Leaf Surface Wettability Research Articles

Influence of Multiple Factors on the Wettability and Surface Free Energy of Leaf Surface

The wettability of plant leaves directly reflects leaf hydrophilicity, which is the key factor that influences the adhesion of liquid pesticide as well as affects plant protection products (PPP) efficacy. Generally, the wettability of leaf surface is quantified by the contact angle and surface free energy (SFE), which are mainly dependent on leaf surface properties, liquid properties and other spraying parameters. Therefore, the aim of this paper was to investigate the SFE of rice and rape leaves with the variation of leaf status, leaf surface, and probe liquid as well as the influence of droplet falling height, solid surface, and PPP concentration on the wettability. The results showed that: (1) the dispersive components of SFE of rice and rape account for a large proportion which are closely related to their hydrophobicity—the abaxial of rape new leaf and the adaxial of rape old leaf are easier to wet comparing with rice and rape leaves in other statuses; (2) the increase of droplet falling height had a significant effect on improving the wettability between wax surface and adjuvant solution, while it had little improving effect on the wettability between wax surface and water; (3) the wettability of different solid surface varied greatly, and the order of wettability from good to bad is water-sensitive paper (WSP), wax, rape leaf, and rice leaf; (4) the effect of PPP concentration on the leaf surface wettability is significant, the contact angle decreased with the increase of PPP concentration, and the wettability of microemulsion is better than that of suspending agent and wettable powder. In conclusion, the SFE and wettability of crop leaf surface determine the suitable type of PPP, studying the influence of multiple factors on leaf surface wettability can provide a reliable reference for providing scientific guidance as well as improving the effective utilization of PPP.

Wettability of pear leaves from three regions characterized at different stages after flowering using the OWRK method.

A better understanding of leaf surface wettability is critical to improve the adhesion of liquid pesticides. Leaf surface wettability is dependent on the property of the liquid as well as the physical and chemical properties of the leaf, which vary with climate and growth stage. The aim of this study was to characterize the wettability of pear leaves from three different climatic regions at different stages after flowering. The contact angles of different test liquids were measured on both adaxial and abaxial pear leaf surfaces and the Owens-Wendt-Rabel-Kaelble (OWRK) method was used to calculate surface free energy (SFE) and its polar and non-polar components. The results demonstrated that the SFE of both the adaxial and abaxial surface of the pear leaf, and the proportion of polar component, increased with increasing time after flowering. At early growth stages, pear leaves were highly hydrophobic, similar to a polytetrafluoroethylene surface, whereas at later growth stages, pear leaves were hydrophobic, more similar to a polymethylmethacrylate surface. Also, the SFE differed with climatic region. Factors influencing these changes are discussed. Changes in contact angles and SFE correlated with the change of the leaf surface wettability. Leaves became easier to wet (higher SFE), with an overall increasing polar component to the surface, with increasing age after flowering. As expected, changes in wettability were found in pear leaves at different stages after flowering and in different regions (P < 0.05). Pear leaves from Yuanping were easier to wet than leaves from Yuci and Linyi, and adaxial surfaces were easier to wet than abaxial surfaces. These results provide beneficial information for the application of agrochemicals for improved wetting and spreading behavior. © 2018 Society of Chemical Industry.

Leaf physico-chemical and physiological properties of maize (Zea mays L.) populations from different origins

In this study we evaluated the leaf surface properties of maize populations native to different water availability environments. Leaf surface topography, wettability and gas exchange performance of five maize populations from the Sahara desert, dry (south) and humid (north-western) areas of Spain were analysed. Differences in wettability, stomatal and trichome densities, surface free energy and solubility parameter values were recorded between populations and leaf sides. Leaves from the humid Spanish population with special regard to the abaxial side, were less wettable and less susceptible to polar interactions. The higher wettability and hydrophilicity of Sahara populations with emphasis on the abaxial leaf surfaces, may favour dew deposition and foliar water absorption, hence improving water use efficiency under extremely dry conditions. Compared to the other Saharan populations, the dwarf one had a higher photosynthesis rate suggesting that dwarfism may be a strategy for improving plant tolerance to arid conditions. The results obtained for different maize populations suggest that leaf surfaces may vary in response to drought, but further studies will be required to examine the potential relationship between leaf surface properties and plant stress tolerance.

