Measurement methods and water droplet sizes used to calculate leaf contact angles.
Leaf wettability, indicating the affinity for water on leaf surfaces, is a common phenomenon for plants in a wide variety of habitats. The contact angle (θ) of water on leaves measured at the gas, solid and liquid interface is an index of surface wettability. Leaves are termed as “super-hydrophilic” if θ < 40°, “highly wettable” if θ < 90°, and “wettable” if θ < 110°. If θ > 110°, the leaves are classified as being non-wettable, while θ > 130° for highly non-wettable and θ > 150° for super-hydrophobic. Both internal and external factors can influence leaf wettability. The chemical composition and structure of leaf surfaces are internal causes, but the external environment can also influence wettability by affecting the structure and composition of the surface. The main internal factors that affecting leaf wettability include the content and microstructure of the epidermal wax, the number, size and pattern of trichomes, stomatal density, the shape of epidermal cells, and leaf water status. The leaf contact angles increased with the increasing of leaf wax content. However, studies have shown that the contact angles were more dependent on the complexity of wax structure than on the absolute amount. For trichomes, there are three types of interaction between trichomes and water droplets, including (1) low trichomes density: no apparent influence of trichomes on the location of surface moisture, droplet formation and retention ; (2) medium trichomes density: trichomes appear to circle surface moisture into patches; (3) high trichomes density: trichomes appear to hold water droplets above the trichomes. In some cases, a higher stomatal density was accompanied with a higher contact angles. While, it was also observed that there was no significant correlation between contact angle and stomatal density for some species. For the effects of epidermal cells on leaf wettability, it was generally considered that the combination of a dense layer of surface wax and the convex epidermal cells was what created a hydrophobic leaf surface. However, the influence of leaf water content on contact angle of water droplets on different leaf surfaces was complex, e.g., contact angles increased with decreasing of leaf water content, contact angle remained to be constant with different leaf water content.
- Leaf surface
- wax crystal
- ecological significances
Leaves are covered by a layer of cuticular wax, serving to decrease surface wetting and moisture loss. The epicuticular wax layer may be classified into two main types: a thin wax film that appears to be ubiquitous and a highly crystalline epicuticular wax consisting of wax crystals that is not present on all species [1, 2]. These outer layers control the wetting of leaves, which have been studied by many researchers in recent years because of the “lotus effect” (i.e., the self-cleaning properties that are a result of very high water repellence, as exhibited by the leaves of the lotus flower) [3–7].
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together . The degree of wetting is determined by a force balance between adhesive and cohesive forces, which is often characterized by contact angle of water measured at the gas, solid, and liquid interface [9–13]. Contact angle gives an inverse measurement of adhesion between a liquid and a solid. A lower contact angle indicates the liquid will spread over a larger area of the surface. A greater contact angle indicates the liquid will minimize contact with the surface and form a more spherical water droplet.
In a natural environment, leaf surfaces of a large variety of plants are frequently wetted by rainfall, dewfall, ground fog, and cloud mist, and both adaxial and abaxial surfaces are frequently affected. Depending on the tissue hygroscopicity, it may consist of individual drops, or of water films of thickness between a few nanometers and a few micrometers . Several studies have shown that the contact angles between leaf surfaces and water droplets range from 0° to 180°, depending on plant species [3, 4, 9, 10, 12–25]. The differences in physical and chemical properties of leaf surfaces possibly lead to the different contact angles of leaves. These surface properties include the number and pattern of trichomes [12–14], the three-dimensional microstructures of epicuticular cells [3, 4, 15], the microstructures and compositions of wax crystals/films [3, 4, 16–18], and the number and distribution of stomata [12, 13, 19].
Wettability is a comprehensive response at the solid, gas, and liquid phase interface of leaves and it significantly affects physiological and ecological functions of plants. For example, leaf wettability influences pollutant deposition such as acid rain [20, 26, 27], ozone [20, 21], and particulate matter [4, 18, 28, 29], for foliar nutrient leaching , in the control of plant disease [23, 30–36], for plant photosynthesis and yield [24, 37, 38], and in the interception of precipitation [25, 39–42]. Recent advances in the area of the wetting of leaf surfaces and its ecological significances are reviewed at present. In Section 2, we discuss the contact angles on ideal and rough surfaces (the Young equation, Wenzel model, Cassie–Baxter model, and Cassie–Baxter to Wenzel transition), the classification of leaf surface wetting, and the methodologies used to measure leaf contact angles. In Section 3, we discuss how the wax content and structure; the number, shape, and pattern of trichomes; the stomatal density; the shape of epidermal cells; and the leaf water status, affect leaf wettability. In Section 4, we discuss the influence of leaf wettability on rainfall interception, photosynthesis rate, pathogen infection, and environmental quality.
