Plant–Microbe Interactions: Wetting of Ivy (Hedera helix L.) Leaf Surfaces in Relation to Colonization by Epiphytic Microorganisms

Abstract Leaf wettability, cuticular wax composition, and microbial colonization of upper and lower leaf surfaces of ivy (Hedera helix L.) was investigated for young and old leaves sampled in June and September. Contact angles of aqueous buffered solutions measured on young leaf surfaces ranged between 76° and 86° and were not dependent on the pH value of the applied droplets. Contact angles measured on old leaf surfaces were up to 32°, significantly lower than on young leaf surfaces. Furthermore, contact angles were significantly lower using aqueous solutions of pH 9.0 compared to pH 3.0, indicating the influence of ionizable functional groups on leaf surface wetting properties. Observed changes in leaf wetting properties did not correlate with different levels of alkanoic acids in cuticular waxes. However, microscopic examination of the leaf surfaces indicated the influence of epiphytic microorganisms on wetting properties of old leaves, since their surfaces were always colonized by epiphytic microorganisms (filamentous fungi, yeasts, and bacteria), whereas surfaces of young leaves were basically clean. In order to analyze the effect of epiphytic microorganisms on leaf surface wetting, surfaces of young and clean ivy leaves were artificially colonized with Pseudomonas fluorescens. This resulted in a significant increase and a pH dependence of leaf surface wetting in the same way as it was observed on old ivy leaf surfaces. From these results it can be deduced that the native wetting properties of leaf surfaces can be significantly masked by the presence of epiphytic microorganisms. The ecological implications of altered wetting properties for microorganisms using the leaf/atmosphere interface as habitat are discussed. Received: 20 March 1999; Accepted: 5 July 1999; Online Publication: 18 July 2000     

Host surface properties affect prepenetration processes in the barley powdery mildew fungus

The initial contact between Blumeria graminis f.sp. hordei and its host barley (Hordeum vulgare) takes place on epicuticular waxes at the surfaces of aerial plant organs. Here, the extent to which chemical composition, crystal structure and hydrophobicity of cuticular waxes affect fungal prepenetration processes was explored. The leaf surface properties of barley eceriferum (cer) wax mutants were characterized in detail. Barley leaves and artificial surfaces were used to investigate the early events of fungal infection. Even after epicuticular waxes had been stripped away, cer mutant leaf surfaces did not affect fungal prepenetration properties. Removal of total leaf cuticular waxes, however, resulted in a 20% reduction in conidial germination and differentiation. Two major components of barley leaf wax, hexacosanol and hexacosanal, differed considerably in their ability to effectively trigger conidial differentiation on glass surfaces. While hexacosanol, attaining a maximum hydrophobicity with contact angles of no more than 80 degrees, proved to be noninductive, hexacosanal significantly stimulated differentiation in c. 50% of B. graminis conidia, but only at contact angles > 80 degrees. These results, together with an observed inductive effect of highly hydrophobic, wax-free artificial surfaces, provide new insights into the interplay of physical and chemical surface cues involved in triggering prepenetration processes in B. graminis.     

Characterization of the Topography and Wettability of English Weed Leaves and Biomimetic Replicas

The topography and wettability of the underside of English weed （Oxalis pes-caprae） leaves and of their biomimetic replicas are investigated. Polyvinyl siloxane molds were cast from the leaves and then filled with an epoxy pre-polymer to produce replicas. The particular topographical structures of leaves and replicas were evaluated by optical microscopy and Scanning Electron Microscopy （SEM） analysis. The static wettability of leaves and replicas was assessed by contact angle measurements, while the dynamic wettability was characterized by estimating contact angle hysteresis and studying the dynamic behavior of impacting water droplets. A smooth glass slip and its replica were used as control surfaces. The replica moulding method used was able to transfer the characteristic pattern of irregular 100 μm - 200 μm × 60 μm convex papillae interspersed with stomata of the original leaf to the epoxy replicas. The static contact angle of 143°± 3° and the contact angle hysteresis of 2~ indicate that the underside of the English weed leaf is close to superhydrophobic. The lower contact angles （130° ± 4°） and higher hysteresis （31°） observed for the replica when compared with the original leaves were associated to an inaccurate replication of the chemistry and structures of the three-dimensional wax projections covering the plant surface. Also, trichomes in the original leaves could not be accurately reproduced due to their flexibility and fragility. Differences in wetting behavior were also evident from droplet impact experiments, with rebound regimes prevailing in the original leaves and regimes characterized by higher adhesion and larger dissipation predominating in the replicas. Nevertheless, the morphological features of the leaf transferred to the replica were sufficient to promote a clear hydrophobic behavior of the replica when compared with the smooth epoxy reference surface.     

