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Green Symphonies

Plants may be just as noisy as other organisms.

Plants may be just as noisy as other organisms.

By Monica Gagliano

New research reveals plants emitting and responding to sounds.

For thousands of years, indigenous healers and shamans around the globe have been learning the “songs” of plants, through which they are taught how to diagnose and treat specific illnesses (see box). This may sound as crazy as the inventor in Roald Dahl’s The Sound Machine, yet in a recent paper in Trends in Plant Science I have confirmed with colleagues from the UK and Italy that plants do indeed generate their very own cacophony of sounds. Furthermore, there is every reason to think that plants would have evolved to respond to sounds given that they, just like all of us, live in a very noisy world.

The idea that plants are capable of producing, detecting and using acoustic signals is not new. The study of plant bio­acoustics, however, has suffered from the methodological and technological problems of early investigations in the 1940s, and this historical baggage, in conjunction with folkloric and at times esoteric reports of the influence of sound and even music on plants have severely hindered investigations of this aspect of plant ecology – until very recently.

We have taken a fresh look at the idea and carried out a series of experiments testing whether plants can produce, detect and respond to acoustic vibrations. By using modern laser technology and a non-contact method of measuring vibrations, we were able to detect a Doppler shift in the frequency of our laser beam as it hit the roots of young corn plants – the vibration of the root itself caused the observed shift! This enabled us to capture the loud and frequent “clicking” sounds emitted by the corn roots at around 200 Hz.

Clearly these plants emit their own sounds, but what do they detect if anything? We broadcasted acoustic signals at different frequencies back to the corn roots. Remarkably, these roots were very frequency-selective and only reacted to the specific frequency they emit themselves (i.e. around 200 Hz) by bending towards the sound source.

This provides the first rigorous, experimental and quantitative evidence of a plant’s ability to produce, detect and respond to acoustic vibrations. In doing so it helps to overturn years of ostracism of plants from a world full of sounds.

At this stage, our understanding of how plants produce and respond to sound is still limited. Previously, acoustic emissions have been interpreted as the result of the abrupt release of tension in the water transport system of plants following cavitation, a process that commonly hits drought-stressed plants. Cavitation occurs when dissolved air within the water expands in the xylem, eventually generating air bubbles that occlude its conduits so that they are unavailable to transport water. In this context, acoustic signals are simply emitted by plants as an incidental physiological by-product of this process of cavitation.

While it is undisputed that cavitation can induce acoustic emissions, it is now clear that not all acoustic events are attributable to cavitation alone. The truth is that we don’t really know how plants produce and emit these sounds.

But the fact remains that plants are noisy organisms, and it is going to be very exciting to discover how they might use these sounds and what potential ecological role sound plays in a plant’s life.

Plants Emit and Respond to Sounds, But Do They Mean Anything?
Communication is generally defined as the intentional transfer of signals that benefit both the emitter and the receiver. The flow of information between species (or individuals within species) relies on the sender’s encoding mechanisms and the receiver’s decoding mechanisms; indeed, it is the relationship between how information is packaged (encoding) and the content translated (decoding) that determines the outcome of communicative interactions.

Any such exchange of information between living organisms can be considered communication, yet the notion of communication by plants has long been regarded as a fringe idea because the exchange of information in plants has always been thought to involve simple involuntary cues rather than intentional signals that cause behavioural changes.

However, this attitude is rapidly changing as evidence reveals that plants can perceive and respond to cues emitted by their immediate neighbours and signal information about forthcoming conditions to other plants located further away.

But is it truly necessary to assume intent or benefit for the emitter or receiver? Indeed, it has been difficult to determine benefit or intent in communications between animals, let alone plants, yet there are cases where even involuntary acoustic emissions by animals serve as a means of communicating with other individuals.

