Sound and its use in communication have shaped the ecology, evolution, behaviour, and ultimately the success of many animal species. But are animals the only lifeforms to communicate with sound? Do plants also use sound to pass information, and if so, what might this mean in a plant’s life?
The idea of plants being capable of producing, detecting, and using acoustic signals is certainly 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 reports of the phenomenon, have severely hindered prospects of investigating this aspect of plant ecology.
But we know that plants have evolved to detect and respond to sound waves or vibrations in their environment. A well-known example of this ability is displayed by the rapid leaf-folding action of the sensitive plant, Mimosa pudica. By reducing the leaf area presented to herbivores, while simultaneously making its defensive thorns more visible, this response is widely believed to have evolved to prevent or reduce predation risk.
The ability to respond to vibrations is not unique to M. pudica and many plant species have evolved a range of adaptive strategies to exploit sound. For example, a number of flower families, including that of tomatoes and blueberries, use “buzz” pollination where the pollen is released from flowers only when they are vibrated at the correct ultrasonic frequency, a feat achieved by bees that have co-evolved to vibrate their flight muscles appropriately. Despite the ecological and evolutionary significance of sound in plant-animal interactions, no quantitative information on the mechanisms through which plants detect and respond to sound, and modify their growth accordingly, was available until very recently.
In a recent article published in Trends in Plant Science, my colleagues and I show that roots of young corn plants emit loud and frequent “clicking” sounds. Remarkably, these roots also react to specific sounds by exhibiting frequency-selective sensitivity that causes them to bend towards the sound source.
This provides the first rigorous, experimental evidence of a plant’s ability to produce, detect and respond to acoustic vibrations. It does, however, leave open the next obvious question of why plants should do so.
Why should plants emit and receive sound, and do they mean anything?
From an evolutionary perspective, the reception and processing of energy embedded in acoustic waves is advantageous, as it provides information about the environment, whether close by or distant. Given the ease with which it transmits through the environment, especially in dense substrates like soil, sound offers a particularly effective transmission channel for short range signalling. However, it may be useful for long range signalling too; acoustic signals could mediate interactions, such as competition between plants for the resources available within the substrate.
In trying to understand how and why plants sense sound in their environments, the key questions clearly reside in the nature of the sounds and the information it carries. The answer to these questions is important to better understand the processes underlying species interactions and co-evolution. Yet, the potential application of this knowledge to real world problems could be just as remarkable. For example, take the devastating effects that droughts bring on the environment, particularly through vegetation loss, and to the economy of affected communities. Drought cannot be prevented but its effects can be mitigated through effective management strategies and a better understanding of the processes underlying plants' responses to drought.
Recent research in this context has demonstrated that unstressed plants are able to respond to stress cues emitted by their drought-stressed neighbours, and relay this “drought alarm” signal to unstressed plants further away. Could the “talkative” nature of plants help us to cope better with drought? Aspects of plant acoustic emissions have already been used as a crude indicator of drought-related stress and tolerance in different species. By determining the ecological role of sound in plant communication, we could significantly advance our knowledge of plant ecology, and thus contribute towards a better understanding of the processes underlying plant responses to stressful environmental conditions.
Where to from here?
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 automata-like passive and insensitive creatures is evidently obsolete and inappropriate.
A shift in our perception of plants is not only important for advancing our scientific knowledge into the world of plants in its full complexity. Knowledge of plant autonomy also has critical ecological consequences as it opens up a new debate on the perception and action of people towards plants.
Indeed, such a debate is important and urgent as it concerns our current ecologically inappropriate behaviour towards plant life, where plants are treated 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.