What does a stressed plant sound like? A bit like bubble-wrap being popped. Researchers in Israel report in the journal Cell on March 30 that tomato and tobacco plants that are stressed—from dehydration or having their stems severed—emit sounds that are comparable in volume to normal human conversation. The frequency of these noises is too high for our ears to detect, but they can probably be heard by insects, other mammals, and possibly other plants.
“Even in a quiet field, there are actually sounds that we don’t hear, and those sounds carry information,” says senior author Lilach Hadany, an evolutionary biologist and theoretician at Tel Aviv University. “There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring.”
Although ultrasonic vibrations have been recorded from plants before, this is the first evidence that they are airborne, a fact that makes them more relevant for other organisms in the environment. “Plants interact with insects and other animals all the time, and many of these organisms use sound for communication, so it would be very suboptimal for plants to not use sound at all,” says Hadany.
The researchers used microphones to record healthy and stressed tomato and tobacco plants, first in a soundproofed acoustic chamber and then in a noisier greenhouse environment. They stressed the plants via two methods: by not watering them for several days and by cutting their stems. After recording the plants, the researchers trained a machine-learning algorithm to differentiate between unstressed plants, thirsty plants, and cut plants.
The team found that stressed plants emit more sounds than unstressed plants. The plant sounds resemble pops or clicks, and a single stressed plant emits around 30–50 of these clicks per hour at seemingly random intervals, but unstressed plants emit far fewer sounds. “When tomatoes are not stressed at all, they are very quiet,” says Hadany.
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An audio recording of plant sounds. The frequency was lowered so that it is audible to human ears. Credit: Khait et al.
Water-stressed plants began emitting noises before they were visibly dehydrated, and the frequency of sounds peaked after five days with no water before decreasing again as the plants dried up completely. The types of sound emitted differed with the cause of stress. The machine-learning algorithm was able to accurately differentiate between dehydration and stress from cutting and could also discern whether the sounds came from a tomato or tobacco plant.
Although the study focused on tomato and tobacco plants because of their ease to grow and standardize in the laboratory, the research team also recorded a variety of other plant species. “We found that many plants—corn, wheat, grape, and cactus plants, for example—emit sounds when they are stressed,” says Hadany.
A photo of a cactus being recorded. Credit: Itzhak Khait
The exact mechanism behind these noises is unclear, but the researchers suggest that it might be due to the formation and bursting of air bubbles in the plant’s vascular system, a process called cavitation.
Whether or not the plants are producing these sounds in order to communicate with other organisms is also unclear, but the fact that these sounds exist has big ecological and evolutionary implications. “It’s possible that other organisms could have evolved to hear and respond to these sounds,” says Hadany. “For example, a moth that intends to lay eggs on a plant or an animal that intends to eat a plant could use the sounds to help guide their decision.”
Other plants could also be listening in and benefiting from the sounds. We know from previous research that plants can respond to sounds and vibrations: Hadany and several other members of the team previously showed that plants increase the concentration of sugar in their nectar when they “hear” the sounds made by pollinators, and other studies have shown that plants change their gene expression in response to sounds. “If other plants have information about stress before it actually occurs, they could prepare,” says Hadany.
An illustration of a dehydrated tomato plant being recorded using a microphone. Credit: Liana Wait
Sound recordings of plants could be used in agricultural irrigation systems to monitor crop hydration status and help distribute water more efficiently, the authors say.
“We know that there’s a lot of ultrasound out there—every time you use a microphone, you find that a lot of stuff produces sounds that we humans cannot hear—but the fact that plants are making these sounds opens a whole new avenue of opportunities for communication, eavesdropping, and exploitation of these sounds,” says co-senior author Yossi Yovel, a neuro-ecologist at Tel Aviv University.
“So now that we know that plants do emit sounds, the next question is—’who might be listening?'” says Hadany. “We are currently investigating the responses of other organisms, both animals and plants, to these sounds, and we’re also exploring our ability to identify and interpret the sounds in completely natural environments.”
This is not a newly discovered phenomenon, plants, grasses and trees are very good at detecting and warning, eg
Stressed plants show altered phenotypes, including changes in color, smell, and shape. Yet, the possibility that plants emit airborne sounds when stressed – similarly to many animals – has not been investigated. Here we show, to our knowledge for the first time, that stressed plants emit airborne sounds that can be recorded remotely, both in acoustic chambers and in greenhouses. We recorded ∼65 dBSPL ultrasonic sounds 10 cm from tomato and tobacco plants, implying that these sounds could be detected by some organisms from up to several meters away. We developed machine learning models that were capable of distinguishing between plant sounds and general noises, and identifying the condition of the plants – dry, cut, or intact – based solely on the emitted sounds. Our results suggest that animals, humans, and possibly even other plants, could use sounds emitted by a plant to gain information about the plant’s condition. More investigation on plant bioacoustics in general and on sound emission in plants in particular may open new avenues for understanding plants and their interactions with the environment, and it may also have a significant impact on agriculture.
Source: Plants emit informative airborne sounds under stress
The remarkable ability of plants to respond to their environment has led some scientists to believe it’s a sign of conscious awareness. A new opinion paper argues against this position, saying plants “neither possess nor require consciousness.”
To explain these apparent behaviors, a subset of scientists known as plant neurobiologists has argued that plants possess a form of consciousness. Most notably, evolutionary ecologist Monica Gagliano has performed experiments that allegedly hint at capacities such as habituation (learning from experience) and classical conditioning (like Pavlov’s salivating dogs). In these experiments, plants apparently “learned” to stop curling their leaves after being dropped repeatedly or to spread their leaves in anticipation of a light source. Armed with this experimental evidence, Gagliano and others have claimed, quite controversially, that because plants can learn and exhibit other forms of intelligence, they must be conscious.
Nonsense, argues a new paper published today in Trends in Plant Science. The lead author of the new paper, biologist Lincoln Taiz from the University of California at Santa Cruz, isn’t denying plant intelligence, but makes a strong case against their being conscious.
Ultrasonic acoustic emission (UAE) in trees is often related to collapsing water columns in the flow path as a result of tensions that are too strong (cavitation). However, in a decibel (dB) range below that associated with cavitation, a close relationship was found between UAE intensities and stem radius changes.
UAE was continuously recorded on the stems of mature field-grown trees of Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) at a dry inner-Alpine site in Switzerland over two seasons. The averaged 20-Hz records were related to microclimatic conditions in air and soil, sap-flow rates and stem-radius fluctuations de-trended for growth (ΔW).•
Within a low-dB range (27 ± 1 dB), UAE regularly increased and decreased in a diurnal rhythm in parallel with ΔW on cloudy days and at night. These low-dB emissions were interrupted by UAE abruptly switching between the low-dB range and a high-dB range (36 ± 1 dB) on clear, sunny days, corresponding to the widely supported interpretation of UAE as sound from cavitations.•
It is hypothesized that the low-dB signals in drought-stressed trees are caused by respiration and/or cambial growth as these physiological activities are tissue water-content dependent and have been shown to produce courses of CO2 efflux similar to our courses of ΔW and low-dB UAE.
