Plants emit acoustic signals when they are in stressful situations, according to research led by Tel Aviv University (Israel) that reveals a possible communication mechanism between plants and animals.
Although the signals are not perceived by the human ear, since their frequency is in the ultrasound range, they can be informative for other species and have a role in ecosystems, maintain the authors of the research in the journal Cell, where they present their results today.
The ultrasounds emitted by plants have a volume similar to that of a human conversation, report the authors of the research. They sound, to those who can hear them, similar to the popping of the small plastic bubbles used in packaging when they break.
In situations of stress, such as drought or infection, plants change color or emit molecules that indicate to other species whether they are interested in the plant or not, for example to lay their eggs or eat its leaves, the researchers recall. But until now it has not been studied whether they also produce acoustic signals, beyond the sound of the wind in the plant.
“Plants interact with insects and other animals on a constant basis, and many of these organisms use sound to communicate; it would be very sub-optimal for plants not to use sounds at all," says Lilach Hadany, director of the research, in a statement.
To clarify, Hadany and his team have exposed tomato and tobacco plants to two different stress situations, in one case by depriving them of water and in another by cutting their stems. The experiments have been completed with microphones capable of recording ultrasound that have been placed at a distance of between 3 and 5 meters from the plants.
The results show that both tomato plants and tobacco plants make more sounds when they are stressed than when they are well cared for, and that the sounds they make change depending on the type of stress they experience.
Tomato plants emit an average of 35 clicks per hour when they lack water, 25 when a stem is cut and less than 1 when they are well. The tobacco plants, which suffer the most, emit 11 when they lack water, 3 when a stem is cut and also less than 1 when they are well.
An artificial intelligence program has learned to correctly identify, when faced with a specific click, which of the two plants it came from and what type of stress affects it, indicating that "sounds transmit information," the researchers write in Cell.
Experiments carried out with wheat, corn, vines, nettles and a species of cactus have shown that these species also emit more sounds in stressful situations. Given these results, "we expect that many plants emit sounds, but the diversity and characteristics of these sounds still have to be investigated," the researchers point out.
The sound comes from the stem of the plants and seems to be produced by a phenomenon of cavitation -that is, the formation of cavities within the stem-. On the contrary, no sound has been recorded from the wooden parts of the vine or from the trunk of almond trees.
"It is an emerging and controversial issue," says Maria Lois, director of the Center for Research in Agrigenomics (CRAG), who has not participated in the research. "That plants emit sound in response to stress seems to be related to a mechanical issue."
The authors of the research do not suggest at any time that plants have the will to produce sounds - something that would be inexplicable, since the will is a function of the nervous system, which plants lack.
But "it is possible that other organisms have evolved to hear and respond to these sounds," says Lilach Hadany. "For example, a moth that is going to lay eggs on a plant or an animal that is going to eat a plant might use sound to guide its decision."
According to the researcher, "even in a silent field there are sounds that we don't hear," says Hadany. "There are animals that can hear these sounds, so there's a chance that there's a lot of acoustic interaction going on."
Also other plants could react to sounds of plant origin. Several recent investigations have discovered that some plants regulate the activity of their genes in response to sounds produced by insects, for example, increasing nectar production in flowers when pollinators approach.
According to the hypothesis proposed by the researchers from the University of Tel Aviv, the sounds that plants emit in stressful situations have the function of warning their peers, for example so that they begin to prepare for a situation of drought.
In the future, farmers may also be interested in listening to the sound of plants to identify those that are in a situation of stress and optimize the use of water, the researchers say.
"The most direct potential application of this research would be a sensor that could be very important for precision agriculture," says Maria Lois. "It's definitely a theme to watch."
