In new research, biologists at Tel Aviv University recorded ultrasonic sounds emitted by tomato and tobacco plants inside an acoustic chamber, and in a greenhouse, while monitoring the plant’s physiological parameters.
They developed machine learning models that succeeded in identifying the condition of the plants, including dehydration level and injury, based solely on the emitted sounds.
The frequency of these sounds is too high for human ears to detect, but they can probably be detectable by other organisms such as insects, mammals, and possibly other plants.
Plants show significant changes in their phenotypes in response to stress. They differ visually, with respect to both color and shape, from unstressed plants.
They also emit volatile organic compounds, e.g. when exposed to drought or herbivores. These compounds can also affect neighboring plants, resulting in increased resistance in these plants.
Altogether, plants have been demonstrated to produce visual, chemical, and tactile cues, which other organisms can respond to.
Nevertheless, the ability of plants to emit airborne sounds — that could potentially be heard by other organisms — has not been sufficiently explored.
“Even in a quiet field, there are actually sounds that we don’t hear, and those sounds carry information,” said Tel Aviv University’s Professor Lilach Hadany, senior author of the study.
“There are animals that can hear these sounds, so there is the possibility that a lot of acoustic interaction is occurring.”
“Although ultrasonic vibrations have been recorded from plants before, this is the first evidence that they are airborne, a fact that makes them more relevant for other organisms in the environment.”
In their research, Professor Hadany and colleagues used microphones to record healthy and stressed tomato and tobacco plants, first in a soundproofed acoustic chamber and then in a noisier greenhouse environment.
They stressed the plants via two methods: by not watering them for several days and by cutting their stems.
After recording the plants, the researchers trained a machine-learning algorithm to differentiate between unstressed plants, thirsty plants, and cut plants.
They found that stressed plants emit more sounds than unstressed plants.
The plant sounds resembled pops or clicks, and a single stressed plant emitted around 30-50 of these clicks per hour at frequencies of 40-80 kHz and seemingly random intervals, but unstressed plants emitted far fewer sounds.
“When tomatoes are not stressed at all, they are very quiet,” Professor Hadany said.
Water-stressed plants began emitting noises before they were visibly dehydrated, and the frequency of sounds peaked after 5 days with no water before decreasing again as the plants dried up completely. The types of sound emitted differed with the cause of stress.
The machine-learning algorithm was able to accurately differentiate between dehydration and stress from cutting and could also discern whether the sounds came from a tomato or tobacco plant.
Although the study focused on tomato and tobacco plants because of their ease to grow and standardize in the laboratory, the researchers also recorded a variety of other plant species.
“We found that many plants — corn, wheat, grape, and cactus plants, for example — emit sounds when they are stressed,” Professor Hadany said.
The exact mechanism behind these noises is unclear, but the authors suggest that it might be due to the formation and bursting of air bubbles in the plant’s vascular system, a process called cavitation.
Whether or not the plants are producing these sounds in order to communicate with other organisms is also unclear, but the fact that these sounds exist has big ecological and evolutionary implications.
“It’s possible that other organisms could have evolved to hear and respond to these sounds,” Professor Hadany said.
“For example, a moth that intends to lay eggs on a plant or an animal that intends to eat a plant could use the sounds to help guide their decision.”
Sources:
Itzhak Khait et al. 2023. Sounds emitted by plants under stress are airborne and informative. Cell 186 (7): P1328-1336.e10; doi: 10.1016/j.cell.2023.03.009
