- Tobacco and tomato plants emit sounds in the ultrasonic range of 20-100 kHz.
- They emit sounds under certain stress conditions (cut and dry).
- These sounds can be distinguished to know the type of stress.
It has been already proven that plants can see, hear, and smell. They even fight for territory, seek out food, trap prey, and evade predators. They are as alive as any animal, and like animals, plants exhibit several behaviors.
Recently, Israeli researchers at Tel Aviv University made an interesting discovery: plants emit airborne sounds in response to stress. Stressed plants differ visually, with respect to both shape and color, from unstressed ones.
Plants also emit volatile organic compounds when exposed to herbivores or drought. These compounds affect neighboring plants, increasing their resistance. The findings can change the way we think about the plant kingdom and open new avenues for precision agriculture.
Sounds Can Be Recorded Remotely
Researchers placed microphones 10 cm from the plants to see whether they emit informative airborne sounds, which might serve as potential signals to their surroundings.
Surprisingly, microphones picked up sounds in the ultrasonic range of 20-100 kHz from tobacco and tomato plants. They emit sounds when their stems are cut or when they are stressed by a lack of water. They may not produce any sound under other stresses, such as temperature or salt.
Although humans cannot hear these sounds, some mammals and insects can hear them from as far as 5 meters away. Researchers even speculate that plants could hear other plants and react accordingly. A moth may more likely to lay eggs on a water-stressed plant.
Reference: BioRxiv | DOI:10.1101/507590
When stems were cut, tobacco plants emitted an average of 15 sounds in the following hour, while tomato plants made 25. Similarly, the water-stressed tobacco plants emitted 11 sounds in an hour, while tomato plants emitted 35. Unstressed plants, on the other hand, made fewer than 1 sound/hour, on average.
The sound can be distinguished to know the type of stress. The team developed AI (artificial intelligence) models to differentiate between general noises (such as rain and wind) and plant sounds. They also trained the model to identify the plants’ condition (intact, cut, or dry) based solely on the sound they produce.
Although researchers analyzed only tobacco and tomato plants, other plants were also found to be making sounds. They captured ultrasonic sounds coming out of a weed henbit dead-nettle and a spiny pincushion cactus.
But why plants make such sounds under stress? Researchers don’t know the exact answer but they believe cavitation could be a possible explanation.
More study in the plant bioacoustics field is required, particularly in the ability of plants to produce and react to sounds under various conditions and environments. This will reveal a new pathway of signaling, parallel to volatile organic compounds, between plants and their surroundings.