The worst, the loudest
To check whether plants can make sounds, Israeli scientists led by prof. Lilach Hadany, an evolutionary biologist, planned a series of experiments. They grew dozens of seedlings from two plants—tobacco and tomato—first in a soundproofed tunnel, then in an even noisier greenhouse. They set up microphones around the seedlings, which transmit the recorded sounds to a computer. Scientists installed an artificial intelligence algorithm in it, which they trained to distinguish sounds, so that it could pick out from the recording those produced by plants.
Now it was necessary to provoke the plants to make sounds. Scientists did it two ways – they cut off single stems from some plants, and stopped watering others. Then they checked to see if the microphones picked up any unusual sounds that the algorithm recognized. It turns out that there were a lot of them! According to the team of Prof. Dr. Hadana writes in the scientific journal The Cell, plants under the influence of negative stimuli — stressors — make more vocalizations than plants left alone. A computer program calculated that one plant made 30-50 such clicks per hour after cutting the stem, and the untouched plants made much less noise. “When the tomatoes are not under stress, they are very calm,” Hadani says.
A computer algorithm has shown that the sounds produced by dried plants differ from those produced by a plant whose stem has been cut off
The algorithm also worked to identify the pattern of sounds produced by plants that were not watered. They begin to make their “clicks” before they get too dehydrated. “The sound frequency peaked after five days without water,” the researchers stated. The sounds were in the ultrasonic range, that is, outside the range audible to the human ear. However, these are sounds of similar volume to human conversation, for organisms that receive ultrasound audible from about a meter away.
This was not the first experience of recording such clicks. The fact that plants are not completely silent was first reported 10 years ago by Dr Monica Gagliano of the University of Western Australia. Recorded the sounds made by the roots of young corn seedlings. They released a wave with a frequency of 220 Hz, causing vibrations in the soil. And Dr. Gagliano noticed that the roots of other seedlings placed in transparent containers in the water indicated the source of the waves at this frequency.
Professor team. Hadani went a step further. Israeli scientists have proven that this “vegetable speech” can spread through the air like sounds made by animals or humans. And it’s really like talking – a computer algorithm has shown that the sounds made by dried plants differ from those made by a plant whose stem has been cut off. It can also tell if tomatoes or tobacco are “talking”.
Scientists have a hypothesis about how plants make these sounds. This may be due to a process called cavitation, which is the formation and bursting of air bubbles in a plant’s vascular system. a. Hadani notes that there is no evidence for the communicative role of plant clicks, but she is convinced that they must be some kind of message to other plants or animals. – The next question is who or what hears them? – says the prof. Hadance. Plants can almost certainly hear each other, because botanists have noted that at least some plants are sensitive to air vibrations – the effect caused by sound waves. This ability has, for example, flowers produced by tomato or berry bushes, which secrete nectar only in response to a certain frequency of vibrations produced by the wings of bees that pollinate these plants. According to prof. Hadana, many animal species may have evolved the evolutionary ability to hear “plant speech”, such as moths that lay eggs on them, or animals for which a particular plant is food.
According to Dr. Gagliano, plants can also recognize each other thanks to these sounds. A researcher conducted an experiment in 2013 in which she showed that hot peppers grow better in the presence of basil, and that their growth is inhibited if fennel is grown nearby. This relationship has also been noted by gardeners for years. In the lab, Dr. Gagliano’s team tried to grow hot pepper seeds in the presence of adult basil and fennel plants. It turned out that the seeds near the dill germinated worse. The only way they could identify this unwanted neighbor was by listening to the sounds it made, Dr. Gagliano believes, although those sounds could not be recorded at the time.
Smells like disaster
It is not without reason that Dr. Gagliano isolated the plants in her experiment so that they could not “smell” each other. Smell communication is their primary language. The function of information carriers is performed by volatile or soil-penetrating chemical compounds. With their help, plants can warn of predators or drought and recognize their relatives among other plants.
Richard Karban, an entomologist at the University of California, conducted one of the first studies to show that plants can communicate through secreted chemicals. For the purposes of the experiment, he transplants Artemisia vulgaris, composed of genetically identical seedlings, to a cattle pasture. When the plants grew, the scientist damaged one plant and then watched how quickly the surviving animals ate sagebrush. It turned out that within a radius of 60 cm from the affected plant, the rest of the sagebrush had been mysteriously “vaccinated” for eating. Most likely, they have become less palatable to herbivores. The scientist believes that the damaged sagebrush plant released unidentified volatile chemicals into the air, which its neighbors “sniffed”, thanks to which they can produce chemicals that spoil their taste over time. Therefore, animals did not eat them.
It turns out that not only the above-ground parts of plants have the ability to communicate, but also their roots. They also release chemicals into the soil that change its composition – this is the kind of chemical information other plants are able to pick up on. As scientists from the Hebrew University of Jerusalem have proven, thanks to these chemical signals emitted on the ground, plants can warn each other about the upcoming drought. In their experiment, the scientists planted six seedlings in pots standing in a row, so that each subsequent plant would have half of its roots in one pot and the other half in another. Then they stop watering the land in the first pot. After a quarter of an hour of drying the soil in this pot, the plants in the other pots knew what was happening – they all reacted to the coming drought. They have their stomata closed, which limits the evaporation of water. The first plant, after sensing dry land, seemed to secrete chemicals into the soil that had picked up on its neighbor’s roots, and it passed on the information.
Botanicals have mastered the art of aromatic communication to perfection. They can emit odors that provide clear information about other species, such as insects. Entomologists at Penn State University found that goldenrod can alter its scent to repel hostile fruit flies. Goldenrod does not like fruit flies, because these flies lay eggs on goldenrod stems, which impair plant growth. They make it difficult to produce and plant seeds. Goldenrods have learned to recognize the scent of an approaching male and then produce aromatic compounds to repel female flies. Date and spawning does not happen, Goldenrod is saved.
Recently, scientists have discovered another type of signal that plants can use to communicate – electrical impulses, like those running between neurons in our brain. This type of communication between plants is being studied by a Polish scientist, prof. Stanisław Karpiński of the Warsaw University of Life Sciences. And in collaboration with scientists from the University of Missouri, he determined that plants transmit information to each other using electrical impulses sent through the leaves. Such signals can spread between plants of different species. a. Karpiński demonstrated this in an experiment in which he placed two dandelions next to each other, and the leaves of one of them touched the mimosa leaves. It turned out that after applying a copper wire to a dandelion leaf, mimosa immediately began to curl the leaves.
Electrical signals can be transmitted not only by touching the leaves, but also by the roots, including those that do not touch at all. Such electrical signals between plants — and especially trees — are mycelial hyphae of mycorrhizal fungi entering into symbiosis with trees. Scientists have shown that the potential differences between the hyphae of fungi growing on tree roots and the entire soil in the forest are similar to the potential differences in the human brain. As explained by A.D. Andrew Adamatsky of the University of West Bristol in England, some of the chains of electrical impulses running through mushrooms are similar to sentences in human language. The scientist counted about 50 words that mycorrhizal fungi exchange with each other – however, their content is still unknown.
However, there are other known signals that trees exchange with each other via fungi. These are different chemicals secreted by plant roots and transported by fungi. Thanks to them, plants can inform each other about threats, send each other nutrients, or even fight each other. One of the first substances a plant uses to notify others of its presence is juglone, a chemical molecule produced by the black walnut and transmitted via the mycelium to surrounding plants. The message isn’t cute – it’s something like “get out of here”. And the plants are moving. Gardeners know very well that nothing grows under a black walnut tree.
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