Plants Can Respond to Sound: Are You Listening?

Are plants really unlike us as the biological system of classification dictates, or could they be similar to us?

Do plants too enjoy the beauty of the morning sun, smelling the fragrance of their flowers, listening to the lovely buzzing bees, consuming the meal of minerals and water, and most crucially, conveying their feelings through sound?

To many, this might look like a fictional story out of a school kids comic-book, but apparently extensive research in the field of plant physiology has evinced that many of these things truly occur in the plant world.

Plants and animals both arose from prokaryotes through the process of endosymbiosis.

Endosymbiosis refers to a close, long-term relationship between two organisms where one lives inside the other. This partnership can be mutually beneficial, meaning both organisms gain something from the arrangement.

This process took place in the primitive atmosphere, which lacked free oxygen, a reducing atmosphere favored the origin of life since organic compounds are easily destroyed in the presence of oxygen.

Thus, the earliest microbes that arose in the primordial soup were anaerobic, flourishing in the absence of oxygen.

Some early, oxygen-hating (anaerobic) organisms—likely fermentative bacteria, which convert sugars and other compounds into simpler substances like alcohol or acid, without using oxygen—engulfed some oxygen-loving (aerobic) bacteria.

Instead of digesting them, they formed a partnership (symbiosis), where both sides benefited (endosymbiosis).

The ingested prokaryotes that could respire oxygen transformed into mitochondria, and those that could perform photosynthesis turned into chloroplasts.

This is how we had our earliest endosymbiotic eukaryotes, which could synthesize their own food and respire, releasing carbon dioxide into the atmosphere.

But here the similarity ended, the two types of cells, prokaryotes and eukaryotes, took different evolutionary routes.

Prokaryotes (like bacteria and archaea) evolved specialized functions that allowed them to inhabit extreme environments, among other traits. In contrast, eukaryotes developed into more complex organisms, leading to plants, animals, fungi, and protists. 

Plants evolved as forms that were non-motile, multicellular, contained photosynthetic pigments called chlorophyll inside special organelles known as plastids, and could make their own food. 

Other distinct features that differentiated them from animals were the absence of a typical nervous system and sense organs.

However, experiments over several years have revealed that plants can perceive light, scent, touch, wind, and even gravity, just like us, and can respond to sounds, too.

Plants don’t always grow in the ideal environment and are subjected to external situations that negatively affect them physiologically and metabolically.

Thus, stress can negatively affect the productivity of plants.

Not only biotic and abiotic factors (e.g., animals, pathogens, temperature, salinity) but also physical and mechanical factors (e.g., wind, rain, vibrations) have been found to stimulate plant sensitivity.

They primarily rely on chemical and physical signals for communication and response to their environment rather than auditory perception, as was thought earlier.

Communication through sound is commonly found only in humans and certain terrestrial mammals.

In humans, when sound waves enter the ear canal, they vibrate the tympanic membrane (eardrum), this vibration passes on to three tiny bones (ossicles) in the middle ear from where they reach the inner ear.

Tiny hair cells called stereocilia in the inner ear transform the vibrations into electrical energy and send it along nerve fibers to the brain, and we end up listening to the humdrum around us.

Only waves with frequencies between about 20Hz (infrasound) and 20kHz (ultrasound), the range of audio frequencies, arouse an auditory perception in humans.

Different animal species have different auditory ranges, for instance, ultrasounds are perceived by some animal species, such as dolphins and bats, while infrasounds are perceived by elephants, fish, and cetaceans.

Even insects emit species-specific sounds to attract mates or to escape potential predators, and fruit flies, snakes, frogs, and birds can perceive sound vibrations without an eardrum.

The absence of any specialized organ for hearing in plants makes it very mysterious how plants might respond to any sound stimulus.

Plants Can Respond Positively to Audible Sound

Several experiments to determine the hearing abilities of the plants have revealed that certain audio prompts in the audible range of 20 Hz-20 kHz can have a beneficial effect on plant growth. 

Recently, its application in agricultural plants has been investigated.

Green Music (comprising the natural sound of flowing river water, insect chatter, birds whistling, and rainfall) and classical music positively impacted seed germination, growth, yield, chlorophyll accumulation, starch content, and stress tolerance in different species.

When tomato plants were exposed to the sound of different frequencies, their total phenol and lycopene content, as well as ascorbic acid content, increased to a maximum of 70%.^[1]

Scientists have even suggested the term ‘phytoacoustics’ to describe the emerging field exploring sound emission and sound detection in plants.