Effects of the natural microstructures on the wettability of leaf surfaces

The effects of natural microstructures on the wettability are investigated based on the systematic analysis on the contact angles and morphology of the leaf surfaces of four kinds of plants, Photinia serrulata, Ginkgo, Aloe vera and Hypericum monogynum. P. serrulata possesses the most wettable leaf surface due to the small corrugation and raised boundary of the microstructures, while H. monogynum leaf shows the largest contact angle as it exhibits corrugated microstructures with smaller pitch value and larger height compared with that of Aloe vera. The long-shaped and well aligned microstructures, which are beneficial for the diffusion of water, make the Ginkgo leaf surface to be hydrophilic. The study elaborates the effects of microstructures on the surface wettability, which shed light on the design of surfaces for different wettable needs.

Comparison of leaf surface roughness analysis methods by sensitivity to noise analysis

Surface roughness is of great interest in agricultural spraying because it is used to characterise leaf surface wettability to predict the behaviour of droplets on a leaf surface. In recent years, the use of texture analysis to estimate surface roughness has emerged. In this paper we propose to estimate leaf surface roughness by using an optimisation of the Generalized Fourier Descriptors method. This approach is then compared with two other standard methods in the literature, one based on grey level intensity variation and the other on wavelet decomposition. Since roughness has many definitions and each method is calculated differently, we propose a new approach to compare the results based on the sensitivity of each method according to surface roughness variations. These variations were introduced by adding different kinds of noise to the image. Gaussian and salt & pepper noise are added to simulate rapid changes and peak impulses on the surface topography, whereas a Structural noise (sinusoidal signal) is added to simulate depth on the surface topography. The novelty of this contribution is the use of a new approach and procedure for agronomic application (leaf surface roughness). The results obtained are expected to be used to characterise the adhesion mechanisms of liquid droplets on a leaf surface.

Leaf surface wettability and fatty acid composition of Arbutus unedo and Arbutus andrachne grown under ambient conditions in a natural macchia

Features of the adaxial and abaxial surface microstructure of Arbutus unedo L. and Arbutus andrachne L. are evaluated as possible consistent parameters contributing to the wetness of leaves. The abaxial leaf surface of A. andrachne and A. unedo was determined to be more hydrophobic than the adaxial leaf surface. Hydrophobicity may be of particular importance for the ecophysiological status of the hypostomatic leaves of both Arbutus species, which exhibit a long lifespan and are exposed to various environmental stimuli. Water repellence may also be correlated to the increased presence of surface wax. Lipid analysis of the leaves of both plant species revealed an abundance of α-linolenic acid, with palmitic acid as the second major contributor, followed by linoleic and oleic acid. Oleic and linoleic acid were present in slightly larger percentages in A. andrachne compared with A. unedo, whereas myristic, palmitic and α-linolenic acids were found in elevated percentages in A. unedo. The fatty acid composition analysis of the leaf wax of A. andrachne and A. unedo ranged from C16 to C26, with fatty acids of an even longer chain length detected in the case of A. andrachne. Despite the similar fatty acid composition of total lipids, the composition of the wax fraction showed differences between the two Arbutus species, which may partly contribute to the foliar surface properties of the two species.

Research on the changes in wettability of rice (Oryza sativa.) leaf surfaces at different development stages using the OWRK method

A good knowledge in wetting behavior of pesticide spray liquid on plant surface is crucial to spray applications. Difference in leaf surface wettability would result in obvious changes in spray wetting behavior. The aim of this paper is to obtain the changes of wettability during different growth periods. The contact angle (CA) of rice leaf for each liquid increased with rice growth. No significant difference was found between cultivars. The CA was found to be correlated with the polar component of liquid surface tension. The square of the polar component was also found to be highly significant indicating that the relationship between these two properties was not a simple linear one. The surface energy of each plant surface decreased as the plants aged. This was also true of each part of the surface energy. However, no obvious difference on the proportion of the components was found among different cultivars and stages. The changes in value of CA and surface free energy (SFE) both reflect the changes of the leaf surface wettability, while the SFE value shows better in wettability characterizing. Obvious rice leaf wettability changes were found on different development stages, which may be beneficial for researches in agrochemical sprays wetting and spreading behavior. Factors influencing these alterations were discussed.

