Ouch! Help! Stressed plants make noise

According to a study, stressed plants can make a lot of noise - but people don't hear it. The noises are about as loud as a normal conversation, but the frequency of the tones in the ultrasonic range is too high for humans, write scientists from Tel Aviv University in the journal Cell.

“The sounds in the ultrasonic range could be heard by many mammals and insects from a distance of three to five meters,” the researchers assume. (You can hear them here.)

Plants such as tomatoes and tobacco become noisy when they suffer from drought stress or when their stems are cut, the research team further reports. They sound as if the small capsules were being crushed by bubble wrap.

Whether the plants produce such sounds to communicate with other organisms is unclear. Other studies have already shown that plants, for example, increase the sugar concentration in their nectar in response to noises made by pollinators.

And what if an entire wheat field is about to be harvested? The scientists explain that crop plants such as corn or wheat also make noises under stress. "Therefore, it's likely that sounds are also emitted during harvesting (in the form of cutting)," shares Lilach Hadany, an evolutionary biologist at Tel Aviv University. The team was also able to show that cacti, vines and deadnettles also make noises.

Some allotment gardeners may be horrified at the thought that the stress on their grown vegetables will start popping when they are cut off. However, the researchers see a practical potential use for agriculture in their findings: With the help of sound recordings, for example, the irrigation of plants in the field or in the greenhouse can be monitored and made more effective.

For the study, the researchers examined tomato and tobacco plants under different conditions. In one of the experiments, the plants had too little water, in another their stems were cut. For comparison, the team also looked at undisturbed specimens. The scientists used microphones to record sounds in a soundproof room and in a greenhouse.

The result: According to the study, stressed plants emitted significantly more noise than healthy ones. Under stress, they made around 30 to 50 tones per hour. "If tomatoes aren't stressed at all, they're very quiet," says Hadany. Using an algorithm, the team was able to see how the sounds differed depending on the type of stress.

The researchers assume that the cause of this phenomenon takes place inside a plant. Studies have shown that so-called cavitation occurs in plants that suffer from drought stress. Roughly speaking, air bubbles form in the vascular system, which expand and collapse again. This leads to vibrations.

"The design of the study is good," says Sibaji Kumar Sanyal, a molecular biologist at Heinrich Heine University in Düsseldorf, who was not involved in the study. You can quickly understand from the sounds if the plants were not watered properly. For future studies, however, it is important to examine other plant species in addition to tomatoes and tobacco.

So do people get the wrong idea about the quiet nature – when, for example, the domestic houseplants need water again or the vegetables are harvested in the garden? "That's an interesting idea. But if you look at the frequency of the plant sounds, it is in the ultrasonic range. That's why they are still silent for us," explains Sanyal.