Leaf Wettability Research Articles

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.

Effects of evening and nighttime leaf wetting on stomatal behavior of Cryptomeria japonica growing in dry soil

To examine the hypothesis that stomatal behavior of plants in dry soil is influenced by a slow recovery from daytime water deficit, we studied the effect of repeated wetting of leaves during evening and night in Cryptomeria japonica seedlings grown in dry soil. After 7 and 10 days of leaf wetting treatment the midday leaf water potential decreased and the transpiration rate increased, respectively. Therefore, we suggest that rapid recovery from daytime water deficit could weaken the water conserving stomatal behavior that adapts to drought conditions in the roots.

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

Structures of the leaf surface waxes and the leaf wettability

Structures of the leaf surface waxes and the leaf wettability

Patterns of Leaf Wettability along an Extreme Moisture Gradient in Western Patagonia, Argentina

The objective of this study was to investigate patterns of surface features of leaves related to susceptibility to wetness for plants along a strong precipitation gradient. Leaf wettability and droplet retention were examined for leaves of 37 species (representing 28 families) occurring in steppe, ecotone, and temperate rain forest habitats along a steep moisture gradient in northwestern Patagonia, Argentina. Morphological and structural characteristics of leaves significantly affected leaf surface wetness, and these characteristics varied between habitats. Typically, leaves had more stomata on abaxial than adaxial leaf surfaces ($$P< 0.0001$$). Droplet retention and leaf wettability (θ) were negatively correlated (Spearman $$r=-0.694$$; $$P< 0.0001$$). Leaves in dry habitats tended to be less wettable and less likely to retain droplets on the leaf surface; however, overall differences were not significant. The presence of trichomes significantly reduced wettability ($$P< 0.05$$). The high frequency and natural variability of wetting events in these and a great variety of other habitats, coupled with the large range in surface wettability among plant species and the potentially strong effects on photosynthesis, growth, and pathogen infections, suggest an important evolutionary avenue related to the dynamics of water on leaf surfaces.

A surface microrelief on the leaves of Glossophyllum florinii (?Ginkgoales) from the Upper Triassic of Lunz, Austria

The cuticles of many extant seed plants display distinct surface microreliefs, which represent adaptations to certain habitat conditions or mechanical defences against herbivores and phytopathogenic microorganisms. Although microreliefs have variously been noted in fossil cuticles, hypotheses relating to the effectiveness of these structures in fossil plants have not been advanced to date. A surface microrelief composed of longitudinally orientated idiocuticular striae occurs on the leaves of the enigmatic Carnian (Late Triassic) gymnosperm (?ginkgophyte) Glossophyllum florinii Krausel from the Northern Calcareous Alps of lower Austria. Most striae originate from the tips of the papillae on the stomatal subsidiary cells. The G. florinii surface microrelief may have (1) reduced leaf wettability, (2) produced or enhanced the self-cleaning effect of the leaf, (3) prevented the formation of a water film on the leaf surface, and/or (4) mechanically stabilized the leaf. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 87–95.

Early Responses to Acute Ozone Exposure in Two Fagus Sylvatica Clones Differing in Xeromorphic Adaptations: Photosynthetic and Stomatal Processes, Membrane and Epicuticular Characteristics

Two Fagus sylvatica L. clones were used to investigate the early responses to acute O3 exposure (150 nL L(-1), i.e., 1.35x ambient hourly peak in rural Italy) and whether xeromorphic adaptations affect gas exchange, membrane, and epicuticular responses. One clone originated in a wet and temperate climate in Central Italy (Tuscany); the other clone originated in a warmer and drier climate in the southern-most part of the F. sylvatica distribution (Sicily). Because of higher base gas exchange rates, the most negative effects of O3 exposure (gas exchange impairment, uncoupling between net photosynthesis and stomatal conductance, increased membrane lipid peroxidation) were found in the southern clone. Xeromorphic adaptations (higher epicuticular waxes and stomatal density, lower leaf wettability and size) were found in this clone. Our results suggest that xeromorphism may increase O3 sensitivity in species not adapted to face water stress, like the mesophilic F. sylvatica, when experiments are carried out with full irrigation. We present evidence describing the relationship between gas exchange and number and status of stomata. Stomatal density and the structural damage to stomata resulting from O3 exposure did not affect gas exchange: In fact, non-stomatal limitations to photosynthesis prevailed over stomatal limitations.