Die Wasserbenetzbarkeit von Blattoberflächen und ihre submikroskopische Wachsstruktur

Summary The wettability of a leaf surface is defined by the contact angle between a water-droplet and the surface of the leaf.Contact angles of 60–80° were measured on easily wettable leaves. These leaves have no wax on the outer cuticular layer.Contact angles of 130–160° were measured on leaves with a low wettability. These leaves have wax on the outer cuticular layer, which shows submicroscopic structures characteristic of the particular plants.A comparison of the wettability and the different structures of the wax showed no true distinctions.The wax does not adhere strongly to the outer cuticular layer and is cast off in the case of old leaves. Therefore the wetability alters with the age of the leaves. If the wax on young leaves is destroyed or removed by outer influences it can be produced again within a few hours.The rebuilt structure of the wax is not always similar to the original one. When the wax was dissolved by organic solvents no new wax formation was observed.Cigarette smoke greatly increases the wettability of leaves of Tropaeolum majus, parochetus communis and Chelidonium majus without causing a visible alteration of the submicroscopic structure. In this case the leaves recover their water-repellency within a few hours, but not if they were treated with Diesel smoke.When the wax structure was destroyed by fungus or Aleurodina no new wax formation was observed.

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Herrn Professor Dr. Walter Rentschler zum 60. Geburtstag     

西安市常见绿化植物叶片润湿性能及其影响因素

利用接触角测定仪测定了西安市21种常见绿化植物叶片表面的接触角,探讨了叶片表面特性如蜡质、绒毛、气孔对接触角的影响。结果表明,植物叶片正背面、物种间的接触角差异均显著,叶片正面和背面接触角大小在40°～140°。接触角大小与变异系数呈负相关,可能由于接触角小的润湿叶片在不同的生境和位置下,受到环境条件的影响较大而出现大的变异;接触角较大的非润湿性叶片,环境物质持留时间较短,对叶片形态和组成影响较小,因而出现小的变异。植物叶片表面的接触角随蜡质含量的升高而增大。表皮蜡质去除后大部分叶片接触角明显降低,尤其是疏水性较强的银杏(Ginkgo biloba)、月季(Ro-sa chinensis)和紫叶小檗(Berberis thunbergii)。女贞(Ligustrum lucidum)正背面、加杨(Popu-lus canadensis)背面等亲水型的叶片蜡质去除后接触角反而增大。叶片绒毛的多少及其形态、分布方式对接触角具有重要的影响,不同的作用方式表现出润湿和不润湿的特征,人为将其去除可以增加叶片的润湿性。背面气孔密度与气孔长度、保卫细胞长度呈负相关;接触角则与气孔密度呈负相关,与气孔长度呈正相关。     

Seasonal changes of leaf surface contamination in beech, oak, and ginkgo in relation to leaf micromorphology and wettability

The leaf surfaces of beech, oak and ginkgo have been investigated with respect to contamination with particles during one growing season. Based on the observation that particles are removed from water-repellent leaves by rain (Lotus effect) the three species were selected because they differ in leaf surface micromorphology and wettability. Leaves of beech are smooth, lacked wax crystals and were ±wettable. Those of ginkgo were rough because their cells were convex and were densely covered by wax crystals, resulting in permanent water repellency. Leaves of oak were covered by waxes and were water repellent when young, but, a few weeks after leaf expansion had ceased the waxes were rapidly eroded. These differences in wettability resulted in different amounts of contamination. Ginkgo collected a very small number of particles during the whole vegetation period. In beech the contamination was significantly higher, but fairly constant, whereas oak leaves accumulated particles with age.     

