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Plants Are Listening, But How?

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Study shows ways that plants may respond to sound waves

plants that are alive

Some gardeners will swear that talking to plants helps them grow. And the word in the garden is that basil is good to have around. Is this all superstition, or is there something noisy happening here?

Plants are known to communicate with each other via shade, aromatic chemicals, and physical touch; they can promote growth as well as defense against disease, and the attraction of bees and other pollinators. A group of researchers from the University of Western Australia reported a new type of unknown mechanism by which some plants communicate. Biologist Monica Gagliano’s team planted common ornamental chili pepper seeds (Capsicum annuum, pictured) near a basil plant, which is known for its ability to encourage plant growth. But this time, the researchers erected barriers around the basil that prevented the plant from deploying its usual bag of growth-promoting tricks.

Despite the separation, the chili seeds germinated faster when basil was a neighbor, suggesting a message was getting through. Since the barriers were up, the researchers could rule out light, touch and chemical “smell.”

So, what was it? How was the messaging getting through? Gagliano’s team proposes a new type of communication between plants, possibly involving nano-scale sound waves, traveling through the dirt to bring encouraging “words” to the growing seeds.

The idea of acoustics in plants is a fairly new one. We know a lot about how animals communicate with sound, from human ears picking up sound waves, to snakes detecting vibrations in the ground. In fact, plants and snakes may have something in common: they both could share the ability to detect vibrations (“sound”) from dirt.

Dirt, it turns out, is a pretty good propagator of sound waves. It’s denser than air or even water, and can readily physically transmit vibrations over a long distance. And, ears aren’t even necessary to pick up these waves. While a normal human conversation is conducted at about 60 decibels, it can transmit a vibration of about 10-50 nm (nanometers, the measure of the amplitude, or “width” of the wave). However, many auditory sensory organs can pick up much, much lower amplitudes—a range between 0.1 nm and 1 micrometer is typical. It doesn’t take much noise to get noticed!

While Gagliano’s study points to this kind of auditory mechanism in plants, it also runs flat into a scientific obstacle: we know practically nothing about acoustics in plants. No known receptors or detection mechanisms are known. But, some research is showing signs of responses. Gagliano cites research in which sound waves could direct growing roots in the direction of the sound origin, and showed a preference for a frequency of about 300 Hz. Both preferences indicate some kind of detection of sound, and possibly a growth orientation created by sound.

Understanding this novel communication could help growers boost crop yields and increase global food supplies. Might be time for plant scientists to, um, speak up.

Gagliano, M., & Renton, M. (2013). Love thy neighbour: facilitation through an alternative signalling modality in plants BMC Ecology, 13 (1) DOI: 10.1186/1472-6785-13-19

Gagliano, M., Mancuso, S., & Robert, D. (2012). Towards understanding plant bioacoustics Trends in Plant Science, 17 (6), 323-325 DOI: 10.1016/j.tplants.2012.03.002

Photo: Flickr

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