Foliar Iron Fertilization of Peach (Prunus persica (L.) Batsch): Effects of Iron Compounds, Surfactants and Other Adjuvants

Experiments to assess the capability of different combinations of iron (Fe) compounds and adjuvants to provide Fe via foliar application to Fe-deficient plants have been carried out. A total of 80 formulations containing (1) one of five Fe-compounds [FeSO4·7H2O, Fe(III)-citrate, Fe(III)-Ethylenediaminetetraacetic acid (EDTA), Fe(III)-Diethylenetriamine pentaacetic acid (DTPA), Fe(III)-Iminodisuccinic acid (IDHA)], (2) a surfactant (Mistol, alkyl-polyglucoside1 or alkyl-polyglucoside2), and (3) an adjuvant (glycerol, methanol or glycine–betaine) were studied with respect to leaf wetting ability and surface tension. From the initial formulations only 26 resulted in adequate leaf wetting, 20 with alkyl-polyglucoside2 and 3 each with Mistol and alkyl-polyglucoside1, and some of them (four with alkyl-polyglucoside2, one with Mistol, and three with alkyl-polyglucoside1) were found to have inadequate surface tension values for use as foliar fertilizers. In a second experiment, 20 formulations containing alkyl-polyglucoside2 and one each of the five Fe-compounds and adjuvants listed above, were used for a foliar experiment with Fe-deficient peach trees [Prunus persica (L.) Batsch] grown under field conditions. Iron-deficient shoots were sprayed only once and leaf re-greening was assessed over 6 weeks for leaf chlorophyll content (via SPAD measurements) and percentage of green leaf area (via image analysis). Foliar Fe application always resulted in leaf Chl increases, although different degrees of re-greening were observed for the various Fe-compounds tested. Best results were obtained after treatment with formulations containing (in a decreasing order): Fe(II)-sulfate, Fe(III)-citrate, Fe(III)-EDTA, Fe(III)-IDHA, and Fe(III)-DTPA. A positive effect of adding glycerol, methanol or glycine–betaine was often observed, although the effect depended on each Fe-containing compound, indicating the existence of significant interactions between spray components. Results are of importance while trying to critically evaluate the potential of Fe sprays as a viable strategy to remedy plant Fe deficiency under field conditions.

Effect of environment on giant foxtail (Setaria faberi) leaf wax and fluazifop-P absorption

Controlled-environment experiments were conducted to determine giant foxtail epicuticular wax (ECW) deposition and fluazifop-P absorption under different environmental conditions and with two adjuvants. Drought stress and low temperature increased leaf ECW content, whereas low light intensity decreased ECW content compared with medium light intensity. Drought stress conditions decreased the fatty acid and primary alcohol content of ECW and increased the hydrocarbon content compared with field capacity. Compositional changes would make the ECW more hydrophobic and reduce leaf wetting by herbicide spray. Increasing air temperature decreased the aldehyde content of ECW, whereas decreasing light intensity increased ECW fatty acid and aldehyde content while decreasing primary alcohols and esters. Compositional changes under low light intensity would make the ECW more hydrophilic and increase leaf wetting by a herbicide spray. Drought stress reduced fluazifop-P absorption regardless of the temperature but could not further reduce fluazifop-P absorption under low light intensity. Fluazifop-P absorption by plants under low light and drought stress conditions was similar to plants under low or medium light intensity and field capacity conditions. Similarly, the rate of fluazifop-P absorption was less under drought stress and low light conditions. Fluazifop-P absorption was greater when crop oil concentrate was added compared with 28% urea ammonium nitrate or no additive. Crop oil concentrate, added to the herbicide solution, overcame reduced fluazifop-P absorption under the low light conditions and in one of the two drought stress regimes but could not overcome reduced fluazifop-P absorption with the high temperature regime.

