with those of seeds in a still-water environment. The experiment found that slight "drizzle" had little effect on germination, while louder rain significantly increased the germination rate. Under the strongest simulated rainstorm conditions, the germination rate increased by more than 30%.
The research team also found key clues from an earlier work. A 2002 study reported that Arabidopsis mutants unable to synthesize starch responded significantly differently to normal plants when exposed to vibration. Sound waves are essentially vibrational energy that travels through gases, liquids, or solids, causing structures such as the human eardrum to vibrate, which we perceive as sound. The MIT team hypothesized based on this: Plants may need to have the ability to synthesize starch in order to "hear" sound waves.
Following this idea, the researchers focused on a type of structure called "statolith" in plant cells. The term statolith is derived from the Greek word for "standing stone" and is a key device used by plants to sense the direction of gravity. Cells that can sense gravity are filled with small bodies filled with high-density starch. They sink inside the cells and "report" to the plant which direction is "down" through contact with surrounding structures and final resting position. The researchers modeled how recorded rain sounds acted on balance stones in rice seeds and found that the sounds were enough to shake up a layer of balance stones that had sunk at the bottom of the cells like beads on a drum. The balance stones were barely affected by the sound of light rain; as the rain intensified, they were continuously thrown up and accelerated, a behavior consistent with the observed stimulation of germination.
The model also shows that the stacked state of this layer of equilibrium stones at the bottom of the cell behaves almost like a liquid under the influence of sound waves, similar to a ball pit filled with plastic balls in a children's playground. In this case, sound energy continuously agitates this layer of "liquid," helping to diffuse chemical signals more efficiently to other parts of the plant. The reason why the starch-deficient Arabidopsis thaliana mentioned above has difficulty in responding normally to vibrations is probably because they cannot make the starch required for statoliths, which prevents this sensory system from functioning. This shows that balance stones are likely to be an important mechanism for plants to "hear" external vibrations.
As evidence continues to accumulate, the scientific community generally accepts the fact that plants can sense and respond to sound. But whether this means that plants are actually "listening", that is, whether some kind of consciousness or mind is required to perceive the signal, is still debated. Plants, unlike humans and most animals, do not have similar nervous systems and centralized brains. In recent years, there has been a fierce debate over whether plants possess some form of "intelligence". Some researchers believe that plants exhibit intelligent behavior to a certain extent, while others hold a negative attitude towards this.
One piece of evidence supporting the idea of "plant intelligence" comes from a 2017 study: pea roots appear to be able to "find" water in a simple maze, following the sound of water flowing. Another 2016 study claimed that pea seedlings can learn to associate the direction of wind from a fan with the direction of light, thereby "predicting" where the light source is. In plants, electrical signals similar to those in animals have also been observed, although these signals are not propagated through specialized structures that correspond exactly to the nervous system. In many cases, scientists don't yet know the exact function of these electrical signals, most likely because the way plants respond isn't always intuitive.
Clear examples include the Venus flytrap, which uses electrical signals to trigger its leaves to quickly close and then "crushes" its prey, and the mimosa, which uses electrical signals to quickly close its leaves when touched. These phenomena leave room for imagination for a more “decentralized” form of intelligence, in which information processing and response may be distributed throughout the plant system rather than concentrated in a single brain-like structure. However, it is still arbitrary to directly equate this distributed response with "hearing" or "consciousness" in the human sense.
Beyond the question of hearing, consciousness itself also poses philosophical challenges to research. There are many different opinions on the definition of consciousness. Biologist Lynne Margulies and her collaborator Dorian Sagan have proposed that at its most basic level, consciousness can be understood as an awareness of the external world. If this is used as a criterion, then all species that are to survive and respond to their environment presumably must possess some form of consciousness, although its complexity and specific expressions vary widely.
Perhaps the world "perceived" by rice seedlings is so far removed from human experience that it is difficult for us to truly understand how they "experience" sound waves. But judging from this MIT study and existing evidence, it is probably not an unfounded metaphor to say that they "hear" the sound of rain in a sense.