Based on the findings so far, Plant Acoustic Frequency Technology (PAFT) ranging from 60 Hz to 2 kHz and intensities ranging from 50 to 120 dB was developed to increase crop productivity and quality through exposure to sound waves.^[2]

PAFT-treated spinach showed significant improvement in yield, Vitamin A, B, and C content.

In a study, exposure to different sound frequencies was found to increase flavonoid content in several vegetable sprouts. 

Flavonoids and isoflavonoids are natural compounds in plants that show immunomodulatory, antiviral, and antioxidant properties, which contribute to the plant’s defense system for protection against insects and disease.

What Happens When a Plant is Exposed to Stress-Induced Sound?

The next logical question was: just as sound at certain frequencies stimulated plant growth, can a stress-induced sound, maybe at a different frequency, evoke a defensive response in plants? 

Scientists Appel and Cocroft devised an experiment in which caterpillars (biotic stress) were allowed to feed on the leaves of Arabidopsis and the sound made by the fine movement in the leaves due to the chewing of the caterpillar was recorded using a laser vibrometer system.^[3]

These recorded vibrations were then played back to another set of healthy Arabidopsis plants and a third control set was subjected to only silence. 

The researchers found that plants in the second set, when later infested with caterpillars, produced more offensive insecticide-like chemicals called glucosinolates and anthocyanins.

Caterpillars react to this chemical defense by crawling away, so this experiment revealed that plants can respond to sound alone even without being touched or damaged by insects, and that the response is more rapid and potent. 

This experiment demonstrates that exposure to stress-induced sound beforehand steps up the immunity in plants just as a dose of vaccine steps up our immune system.

Simultaneously, a variety of recordings, including wind and mating sounds of different insects, which had a similar frequency spectrum but different temporal presentation, were not able to evoke similar responses from Arabidopsis plants.

This indicates that the plants can distinguish vibrations from other common sources of environmental vibration.

Thus, while plants may not exactly be hearing, they do sense sound vibrations and can also differentiate between them.

This opens up further avenues of using vibrations to enhance plant defenses, which could be highly economical and eco-friendly for agriculture as opposed to spraying costly and harmful pesticides.

Cellular Changes in Plants When Stimulated by Sound

To understand the cellular events triggered by sound stimulation, a study investigated the global transcriptomic and proteomic changes in Arabidopsis upon exposure to sound vibration at five single frequencies.^[4]

Microarray analysis revealed that upon sound stimulation, several plant genes, including those involved in calcium signaling, redox homeostasis, signal transduction, and transcription, exhibited differential expression.

Also, a large number of genes involved in hormonal signaling and respiratory pathways were also up-regulated.

Hormones controlling growth (Indole-3-acetic acid and Gibberellin) and hormone-strengthening defense (Salicylic acid and jasmonic acid) were found to be synthesized in greater concentration.

Calcium acts as a second messenger and plays an important role in transducing messages along a signaling cascade.

Sound vibrations, being pressure waves, have been postulated to influence plant cells mechanically.

Thus, the signal transduction mechanisms of sound and touch might be similar, and this overlap of pathways may be useful for creating precise tools in engineering.

Upregulation in the expression of ion transporters like H+-ATPase, which plays an important role in active transport, suggests that these transporters may have some mechano-sensitive role in mediating sound vibration-induced cellular responses.

Several enzymes involved in the light reaction of photosynthesis—Calvin cycle, glycolysis, and TCA cycle—showed altered expressions with a majority of them being up-regulated.

RuBisCO is the key regulatory enzyme of ‘C3 carbon fixation’, and its up-regulation reflects the enhanced photosynthetic state of plants after sound exposure.

Based on these findings, a model of how sound vibrations affect plant cells has been proposed.

These sound vibrations, through mechanical input, increase cell membrane tension by upregulating a cell wall-modifying xyloglucan endotransglucosylase.

Due to alteration in the elasticity of the cell wall, membrane-bound mechanosensitive ion channels are activated. The signals are transduced into the cell by binding to calcium ions.

Kinases transduce the message downstream through phosphorylation/de-phosphorylation of different signaling proteins or transcription factors, eventually resulting in altered gene expression.

Genes and corresponding proteins involved in vital processes like photosynthesis, glycolysis, amino acid metabolism, and sulfur metabolism are upregulated.

These changes, along with hormonal adjustments, result in enhanced growth and defense in plants.

Plants Emit Ultrasonic Sounds during Stress

However, despite such extensive knowledge regarding the cellular changes resulting from the sound stimulus, there’s a dearth of information regarding sound emission by the plants themselves and whether this sound could be heard at some distance from the plant.

