17 April 2012

Plants respond to sounds, and may talk

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Plants may be able to respond to sounds and even make clicking noises to communicate with each other, according to new research.
tree plant communication sound

PHILADELPHIA: Plants may be able to respond to sounds and even make clicking noises to communicate with each other, according to new research.

If initial results can be confirmed and replicated, the latest findings could change our understanding of how plants sense their environment and communicate with each other. It would also overturn years of scepticism that plants can ‘talk’ in this way and give lie to the idea that animals are the only organisms capable of such feats.

“There is a lot of historical baggage attached to these sorts of questions but we are now able to test them properly, and we have produced initial evidence to support the idea that plants can produce, perceive and change their behaviour in response to sound,” said Monica Gagliano, plant scientist at the University of Western Australia in Perth and co-author of a recent review paper in Trends in Plant Science. “We’re proposing that sound would be a good way of mediating the behaviours that we see in plants.”

Plant communications

Plant communications have long interested scientists. In the last 20 years researchers established that plants are able to detect, react to and even communicate using chemical signals. Physiological responses such as flowering, fruit ripening, germination and shoot or root development can all be mediated by chemical signals. Plants can even produce volatile chemicals to communicate with each other when in danger from an approaching herbivore, for example.

Over the years some people also suggested that plants might be capable of using acoustic signals for similar purposes. But such claims have always been treated with great suspicion, largely because there was no evidence but also because they were seen as ‘hippy pseudoscience’. This discouraged scientists from properly investigating the ideas for fear of damaging their reputations, said Gagliano. Technological limitations also held back such research, so the ideas remained largely untested.

Gagliano and colleagues decided to take a fresh look at the question and carried out a series of experiments at the University of Bristol in Britain, using highly sensitive instruments normally used for testing the acoustic abilities of insect antennae. The team now presented some of their results in an attempt to generate interest in the subject and encourage further research.

Signalling with sounds

The researchers showed that the young roots of corn plants suspended in water make “loud and frequent” clicking noises. They also found that when subjected to sounds at frequencies of 220Hz – within the range that the roots themselves emitted – the roots responded by bending towards the source of that sound. This is the first rigorous experimental evidence of a plant’s ability to produce, detect and respond to acoustic cues, said Gagliano. “Those roots would normally be growing down with gravity at that stage in their development, so the fact that they change to bend towards the sound source is very interesting.”

The team also addressed the question of why plants would have evolved such abilities. They point out that from an evolutionary perspective, the perception and processing of sounds would be advantageous because it provides crucial information about the surrounding environment. What’s more, sounds transmits easily through soil, so it would be an effective way to signal across short ranges, like that required for interactions between plants competing for the same resources within the soil.

Worth following up

Although the team presented initial evidence, they point out that there are huge gaps in our understanding of the acoustic sensory and communication abilities of plants. “The next step is to see how plants differ in their abilities to emit and receive sounds,” says Gagliano. “Then we need to find out what information is encoded in those sounds. So we’ll look at if they can communicate warning signals as they do with chemicals or if they might use acoustic cues to arrange themselves spatially.”

Richard Karban, who studies plant communication at the University of California Davis, USA, commented that the proposals were interesting but pointed out that, at this stage, evidence was still thin on the ground. “There is not a whole lot there so I don’t know that I would say it’s convincing but it’s worth following up for sure,” he commented. “The evidence presented references an unpublished thesis, so we don’t really get to look at the experimental design, to see whether this was replicated, to see if it’s credible.”

“If this is a real phenomenon, and it is something that other people can repeat, the results have the potential to be really exciting and novel. It would mean that a realm [of communication] that we imagine to be limited to animals would be something that plants are capable of as well.”

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