Do Plants Emit Ultrasonic Radiation? Why?

Roald Dahl published a short story titled ‘The Sound Machine’ in 1949. It talked about the level of crazy a person can go to on hearing sound frequencies from plants using a machine he made.

The level of his madness was such that he called for a doctor and asked him to put iodine and a bandage on a tree trunk that had fallen due to a storm. 

Fast forward to 70 years, in 2019, turns out the short story meant for fiction isn’t just fiction anymore. Plants do emit ultrasonic radiation!

Surprisingly wild, isn’t it?

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What are ultrasonic radiation? Can we hear them?

In fact, these are sound waves. The human ear is equipped to hear and handle frequencies ranging from 20 hertz to 20,000 hertz. 

The sound frequencies belonging to the bracket below 20 Hz represent the infrasonic range of sound waves. Similarly, the ultrasonic waves belong to the frequency range higher than 20,000 Hz. Naturally, the human ear would not be able to hear ultrasonic frequencies. 

Sonar wave echo sound conceptual line abstract background

The ultrasonic waves are beyond the audible range of humans. (Photo Credit : ioat/Shutterstock)

However, this does not imply that other organisms cannot detect or hear ultrasonic or infrasonic waves. For example, bats and whales have been found to navigate using ultrasonic frequencies. Animals like rhinoceros and elephants can even hear infrasonic radiation. 

Ultrasonic radiation emitted by plants

Plants, being mute and sessile (unable to perform locomotion) have been found to respond to a lot of signals including tactile and optical signals. For example, the touch-me-not plant responding tactile signals.

But, for the first time, scientists may have found evidence that plants produce sound.

Researchers from Tel-Aviv University studied two plants in particular: tobacco and tomato plants. To record such sounds, an acoustic device was specially designed. The reaction of plants to stress was tested. The researchers exposed the plants in question to two different categories of stress: drought and physical damage (cuts), and recorded what sound the plants may produce.

The research was printed in a pre-print publication and is therefore not peer reviewed.

The experiments were conducted on tomato and tobacco plants

The experiments were conducted on tomato and tobacco plants. (Photo Credit : twenty20)

The results of the experiment

The researchers found that the average number of sounds emitted by a drought-stressed tomato plant was 35.4 per hour. The same for a tobacco plant was 11.0 per hour. In case of the stress due to stems being cut, the average stood as 25.2 number of sounds per hour for the tomato plant. For tobacco, however, it was 15.2 per hour.

Additionally, the tomato sounds were observed for 10 days starting from the day when it was properly irrigated. The plants released fewer sounds when they were watered fully. Then, as the water content decreased, the number of sounds started increasing. The sounds finally decreased as the plant neared drying up.  

A possible explanation of these agony sounds 

There no explanations as yet for how or why the plants produce sound. The researchers speculate that when plants suffer from dehydration, the xylem gets impacted. Xylem, consisting of dead tube-shaped cells lined one after the other is the tissue responsible for water conduction in plants. 

Cross section of some xylem cells. Xylem is a type of transport tissue in vascular plants.

The popping of an air bubble trapped in the cells of xylem might cause ultrasonic vibrations. (Photo Credit : Sakurra/Shutterstock)

As the xylem dries up, a phenomenon called cavitation usually takes place. In this, air bubbles are formed in the xylem and these air bubbles gradually expand and then explode. This popping up of air bubbles may cause vibrations in the form of acoustic emissions.  

Research published in 2008 that also concluded ultrasonic acoustic emissions (UAE) from pine and oak trees found in the Alps due to drought conditions, changes in the stem radius, immediate air and soil conditions, hypotheised a similar reason for the plant emitting sound.

The research has its limitations. More work needs to be on other plant species too. Until there is more evidence and other scientists attempt to reproduce the results, no relation can be established between the sounds from different species.

Moreover, the conditions in response to which the sounds are emitted might also vary depending upon plants and the types of stress they are exposed to. Stress could be due to excessive sunlight (UV radiation), herbivores, wind, cold or diseases that the plant might be suffering due to.

How ultrasonic plant signals can mean in the future

The range in which plants emit signals might not be audible to humans but these frequencies fall in the audible range of other mammals and insects. Moths that use tobacco and tomato as hosts for keeping their larvae can react to these ultrasonic frequencies. The researchers hypothesize that the moth may avoid laying eggs in plants that are stressed.

Plants are silent beings meme

Moths might be able to hear the ultrasonic signals released by plants under stress.

They also speculate that the neighboring plants might hear these signals and react accordingly. The signals coming from drought-affected plants could therefore act as alarms and can signal the ‘unstressed plants’ to close their stomata.

More importantly, as the water crisis in the world increases, the proper understanding and utilization of these ultrasonic signals could help in the more prudent use of our resources by farmers. Plants might get customized treatment in the form of more accurate and precise irrigation, so to say.

The other way round: Instances of plants responding to sound signals 

Sound vibrations of a certain frequency and amplitude can significantly enhance cell division of the callus. Not just this, the rates of enzymatic and hormonal activity have also been found to rise. Plants like cucumber, tomato, lettuce, spinach showed improved growth rates on exposure to sound waves. In addition to this, diseases like spider mite, grey mold, and some viral diseases of tomato were found to drop in their rates of occurrences with plants exposed to sound. 

Interestingly, even the microscopic world isn’t less impressed by sound signals. Escherichia coli shows a marked increase in colony formation on perceiving sound vibrations. Even yeast shows a boosted rate of growth on exposure to sound signals. 

Natural sound vibrations like the chirping of a bird, the buzzing of bees, the stridulation of crickets can be accounted for the improved rate of seed germination in Abelmoschus esculentus and Cucurbita pepo. To know more about whether plants can hear, click “hear“. 

Sound has been helping the animal world in clear communication for ages. Likewise, as the researches advance, it can also help in communication and understanding the silent plant world also and it will prove greatly beneficial to the human race, ultimately improving harmony among ecosystems.

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About the Author

Pragyanshi is pursuing Bachelors in Life Science Chemistry and Botany from St Xavier’s College, Mumbai. She developed an interest in science right when she was introduced to it and since then has been eagerly trying to find scientific answers to all the conceptions-misconceptions-superstitions which tend to float around. She’s a bibliophile and wouldn’t complain if you send her to a dungeon with books.

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