The sound of plants, a less silent realm than we imagine

Many types of objects, when properly stimulated, can make a sound in response.

If we hit a wooden surface, or a crystal glass, or a wall, we will get a sound in response which depends on the characteristics of the object and on the mechanical energy we have transferred to it.

Even without being stimulated through a transfer of mechanical energy, physical objects can emit sounds: for example, the sizzle of burning wood or the crash of a tree that yields correspond to the transfer of mechanical energy from the considered object to the air, in response to various phenomena.

The interpretation of the sounds that accompany various phenomena can provide useful information to living beings about the occurrence of those phenomena: the rumble of thunder can signal the arrival of rain, the sound of falling water can herald a waterfall and etc.

For this reason, a recent work demonstrating how is extremely interesting plants emit specific sounds associated with different types of phenomena that may interest them.

In particular, tobacco and tomato plants subjected to two different types of stress, namely dehydration or cutting, produce sounds of different types. The results, in particular, showed that plants under stress emit sounds in the air detectable from a distance. Plant emissions, in the ultrasonic range of ∼20–100 kHz, resulted audible up to five meters away by many mammals and insects, taking into account their auditory sensitivity.

The use of an artificial intelligence has made it possible to classify the different sound patterns; this demonstrates that, in principle, there is enough information encased in the generated noise, to be able to distinguish different states of a plant.

The researchers were thus able to distinguish between the sounds emitted in two different stress conditions – dry or cut plants – with an accuracy of 70% and were able to differentiate dehydrated plants and control plants, based only on the sounds they emit, with a 84% accuracy. The noise emission was correlated to the transpiration rate of the plant – hence the sensitivity to dehydration – while the daily number of emissions increases during the first days of dehydration and decreases when the plant dries up. The sounds emitted by plants at high and low levels of dehydration are also diverse and distinguished by researchers with 81% accuracy.

The mechanism for the production of sound has been identified by researchers in the cavitation that occurs inside the system of lymphatic vessels of a plant, and in fact the sounds emitted depend on the size of the tracheas: plants of different sizes, consequently, they make different sounds.

The data obtained immediately suggest useful applications in agriculture, in which listening by special sensors to the sound emitted by plants can be used to detect stress of different types and to act accordingly in a targeted manner; even more, however, they solicit ecological investigations, for the identification of those who may, in nature, be “listening”.

The frequencies and intensities emitted, as mentioned, are audible by different types of animals, such as mice and moths, and they can then be used to distinguish the status of a plant for consumption or spawning, or for other interactions by different animals; there is no reason to think, in fact, that such obvious sound patterns are not used like many others to extract useful information from the environment by different species.

A dry plant, or a cut plant, may be of more or less interest to different animals; and if this information is available at distances of a few meters, even at night, its use can be interesting, especially for small animals.

Even more, the authors indulge in conclusions about considerations about the ability of the plants themselves to receive signals emitted by conspecifics or from different plants, to metabolically adapt their state, for example by rapidly mobilizing toxins in the case of severed plants or by activating mechanisms of resistance to drought in the case of plants in a situation of water stress; however, although sonoception occurs at the cellular level also in animals, and therefore is accessible to a certain extent without organs specialized for the purpose through mechanosensors with which many cells are equipped for various purposes, before the identification of one or more specific mechanisms for reception of stimuli in the frequencies and intensities demonstrated in this study, it is necessary to exercise a healthy skepticism regarding a proposed sound communication between plants.

It may well be that certain particular sound stimuli, as demonstrated in at least one case, are capable of activating specific responses in plants through mechanisms specifically adapted for the purpose; but before going too far to imagine something that has a function comparable to an auditory system instead of a mechanosensor for specific frequencies, we have to wait for data that, to date, are missing.

The kingdom of plants is certainly much less silent than we imagine, and many animals are probably able to hear the voices that populate it; but before launching into dreaming of green conversations, we have to wait for science.