For example, the ultrasonic vocalisations that infant rodents emit when they are isolated from their nest and experiencing extreme cold exposure are not driven by any motivation to communicate acoustically or even to emit sounds, but are the result of a reflexive physiological and/or biomechanical process that produces sound as a by-product. Yet, the involuntary ultrasonic vocalisations of a cold pup outside the nest reliably prompt the mother to the pup’s rescue, hence triggering a behavioural response in the mother that is beneficial to the signalling pup.

Whether intentional signals or involuntary cues, can sounds emitted by one plant affect the behaviour of surrounding plants? If we assume neither intent nor benefit for the emitter or receiver, then we can really start asking questions about plant communication and the potential role of acoustic emissions in plant assemblages by studying whether the acoustic cues have a function and, if so, how they affect responses in other organisms, plants and animals alike.

The potentially adaptive functions of sound in the life of plants are awaiting to be explored!

The Future Is Greener
To understand how and why plants sense sound in their environments, we need to learn about the nature of their sounds and the information they carry. The answers are clearly important for understanding the processes that regulate species interactions and co-evolution, and ultimately shape thriving and diverse biological communities. Yet, the potential application of this knowledge to the real world could be just as remarkable.

For example, the phonobiological wine selected by the Euro-Brazilian Sustainable Development Council and UN-Habitat as one of the 100 world’s most important initiatives in consolidating the Green Economy in the past 10 years was produced organically using sound to enhance plant growth, quality of harvest and, most interestingly, support pest management of the vineyard (i.e. no pesticides were needed). A better understanding of the effect of sound on the biology of plants and how we may use this knowledge to establish “clean” agronomy practices can truly open up significant opportunities for a greener future.

We are increasingly discovering that plants are highly sensitive organisms that actively process and evaluate information about their neighbours as well as about the resources available in their surroundings, and modify their behaviour accordingly. Our new findings confirm that the prevailing Aristotelian view of plants as passive and insensitive creatures is outdated and incorrect.

A shift in our perception of plants is clearly important for advancing our scientific knowledge into the world of plants in its full complexity. However, such knowledge also has major ecological implications as it opens up questions on how we really perceive plants and the way we use them. Indeed, these questions are important as they concern our current ecologically inappropriate behaviour towards plant life, where plants are mostly exploited as mere resource objects and materials. This attitude has paved the way to the relentless alteration and destruction of natural habitats, which are predominantly plants.

At a time of environmental crisis, promoting a new perception of plants as living beings in their own right means creating the conditions for the well-being of those who truly make life on Earth possible: plants.

Box: A World Full of Sounds
From the sub-microscopic world of atoms and molecules to the macroscopic world of earthquakes and tsunamis, energy exists everywhere in the form of vibrations, and often exhibits a wave-like behaviour as it moves throughout space and time. As waves propagate, they transport energy as well as a varying amount of information about the things they encounter.

Living organisms have evolved ingenious ways of utilising wave motion of various kinds as couriers of information. Sound waves of many different frequencies and sources constantly crisscross the environment we live in and tell us a great deal about the surrounding world.

Generally, our awareness of a sound depends on its loudness, which is strongly correlated with the intensity of the sound and its frequency. The intensity of the signal is a measure of the amplitude of the sound wave (i.e. the amount of energy in the wave), and determines how far that acoustic wave can travel. The frequency of the vibration is responsible for the “pitch” of the acoustic signal, determining whether the sound will be heard at all.

Because the perception of sound in humans is limited to audio frequencies in the range of 20 Hz to 20 kHz, species that exploit acoustic frequencies outside the pitch of the human ear seem silent to us. Nonetheless, from the very low infrasonic (<20 Hz) long-distance calls of the great African elephant to the ultrasonic (>20 kHz) “conversations” of tiny bacteria and the very high pitched vocalisations of many rodents, bats and some singing frogs, we now know that many of them are actually quite noisy.

The ability to sense sound and vibrations is an ancient sensory modality behind the behavioural organisation of all living organisms and their relationship with their environment. Hence it should not come as a real surprise to find that plants may be just as noisy as other organisms.

Monica Gagliano is a Postdoctoral Research Fellow in the Centre for Evolutionary Biology at the University of Western Australia.