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Plants emit sounds when stressed

At first considered more or less inert by science, plants have in fact turned out to be very dynamic entities that can detect and interact with their environment as animals do. After showing that plants can communicate with each other using a universal chemical language, and even travel short distances, researchers have recently discovered that they are also capable of producing sounds in response to different types of stress.

Although it has been revealed in recent years that plants are able to see, hear and smell, they are still considered silent. But, for the first time, they were recorded producing sounds when stressed, which researchers say could open a new field for precision farming, where farmers would listen to crops lacking water or nutrients.

Itzhak Khait and colleagues at Israel’s Tel Aviv University discovered that tomato and tobacco plants emit sounds when they are stressed by lack of water or when their stems are cut off at frequencies that humans can not hear. Microphones placed 10 centimeters from the plants received sounds in the ultrasonic range of 20 to 100 kilohertz, which insects and some mammals would be able to hear and detect within 5 meters.

Researchers even suggest that butterflies may not lay their eggs on a plant that seems stressed by lack of water. Plants could even hear that others lack water, and react accordingly. Previously, devices were installed on plants to record the vibrations caused by the formation and explosion of air bubbles – a process known as cavitation – inside xylem tubes used for transporting ‘water.

Sounds produced in response to different types of stress

But this new study is the first to record plant sounds emitted from a distance. On average, drought-stressed tomato plants emitted 35 sounds per hour, while tobacco plants produced 11. When plant stems were cut, tomato plants averaged 25 sounds per hour. and those of tobacco 15. Unstressed plants produced less than one noise per hour, on average.

Plants emit sounds when stressed

a) Experimental protocol used by the researchers. b), c) and d): Amplitudes and number of sounds emitted by tobacco plants and tomatoes lacking water or cut. Credits: I. Khait et al. 2019

It is even possible to distinguish the sounds to know what is the source of the stress. Researchers conducted a deep-learning algorithm to distinguish between plant sounds and wind, rain, and other noise from the greenhouse, correctly identifying in most cases whether the stress was due to drought or at a break, depending on the intensity and frequency of the sound. Tobacco stressed by lack of water seems to produce louder sounds than cut tobacco, for example.

Plants emit sounds when stressed

Although Khait and his colleagues are only interested in tomato and tobacco plants, they think that other plants can also make sounds when stressed. In a preliminary study, they also recorded ultrasonic sounds from a cactus ( Mammillaria spinosissima ) and amoxicillam ( Lamium amplexicaule ). Cavitation is a possible explanation of how plants generate sounds.

Better understanding plant stress: towards micro-controlled agriculture?

Enabling farmers to listen to water stressed plants could “open a new path in the field of precision agriculture,” the researchers suggest. They add that such capacity will become increasingly important as climate change exposes more areas to drought.

The authors warn that the results can not yet be extended to other stresses, such as salt or temperature, as they do not lead to sounds. In addition, no experiment was conducted to show whether a butterfly or any other animal could hear and respond to the sounds emitted by the plants. This idea remains hypothetical, for the moment.

If plants emit sounds when stressed, cavitation is the most likely mechanism, says Edward Farmer of the University of Lausanne, Switzerland. But he is skeptical about the results and would like to see more controls, such as the sounds of a soil that dries without plants.

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