Wetting of the upper needle surface of Abies grandis: influence of pH, wax chemistry and epiphyllic microflora on contact angles

Wetting of the upper needle surface of Abies grandis Lindl. by aqueous solutions of different pH values was investigated. With increasing needle age, contact angles decreased significantly from about 75° on current-year needles to values lower than 30° on 4-year-old needles. On older needles, contact angles were significantly lower, by more than 10°, when aqueous solutions of pH9-0 were used compared with those of pH3-0. On the surfaces of older needles, contact angle titrations were carried out, contact angles being measured with aqueous solutions covering a pH range from 3.0 to 11.0. Measured titration curves showed clear inflection points around pH 7.0, indicating the existence of ionizable carboxylie groups in the interface between needle surface and atmosphere. The evidence seems convincing that the pronounced pH dependence of wetting is mainly due to the presence and/or activity of epiphyllic micro-organisms, whereas the cuticular wax composition of Abies grandis needles does not appear to contribute significantly to this phenomenon. Thus, the results presented here allow the general conclusion that changes of contact angles measured on leaf surfaces may not always be due to changes in the leaf surface chemistry and/or the fine structure of leaf surface waxes, but may also be due to increased amounts of epiphyllic micro-organisms significantly altering the leaf surface wetting properties.     

Effect of alterations in cuticular wax biosynthesis on the physicochemical properties and topography of maize leaf surfaces

The leaf surface properties of 11 cuticular wax mutants of maize were characterized, and this information was used to identify the quantitative relations among distinct leaf surface traits. Compared with the wild‐type maize, these mutants were reduced 3–24% in their leaf surface hydrophobicity, 20–88% in the mass of cuticular waxes on their leaves, and 52–94% in the percentage of planar leaf surface area covered with epicuticular crystalline waxes. They also differed in the presence and abundance of the epicuticular crystalline waxes in each of seven structural classes. With the exception of one mutant, the mass of cuticular waxes produced by these mutants was positively correlated with the number of epicuticular crystalline waxes per unit area on their leaves. Furthermore, an increase of 0·4 mg of cuticular wax per gram of leaf (dry weight) was associated with a 1% increase in leaf surface area covered by epicuticular crystalline waxes, and this 1% increase was associated with a 2° increase in the contact angle of a water droplet on the leaf surface. Linear differences in the leaf surface hydrophobicity were associated with exponential differences in the mass of the cuticular waxes produced. Quantitative knowledge of these leaf surface properties is highly relevant to the interactions of leaves with environmental factors such as microbes, insects, agricultural chemicals, and pollutants.     

Epicuticular wax structures on stems and comparison between stems and leaves – A survey

The cuticle, forming the outermost layer of plant tissues and being in direct contact with the environment, consists of waxes and cutin. Waxes are hydrophobic substances that are divided in two groups: intra- and epicuticular, depending on their localisation. Epicuticular waxes appear as smooth coverings, however, many plants also produce superimposed wax structures of a crystalline nature. While studies of waxes have almost exclusively focused on leaves, here a survey of epicuticular wax structures on stems is presented. The stem surface of 343 higher plant taxa, representing 80 families, was examined using scanning electron microscopy. The adaxial and abaxial surfaces of leaves of 319 taxa were also examined to determine the relationship between wax structures on stems and leaves. Wax structures are classified, described and discussed. The results of the study indicate that stems exhibit the same main wax crystal types that have been described for leaves. Seventy percent of the examined taxa produced wax crystals on their stems. In ∼24% of the taxa, wax crystals were absent on leaves and found only on stems. In plant taxa that produce wax crystals, 40% exhibit the same type on either side of their leaves and on their stem. However, a much stronger morphological similarity exists between crystal shapes present on the adaxial and abaxial surfaces of leaves than between those present on the stem and those on leaves. In general, these observations suggest that stems are quite different than leaves in terms of their epicuticular wax structures.     