Seasonal variations in leaf capturing of particulate matter, surface wettability and micromorphology in urban tree species

The seasonal changes in leaf particulate matter (PM) accumulation, surface wettability and micromorphology in urban tree species, including Sophora japonica (S. japonica), Platanus acerifolia (P. acerifolia) and Cedrus deodara (C. deodara), were studied during a single growing season. The three species showed distinct seasonal trends in PM accumulation, increasing from spring to autumn (or winter) even during the rainy season, but at different rates. During the study, the leaf PM retention amount of P. acerifolia, a species with ridged leaf surfaces, was significantly higher than that of S. japonica and C. deodara, species with waxy leaf surfaces. The contact angles of water droplets on leaves decreased with leaf age except on the abaxial surface of S. japonica, which remained non-wettable or highly non-wettable throughout the growing season; the decrease in the contact angle on adaxial surface of S. japonica was greater when compared with P. acerifolia and C. deodara. A significant and negative relationship existed between leaf PM retention amounts and contact angles on adaxial surface of leaves of all three species. The increase in wettability, probably caused when epicuticular wax was destroyed by mechanical and chemical abrasion, seemed to be the main factor leading to seasonal variations in leaf PM accumulation.

Leaf Surface Wettability and Implications for Drop Shedding and Evaporation from Forest Canopies

Wettability and retention capacity of leaf surfaces are parameters that contribute to interception of rain, fog or dew by forest canopies. Contrary to common expectation, hydrophobicity or wettability of a leaf do not dictate the stickiness of drops to leaves. Crucial for the adhesion of drops is the contact angle hysteresis, the difference between leading edge contact angle and trailing edge contact angle for a running drop. Other parameters that are dependent on the static contact angle are the maximum volume of drops that can stick to the surface and the persistence of an adhering drop with respect to evaporation. Adaption of contact angle and contact angle hysteresis allow one to pursue different strategies of drop control, for example efficient water shedding or maximum retention of adhering water. Efficient water shedding is achieved if contact angle hysteresis is low. Retention of (isolated) large drops requires a high contact angle hysteresis and a static contact angle of 65.5°, while maximum retention by optimum spacing of drops necessitates a high contact angle hysteresis and a static contact angle of 111.6°. Maximum persistence with respect to evaporation is obtained if the static contact angle amounts to 77.5°, together with a high contact angle hysteresis. It is to be expected that knowledge of these parameters can contribute to the capacity of a forest to intercept water.

Wettability of soybean (Glycine max L.) leaves by foliar sprays with respect to developmental changes

Leaf wettability considerably defines the degree of retention of water and agrochemical sprays on crop and non-target plant surfaces. Plant surface structure varies with development therefore the goal was to characterise the wettability of soybean leaf surfaces as a function of growth stage (GS). Adaxial surfaces of leaves developed at GS 16 (BBCH) were 10 times more wettable with water than leaves at the lower canopy (GS 13). By measuring contact angles of a liquid having an intermediate surface tension on different leaf patches, an illustrative wetting profile was elucidated, showing to what degree wetting varies (from > 120° to < 20°) depending on leaf patch and GS. While the critical surface tension of leaf surfaces at different GSs did not correlate with the observed changes, the slope of the Zisman plot accurately represented the increase in wettability of leaves at the upper canopy and lateral shoots (GSs 17 to 19, 21 and 24). The discrimination given by the slopes was even better than that by water contact angles. SEM observations revealed that the low wettability observed at early GSs is mainly due to a dense layer of epicuticular wax crystals. The Zisman plot slope does not represent the changes in leaf roughness (i.e. epicuticular wax deposition), but provides an insight into chemical and compositional surface characteristics at the droplet-leaf interface. The results with different wettability measurement methods demonstrated that wetting is a feature that characterises each developmental stage of soybean leaves. Positional wettability differences among leaves at the same plant and within the same leaf are relevant for performance, selectivity and plant compatibility of agrochemicals. Implications are discussed.

Surface morphology and chemical composition of lamb’s lettuce ( Valerianella locusta) after exposure to a low-pressure oxygen plasma

The influence of a low-pressure oxygen glow discharge plasma on secondary metabolites of lamb’s lettuce was examined. After extraction and hydrolysis the compounds were analysed by means of RP-HPLC. Whereas pure compounds showed a time-dependent degradation (flavonoids) or remained unchanged (phenolic acids), the exposure of lamb’s lettuce to the oxygen plasma led to a significant increase of protocatechuic acid, luteolin, and disometin after 120 s treatment time, independent of the applied plasma driving voltage. Contact angle (CA) measurements show an increased wettability of adaxial leaf surfaces after plasma exposure. A successive degradation of epicuticular waxes and cutin of the plant’s epidermis has been indicated by means of FTIR (ATR) and scanning electron microscopy (SEM). These findings suggest that reactive species in the plasma play a crucial role for the observed changes upon exposure to non-thermal plasma.

