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Jan 17th 2020

Can Plants Hear? The Science of Sound-Sensing Flora

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Can plants hear? For decades, devotees have claimed that playing the right music mix could boost garden growth. Despite limited success of this Depeche Mode mandate for plant modification, however, recent evidence suggests that plants do “hear” — just not quite like us.

So let’s break it down: What’s the effect of sound on plants? What are they listening to? Can they make us a mix tape? Here’s a quick look at the science of sound-sensing flora.

Ear-ly Warning Systems

Shhhhhh. They’re listening.

That’s the overarching conclusion from multiple research studies: While plants don’t have ears, they can “hear” sounds in their local environment. More importantly, they can react.

According to research from Tel Aviv University, Oenothera drummondii flowers “exposed to playback sound of a flying bee or to synthetic sound signals at similar frequencies, produce sweeter nectar within 3 minutes, potentially increasing the chances of cross pollination.” The flowers also vibrated mechanically in response to these sounds, “suggesting a plausible mechanism where the flower serves as an auditory sensory organ.” And this wasn’t simply a case of flower + sound = response — while the plants vibrated and made sweet, sweet nectar in response to pollinator sounds, they showed no response to random, higher-frequency noise.

Scientific American, meanwhile, highlighted work from the University of Western Australia. Evolutionary biologist Monica Gagliano and her team put pea seedlings in pots with two “arms” — one led to a tray of water or plastic tube with water flowing inside, while the other led to dry soil. In all cases, the peas grew toward the water, suggesting they could “hear” it moving through the pipe. The Scientific American piece also pointed to a 2014 study that discovered the rock cress arabidopsis can tell the difference between blowing winds and chomping caterpillars. If it’s the latter, the plant pumps up its chemical toxin production to frustrate flora feeders.

Sound Science

Why haven’t we heard about this until now? As noted by a study about the impact of sound on mung bean sprouts, investigating the effect of sound on plants has typically focused on noise outside the audible range (20-20,000 Hz). Leveraging more common noise levels and frequencies, the researchers found that sound intensity of 90 dB and frequencies of around 2,000 Hz “indicated that the sound wave can reduce the germination period” of mung bean sprouts and drive a “significant increase in growth.”

As noted by Beyond Chemical Triggers: Evidence for Sound-Evoked Physiological Reactions in Plants, meanwhile, exposing rice to 0.8-1.5 kHz sound waves for one hour produced increased drought tolerance and stomatal conductance. And when crop plants such as pepper, cucumber, tomato and strawberry were exposed to 1,000 kHz sound, systemic immune responses occurred — specifically, “the Ca2+ ions influx cytosol from outside the plants membrane.” Researchers suggested that these ions may act as secondary messengers to help boost plant resistance to microbial pathogens.

This also helps explain why the supposed success that comes from supplying plants a steady stream of classical, pop or rock music to promote growth are so hard to pin down scientifically — since plants aren’t normally exposed to the melodious strains of Beethoven or the prog-rock pugnacity of Pink Floyd, it’s hard to garner reliable results.

Listening In

If plants can hear, should we also be paying attention to what they say?

While the so-called “wood wide web” has now been mapped, this underground network of microbes doesn’t really speak a language we can leverage to improve plant protection and production. But new research suggests that plants may emit ultra-sonic sounds in response to specific stimuli, such as a lack of water or their stems being cut. While there’s speculation here that insects may avoid stressed plants in favor of more mellow alternatives, that’s putting the stem before the stamen — there’s just not enough data yet to yield a clear conclusion.

Still, there’s potential benefit to listening in: Water-stressed plants appear to make louder sounds than their stem-cut companions. If further research bears out this observation, it could open the door to plant listening posts in commercial farming and greenhouse operations that would let growers pinpoint areas of potential drought to take appropriate action.

Can You Hear Me Now?

Humans love plants. Though not always the best stewards of seed-based life, we’re nonetheless compelled to find fundamental connections between flora and fauna. While human evolution is unlikely to align with plant-based powers like photosynthesis, it turns out there’s natural commonality in noise.

Can plants hear? Not exactly. But it sounds like they’re paying attention.

Are you interested science and innovation?  We are, too. Check out Northrop Grumman career opportunities to see how you can participate in this fascinating time of discovery.

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