广州市常见行道树种叶片表面形态与滞尘能力

以广州市常见的18种行道树为对象,通过扫描电镜观察比较了行道树的叶表面形态结构、应用接触角测定仪测定了绿化树种叶片的接触角对滞尘能力的影响.结果表明:不同树种的滞尘量差异显著,18种植物叶片雨后第26天的最大滞尘量在0.066-1.831 g/m2,物种间相差达27倍以上.叶表面具有网状结构,气孔密度较大(20＜气孔密度＜60个)且气孔开口较大(如芒果)容易滞留粉尘;叶表面平滑具有蜡质层,气孔排列整齐,无明显起伏(如红花羊蹄甲、桃花心木、大叶紫薇、鹅掌藤),滞尘能力较弱.植物叶片接触角与滞尘量呈负相关(r=-0.614),接触角＜90°的表现为亲水性.易润湿的植物叶片雨后第26天最大滞尘量在1.0-1.831 g/m2,叶片表面的形态结构凹凸不平,具有钩状或脊状褶皱、突起等且20＜气孔密度＜60范围内,测得的接触角较小(芒果、重阳木、高山榕),使得粉尘与植物叶片接触面积较大,粉尘不易从叶面脱落,滞尘能力较强.而接触角较大的盆架树、麻楝、大叶紫薇、鹅掌藤和红花羊蹄甲的滞尘量均＜1.0g/m2,其特殊的表面结构和疏水的蜡质使颗粒物不易吸附在植物叶片上,因此滞尘能力较弱.由此可见,植物叶表面蜡质含量和气孔密度及其叶片接触角的大小是影响植物叶片滞尘能力的主要因素,在进行城市绿化时,适当考虑选择叶表面形态有利于滞尘的绿化树种,将可提高城市植被的环境效应.     

Cuticular wax load and surface wettability of leaves and fruits collected from sweet cherry (<Emphasis Type="Italic">Prunus avium</Emphasis>) trees grown under field conditions or inside a polytunnel

In this work, the effect of microclimate at the tree level inside a polytunnel and in an uncovered orchard of Prunus avium was studied through the observation of leaf and fruit micromorphology and the accumulation of cuticular waxes. Records of environmental parameters showed a significantly higher daytime temperature in the polytunnel (on an average 4°C), whereas the night temperature did not differ between treatments. Furthermore, photosynthetic active radiation and UV-B radiation inside the polytunnel were 43 and 97% lower, respectively, than the radiation measured outside. Leaves grown in the polytunnel had lower contact angles and higher load of cuticular wax per unit area, irrespective of cultivar. The impact of the microclimate was observed at the first sampling and did not increase with exposure time of leaves. Fruits of the cultivars ‘Souvenir’ and ‘Prime Giant’ grown outside had significantly less cuticular wax despite their delayed harvest time of 11 and 5 days, respectively. The relationship of higher cuticular wax load and lower contact angles of protected leaves might have practical importance for plant protection activities and the retention and uptake of leaf-applied agrochemicals.     

Ontogenetic variation in chemical and physical characteristics of adaxial apple leaf surfaces

The reaction of plants to environmental factors often varies with developmental stage. It was hypothesized, that also the cuticle, the outer surface layer of plants is modified during ontogenesis. Apple plantlets, cv. Golden Delicious, were grown under controlled conditions avoiding biotic and abiotic stress factors. The cuticular wax surface of adaxial apple leaves was analyzed for its chemical composition as well as for its micromorphology and hydrophobicity just after unfolding of leaves ending in the seventh leaf insertion. The outer surface of apple leaves was formed by a thin amorphous layer of epicuticular waxes. Epidermal cells of young leaves exhibited a distinctive curvature of the periclinal cell walls resulting in an undulated surface of the cuticle including pronounced lamellae, with the highest density at the centre of cells. As epidermal cells expanded during ontogenesis, the upper surface showed only minor surface sculpturing and a decrease in lamellae. With increasing leaf age the hydrophobicity of adaxial leaf side decreased significantly indicated by a decrease in contact angle. Extracted from plants, the amount of apolar cuticular wax per area unit ranged from only 0.9 microgcm(-2) for the oldest studied leaf to 1.5 microgcm(-2) for the youngest studied leaf. Differences in the total amount of cuticular waxes per leaf were not significant for older leaves. For young leaves, triterpenes (ursolic acid and oleanolic acid), esters and alcohols were the main wax components. During ontogenesis, the proportion of triterpenes in total mass of apolar waxes decreased from 32% (leaf 1) to 13% (leaf 7); absolute amounts decreased by more than 50%. The proportion of wax alcohols and esters, and alkanes to a lesser degree, increased with leaf age, whereas the proportion of acids decreased. The epicuticular wax layer also contained alpha-tocopherol described for the first time to be present also in the epicuticular wax. The modifications in the chemical composition of cuticular waxes are discussed in relation to the varying physical characteristics of the cuticle during ontogenesis of apple leaves.     

