One of the most cited papers ever in Annals of Botany is about an amazing nanotechnology that plants have ‘built in’. Don’t you get frustrated with every surface getting coated with dust and grime — my sticky keyboard is annoying me at this moment, I know the car needs washing, and it’s not just the dull weather that stops me seeing through the windows, while somebody thought my just cleaned and polished bicycle (Dawes Karakum serial J20400801 should you come across it) looked so nice last week that they decided to cut the lock and steal it from the front of the Biology Department.
So the null hypothesis is that “plants have daily servants that come out at night and polish them” so they can catch the sun and don’t get grimy, sticky leaves covered with nasty fungal spores and bacteria. Stand in a city street, or look out from a third-floor hotel room as I did at #Solo10 Science Online last week, and the leaves of the London plane trees will be as fresh as they were when they first opened six months ago. The null hypothesis is wrong!
Neinhuis and Barthlott pioneered the study of the mechanisms of the non-wettability of plant leaf surfaces, and their classic paper in Annals of Botany (free PDF doi: 10.1006/anbo.1997.0400) shows how the surface features exploit the surface tension of water to be not only non-wettable, but also self-cleaning, so that rain water removes from the leaf surface any dust, spores or other deposits with very high efficiency.
Both the Annals authors, Christoph Neinhuis and Wilhelm Barthlott continue their research work on self-cleaning properties of plants. Wilhelm Barthlott took out the trade-name Lotus-Effect® for the self-cleaning super-hydrophobic micro –to nano-structured products, copyrighting the phrase in 1997, and since then has been developing a portfolio of patents. In the last year, as the company website shows, a range of products from roofing tiles to wall surface paints. More recently, Barthlott and his colleagues published a paper in the journal Advanced Materials
showing that ships coated with a leaf-like surface coating that traps air might use 10% less fuel — see http://www.lotus-effekt.de/en/news/Salvinia.pdf for the press release.
For visual demonstrations of the phenomenon, just look at youtube using this link or searching for ‘Lotus effect’
Like ducks and sheep, detergents do overcome the water repellent properties of plants, and organic solvents with low surface tension also wet the whole surface. But unlike the animals, washing or rainfall can restore the surface — although the waxes and oils have some role, the physical surface characters are much more important, and do not require secretions like the animals waxes and oils. Of course, it also means that the plants are more resistant to pollution, whether on city streets in the air, or oil in the water, compared to animals.
Research on plant leaf surfaces is a regular feature in Annals of Botany, although there has been little about the water repellent properties recently. In fact, Uwe Winkler and Gerhard Zotz had an article in the July 2010 issue discussing not repellency but attraction in “‘And then there were three’: highly efficient uptake of potassium by foliar trichomes of epiphytic bromeliads Ann Bot (2010) 106(3): 421–427. Other recent papers discuss the roles of leaf surfaces in water relations — uptake and regulation of water in the plant — and, of course, in photosynthesis. I’ll note here that my own first submission to Annals of Botany’s sister Journal AoB Plants wiil probably be a paper showing the contrasting leaf surface waxes in a range of different banana (Musa) genotypes — a good example of leaves with water repellency, but also where accessions vary in water use and foliar disease susceptibility, properties that do relate to the surfaces.