What! Plants can hear water???

Traditionally, humans have five main senses – hearing, touch, taste, smell, and sight. Ever-curious about the senses of other organisms, there has been much ‘debate’ amongst human beings about the sensory capabilities of plants. Although we don’t have the space to go into in-depth analysis of this subject here, we are happy to add to the discussion by alerting readers to Monica Gagliano et al’s ‘hydro-phytoacoustics’ study of plant roots (and its supporting video here).

Arguably controversially entitled “Tuned in: plant roots use sound to locate water”, this research article proposes that roots of Pisum sativum (garden pea) were able to locate a water source by sensing the vibrations generated by water moving inside pipes. Since this happened in the absence of substrate moisture, it would appear to rule out involvement of the plant roots’ hydrotropism response. Interestingly, when presented with both moisture and acoustic cues, the roots preferentially responded to the moisture in the soil over acoustic vibrations. This behaviour is interpreted as indicating that roots use acoustic gradients broadly to detect water sources at a distance, but exploit moisture gradients to home-in on a specific source.

This is an intriguing bit of plant behaviour that apparently using two senses – hearing * and taste [of the water] – in the battle to acquire that most precious of resources. But, although this seems to work well in single plant experiments, how might it work in the mixed-species environment outside in the wide world? Are there differences in water-hearing abilities between plant species which might give the ‘more-acute-of-hearing’ the edge when it comes to detecting and reaching water sources before competing species? Fascinating work, and which highlights a more general concern about sound in the natural environment as the authors also argue for more research into the role of sound in biology and ecology more generally. In particular, they raise concerns about the contribution that noise pollution might make to organisms’ – plants and animals – ability to respond appropriately to their surrounding ‘soundscape’ **.

But, until this pea-root behaviour is demonstrated in Arabidopsis, will sceptics consider this just to be a one-off curiosity rather than a widespread legitimate plant behavioural phenomenon..? If you prefer not to dwell on this aspect of plant biology – which might make readers of a non-open-mind persuasion uncomfortable with its suggestion of ‘green intelligence’ – watch this space for new insights into the apparently less controversial phenomenon of hydrotropism [coming soon to a blog near you…].

* This isn’t the only example of a plant sense of hearing. Readers may also recall the work of Heidi Appel and Rex Cocraft where the sound of caterpillars chewing on plant material was sufficient to initiate a herbivory-defence response in otherwise undamaged Arabidopsis plants. And the phenomenon of buzz pollination is also an example of a plant sense of hearing…

** For more on the ecological effects of noise pollution, you could listen to David Biello’s 60-second podcast at Scientific American, which summarises the research of Clinton Francis et al. on the acoustic consequences on local flora and fauna of gas wells in New Mexico (USA). Do also read the Commentary on this article by Alvin Chan and Daniel Blumstein.