Attachment force of the beetle Cryptolaemus montrouzieri (Coleoptera, Coccinellidae) on leaflet surfaces of mutants of the pea Pisum sativum (Fabaceae) with regular and reduced wax coverage

This study represents an investigation of surface-related plant–insect interactions. Surface micro-morphology of leaflets in pea (Pisum sativum) with wild-type crystalline surface waxes (waxy) and with reduced crystalline surface waxes (glossy) caused by a mutation (wel) were studied using various microscopy techniques. The free surface energy of these plant surfaces was estimated using contact angles of droplets of three different liquids. The morphological study of the attachment system in the ladybird beetle Cryptolaemus montrouzieri was combined with measurements of attachment (traction) forces, generated by beetles on these plant substrates. Differences were found in wax crystal shape, dimensions, and density between the adaxial and abaxial surfaces of waxy and glossy plants. The crystalline wax was not completely eliminated in the glossy plant: it was only slightly reduced on the adaxial side and underwent greater changes on the abaxial side. The free surface energy for both surfaces of both pea types was rather low with strongly predominating dispersion component. Insects generated low traction forces on all intact plant surfaces studied, except the abaxial surface of the glossy plant, on which the force was greater. After being treated with chloroform, all the surfaces allowed much higher traction forces. It is demonstrated that the difference in the crystal length and density of the epicuticular wax coverage within the observed range did not influence wettability of surfaces, but affected insect attachment. The reduction in insect attachment force on plant surfaces, covered with the crystalline wax, is explained by the decrease of the real contact area between setal tips of beetles and the substrate. Handling editor: Lars Chittka.     

Contact Areas between Waxy Leaf Surfaces and Aqueous Solutions: Quantitative Determination of Specific Leaf Surface Contact Areas

The contact areas (on a molecular scale) between aqueous solutionsand leaf surfaces have been measured in order to obtain informationon the actual areas effective in cuticular transport. For thispurpose, monolayer adsorption of pentachlorophenol (PCP) wasused on leaf surfaces and on artificial surfaces. The adsorptionisotherms were analysed according to the Brunauer-Emmett andTeller (BET) formalism leading to BET constants and actual contactareas between the aqueous PCP solution and the surfaces. BETconstants ranged from 4 to 11, with an arithmetic mean of 6·5.No relationship was found between the chemical composition ofthe surface waxesand the BET constants, and there were no significantdifferences between BET constants measured using isolated waxesor intact leaves. This indicates that the surfaces of the waxesare made up mainly of methylene and methyl groups, while mostfunctional groups appear not to contribute significantly tosurface sorption of PCP. This conclusion is supported by theobservation that the BET constant measured on silanized glasshaving a pure CH_3–surface was 5·7, which was notsignificantly different from 6·5. Specific leaf surfacecontact areas calculated from the BET constant and the areaof an adsorbed PCP molecule ranged from 1·6 (Hordeumvulgare) to 137 (Abies koreana). This means that the areas ofcontact between aqueous solutions and surface waxes were 1·6to 137 times larger than the apparent surface areas, which mightbe the consequence of a fractal leaf surface morphology. A leafsurface contact area greater than unity can result only froma rough surface that is wetted by the solution. It is argued,that interspecific differences in the rates of cuticular permeationmay be largely due to differences in leaf surface contact areasand not to differences in cuticular permeabilities. Consequently,differences in the rates of cuticular permeation can be interpretedonly if the leaf surface contact area that is effective in transportis known. Key words: Cuticular transport, epicuticular wax, fractality, specific leaf surface contact area, plant cuticle     

Chemical composition of the epicuticular and intracuticular wax layers on adaxial sides of Rosa canina leaves

