Plants can also become noisy when stressed
In “Harry Potter and the Chamber of Secrets” it certainly has its best-known appearance today: the mandrake, a herbaceous plant with a taproot in the shape of a human-like creature, which requires ear protection to be worn when uprooted – or repotted – because it then causes a let out a deathly scream.
The legend probably goes back to a passage by the ancient historian Flavius Josephus. In his work on the Jewish war in the 1970s AD, he reports on a root called Baáras, which liberates from demons but kills anyone who pulls it out of the ground to do so. But you can tie a dog to the plant and then lure it away with food, which will successfully uproot the plant and only kill the dog. However, there is not a word in Josephus about screams. Someone in modern times probably thought that the causality was an acoustic one when he discovered the Baáras root and the mandrake, which has been considered to have healing powers since antiquity and the Middle Ages Mandragora officinarum mixed.
Screaming green stuff, that was still missing. However: mechanical vibrations in plant tissues under drought stress have already been observed; This caused cavitation in the sap of trees, for example, i.e. the formation of small bubbles that implode and emit ultrasound. However, until now this could only be measured with receivers that were connected directly to the plants.
But now a team led by Itzhak Khait from Tel-Aviv University in Cell Plant sound signals that were transmitted through air, “screams”, have been described, although they are only a few tenths of a millisecond long and in the ultrasonic range between 20 and 100 kilohertz, but with volumes of up to 60 decibels – as loud as human conversation is – at a distance of ten centimetres. Tomatoes and tobacco plants, which Khait and colleagues had subjected to two stresses: water deprivation and pruning, have roared.
The effect is clear, the authors write: Thirsty tomato plants, for example, emitted between 29 and 41 sound signals per hour – the well-watered control group less than one. The number increased with every day that the plants were not watered – until the “screams” decreased again after five days because the test subjects then dried up so slowly.
The sound spectra differed depending on the type of plant and the load: dying tobacco screamed louder at the high-frequency end than a tomato that was suffering in the same way, but if you pruned it instead of not watering it, some high frequencies were missing. The researchers even managed to train machine-learning software to recognize the type and degree of plant stress from the ultrasonic roar of the stressed people.
Of course, the authors do not forget to mention that the effect could possibly be useful for agriculture in times of climate change. At least they managed to receive acoustic signals through the air from wheat, corn and grapevines of the Cabernet Sauvignon variety, among other things. However, it is not really clear how the plants do this, except that it is probably also based on cavitation.
And evolutionary biologists are now likely to start pondering what selects plants for their ability to call for water when no one who can operate a watering can is listening – at most which, ultrasonically sensitive pests, for example, would only exploit such a plant emergency for their own benefit . But maybe the acoustic defense by plants is still in the evolutionary future. The Screaming Mandrake may not be fantasy at all, but botanical science fiction.