Stressed Plants Also Produce Sounds, Give Signals

Can plants produce sounds or give some signals when in distress just like any other species of the animal kingdom? Well, it is quite possible that plants may produce sounds while facing such situations.

In a recent study published in Cell, scientists have proposed that plants emanate airborne sounds while in stress like deprived of water or stems cut. 

According to the study, plants can produce up to 35 sounds an hour in such situations. On the other hand, well-hydrated plants or plants that don’t suffer cuts can produce as low as one sound an hour.

Humans may never hear such sounds because they are ultrasonic, meaning sounds with very high frequency—around 20-100 kilohertz. Such high-pitched sounds are not detectable by human ears. But some animals can hear these sounds of high frequencies.

To detect such sounds, the researchers observed tomato and tobacco plants kept in an acoustic chamber inside a greenhouse. They also monitored the physiological conditions of the plants. The microphone used in the chambers could detect sounds coming from the plants even if humans could not.

The sounds were made by plants deprived of water or those that had recent cuts in their stems. In an article published in Nature, Emma Maris, an environmental writer based in Oregon, USA, quoted Lilach Hadany, the corresponding author of the research, as saying, “It is a bit like popcorn—very short clicks, it is not singing.” The sounds made by the stressed plants were processed so that they became audible to human, which can be found in Maris’s piece.

The question is how plants can make sounds when they don’t have any organ like the vocal cord or lungs? According to Hadany, as mentioned in the Emma’s piece, it probably is centred in xylems, which are the transport tissues in plants. They transport water absorbed by roots from soil to the leaves and stems. In this process they also transport nutrients to various parts of a plant.

When a plant is exposed to drought, air bubbles formed in the xylem can expand and collapse inside it. This creates vibrations and the entire process is known as cavitation, the authors wrote in their paper.

Previous studies showed that these vibrations can be recorded by connecting recording devices directly to the plants. However, these previous studies, according to Hadany and her co-authors of the study, could not say anything about whether these sounds are detectable at a distance from the plant.

This made the question open—whether plants can emanate airborne sounds or not? Hadany’s team was intrigued and advanced our understanding about sounds made by plants.

Notably, before Hadany and her team’s findings were published in Cell on March 30, the research was also made online in the preprint server biorxiv back in 2019, which can be found here. The preprint servers’ research papers are not peer reviewed and open to comments and questions. These kinds of research articles are modified and can further be published in other peer reviewed journals.

Along with detecting sounds from the plants, the authors also developed a machine-learning model that can predict whether a plant is water deprived or cut with an accuracy of 70%. This means that if someone can detect the sounds of a plant in the farm or in any horticulture capacity, those sounds can be fed to the machine-learning model and the model in return will say whether the plants are suffering drought or they have recent cuts—that too with 70% accuracy. Combined, the results suggest that it may be useful for farming or horticulture.

Hadany’s team had also reported in a 2019 research paper published in the journal Ecology Letters that plants can hear sounds of other organisms and can respond. They reported that beach evening primroses can release sweet nectar when they hear the sound of bees flying around.

However, experts have other opinions. They say that there is a need for further investigation on the topic before coming to any solid conclusion. In Emma’s article, Graham Pyke, a biologist retired from the Macquarie University, Sydney, Australia, was quoted as saying, “It is unlikely that these animals are really able to hear the sound at such distances.” He added that further research is necessary on this matter.