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Plants emit a high frequency distress sound when they experience stress

Some plants emit a high frequency distress sound when they are placed under environmental stress

In a new study, researchers have observed plant distress cries for the first time. This discovery may be good news for precision agriculture; farmers will be able to find out which plants need water simply by listening.

Researchers at the University of Tel Aviv in Israel discovered that tomato and tobacco plants make noises inaudible to humans when they are stressed by lack of water or when their stems are cut off.

Ultrasonic distress signals

Microphones 10 centimeters away from the plants picked up ultrasonic sounds between 20 and 100 kilohertz. According to the researchers, insects and some mammals can hear and respond to these sounds up to 5 meters away. For example, a moth might decide not to lay its eggs on a plant that sounds as if it is short of water. Plants can also hear when other plants are short of water and act accordingly.

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

In previous research in this field, scientists attached devices to plants and recorded vibrations. These vibrations are caused by the formation and cracking of air bubbles in the tubes that plants use for water transport - a process known as 'cavitation'. In this new research, however, the sound produced by plants was measured from a distance for the first time.

On average, a drought stricken tomato plant makes 35 noises per hour, while a tobacco plant makes 11. When cutting stems, tomato plants made an average of 25 noises in the following hour, compared to 15 in the case of tobacco plants. Plants that did not suffer from stress produced an average of less than 1 sound per hour.

Environmental stress: draught and stem cutting

The sounds even give an indication of the cause of the stress. The researchers created a computer model that differentiates between plant sounds and the sounds of wind, rain and other noises in the greenhouse. Based on the intensity and frequency of the noise, the model could in most cases successfully determine whether the plant sounds were the result of water shortage or a stem cut. It turns out that a dry tobacco plant sounds louder than a plant with a cut.

Although researchers looked only at tomato and tobacco plants, they believe that other plants also produce sounds following a stressful event. In a preliminary study, they recorded ultrasonic sound from a Pincushion cactus (Mammillaria spinosissima) and Henbit deadnettle (Lamium amplexicaule). According to the researchers, cavitation is a possible explanation of the sounds observed in plants.

What are the benefits of the discovery for precision agriculture?

For farmers, the possibility to hear whether plants are dry offers new opportunities in the field of precision agriculture, the researchers indicate. They add that this ability is becoming increasingly important as more areas are affected by drought as a result of climate change.

Using the sounds of drought-stressed plants in precision farming can work if setting up a recording system near the fields is not too costly.

However, the results cannot be generalised to other stress factors such as salinity or temperature, which may not lead to distress sounds. In addition, no experiments have yet been carried out that show that moths or other animals can actually hear the sounds of plants and react to them. This idea therefore remains speculation for the time being.

If plants actually make noise when they are stressed, then cavitation is the most likely underlying mechanism, says Edward Farmer of the University of Lausanne in Switzerland. But the results make him skeptical. He would like to see more variables checked.

Farmer adds that the idea that moths listen to plants and avoid stressed individuals is somewhat too speculative. He says there are plenty of other explanations for why insects avoid some plants and not others.