Recently, researchers at Tel Aviv University in Israel have proved that plants indeed emit airborne sounds, which can be detected from several meters away and may serve as potential signals to their physiological state.

However, this sound falls in the ultrasonic range from 20 to 100 KHz and beyond the range of human hearing.^[5]

They placed highly sensitive microphones near tomato and tobacco plants under different stress conditions.

One set of plants was treated as a control to record a baseline sound emission, and a second set was stressed by depriving it of water for a prolonged period, and a third set was subjected to stem cutting.

The plants were kept in a highly controlled environment, like in specially designed acoustic boxes or greenhouses, to negate interfering background noises.

The instruments picked up the crops’ squeals, which, by their ultrasonic range, can also be detected by some mammals and insects (mice and moths).

Most surprisingly, the sounds were picked up by microphones even at a distance of 3-5 meters.

The recordings revealed that the different plant species made distinct sounds at varying rates, depending on their stressor. 

Drought-stressed tomato plants emitted about 35 ultrasonic squeals per hour, on average, while those with cut stems emitted about 25 squeals. 

Drought-stressed tobacco plants let out about 11 screams per hour and cut crops produced about 15 screams per hour. In comparison, untreated plants remained more or less silent.

Though it is not clear how the sound is being produced, it has been shown that transpiration produces tension in xylem vessels, and simultaneously, gas bubbles (cavitation) are produced in xylem vessels during this process.

In dehydrated plants, this cavitation is more pronounced, whereby air bubbles in the stem form, expand, and collapse, producing a pop-like sound that is difficult for humans to hear.

Sound emissions exhibited a ‘midday depression’ showing one major peak in the morning and another major peak in the late afternoon.

The pattern is strikingly similar to how transpiration shows a midday depression.

This suggests that there is a close link between the formation of bubbles within the xylem and the closure of stomata. 

This stomatal closure allows sufficient time for equalizing the water potential in the xylem, avoiding cavitation, and repairing xylem embolism.

It was also found that the frequencies of the sounds emitted by different plant species correspond to their trachea diameter, with wider tracheas in plants emitting lower sounds. 

The variation in the number of squeals emitted under drying and cutting conditions is due to the different rates at which gas flows inside the trachea.

Drying is gradual, due to which outside air enters the trachea at a slow rate, while cutting involves rapid air-seeding through all the trachea.

In accordance, cut plants emit sound for a shorter period than dry plants.

Using machine learning—a type of artificial intelligence algorithm—it was possible to ascribe each unique type of sound to a particular category of plants: dry, cut, or intact, with an accuracy of 70%. 

A study tested a variety of plants, like wheat, corn, grape, cactus, and henbit, and all of these were found to produce these sounds.

Scientists think that if we can figure out what the plant sounds mean, then insects like moths might be able to hear them too, this could help them find plants with higher levels of distress, which are easier to attack.

Someday, farmers could use a similar technology to listen for drought-stressed crops in their fields and water them accordingly before wilting sets in and before the plant becomes susceptible to easy attack by an infesting moth.

More precise irrigation can save up to 50% of the water expenditure and increase the yield, with dramatic economic implications.

Neighboring plants may also respond to the sounds emitted by stressed plants.

Plants like Arabidopsis, pepper, cucumber, and tomato have already been shown to react to audible sounds and accordingly increase their defense mechanisms against pathogens and abiotic stresses such as drought.

Plants are letting out ultrasonic noise, but the question is, “Who might be listening?”

Identifying the response of nearby plants, explaining their underlying cellular and physiological processes, and checking the response of the infesting bugs to these mysterious sounds will have great implications for increasing food security and conserving ecology.

References

1. O. Altuntas et al, ‘The assessment of tomato fruit quality parameters under different sound waves’, J Food Sci Technol, Apr 2019, doi: 10.1007/s13197-019-03701-0[↩]
2. R. H. Hassanien et al, ‘Advances in effects of sound waves on plants’, J Integrative Agric,  Feb 2014, https://doi.org/10.1016/S2095-3119(13)60492-X[↩]
3. H. M. Appel et al, ‘Plants respond to leaf vibrations caused by insect herbivore chewing’, Jul 2014, Oecologia, doi: 10.1007/s00442-014-2995-6[↩]
4. R.Ghosh et al, ‘Exposure to Sound Vibrations Lead to Transcriptomic, Proteomic and Hormonal Changes in Arabidopsis’, Sci Rep. Sept 2016, doi: 10.1038/srep33370[↩]
5. I. Khait et al, ‘Sounds emitted by plants under stress are airborne and informative’, Cell, Mar 2023, DOI: 10.1016/j.cell.2023.03.009[↩]