When there is a lot of stress, plants will scream, and it will be super loud.

Don't think that plants are quiet and silent under any blow. Actually, it's our ears. We don't hear their voices.

When human beings are under pressure and have nowhere to release, they may run to an open place and shout loudly, as if they can get the backlog of emotions out of the body and dissipate in the air.

If a plant encounters the pressure of survival, it may not be realistic to want to run around. But even if it can't move easily, the sound it makes can still float through the air to farther away.

Usually, it is difficult for human beings to hear these sounds only with their own ears, so that they think that plants are a group of silent life. But with the help of the equipment, the scientists in the laboratory collected the sound waves of the plant "shouting".

Of course, before that, the researchers had to inflict some damage before they could force the plants to shout. So, what exactly did they do?

When you get hurt, scream.

Itzhak Khait, from Tel Aviv University in Israel, and his colleagues set two dilemmas for plants to capture their voices: drought and cut stems.

The contestants who take these tests are our more common plants, tomatoes and tobacco. Before the experiment began, they all grew healthily in the wet soil. After the experiment began, the fate of the plant was different: some were no longer watered, some were cut off the stem, and some continued to live under normal conditions, as the control group.

The experiment was carried out in a soundproof box, and the radio equipment was placed 10 centimeters away from the plant to listen to the sound made by the plant after different injuries.

As a result, both drought-stressed plants and plants with severed stems make a lot of noise: the volume is about 65 decibels (usually the two people talk face-to-face at about 60 decibels) and the frequency is between 20, 000 and 100, 000 hertz. It's ultrasound.

Human ears can only hear sounds between 20 and 20, 000 hertz, and this range narrows with age. Therefore, if you replace the radio equipment with a person and stand 10 centimeters away from the plant, it is unlikely to catch such a high-frequency "scream".

These shouts still come very frequently. Tomato plants under drought stress emit an average of 35 sounds per hour, while tobacco plants emit 11 sounds per hour. When the stem was cut, tomato plants made an average of 25 sounds over the next hour, while tobacco plants made 15 sounds. By contrast, plants that are not exposed to drought and are not cut only make occasional sounds, on average, less than once an hour.

So scientists believe that these ultrasound waves are the response of plants to the pressure of survival. Of course, in addition to lack of water and being cut, plants face many other pressures, not all of which can form screams.

How did this sound come from?

Sound is nothing but the vibration of an object.

As early as the 1960s, scientists detected vibrations during plant drought and believed that those vibrations were caused by cavitation. When there is a lack of water, the air dissolved in the xylem of the plant will form bubbles, and then the bubbles will continue to expand and even burst, which is a kind of hole phenomenon.

The hole phenomenon will redistribute the stress in the plant, when the stress is concentrated in one place, it is possible to quickly release a large amount of energy: mechanical energy is converted into sound energy, so this process is called "acoustic emission" (Acoustic Emission,AE).

More than half a century ago, researchers found that the plant was vibrating by detecting the waveform emitted by the plant. Nowadays, in industry, people often use the principle of acoustic emission to test whether a material is defective. If multiple sensors are used, the specific location of the defect can also be determined.

However, when using acoustic emission to detect a piece of material or a plant, it is usually necessary to connect the sensor directly to the object being tested. Therefore, in previous studies, although scientists have captured the sound of plants, sound waves are transmitted by the plant itself and received by the equipment, which does not prove that the outside world has a chance to hear those sounds.

But this time, the Tel Aviv University team placed the radio equipment 10 centimeters away, proving that ultrasonic waves emitted by plants do spread in the air when they are short of water or cut. And this means that even if humans can't hear it, other animals may be able to hear it.

Scientists say the ultrasound they collect may also be heard by many mammals (such as mice) and insects (such as moths) standing 3 to 5 meters away. This is very important for researchers.

There's a difference between ultrasound and ultrasound.

The team found that although plants emit ultrasound during drought and stem cutting, they emit ultrasound differently under two different pressures.

Specifically, they wrote a machine learning algorithm, or AI, which fed sound waves collected in both cases to AI to train it to distinguish between dry plants and cut plants. After the training, the correct rate of AI to distinguish between the two was more than 70%.

This means that the sound waves emitted by dry plants are different from those emitted by plants with severed stems for AI. AI can tell the difference between the two, and it is possible for other animals to tell the difference.

Scientists say that if an insect that lives on a plant can hear the difference in sound waves, it is possible to use this information to prevent itself from laying eggs on a dry plant. If there is an insect that feeds on a plant, its natural enemies can hear the difference in the sound waves and have the opportunity to run in the direction of the sound waves to catch more insects and protect the safety of the plants.

In addition, it is not just animals that can hear sounds. Previous studies have shown that plants also have strong perceptions, responding not only to touch (such as mimosa closes), but also to many stimuli, including sound. If one plant is caught in a drought, and another plant hears its screams, it may also trigger a mechanism to protect itself.

Of course, scientists still have a long way to go before these ideas are confirmed.