For the first time, researchers have apparently listened to plants “correctly”: their study shows that tomatoes and the like emit species-specific ultrasonic sounds with special information content. Whether the plants are “satisfied” or injured or “thirsty” is reflected in special patterns of these clicking sounds. Unlike us, some creatures could hear, and perhaps even “understand,” the plant’s ultrasonic tones. The scientists say that the noises may therefore have an ecological significance.
Wind makes leaves rustle and branches squeak – but plants don’t seem to make any sounds of their own accord. But maybe we just don’t hear them? This question was at the heart of the team of Israeli and US researchers. “It was known from previous studies that vibrometers attached to plants record vibrations. But it was unclear to what extent these vibrations also lead to airborne sound waves – noises that can be propagated and recorded over distances,” says senior author Lilach Hadany from Tel Aviv University.
In order to investigate this question, the scientists carried out experiments with tomato and tobacco plants and later also with other plants. To do this, they placed ultrasonic microphones in front of the test plants – first in a soundproofed acoustic chamber and later also in a greenhouse environment. To investigate to what extent the condition of a plant could be linked to specific sounds, the researchers also stressed their green subjects: some were not watered for several days, while others were cut.
Characteristic ultrasonic clicking
As the team reports, “listening carefully” revealed that plants actually emit high-pitched sounds with frequencies of 40 to 80 kilohertz. For comparison: The maximum frequency that an adult person perceives is around 16 kilohertz. Transformed into a version we can hear, the tones resemble pops or clicks. Unstressed plants only emit about one click per hour, the researchers found. “When tomatoes are doing well, they are very calm,” says Hadany. However, in the two stress states, individual plants made around 30 to 50 of these clicking noises per hour, the evaluations showed.
Put into a form that we can perceive, a stressed plant sounds like this:
© Khait et al.
In the next step, the scientists analyzed the recordings using an artificial intelligence system: After “training”, the machine learning algorithms were able to distinguish between the sounds of different plants and identify the type and degree of stress based on the noise patterns. This means: A “thirsty” plant sounds characteristically different from an injured one. In addition, the algorithms identified and classified the plant sounds even when the plants were housed in a greenhouse with background noise.
Although the study focused on tomato and tobacco plants because they are easy to cultivate in the laboratory, the research team also included a variety of other plant species. “We found that many plants – for example corn, wheat, vines and cactus plants – emit noises when they are stressed,” says Hadany. The exact mechanism behind these noises has yet to be elucidated. However, the researchers suspect that it could be due to the formation and bursting of air bubbles in the plants’ vascular system – a process known as cavitation.
A phenomenon with meaning?
According to the researchers, the phenomenon could be important in two respects: Sensor systems could potentially be used in crop production to detect exactly when plants should be watered. Above all, however, the scientists see a possible ecological significance in the plant sounds: “It is possible that certain organisms have developed in such a way that they can hear these sounds and react to them,” says Hadany. “For example, an insect that wants to lay its eggs on a plant, or an animal that wants to eat a plant, could use the sounds as a decision-making aid.”
Another exciting possibility is that other plants could also “hear” the sounds and use them as information. In this context, it is already known from previous research that plants can react to sounds and vibrations. “If plants get information about stress from others before it occurs to them themselves, they could prepare for it,” says Hadany.
So the study raised exciting questions that the researchers now want to pursue further: What is the mechanism behind the plant noises? To what extent do certain animals or plants recognize and react to these sounds? “Now that we know that plants emit noises, the main question is: Who could be listening?” Hadany concludes.
Source: Cell Press, Tel-Aviv University, Article: Cell, doi: 10.1016/j.cell.2023.03.009