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 (theta), 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 (theta < 90 degrees). Temperate leaves were non-wettable (110 degrees < theta < 130 degrees). Subalpine and alpine leaves were highly non-wettable (130 degrees < theta < 150 degrees) 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 degrees ; higher values were linked to high non-wettability (theta > 130 degrees) which was more often observed at high altitude. Good droplet run off at +/-10 degrees inclination was found in highly wettable leaves (theta < 90 degrees) 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.

Influences of air humidity during the cultivation of plants on wax chemical composition, morphology and leaf surface wettability

Changes of cuticular wax morphology, chemistry, and wettability of Brassica oleracea (Brassicaceae), Eucalyptus gunnii (Myrtaceae) and Tropaeolum majus (Tropaeolaceae), with different types of surface waxes, cultivated at three levels of relative air humidities (RH) (a: 20–30%, b: 40–75% and c: 98% RH) have been investigated. Wax chemistry was investigated by gas chromatography (GC) and mass spectroscopy (MS), micro morphology by scanning electron microscopy (SEM) and surface wettability by measuring the contact angle. Grown at 98% RH, all species showed decreased total wax mass and wax crystal density but significantly increased leaf surface wettability. In contrast cultivation at 20–30% RH led to increased total amounts of wax and reduced surface wettability. SEM investigations and GC/MS analysis revealed that wax crystal morphology and qualitative wax composition of Brassica were altered after cultivation at 98% RH, whereas chemical composition and morphology of wax crystals in Tropaeolum and Eucalyptus remained unchanged.

Effect of leaf surface wetness and wettability on photosynthesis in bean and pea

ABSTRACTLeaf surface wetness that occurs frequently in natural environments has a significant impact on leaf photosynthesis. However, the physiological mechanisms for the photosynthetic responses to wetness are not well understood. The responses of leaf CO2 assimilation rate (A) to 72 h of artificial mist of a wettable (bean; Phaseolus vulgaris) and a non‐wettable species (pea; Pisum sativum) were compared. Stomatal and non‐stomatal limitations to A were investigated. A 28% inhibition of A was observed in the bean leaves as a result of a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco. The decrease of Rubisco was mainly due to its partial degradation. In contrast to the bean leaves, a 22% stimulation of A was obtained in the 72 h mist‐treated pea leaves. Mist treatment increased stomatal conductance by 12.5% and had no effect on the amount of Rubisco. These results indicated that a positive photosynthetic response to wetness occurred only in non‐wettable species and is due to the change in stomatal regulation.

Patterns of leaf surface wetness in some important medicinal and aromatic plants of Western Himalaya

The present study aims at investigating the role of leaf morphological features and their relation to leaf surface wettability for important medicinal and aromatic plants of Western Himalaya. The surface features related to leaf wettability were studied in 30 plant species representing 21 different families growing under open and shade conditions. The leaf surfaces with the highest density of trichomes and stomata per unit area were found to be the least wettable, regardless of condition type. Most of the species of both conditions were hypostomatic, and per unit area concentration of stomata contributes more than stomatal size to stomatal area index. Leaf surfaces of open condition species were more water repellent with higher stomatal density, and had lower water droplet retention than shade species. It is suggested that leaf morphological features (stomata and trichome) had a strong influence in reducing the leaf area with surface moisture, which could be correlated with the frequency and duration of leaf wettability in a given condition.

Differences in Weed Tolerance to Glyphosate Involve Different Mechanisms1

The cause of differential susceptibility of barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida to glyphosate was examined by measuring the absorption of 14C-glyphosate, quantifying the amount of epicuticular wax, and observing the wettability of leaf surfaces. In greenhouse experiments, the biomass of barnyardgrass and prickly sida was reduced by 95% by Roundup Ultra®. Hemp sesbania and pitted morningglory showed more tolerance, with 66 and 51% average biomass reduction, respectively. Absorption of 14C-glyphosate in a controlled environment did not follow the trend in species susceptibility with barnyardgrass, 30%; prickly sida, 18%; hemp sesbania, 52%; and pitted morningglory, 6%; absorption. The high tolerance of pitted morningglory to glyphosate can be attributed mostly to limited absorption, but the tolerance of hemp sesbania is due to other mechanisms. The addition of nonionic surfactant (NIS) to a low rate of Roundup Ultra® reduced absorption of 14C-glyphosate by barnyardgrass and h...