Wettability of Poplar Leaves Influences Dew Formation and Infection by Melampsora larici-populina.

The wettability of leaves of 60 poplar (Populus spp.) clones from sections Aigeiros (black) and Tacamahaca (balsam) and their hybrids was assessed by measuring the contact angle of calibrated water droplets on their abaxial leaf surface. The frequency and duration of dew on these clones were recorded in two nurseries. Black poplar leaves had the highest wettability, with relatively flat-shaped water droplets and a short drying time, in both the laboratory and nursery. Conversely, round-shaped water droplets on balsam poplars dried more slowly in the laboratory. In the nursery, dew was more frequent and persisted longer. First-generation intersectional hybrids behaved like balsam poplars, but successive backcrosses with black poplar (P. deltoides) resulted in clones with leaves that were more wettable and possessed less frequent and persistent dew. Infection by Melampsora larici-populina on two hybrid clones ('Robusta' and 'Beaupré'), assessed by the number of uredinia in the laboratory, was dependent on the duration of leaf moisture. Maximum infection occurred when water was present on the leaves for 12 h. Leaf wettability should be considered as an additional trait when breeding poplar for durable resistance to M. larici-populina, as this characteristic is likely to be a useful defense against all pathotypes of the pathogen.

Simulating of leaf wetness duration within a potato canopy

A leaf wetness duration experiment was carried out in a potato field in the centre of the Netherlands during the growing season of 2003. A within-canopy dew simulation model was applied to simulate leaf wetness distribution in the canopy caused by dew and rainfall. The dew model is an extension of an earlier-developed energy budget model, distinguishing three layers within the potato canopy. To run the dew model successfully, information on the above-canopy wind speed, air temperature, humidity and net radiation as well as the within-canopy temperature and humidity must be available. In most cases leaf wetting starts in the top layer followed by the centre and the bottom layer, in that order. Leaf drying shortly after sunrise takes place in the same order. Leaf wetness lasted longest in the bottom layer. Rainfall was accounted for by applying an interception model. The results of the dew model agreed well with leaf wetness recorded with a resistance grid.

Susceptibility of clover species to fungal infection: the interaction of leaf surface traits and environment

Many foliar pathogens require free water to germinate; therefore, disease pressure should favor plants that are able to repel water. For a suite of 18 sympatric clover species (Trifolium and Medicago, Fabaceae), we evaluated leaf traits affecting leaf wetness and susceptibility to infection by the fungal pathogen Stemphylium sp., causal agent of Stemphylium leaf spot. Spore germination increased with time in free water, and the relative susceptibility of host plants to infection was proportional to the duration of water retention on leaves. Larger leaves captured more water and retained it longer. Unexpectedly, trichomes and leaf wettability did not affect water capture. For clovers planted within natural clover populations at two sites, infection was threefold greater at the wetter site. At the drier site, water retention on the leaf surface was an important predictor of infection rates across host species, but persistent fog and dew at the wetter site reduced the importance of rapid leaf drying. Our results suggest that plant adaptations that reduce water retention on leaves may also reduce disease incidence, but the selective advantage of these traits will vary among habitats.

Effects of VOCs on herbaceous plants in an open-top chamber experiment

A selection of herbaceous plants representing the ground flora around a typical chemical installation in the UK was exposed continuously for 7 weeks to a mixture of six VOCs (acetone, acetonitrile, dichloromethane, ethanol, methyl t-butyl ether and toluene) in open-top chambers. Exposure concentrations were based on predictions of atmospheric dispersion from a single source, at a distance of approximately 2 km. The effects of continuous exposure, representing a worst-case, were measured in terms of uncontrolled water loss from leaves, leaf wettability, chlorophyll content and fluorescence, dry matter production and detailed observations of changes in plant growth and phenology. There were significant effects of VOC exposure on seed production, leaf water content and photosynthetic efficiency in some plant species. Such effects may be detectable in vegetation close to major industrial point sources of VOCs, or as a result of an accidental release of material during manufacture or transport. Some of the species tested e.g. birdsfoot trefoil ( Lotus corniculatus L.) seem to be promising as potential bioindicators for VOCs, but there may be other even more sensitive species waiting to be discovered. However, the most obvious and conveniently measured response to VOC exposure in the birdsfoot trefoil (premature senescence i.e. advanced timing of seed pod production) could easily be confused in the field with climatic influences. It is also uncertain at this stage whether any of the effects observed would lead to longer term ecological changes in natural plant communities, through biased competition between sensitive and more tolerant species.