2. Criteria and measurement of leaf wettability
2.1. Surface wetting and contact angle
2.1.1. The Young equation
A droplet on a solid surface wets the surface to a certain degree. To what extent a surface gets wet can be described by the contact angle. Contact angle is defined as the angle formed by a liquid at the three-phase boundary where the liquid, gas, and solid intersect (Figure 1). The contact angle directly provides information on three interfacial free energies involved: solid–gas/vapor (
2.1.2. Wenzel model
Young’s equation applies strictly to an ideal surface. However, most leaf surfaces in nature are not perfect smooth, rigidity, or chemical homogeneity because of leaf surface properties [3, 4, 12–18, 45, 46]. Wenzel  modified Young’s equation to account for contact angles formed on rough surfaces (
The roughness factor is defined as the actual contact area to apparent contact area, and acts as an amplification of the effect of the surface chemistry, i.e., smaller changes in
2.1.3. Cassie–Baxter model
Wenzel’s equation accounts for the surface roughness. It describes the homogeneous wetting regime in which water fills the roughness grooves on the surface, as seen in Figure 2a. However, it does not describe contact angle hysteresis that occurs on heterogeneous surfaces . Contact angle hysteresis is defined as the difference between the advancing contact angle (i.e., the contact angle at the advancing edge of a liquid drop,
Eq. (4) implies that with a small
In particular, Wenzel model applies in such case that the leaf surfaces with intermediate hydrophobic and intermediate hydrophilic characteristics, which remain dry beyond the water droplets. If the texture of surfaces is wettable, a film develops in the texture and the droplet sits upon a mixture of solid and liquid, consisting a composite surface (Figure 4) . In this case, the contact angle between the liquid is 0°, and the equation can be written as
2.1.4. Cassie–Baxter to Wenzel transition
Some researchers studied the stability of the composite interface of superhydrophobic surfaces and the transition from composite to homogeneous interface under pressure [52, 53] and vertical vibration . The intermediate state between the Wenzel and the Cassie modes is shown in Figure 5. The penetration condition is given by
2.2. Classification of leaf surface wetting
The larger the contact angle is, the more repellent a leaf surface shows (Figure 6). According to Aryal et al.  and Wang et al. , the judgment criteria for leaf wettability were as follows: if
2.3. Measurement of leaf contact angle
The measuring methods of leaf contact angle include the static sessile drop method, the dynamic sessile drop method, the dynamic Wilhelmy method, and the Washburn equation capillary rise method [13–17, 28–32, 37–38, 56–78]. The sessile drop method is the most common method, which is measured by a contact angle goniometer using an optical subsystem to capture the profile of a pure liquid on a solid substrate. Older systems used a microscope optical system with a back light. Current-generation systems employ high-resolution cameras and software to capture and analyze the contact angle . Water droplet sizes vary between studies from 0.2 to 15 µl (Table 1). Letellier et al.  considered that the contact angle is dependent on not only the nature and structure of the substrate but also the size of the drops. In their study, they found that if interface is a surface and with a thermodynamic dimension (
|Digital camera and image-processing software||7.5||5||52–122||Belgium||17|
|Microscope and image-processing software||Not reported||1||64.7–138.0||Polish||56|
|Digital camera and geometric analysis||0.2||1||50–85||Japan||57|
|Binocular microscope with protractor graticule||1||1||72–115||UK||58|
|Stereomicroscopic photo and image-processing software||5||18||109–136||United States||30|
|Stereomicroscopic photo and image analysis||5||38||<15–>170||United States||59|
|Digital camera and image-processing software||10||36||40.33–144.25||Guatemala and United States||9|
|Digital camera and image-processing software||10||36||40.3–144.3||Guatemala and United States||60|
|Digital camera and image-processing software||10||5||44.68–77.92||Brazil||61|
|Digital camera and image-processing software||10||33||<20–>150||Guatemala and United States||62|
|Automated tension meter||2||1||85–120||Greece||16|
|Digital camera and image-processing software||7.5||1||51.2–97.7||Belgium||37|
|Microprojector and image analysis||2 mm diameter||1||120–150||Australia||64|
|Drop shape analyzer or digital camera and image-processing software||5||227||50–145||Nepal||55|
|Not reported||1.5 mm diameter||52||39.9–136.1||New Zealand||65|
|Geometric analysis with inspection microscope||5||3||85–105||United States||21|
|Geometric analysis with inspection microscope||5||37||0–180||Argentina||12|
|Bench microscope with a protractor graticule||Not reported||1||101–108||Italy||66|
|Geometric analysis with inspection microscope||2||1||75–97.2||India||67|
|Not reported||5||1||United States||38|
|Geometric analysis with inspection microscope||5||50||23.8–180||United States||68|
