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Are plants listening?

Each of us has this one friend, parent, grandmother (-father) or relative, who is so much into growing plants that they even speak to them. Surprisingly, these plants somehow seem to listen and quite often grow bigger and better than other plants. Have you ever wondered why? Are plants really able to “hear” if we talk to them?


As plants are unable move away from the spot they started to grow on, they developed unique and precise ways so sense their surroundings and if needed adapt in order to survive. Plants are not only able to sense light, temperature, rainfall, humidity, wind, vibrations, daylength but also if an annoying insect is chewing on their leaves. Would it be so astonishing if they are able to sense if someone is speaking to them? And if we break it down, it doesn't seem so far-fetched anymore: spoken words are just vibrations traveling through the air and plants might be able to sense that.


In fact, scientist have been asking this question since the 1960s. In 1968 two Canadian scientists (Pearl Weinberger and Mary Measures) examined the effect of two sound frequencies on the germination and growth of two Canadian wheat varieties. The two frequencies they used were 5 and 12 kc/s (kHz, Kilohertz). Plants were exposed to sound continuously for 4 weeks at an intensity level of 92 dB (that’s approximately the noise level of your hairdryer).


Now, what is the bandwidth of human speech? While the human audio output is slightly variable for each person, the general range lies between 100 Hz to 8 kHz. On a side note: the human hearing ranges from approximately 16 Hz to 16kHz and telephones are designed to work between 300 Hz to 3,4 kHz (but a wideband audio call can transmit sounds ranging from 50Hz to 7kHz).


But let’s go back the study of the two Canadian scientists: They showed that both wheat varieties reacted to the two different sound frequencies, however, the effect was depending on the temperature. One of the two wheat varieties showed a strong response in terms of general growth to both sound frequencies. However, the results of the second wheat variety were variable. If you’re interested in the details, keep on reading, if not, you can skip the following paragraph:


For one of the two wheat varieties (“Marquis”) both sound frequencies stimulated germination at 2°C and 10°C. However, this effect was not seen anymore at 25°C. The stimulation of germination was not as strong in the other wheat variety (“Rideau”). 5 kHz stimulated germination at 2°C in “Rideau” wheat, but none of the frequencies affected germination at 10°C or 25°C. This wheat variety grew more proficiently if exposed to both sound frequencies with regard to plant height and weight, number of roots and the weight of the roots. For Marquis wheat, the effect on growth was depending on the application of sound. Both responses where observed: accelerated and depressed growth. Later studies showed that the positive influence on seed germination was due to a strong induction of a specific plant hormone which plays an important role in seed germination. The two sound frequencies were able to directly activate the gene involved in the biosynthesis of the plant hormone, but how temperature interplays with this response is not clear yet.


Are plants really able to react to sound?

In 2007, scientist discovered rice genes that respond to sound. Meaning, that some plant genes can be activated just by sounds. In this study, the researchers used classical music and single frequencies. Interestingly, the classical music included sonatas of Beethoven, Haydn and Vivaldi. Plants were exposed to 4 hours of classical music before scientists sampled and analyzed their leaves to find sound-responsive genes. After these were found, the researchers investigated the effect of single frequencies on the activation of these genes. They found out, that especially frequencies of 125 Hz and 250 Hz activated the genes, while 50 Hz repressed them. This study showed that rice plants do not only react to sound in general, but that they distinguish between frequencies.


Sound waves do not only influence the growth and development of whole plants but they can also affect already harvested fruits. In 2015, scientist conducted experiments to figure out the effect of sound on tomato fruit ripening. The researchers harvested fruits and treated them with 6 hours of 1 kHz sounds in a sound-proof chamber. Then they followed up the ripening of the fruits for the next 2 weeks. Treating harvested tomatoes with sound resulted in changes in color and hardness, which in turn delayed fruit ripening.



But how is that possible? What is sound doing to the tomato fruits?

The sound influenced the expression of specific genes, which are involved in ripening. To go a bit more into detail: the sound treatment affected the production of ethylene, a plant hormone that is involved in fruit ripening (among many other processes) and thus is also known as the “fruit ripening hormone”. It’s a gaseous hormone, meaning if a plant or fruit is producing ethylene, it can be detected in the air. Have you ever heard of the saying to keep your vegetables separated from your fruits? Or to store your apples and bananas in different bowls, because they would go bad faster? This is because ethylene is produced by some (but not all) fruits and is affecting neighboring fruits and vegetables, thus causing them to ripe (and go bad) faster. Ripe apples, Bananas and tomatoes and typical ethylene producers, while e.g. cucumbers, watermelon, cauliflower, broccoli, but also unripe bananas and tomatoes are ethylene sensitive.


Now back to the study: Normally, the amount of ethylene produced in tomatoes is increasing during their ripening. Unripe green tomatoes only produce a small amount of ethylene, while ripe red tomatoes produce a high amount. The tomato fruits that were treated with sound showed a low expression of the genes involved in the biosynthesis of ethylene compared to the untreated fruits. Thus ripening was slowed down due to the application of sound. This is especially interesting for the agricultural industry, as slowed down fruit ripening could extend shelf life and thus prevent post-harvest losses.


If plants are able to react to sound, can they also talk?

Plants can’t talk, but a recent study from 2019 showed that they indeed can emit sounds when under stress. The researchers discovered that stressed plants (here they investigated tomato and tobacco plants) emit ultrasonic sounds, which can be measured at a distance of 10 cm from the plants. Ultrasonic sound ranges from 20 to 150 kHz. Researchers could trace the signal in greenhouses but also in controlled acoustic chambers. They even went further and used machine learning to investigate if there is a difference in sound emitted from tomato or tobacco plants and in stress type (drought stress or wounding). Indeed, the program was able to identify the origin of sounds by plant species and stress type with a 70 % accuracy. While this type of research is just at the beginning, it could later on be used to monitor the health of crops in large greenhouse just by the sound they’re emitting.


The scientist of this study said that the sound could potentially be heard by neighboring plants, which in turn could respond by adapting to the stress they sensed from their neighbors. Furthermore, even insects might be able to “hear” the emitted sounds to decide on which plants they’d lay their eggs. The emitted sound of a plant could help insects chose a healthy plant over a sick plant to guarantee offspring. But much more research is necessary, before we can get answers to these questions.


The reason why plants seem to grow better in the hands of a person who talks to them, might simply be because they keep a close eye on them and take good care, but the old saying that speaking to your plants help them grow might hold a little bit of truth in it.


Sources:


Weinberger, P., & Measures, M. (1968). The effect of two audible sound frequencies on the germination and growth of a spring and winter wheat. Canadian Journal of Botany, 46(9), 1151-1158.


https://en.wikipedia.org/wiki/Voice_frequency


Jeong, M. J., Shim, C. K., Lee, J. O., Kwon, H. B., Kim, Y. H., Lee, S. K., ... & Park, S. C. (2008). Plant gene responses to frequency-specific sound signals. Molecular breeding, 21(2), 217-226.


Kim, J. Y., Lee, J. S., Kwon, T. R., Lee, S. I., Kim, J. A., Lee, G. M., ... & Jeong, M. J. (2015). Sound waves delay tomato fruit ripening by negatively regulating ethylene biosynthesis and signaling genes. Postharvest Biology and Technology, 110, 43-50.


Khait, I., Lewin-Epstein, O., Sharon, R., Saban, K., Perelman, R., Boonman, A., ... & Hadany, L. (2019). Plants emit informative airborne sounds under stress. bioRxiv, 507590.

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