Plants Can Feel Stress Too

 by Hannah Watts

A recent study done by the University of Tel Aviv found that plants living in stressful environments gave off ultrasonic sounds depending on the condition of the plant. They created machine learning models which were able to identify different stresses on these plants including dehydration and injury based only on the sounds emitted.

The frequency of these sounds is too high for human ears but it is thought these ultrasonic waves can be detected by insects, mammals and possibly other neighbouring plants. 

Plant are seen to change their phenotypes in response to stress often changing colour or shape (eg. yellow leaves often suggest a lack of magnesium whilst wilting and drooping is caused by incorrect water levels). Plants also emit VOCs (volatile organic compounds) when experiencing stresses such as drought or threat from herbivores. However until recently no one considered the fact that plants make auditory cues which are able to travel and be interpreted by other organisms. The University of Tel-aViv study aimed to expand on recent discoveries that plants were able to respond to sounds by changing the expression of specific genes of increasing the sugar concentration in their nectar, hypothesising that if plants were able to emit airborne sounds these sounds would possibly be able to trigger a response in neighbouring organisms.

They recorded tomato and tobacco plants in an auditory box under different conditions picking up the sounds using two microphones. They tested two different stresses - drought and cutting (cutting the stem before recording).

Their experiments found that stressed plants emitted significantly more sounds than the plants in the control groups who were under no measurable stress. 

Although there is no clear reason, it is thought that cavitation in the stem - a process where air bubbles form in the xylem because expanding and collapsing causing vibrations which on a large enough scale can create auditory noise. This hypothesis is supported by a correlation between the diameter of the trachea diameter and the frequency of the sound produced (larger trachea diameter, lower frequency sound produced) as well as a link between the amount of water present in the stem of a plant and the length of time it produces a sound for.

It is hoped that this research will help develop methods for monitoring plants in fields or greenhouses and could offer a way of monitoring crop water levels and disease. This would help save water as more precise irrigation can save up to 50% of water used as well as increase the yield. This becomes particularly important in a time when more areas are exposed to drought due to climate change. 

It is also aiding the understanding of relationships between organisms in ecosystems. Many types of moth (some of who use tomato and tobacco as hosts for their larvae) have shown to be able to hear and react to the sounds recorded. As well as this, plants are known to be able to respond to sounds and it is hoped that by using frequencies emitted by plants in drought-stress, it could be used to help increase the drought tolerance of some species and improve food security. 

The full study can be found published in the Cell Journal or by accessing this link: Sounds emitted by plants under stress are airborne and informative: Cell