|Digital camera and image analysis||0.2||1||60–120||Japan||69|
|Digital camera and image analysis||1–42 mm diameter||1||15–100||Sweden||70|
|Goniometer||2–3 mm diameter||200||117–164||Germany||3|
|Goniometer and image-processing software||500 µm diameter||50||<50–141||Germany||71|
|Geometric analysis with inspection microscope||5||5||71–130||United States||72|
|Goniometer and image-processing software||2 or 6||3||67.1–135.9||China||18|
|Goniometer and image-processing software||5||5||43–146||China||73|
|Goniometer and image-processing software||6||21||42.3–134.7||China||13|
|Goniometer and image-processing software||6||21||41.5–136.0||China||28|
|Goniometer and image-processing software||6||18||47.6–142.7||China||29|
|Not reported||10||33||Not reported||Germany||15|
|Microprojector and image analysis||3 mm diameter||7||29–152||UK||76|
3. Main factors influencing leaf wettability
3.1. Leaf wax
Leaves of higher plants are covered by a cuticle consisting of a cutin matrix with waxes embedded in and deposited on the surface of the matrix [80, 81]. The cutin fraction is a polyester-type biopolymer composed of hydroxyl- and hydroxyepoxy fatty acids, whereas the cuticular waxes are a complex mixture of long-chain aliphatic and cyclic compounds . The major compound classes of plant cuticular waxes are
Studies have shown that wax content had distinct effects on leaf wettability [13, 19, 45, 88]. A study conducted by Koch et al.  showed that the contact angle increased with the increasing of leaf wax content for the three investigated species (
Since leaf contact angle was more dependent on the wax structure, recent papers have highlighted the importance of wax structure on leaf wettability [4, 18, 24, 25, 39, 45, 46, 65, 73‒75]. Neinhuis and Barthlott  found that the leaves of
Barthlott et al.  observed surface micromorphology of at least 13,000 species, representing all major groups of seed plants. In total, 23 wax types are classified, for example, granule, platelet, plate, rodlet, thread, and tubule. For water repellency, a classical example is the lotus leaf, which has a very superhydrophobic surface (contact angle over 150°). The SEM images (Figure 7a, b) showed that the surface of the lotus leaf comprises randomly distributed, almost hemispherically topped papillae with sizes 5–10 µm (height to basal radius aspect ratio ~1) decorated with branchlike protrusions with sizes of about 150 nm [46, 90, 91]. Many elliptic protrusions with an average diameter of about 10 µm were uniformly distributed in the nestlike caves, forming a microstructure on taro leaf (
Trichomes, also known as hairs, are fine outgrowths or appendages on plants, which are of diverse structure (e.g., puberulent, hispid, strigose, villous, pilose, strigillose, tomentose, pubescent, downy, and articulate) and function. Plant hairs may be unicellular or multicellular, branched or unbranched. Hairs on plants are extremely variable in their presence across species and even within a species, such as their location on plant organs, size, density. Trichomes can reflect the sunlight, absorb water and nutrients, and reduce transpiration. Besides, trichomes affect the leaf wettability, which has been analyzed in some investigations [12–14, 19, 38, 68].
Some papers have demonstrated that different trichome density and structure may result in different leaf wettability [3, 59, 67, 68, 92]. In a study conducted in the center Rocky Mountain (USA), a positive correlation was observed between contact angle and trichome density for 50 subalpine/montane species . Brewer et al.  and Pandey and Nagar  considered that the leaves with trichomes were more water repellent, especially where trichome density was greater than 25/mm2, at which they may develop trichome canopy. The adaxial surface of leaves of
Three types of interaction between trichomes and water droplets were evident [38, 68]. On one group of leaf surfaces (Figure 8a), trichomes appeared to have no influence on the location of surface moisture, droplet formation, or retention. In this group, the trichome density was relatively low, and usually a film of water formed on the leaf surface. The leaves of
3.3. Stomatal density
Stomatal density and aperture (length of stomata) vary under a number of environmental factors such as atmospheric CO2 concentration, light intensity, air temperature, photoperiod (daytime duration), and pollutants [85, 95–98]. Previous studies have shown that the contact angle on abaxial surface was higher than that on adaxial surface [9, 12, 13, 65, 68], which was consistent with the stomatal distribution on leaf surfaces. The results of Brewer and Nuñez  showed that the surface with the greatest concentration of stomata was the least wettable. In a study conducted in Xi’an by Wang et al. , they found that a higher stomatal density was accompanied with a higher contact angle. However, the relationship between leaf contact angle and stomatal density was not shown a simple linear relationship. The studies of Juniper and Jeffree , Brewer and Smith , and Kumar et al.  indicated that no significant correlation was observed between contact angle and stomatal density.