BACKGROUND AND AIMS: The waxy cuticle is the first point of contact for many herbivorous and pathogenic organisms on rose plants. Previous studies have reported the average composition of the combined wax extract from both sides of rose leaves. Recently, the compositions of the waxes on the adaxial and abaxial surfaces of Rosa canina leaves were determined separately. In this paper, a first report is made on the compositions of the epicuticular and intracuticular wax layers of Rosa canina leaves. The methods described enable the determination of which compounds are truly available at the surface for plant-organism interactions. METHODS: An adhesive was used to mechanically strip the epicuticular wax from the adaxial leaf surface and the removal was visually confirmed using scanning electron microscopy. After the epicuticular wax had been removed, the intracuticular wax was then isolated using standard chemical extraction. Gas chromatography, flame ionization detection and mass spectrometry were used to identify and quantify compounds in the separated wax mixtures. KEY RESULTS: The epicuticular wax contained higher concentrations of alkanes and alkyl esters but lower concentrations of primary alcohols and alkenols when compared to the intracuticular wax. In addition, the average chain lengths of these compound classes were higher in the epicuticular wax. Secondary alcohols were found only in the epicuticular layer while triterpenoids were restricted mainly to the intracuticular wax. CONCLUSIONS: A gradient exists between the composition of the epi- and intracuticular wax layers of Rosa canina leaves. This gradient may result from polarity differences, in part caused by differences in chain lengths. The outer wax layer accessible to the phyllosphere showed a unique composition of wax compounds. The ecological consequences from such a gradient may now be probed.     

The physico-chemical basis of leaf wettability in wheat

Summary Wild type wheat (Triticum aestivum L.) and three mutant lines that have reduced glaucousness on the flag leaf sheath have been examined for variations in glaucousness, contact angles, wax chemistry and wax morphology. On the sheath and culm, organs that are glaucous in the wild type, increasing glaucousness is correlated with increasing contact angles, an increasing proportion of -diketones plus hydroxy--diketones in the was and an increasing proportion of wax tubes. Organs that were non-glaucous in all four lines, namely both surfaces of the vegetative leaves and the adaxial surface of the flag leaf, had high contact angles, a dense covering of wax plates and waxes rich in primary alcohols but devoid of -diketones and hydroxy--diketones. The abaxial surface of the flag leaf was the most complex of the organ surfaces studied. In the wild type the glaucousness of the sheath continued onto this surface for 1–2 cm and this was correlated with the other characters studied as it was on the sheath. In the mutants, however, the tubes were absent. Flat ribbons of varying widths, a new wax structure in wheat, as well as various types of plates were found instead. These structures continued to the flag leaf tip and were also present on the abaxial surface of the wild type flag leaf. Changes in contact angle at the tip could not be correlated with the other measured parameters.     

A new technique for measurement of water permeability of stomatous cuticular membranes isolated from Hedera helix leaves

Transpiration of cuticular membranes isolated from the lower stomatous surface of Hedera helix (ivy) leaves was measured using a novel approach which allowed a distinction to be made between gas phase diffusion (through stomatal pores) and solid phase diffusion (transport through the polymer matrix membrane and cuticular waxes) of water molecules. This approach is based on the principle that the diffusivity of water vapour in the gas phase can be manipulated by using different gases (helium, nitrogen, or carbon dioxide) while diffusivity of water in the solid phase is not affected. This approach allowed the flow of water across stomatal pores ('stomatal transpiration') to be calculated separately from the flow across the cuticle (cuticular transpiration) on the stomatous leaf surface. As expected, water flux across the cuticle isolated from the astomatous leaf surface was not affected by the gas composition since there are no gas-filled pores. Resistance to flux of water through the solid cuticle on the stomatous leaf surface was about 11 times lower than cuticular resistance on the astomatous leaf surface, indicating pronounced differences in barrier properties between cuticles isolated from both leaf surfaces. In order to check whether this difference in resistance was due to different barrier properties of cuticular waxes on both leaf sides, mobility of 14C-labelled 2,4-dichlorophenoxy-butyric acid 14C-2,4-DB) in reconstituted cuticular wax isolated from both leaf surfaces was measured separately. However, mobility of 14C-2,4-DB in reconstituted wax isolated from the lower leaf surface was 2.6 times lower compared with the upper leaf side. The significantly higher permeability of the ivy cuticle on the lower stomatous leaf surface compared with the astomatous surface might result from lateral heterogeneity in permeability of the cuticle covering normal epidermal cells compared with the cuticle covering the stomatal cell surface.     

