You may be familiar with the growing body of work that provides evidence that plants are able to sense the sounds around them. Now new research suggests they can also generate airborne sounds in response to stress (such as drought or cutting).
A team led by experts from Tel Aviv University has shown that tomato and tobacco plants, among others, not only make sounds, but do so loud enough for other creatures to hear.
Their findings, published today in the journal Cell, help us tune into the rich acoustic world of plants—one that’s happening all around us but is never quite within human earshot.
Plants are “sedentary” organisms. They cannot escape from stressors such as herbivores or drought.
Instead, they evolved complex biochemical reactions and the ability to dynamically alter their growth (and regrow body parts) in response to environmental cues, including light, gravity, temperature, touch, and volatile chemicals produced by surrounding organisms.
These signals help them maximize their growth and reproductive success, prepare for and withstand stress, and form mutually beneficial relationships with other organisms such as fungi and bacteria.
In 2019, researchers showed that humming bees can cause plants to produce sweeter nectar. Others have shown that white noise played on Arabidopsis, a flowering plant in the mustard family, can induce a drought response.
Now, a team led by Lilach Hadany, who also led the aforementioned bee nectar study, has recorded the airborne sounds produced by tomato and tobacco plants and five other species (vine, hen nettle, needle cactus, corn and wheat).
These sounds were ultrasonic, in the 20-100 kilohertz range, and therefore undetectable by human ears.
To conduct their research, the team placed microphones 10 cm from the stems of plants that had either been exposed to drought (less than 5 percent soil moisture) or had been cut off near the soil.
They then compared the recorded sounds with those of non-stressed plants as well as empty pots and found that stressed plants made significantly more sounds than non-stressed plants.
As a great addition to their post, they also included an audio recording of the recording, resampled to an audible range and sped up. The result is a distinguishable “pop” sound.
The number of buds increased as drought stress increased (before decreasing as the plant dried). In addition, sounds could be detected from a distance of 3-5 meters – suggesting the potential for long-distance communication.
Although it remains unconfirmed, the team’s findings suggest that “cavitation” may be at least partially responsible for the sounds. Cavitation is the process by which air bubbles expand and burst inside the plant’s water-conducting tissue, or “xylem.”
This explanation makes sense if we consider that drought stress and cutting will change the water dynamics in the plant stem.
Regardless of the mechanism, it appears that the sounds produced by stressed plants were informative. Using machine learning algorithms, the scientists were able to distinguish not only which species was producing the sound, but also what type of stress it was suffering from.
It remains to be seen whether and how these sound signals may be involved in plant-to-plant or plant-to-environment communication.
Research has so far been unable to record any sounds from the woody stems of woody plants (which include many species of trees), although they have been able to detect sounds from the non-woody parts of the vine (the wood).
It is tempting to speculate that these airborne sounds could help plants communicate their stress more widely. Could this form of communication help plants, and perhaps wider ecosystems, better adapt to change?
Or perhaps the sounds are used by other organisms to detect the plant’s health. Moths, for example, can hear in the ultrasonic range and lay eggs on leaves, as the scientists point out.
Then there’s the question of whether such findings could help with future food production. Global demand for food will only increase. Tailoring water use to target individual plants or parts of the field that create the most “noise” could help us scale up production more sustainably and minimize waste.
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