Plants sound off when they’re in trouble

Link to Science News Explores article

People often talk, sing, or play music to their plants. They feel such things help them grow. But do plants actually make noise to tell us anything about their condition? Researchers at Tel Aviv University in Israel have just discovered that the answer is yes! Audible not to the human ear, but the sounds are there.

Normally, sound is energy vibrating through solid, liquid, or gas materials. Our voices make sounds by vibrating vocal cords as we breathe and speak. But what about plants? They have no vocal cords.

Water moves up a plant in three ways beginning at the roots and then through special tubes. First, the water is pushed up by pressure near the roots. Water also clings to itself and the walls of containers, which is why you see a U-shaped curve in a glass or straw. Put the straw in water, and this causes the water to move up the straw a little. This is called capillary action, like the capillaries carrying blood between arteries and veins. A third way is when water is drawn up from the leaves when it evaporates there (called transpiration, as opposed to perspiration or sweating). Sometimes, air gaps form in the tubes, and that slows down the flow of water, and that's the key to sounds.

Movement of water in a plant (explained on byjus.com)

In 1914, Irish professor Henry Dixon filled a narrow glass tube almost full with water and sealed both ends. This was then heated until the tiny air bubble dissolved into the water. As it cooled, the air was released to form a new bubble, and he noted a metallic clicking sound at that moment. He repeated the experiment with sap from a beech tree and got the same clicking. (Similar experiments had been done in 1850 in France, but this was the first time that sound had been noted.)

The formation and popping of gas bubbles in a liquid is called cavitation, where the word "cavity" refers to holes or space. The "father of cavitation research" was British professor John Milburn, who studied the clicking sounds in plants more than anyone, beginning 50 years after Dixon's research. 

John Milburn 

Milburn attached probes initially to the side of a leaf stem, and later inserted probes into the plant stem's tube to record cavitation events he called audible acoustic emissions, AAEs.  

Milburn's apparatus with probes outside (left) and inside (right) plants

Here are results of 3 experiments Milburn conducted. On the left, he measured clicks/min as the leaf is allowed to lose its water. At first, there are many clicks as bubbles formed, but then the leaf was just full of air and no way to pop bubbles. In the middle, clicks rose as the plant dried out, but when the leaves were covered in a plastic bag to prevent water loss, the number of clicks decreased because fewer bubbles were formed. Remove the bag, and the counts rose again. On the right, Milburn put a drop of water at the base of a leaf, and as water evaporated from its surface, bubbles formed and click count rose. When the drop was removed 3 times, the leaf content had no water coming in, so no bubbles or clicks were seen.

Milburn's cavitation experiments, 1966

Milburn found that the intensity of the clicks was different depending on which plants he used. Medium-Loud sounds came from castor oil plants, currants, gooseberry, geranium, plantain, fern, sycamore, nasturtium, water lily, and hogweed. Weaker sounds came from coltsfoot (aster), fireweed (primrose), catnip, couch grass, and laurel. No sounds came from small seedlings of mustard, but Milburn suggested that maybe his equipment just wasn't sensitive enough.

Over the years, people injected a dye into the plant to see where the air bubbles has been created, but that meant cutting open the plant. The 2023 report from Israel measured sound at a distance from the plants. Instead of attaching or inserting sensors to the plants directly, they set up a soundproof box with 2 microphones about 10 cm (5 inches) away from the plants. 

Israeli acoustic box setup, 2023

But even in a soundproof box, regular microphones are not going to detect bubbles popping (cavitating) inside plants. You need the type that measures ultrasonic noise, whether it's coming from cavitation or the vibration of nearby fibers in the plants. Milburn pioneered the ultrasonic emissions from plants in 1988. The Israeli researchers tested tomatoes and tobacco plants. Like Milburn with probes directly attached, the Israeli group with microphones not touching the plants monitored how clicks rose after plants were watered and bubbles formed, then fell off as they dried out and bubbles presumably stopped forming.

Measuring clicks from tomato plants

Why is all of this important? Well, the Israeli researchers speculated that animals may be attracted to certain sounds, so this research would potentially tell us more about how that works related to pollination. The results might also be important for farmers if recorders were sensitive enough to measure crop clicking in the fields and filter out background noise. Farmers would know when to water the plants, or set up an automatic system to do it.

Here's a YouTube video showing how a new device called PlantWave can be used to change the sounds from plants into music.