Representative image. Photo: Davor Denkovski
A century ago, in eastern India, a renowned physicist toiled to discover the secrets of the plant kingdom. His pioneering research was initially met with disbelief by other plant physiologists globally. However, he continued his research on the plant nervous system and devised numerous ingenious instruments to record the response of plants to various stimuli. He was none other than Sir Jagdish Chandra Bose. Through his groundbreaking research, Bose concluded that plants also have a nervous system that enables them to carry out various physiological functions and respond to different stimuli.
Today, more than a hundred years later, researchers are building upon Bose’s work. A recent study published in the journal Cell suggests that plants can whisper and make their presence felt. They also emit sounds when they do not have sufficient or are being cut, the research showed.
The researchers at Tel Aviv University in Israel used a recording system in a greenhouse setting and in an acoustic chamber to identify the voices of plants. They believe that these airborne sounds from the plants provide information about the physiological status of the plants.
The results of the study demonstrate that plant acoustic emissions may play an important role in ecology and evolution, and could have direct implications for plant monitoring in agriculture.
Identification of these voices
Although Bose predicted the responses of plants to various stimuli, these plant voices have remained unheard for generations.
Plants that are under stress produce visual, chemical and tactile cues to exhit their stress. “Nevertheless, the ability of plants to emit airborne sounds—that could potentially be heard by other organisms—has not been sufficiently explored,” the researchers said.
They devised a unique setup to eavesdrop on plant whispers. They installed two microphones pointing at a plant’s stem in an acoustic chamber [wooden soundproof boxes] and then in a greenhouse setting to detect the voices.
Further, these plants were also subjected to various stress conditions like drought, and stem or leaf cutting. They emitted different sounds under different physiological conditions, which were detected through microphones. Machine learning algorithms were then used to classify these recorded sounds, with a 70% accuracy.
The background noise in the greenhouse such as wind, air conditioning, etc. was also taken into consideration. These noises were first recorded by using microphones in an empty greenhouse and were then cancelled out to identify the acoustic emissions of plants.
Although the plants that were used in the study were tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum), other species also emit sounds. The researchers have since detected sounds made by grapevines and wheat.
According to the New York Times, the emissions sound like popping noise, “not unlike what happens in your joints when you crack your knuckles”.
Why have humans remained oblivious?
When the recorded sounds were analysed, the researchers identified that stressed plants produce over 30 sounds per hour. Notably, even healthy ones produce one sound per hour. Why haven’t humans been able to hear the sounds that plants were producing? These sounds are too high-pitched (in the ultrasonic range of 20-100 kilohertz), making it impossible for the human ear to hear them. But, other mammals and insects like bats and moths, and fellow plants can hear these voices and also respond to them if they are within a distance of 3-5 metres.
In the absence of a vocal cord or lungs, it is difficult to identify organs in plants that produce such sounds. The researchers believe that the xylem is giving a voice to the plants. Xylem is a tube that facilitates water and nutrient exchange from the root to other parts of the plant. The formation or breakage of an air bubble in the xylem, which then produces mini-shock waves inside the plant’s vascular system, might result in such sounds. However, there is no such experimental evidence to back up this hypothesis. The researchers say deciphering the source of these sounds will require more study.
Future perspectives
Plants have always been dubbed as “silent” organisms. Despite Bose’s observations centuries ago, researchers are only currently deciphering the voices of these immobile organisms. This recent study paves the way for a paradigm shift in our perception of the plant kingdom. Plants do produce sounds, and those noises might indeed be an effective way of communication. Stressed plants might inform other fellow plants regarding their condition and also warn them. These voices might also invite pollinators or other organisms.
“That the plants are making different noises that have some information seems like the main contribution of this study,” Richard Karban, an ecologist at the University of California, Davis, told NYT.
Further analysis of these sounds could help farmers and horticulturists detect the stress that a plant is under and devise ways to combat it. The researchers say that plant sound emissions “could offer a way for monitoring crops water and possibly disease states—questions of crucial importance in agriculture”. It could help provide precise irrigation to crops, avoiding unnecessary expenditure. “In times when more and more areas are exposed to drought due to climate change efficient water use becomes even more critical, for both food security and ecology,” they write.
A lot remains to be unearthed as we are just in the incipient stage of understanding plant sounds. However, it is now clear that plants have ways to express themselves. So the next time you harm a plant, remember – they too can cry.
Dipika Mishra has a PhD in Life Sciences from the National Institute of Science Education and Research and is a SciCom enthusiast.