Nanotechnology and self-cleaning from plant leaf surfaces

Self-cleaning nanotechnology from plant leaf surfaces comes of age
Water repellent leaf surfaces of St John's Wort, Hypericum, with convex epidermal cells and dense epicuticular waxes
Water repellent leaf surfaces of St John's Wort, Hypericum, with convex epidermal cells and dense epicuticular waxes

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

Water on a plant leaf - Advanced Materials journal cover
Water on a plant leaf - Advanced Materials journal cover

showing that ships coated with a leaf-like surface coating that traps air might use 10% less fuel – see for the press release.

For visual demonstrations of the phenomenon, just look at youtube using this link or searching for ‘Lotus effect’

Lotus leaf surface. Every cell has a papilla, and water droplets float on these with as little as 0.6% of their surface area in contact.
Lotus leaf surface. Every cell has a papilla, and water droplets float on these with as little as 0.6% of their surface area in contact.

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.

  • Neinhuis and Barthlott plump for self-cleaning as a function of water repellent leaf surfaces. But could this function be incidental to some more important functions of water-repellency? In the case of many aquatic plants, including Nelumbo (lotus) and Salvinia, water-repellent surfaces form an external air-bag when submerged by waves to float the leaf surface back to the surface with already-dry stomatal pores. For land plants there may be another important role for water repellent leaf surfaces: reduction in foliar leaching. If the leaf cannot be wetted, then solutes cannot be washed out. Eucalyptus, characteristically growing in poor soils, has vertically-oriented waxy water-repellent leaves. Is self-cleaning more important than nutrient retention for Eucalyptus?

    The problem of ascribing function to structure has dogged studies of other leaf characters. In an obscure paper in an obscure journal and with the narrow background as a taxonomist I once tried to explain the ecological function of toothed leaf margins (Wood, D. 1970 The role of marginal hydathodes in foliar water absorption. Transactions of the Botanical Society of Edinburgh 41, 61-64). This opened a can of worms. Once you start thinking toothed leaves are adapted to absorb water then you start to look at other leaf characters in a different light. What about patent hairs? These hold water by capillarity of the leaf surface, as do channels over veins. What about surface glands? We know these are absorptive in bromeliads, but what about all the other species with non-secretory glandular hairs? I was a taxonomist working of tropical forest herbs in the genus Chirita in the Gesneriaceae. Species had foliar glands with a cap of cells. Each cap cell had pore in the cell wall beyond the capacity of my light microscope to distinguish. Yet in water, each cap cell exudes a ball of protoplasm through the pore, greatly increasing the area of the cell for water absorption and not constrained by the cell wall.

    I finally argued, with another jump of logic, that these tropical forest herbs, with a syndrome of water-absorbing characters I called ‘potomorphic’, were not constrained by water availability, but by access to nutrients. One source of nutrients was foliar leaching from the canopy (despite canopy leaves having thick cuticle to reduce foliar leaching). Testing this was well beyond my experimental ability. But it does explain a paradox. Why should tropical rain forest herbs often have hairy leaves (for example, the families Gesneriaceae and Melastomataceae)? Answer: to hold water containing canopy leachates and allow foliar absorption.

    I think we still have a long way to go beyond self-cleaning to explain the structure and function of leaf surfaces.

  • I agree entirely that ascribing function to structure is fraught, and easily ends up with anthropocentric absurdities. As you note, uptake of water and nutrients certainly is a property (function?) of leaves: herbicides work so well because leaves take them up. In my first-year undergraduate phylogenetics practical a couple of weeks ago (English oaks and Holm oak) one of the students looked at the toothed and lobed margins and asked me the impossible “why” question, so I am happy to know your answer – even in a paper with a DOI