Plants Emit Ultrasonic Squeals When Stressed, Recordings Reveal
New research shows that plants can actually talk.
But don’t expect them to help you with your homework or sing along with the radio. A new study shows that plants are creating frequencies in responses to their surroundings, or in other words, they are reacting.
These “reactions” in plants, that we could see as analogical to human senses, is actually nothing new. We have learned over the past few years that plants are capable of seeing, hearing, and smelling. And with this newest finding we are just one sense away from completing the five human senses in some species of the plantae kingdom.
For the first time, plants have been recorded making airborne sounds when stressed, which researchers say could open up a new field of precision agriculture where farmers listen for water starved crops.
This study was done by Itzhak Khait and his colleagues at Tel Aviv University in Israel. The researchers found that tomato and tobacco plants made sounds at frequencies humans cannot hear when facing situations such as lack of water or when being cut.
“To investigate plants’ airborne sound emissions, we first constructed a reliable recording system, in which each plant was recorded simultaneously with two microphones within an acoustically isolated anechoic box. We recorded tomato (Solanum lycopersicum) and tobacco (Nicotiana tabacum) plants under different treatments – drought stress, cutting (of the stem), and controls. We focused on the ultrasonic sound range (20-150 kHz), where the background noise is weaker”
On average, drought-stressed tomato plants made 35 sounds per hour, while tobacco plants made 11 and unstressed plants produced fewer than one sound per hour, on average.
The researchers trained a machine-learning model to discriminate between the plant’s sounds and the wind, rain, and other ambient noises of the greenhouse, in order to correctly identify the source of stress.
“After we succeeded filtering greenhouse noises, we could use “clean” plant sounds to classify plant condition in the greenhouse. Results from the acoustic box recordings showed that drought-stressed plants emitted significantly more sounds than control plants”
Sound exists in both low and high frequencies. Humans cannot hear the entire spectrum of frequencies around us. If we could, we would likely become distracted and anxious by always hearing the frequencies around us including radio frequencies from cell towers near us or even cooking a frozen meal in the microwave. Thankfully, the range of human hearing is typically considered to be only 20 Hz to 20 kHz.
With the above in mind, we can now understand why humans are not sensitive to the sounds that plants are emitting.
A possible mechanism by which they generate the sounds is cavitation (the process whereby air bubbles form and explode in the xylem):
“Cavitation explosions have been shown to produce vibrations similar to the ones we recorded… but it has never been tested whether these sounds are transmitted through air at intensities that can be sensed by other organisms. Regardless of the specific mechanism generating them, the sounds we record carry information, and can be heard by many organisms. If these sounds serve for communication a plant could benefit from, natural selection could have favoured traits that would increase their transmission.”
Further research must be conducted to find the potential applications this discovery may have, but these are big steps to understanding the reality of the plants that surround us.