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.

Potentially durable resistance mechanisms in plants to specialised fungal pathogens

Summary Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective propagules should reach the appropriate plant tissue, gain access to the tissue and negate or suppress various kinds of constitutive and inducible resistance mechanisms. The resistance type most frequently deployed in plant breeding is the racespecific resistance, where a hypersensitive response of plant tissue is elicited by an avirulence factor produced by the pathogen. The great disadvantage of this type of resistance is, that it is often ephemeral. Detailed screening of germplasm may result in the discovery of alternative defence mechanisms not associated with hypersensitivity, that may be durable. Avoidance mechanisms may reduce the chance of infection. Upright plant habit has been reported to decrease spore deposition in cereals. Crop architecture may also affect aspects such as humidity and aeration in the crop, and hence the chances for successful infection by pathogen propagules. Other examples of avoidance are leaf surface properties that interfere with leaf wettability, germ tube orientation and finding of stomata to enter the leaf. Stomata in some accessions of Hordeum chilense are excessively covered by cuticular wax that prevent rust fungal germ tubes from perceiving the stomata, resulting in failure of penetration of the pathogen into the leaf. There is evidence that in compatible host species, biotrophic pathogenic fungi induce basic compatibility by suppressing defence mechanisms. Failure of this induction results in abortion of the infection attempt. Several cases of apparently durable resistance are discussed that are based on failure of haustorium formation, and are not associated with hypersensitivity. They may represent cases where the pathogen has problems in establishing basic compatibility.

Ozone exposure affects leaf wettability and tree water balance

• Relatively little is known about the influences of growing-season background ozone (O3 ) concentrations on leaf cuticles and foliar water loss. • Using fumigation chambers, leaf wettability and foliar water loss were studied in two poplar species, Populus nigra and P. euramericana, and a conifer, Pseudotsuga menziesii, under three O3 regimes; control (approx. 1 ppbv O3 ), urban O3 exposure (13-41 ppbv O3 ), and montane O3 exposure (30-45 ppbv O3 ). • Urban O3 exposure delayed a decrease in droplet contact angles over time in Populus leaves by 2-4wk, and decreased droplet contact angles of P. menziesii foliage. Ozone exposure increased foliar water loss and minimal conductance to water vapour for P. euramericana, but not P. nigra and P. menziesii. Both Populus species had lower photosynthetic biomass in O3 treatments, due to production of fewer new leaves, premature leaf abscission and decreased leaf size (P. euramericana only). Leaf abscission was preceded by foliar injury symptoms characteristic of O3 exposure. • Results suggest that exposure to [O3 ] common during the growing season can increase water loss in Populus saplings, but this effect might be offset by decreased foliar biomass. Importantly, responses were highly species specific in a given O3 treatment.

Effects of leaf wetting and high humidity on stomatal function in leafy cuttings and intact plants ofCorylus maxima

