Nanotechnology and self-cleaning from plant leaf surfaces

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  http://​www​.lotus​-effekt​.de/​e​n​/​n​e​w​s​/​S​a​l​v​i​n​i​a​.​pdf 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.

Editor Pat Heslop-Harrison. ORCID 0000-0002-3105-2167

Pat Heslop-Harrison is Professor of Molecular Cytogenetics and Cell Biology at the University of Leicester. He is also Chief Editor of Annals of Botany.

5 Responses

  1. Dave Wood says:

    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.

  2. 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 http://​dx​.doi​.org/​1​0​.​1​0​8​0​/​0​3​7​4​6​6​0​7​0​0​8​6​8​5​202