"Inert" formulation ingredients with activity: toxicity of trisiloxane surfactant solutions to twospotted spider mites (Acari: Tetranychidae).

Organosilicone molecules are important surfactant ingredients used in formulating pesticides. These methylated silicones are considered inert ingredients, but their superior surfactant properties allow them to wet, and either suffocate or disrupt important physiological processes in mites and insects. Aqueous solutions of the tri-siloxane surfactants Silwet L-77, Silwet 408, and Silwet 806 were bioassayed against adult female two-spotted spider mites, Tetranychus urticae Koch, with leaf dip methods to compare their toxicity with organosilicone molecules containing bulkier hydrophobic components. All three tri-siloxanes in aqueous solutions were equivalently toxic (LC50 = 5.5-8.9 ppm), whereas Silwet L-7607 solutions were less toxic (LC50 = 4,800 ppm) and Silwet L-7200 was nontoxic to mites. In another experiment, the toxicity of Silwet L-77 was affected by the wettability of leaf surfaces. The LC50 shifted from 22 to 84 ppm when mites were tested on bean and strawberry leaf disks, respectively. Droplet spreading on paraffin and surface tension were both related to the toxicity of surfactant solutions. Surface tensions of solutions below 23 mN/m caused > 90% mite mortality in leaf dip bioassays. A field test of Conserve SC and its formulation blank, with and without Dyne-Amic adjuvant (a vegetable oil-organosilicone surfactant mixture) revealed that Dyne-Amic had the greatest miticidal contribution, reducing mite populations by 70%, followed by formulation inactive ingredients. Spinosad, the listed active ingredient in Conserve, only contributed miticidal activity when synergized by Dyne-Amic. Researchers should include appropriate surfactant or formulation blank controls when testing insecticides or miticides, especially when using high spray volumes.

Effect of Rainfall Exposure on Leaf Wettability in Near-Isogenic Barley Lines with Different Leaf Wax Content.

Water retention, contact angle of water droplets, and amount of epicuticular wax on leaf surfaces were examined in two lines of near-isogenic barley with normal and glossy leaves. In addition, the effect of rainfall on the wettability of leaf surfaces was examined. Mean water retention of leaves from the 1st to 5th node was 5.6mg/cm2 in normal leaf lines and 12.8mg/cm2 in glossy leaf lines. The mean contact angle of water drops was 125° in normal leaf and 93° in glossy leaf. The mean epicuticular wax content was 17.3μg/cm2 in normal leaf and 9.3μg/cm2 in glossy leaf. In glossy leaves, contact angles decreased with increasing duration of rainfall treatment. After treatment for 5 days, the angle was about 70% of that without rainfall treatment. Reduction of epicuticular wax amount by rainfall exposure occurred in all leaf positions of both barley lines. The reduction rates in the normal leaf line were 10-20%. In the glossy leaf line, the reduction rates were 15% in the 1st leaves and about 50% in the 2nd-3rd leaves.

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.

Patterns of leaf surface wetness for montane and subalpine plants

ABSTRACTThe frequency and duration of water on leaf surfaces have important consequences for plant growth and photosynthetic gas exchange. The objective of the present study was to compare the frequency and duration of leaf wetness under natural field conditions among species and to identify variation in structural features of leaves that may reduce surface wetness. During June–September 1992 in the central Rocky Mountains (USA), natural leaf wetting due to rain and dewfall was observed on 79 of 89 nights in open meadow habitats compared to only 29 of 89 nights in the understorey. Dew formation occurred at relative humidities that were often well below 100% because of radiational heat exchange with cold night skies and low wind speeds (&lt; 0.5 m s−1). A survey of 50 subalpine/montane species showed that structural characteristics associated with the occurrence and duration of leaf surface wetness differed among species and habitats. Both adaxial and abaxial surfaces accumulated moisture during rain and dewfall events. Leaf surfaces of open‐meadow species were less wettable (P= 0.008), and had lower droplet retention (P= 0.015) and more stomata P= 0.017) than adjacent understorey species. Also, leaf trichomes reduced the area of leaf surface covered by moisture. Ecophysiological importance is suggested by the high frequency of leaf wetting events in open microsites, influences on growth and gas exchange, and correspondence between leaf surface wettability and habitat.