When rooted cuttings of Corylus maxima Mill. cv. Purpurea are moved from the wet and humid conditions of the rooting environment, the leaves frequently shrivel and die. Since the newly formed adventitious root system has been shown to be functional in supplying water to the shoot, stomatal behaviour in C. maxima was investigated in relation to the failure to prevent desiccation. Stomatal conductance (gs) in expanding leaves (L3) of cuttings increased almost 10-fold over the first 14 days in the rooting environment (fog), from 70 to 650 mmol m-2 s-1. In contrast, gs of expanded leaves (L1) changed little and was in the region of 300 mmol m-2 s-1. Midday leaf water potential was much higher in cuttings than in leaves on the mother stock-plant (-0.5 versus -1.2 MPa) even before any roots were visible. Despite this, leaf expansion of L3 was inhibited by >50% in cuttings and stomata showed a gradual reduction in their ability to close in response to abscisic acid (ABA). To determine whether the loss of stomatal function in cuttings was due to severance or to unnaturally low vapour pressure deficit and wetting in fog, intact plants were placed alongside cuttings in the rooting environment. The intact plants displayed reductions in leaf expansion and in the ability of stomata to close in response to dark, desiccation and ABA. However, in cuttings, the additional effect of severance resulted in smaller leaves than in intact plants and more severe reduction in stomatal closure, which was associated with a 2.5-fold increase in stomatal density and distinctively rounded stomatal pores. The similarities between stomatal dysfunction in C. maxima and that observed in many species propagated in vitro are discussed, as is the possible mechanism of dysfunction.

Decline in stomatal response to leaf water deficit in Corylus maxima cuttings.

Many woody species can be propagated from leafy cuttings. However, following rooting, cuttings of Corylus maxima Mill. cv. Purpurea do not always survive the transition from a highly supportive rooting environment (e.g., fog) to a more natural environment where evaporative demand is higher. We found that it is not the supply of water to leaves, but stomatal dysfunction that leads to severe water deficits in the rooted cuttings. Two hours after well-rooted cuttings were transferred from the rooting environment, we were able to relate visible signs of leaf water deficit to high stomatal conductance (g(s)) and low relative water content (R). Small expanding leaves (L3) had unusually high g(s) and lower R than fully expanded leaves (L1). Although high cuticular conductances (g(c)) were occasionally observed in L3, SEM confirmed that increased total leaf conductance (g) was mainly a result of abnormally wide stomatal opening. We measured changes in the ability of stomata to control water loss during rooting by determining stomatal responsiveness to leaf water deficit in detached L1 and L3 harvested from cuttings during the first 75 days after severance from stock plants. Reduced stomatal responsiveness was observed within 7 days of severance, prior to adventitious root formation, and was more pronounced in L3 than in L1. A period of acclimatization after rooting (no leaf wetting, but a vapor pressure deficit of 0.20 kPa) reduced g(s) by 50% in L3 but not in L1, and partially restored stomatal responsiveness in L1 but not in L3. After rooting, the original leaves on the cutting retained substantial capacity for photosynthesis (e.g., in L1, 8 micromol m(-2) s(-1) at a photosynthetic photon flux density of 400 micromol m(-2) s(-1)). The implications of the results for post-rooting acclimatization procedures are discussed.

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

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.

Stomatal plugs of Drimys winteri (Winteraceae) protect leaves from mist but not drought.

Two outstanding features of the flowering plant family Winteraceae are the occlusion of their stomatal pores by cutin plugs and the absence of water-conducting xylem vessels. An adaptive relationship between these two unusual features has been suggested whereby stomatal plugs restrict gas exchange to compensate for the presumed poor conductivity of their vesselless wood. This hypothesized connection fueled evolutionary arguments that the vesselless condition is ancestral in angiosperms. Here we show that in Drimys winteri, a tree common to wet forests, these stomatal occlusions pose only a small fixed resistance to water loss. In addition, they modify the humidity response of guard cells such that under high evaporative demand, leaves with plugs lose water at a faster rate than leaves from which the plugs have been experimentally removed. Instead of being adaptations for drought, we present evidence that these cuticular structures function to maintain photosynthetic activity under conditions of excess water on the leaf surface. Stomatal plugs decrease leaf wettability by preventing the formation of a continuous water film that would impede diffusion of CO2 into the leaf. Misting of leaves had no effect on photosynthetic rate of leaves with plugs, but resulted in a marked decrease ( approximately 40%) in leaves from which the plugs had been removed. These findings do not support a functional association between stomatal plugs and hydraulic competence and provide a new perspective on debates surrounding the evolution of vessels in angiosperms.