3.4. Epidermal cells
A gradient in the cell shape directly related to the contact angle of leaf surfaces was observed in some studies conducted by Wagner et al.  (Figure 10), Haines et al. , Neinhuis and Barthlott , Wang et al. , and Wang et al. . Wang et al.  found that the leaves with convex epidermal cells with wax crystals, for example,
3.5. Leaf water status
Leaf water status of plants is also a key factor in the wetting of leaf surfaces. Quantitative studies on the wetting by water of the exterior surfaces of leaves of
4. Ecological significance of leaf wettability
4.1. Interception of precipitation
Rainfall interception of forest areas is an important hydrological process that alters the quantity, timing, and distribution of water input and output on a catchment. On leaf level, leaf surface characteristics contribute to variability in interception between different plant species, resulting in different geometrical shapes of water on leaves (i.e., water film, patches, drops, and spherical droplets) . Wang et al.  investigated leaf water drop adhesion of 60 plant species from Shaanxi, northwest China. The adhesion of water droplets to leaves covered a wide range of area, from 4.09 to 88.87 g/m2 on adaxial surfaces and 0.72 to 93.35 g/m2 on abaxial surfaces. The combined values for adaxial and abaxial surfaces in a single species ranged from 5.67 to 159.59 g/m2. Wilson et al.  found that the leaf maximum water storage capacity of potato was 150 g/m2. They also reported that leaf water accumulation in the upper position of the canopy always exceeded that in the lower canopy, and clumping caused less water accumulation in the upper canopy and greater accumulation in the lower half compared to the random case. Tanakamaru et al.  compared leaf water retention by young and old leaves of
4.2. Photosynthetic rate
The effect of leaf surface wettability on plant photosynthesis stems from the fact that diffusion of CO2 is 10,000 times slower in water than in air [9, 12, 24, 38, 72, 101]. Surface wetness induces different changes in leaf photosynthesis among species because leaf surface wettability varied greatly (i.e., from being covered almost completely by water to being water repellent) [24, 68]. For alpine and subalpine plants, natural dew depressed assimilation by 77% in species having wettable leaves, whereas assimilation was stimulated by 14% in species having nonwettable leaves . For bean and pea, a 22% stimulated assimilation rate was obtained for nonwettable pea leaves in the 72-h mist-treated artificial surface, but the wettable bean leaves were on the contrary, which decreased 28%. They postulated that the photosynthetic responses to wetness are due to the change in stomatal regulation . The results of Brewer and Smith  indicated that surface wetting, either from natural events or spraying irrigation might lead to significant reduction in CO2 exchange and growth potential in agricultural species. Leaf surface wetness caused the greatest decline in photosynthesis for the surfaces with the lowest contact angle, which was due to the fact that water physically blocked stomatal pores.
4.3. Pathogen infection
Different plant species vary widely in pathogen infection, perhaps due to leaf micromorphology, surface chemistry, and degree of leaf wetness [19, 81, 102–104]. Water droplets on the leaf surface can be an important source of water for pathogen infection. Excess leaf wetness promotes pathogen infection in many native and agricultural species . Kuo and Hoch  found that pycnidiospores of
4.4. Environmental quality
It is universally accepted that trees and other vegetation are effective at trapping and absorbing many pollutants, such as particulate matters, CO, NO2, and SO2 [17–18, 56, 105–108], and they can act as biological absorbers or filters of pollutants [18, 105–107]. Leaf as the multifunctional interface between plants and environment is also continuously exposed to high levels of varieties of air pollutants. Air pollutants may cause plants both acute and chronic damages on anatomical and morphological characteristics [17–18, 56, 109, 110], leading variations in leaf wettability. Therefore, leaf wettability is potentially a good indicator to point out differences in urban habitat quality [18, 56]. Adams and Hutchinson  investigated the ability of four species (cabbage:
Leaf surfaces represent the key interfaces between plants and their environment, which influence the biodiversity and biomass, nutrient and water balance, biogeochemical cycle, and productivity of ecosystems. Leaf surface wettability, indicating the affinity for water on the leaf surface, is a common phenomenon for plants in a wide variety of habitats, which directly affect leaf photosynthesis, canopy interception, pathogen infection, and environmental quality. Many studies concentrated on the differences in leaf surface wettability and its relation with leaf microstructure. Leaf surface wettability has been considered to be of great theoretical and academic importance, and studies focused on the ecological significances of leaf wettability should be encouraged in the future.
This work was supported by the National Natural Science Foundation of China (grant no. 41230852).