Leaf wettability decreases along an extreme altitudinal gradient

The duration and amount of water captured on leaves and its functional significance is highly varied. Leaf surface wettability influences water absorption, gas exchange, pathogen infection, nutrient leaching, contamination by pollutants, self-cleaning properties and in freezing environments the probability of extrinsic ice nucleation. To test the impact of environment on the development of leaf wettability, this functional trait was measured in 227 dominant plant species along an extreme altitudinal environment gradient (186–5,268 m) on the wet and dry slopes of the Nepalese Himalayas. Plants from the understorey and open places in woodlands were also compared. Leaf wettability was assessed by droplet contact angle (θ), retention and leaf inclination measurement. With increasing altitude leaf wettability decreased significantly parallel to the observed atmospheric temperature decrease (0.5 K/100 m). Leaves from non-freezing tropical and subtropical origins were highly wettable (θ < 90°). Temperate leaves were non-wettable (110° < θ < 130°). Subalpine and alpine leaves were highly non-wettable (130° < θ < 150°) and adaxial pubescence occurred more frequently. Leaves taken from the understorey were more wettable but had a better droplet run off than leaves sampled in open places. In the semi-arid northern slopes (temperate to alpine) of the Himalayas leaf wettability was decreased in comparison to the southern humid side. The majority of the leaves had a low droplet retention <20°; higher values were linked to high non-wettability (θ > 130°) which was more often observed at high altitude. Good droplet run off at ±10° inclination was found in highly wettable leaves (θ < 90°) of tropical and subtropical origin and on leaves from the forest understorey. Structural properties for low wettability are developed in cold and dry environments and open sites with frequent dew formation as it appears to be an important functional trait to prevent a number of the negative effects adhering surface water may have.     

Determination of diffusion coefficients of octadecanoic acid in isolated cuticular waxes and their relationship to cuticular water permeabilities

Transport properties of cuticular waxes from 40 different plant species were investigated by measuring desorption rates of ¹⁴C-labelled octadecanoic acid from isolated and subsequently reconstituted wax. Diffusion coefficients (D) of octadecanoic acid in reconstituted waxes, calculated from the slopes of the regression lines fitted to the linearized portions of desorption kinetics, ranged from 1.2 × 10^?19 m² s^?1 (Senecio kleinia leaf) to 2.9 × 10^?17 m² s^?1 (Malus cf. domestica fruit). Cuticular water permeabilities (cuticular transpiration) measured with intact cuticular membranes isolated from 24 different species varied from 1.7 × 10^?11 m s^?1 (Vanilla planifolia leaf) up to 2.1 × 10^?9 m s^?1 (Malus cf. domestica fruit), thus covering a range of more than 2 orders of magnitude. Cuticular water permeabilities were highly correlated with diffusion coefficients of octadecanoic acid in isolated cuticular wax of the same species. It is therefore possible to estimate cuticular barrier properties of stomatous leaf surfaces or of leaves where isolation of the cuticle is impossible by measuring D of octadecanoic acid in isolated waxes of these leaves.     

Effect of nanostructure on wettability on copper surface: a molecular dynamic study

Molecular dynamics simulations (MDS) are employed to investigate the effects of interatomic interaction and nanostructure on wettability of water on a copper plate. In the nano scale, these simulation results showed that the contact angle gradually increases with the decreasing of the reaction parameters, which results in the decreasing of free energy on the solid-liquid interface. Therefore, it leads to that the hydrophilic material is turned into hydrophobic, which fits the results that the wettability is changed by low surface energy materials in macro scale. Furthermore, the contact angles on smooth and rough surfaces are 87° and 71.6°, respectively. That is to say that the hydrophilic will increase for hydrophilic material due to the existence of one-layer structure; it agrees with the experimental results in macro scale.