Effect of temperature on cuticular transpiration of isolated cuticular membranes and leaf discs

Measuring residual transpiration in plants: a comparative analysis of different methods

Residual transpiration (RT) is defined as a loss of water through the leaf cuticle while stomata are closed. Reduced RT might be a potentially valuable trait for improving plant performance under water deficit conditions imposed by either drought or salinity. Due to the presence of stomata on the leaf surface, it is technically challenging to measure RT. RT has been estimated by the water loss through either astomatous leaf surface or isolated astomatous cuticular layers. This approach is not suitable for all species (e.g. not applicable to grasses) and is difficult and too time consuming for large-scale screening in breeding programs. Several alternative methods may be used to quantify the extent of RT; each of them comes with its own advantages and limitations. In this study, we have undertaken a comparative assessment of eight various methods of assessing RT, using barley (Hordeum vulgare ) plants as a model species. RT measured by water retention curves and a portable gas exchange (infrared gas analyser; IRGA) system had low resolution and were not able to differentiate between RT rates from young and old leaves. Methods based on quantification of the water loss at several time-points were found to be the easiest and least time-consuming compared to others. Of these, the 'three time-points water loss' method is deemed as the most suitable for the high throughput screening of plant germplasm for RT traits.

An improved representation of the relationship between photosynthesis and stomatal conductance leads to more stable estimation of conductance parameters and improves the goodness-of-fit across diverse data sets

Stomata play a central role in surface-atmosphere exchange by controlling the flux of water and CO₂ between the leaf and the atmosphere. Representation of stomatal conductance (g_sw ) is therefore an essential component of models that seek to simulate water and CO₂ exchange in plants and ecosystems. For given environmental conditions at the leaf surface (CO₂ concentration and vapor pressure deficit or relative humidity), models typically assume a linear relationship between g_sw and photosynthetic CO₂ assimilation (A). However, measurement of leaf-level g_sw response curves to changes in A are rare, particularly in the tropics, resulting in only limited data to evaluate this key assumption. Here, we measured the response of g_sw and A to irradiance in six tropical species at different leaf phenological stages. We showed that the relationship between g_sw and A was not linear, challenging the key assumption upon which optimality theory is based-that the marginal cost of water gain is constant. Our data showed that increasing A resulted in a small increase in g_sw at low irradiance, but a much larger increase at high irradiance. We reformulated the popular Unified Stomatal Optimization (USO) model to account for this phenomenon and to enable consistent estimation of the key conductance parameters g₀ and g₁ . Our modification of the USO model improved the goodness-of-fit and reduced bias, enabling robust estimation of conductance parameters at any irradiance. In addition, our modification revealed previously undetectable relationships between the stomatal slope parameter g₁ and other leaf traits. We also observed nonlinear behavior between A and g_sw in independent data sets that included data collected from attached and detached leaves, and from plants grown at elevated CO₂ concentration. We propose that this empirical modification of the USO model can improve the measurement of g_sw parameters and the estimation of plant and ecosystem-scale water and CO₂  fluxes.

The role of petal transpiration in floral humidity generation

Using petrolatum gel as an antitranspirant on the flowers of California poppy and giant bindweed, we show that transpiration provides a large contribution to floral humidity generation. Floral humidity, an area of elevated humidity in the headspace of flowers, is believed to be produced predominantly through a combination of evaporation of liquid nectar and transpirational water loss from the flower. However, the role of transpiration in floral humidity generation has not been directly tested and is largely inferred by continued humidity production when nectar is removed from flowers. We test whether transpiration contributes to the floral humidity generation of two species previously identified to produce elevated floral humidity, Calystegia silvatica and Eschscholzia californica. Floral humidity production of flowers that underwent an antitranspirant treatment, petrolatum gel which blocks transpiration from treated tissues, is compared to flowers that did not receive such treatments. Gel treatments reduced floral humidity production to approximately a third of that produced by untreated flowers in C. silvatica, and half of that in E. californica. This confirms the previously untested inferences that transpiration has a large contribution to floral humidity generation and that this contribution may vary between species.

Photosynthetic and Morphological Responses of Sacha Inchi (Plukenetia volubilis L.) to Waterlogging Stress

Sacha inchi (Plukenetia volubilis L.) is an important oilseed crop that is rich in fatty acids and protein. Climate-change-related stresses, such as chilling, high temperature, and waterlogging can cause severe production loss in this crop. In this study, we investigated the photosynthetic responses of sacha inchi seedlings to short-term waterlogging and their morphological changes after long-term waterlogging stress. Sacha inchi CO₂ uptake, stomatal conductance, and transpiration rate are affected by temperature and light intensity. The seedlings had a high CO₂ uptake (>10 μmol m^-2s^-1) during the daytime (08:00 to 15:00), and at 32 and 36 °C. At 32 °C, CO₂ uptake peaked at irradiations of 1000 and 1500 µmol m^-2s^-1, and plants could still perform photosynthesis at high-intensity radiation of 2000-3000 µmol m^-2s^-1. However, after 5 days of waterlogging (5 DAF) sacha inchi seedlings significantly reduced their photosynthetic ability. The CO₂ uptake, stomatal conductance, Fv/Fm, ETR, and qP, etc., of the susceptible genotypes, were significantly decreased and their wilting percentage was higher than 50% at 5 DAF. This led to a higher wilting percentage at 7 days post-recovery. Among the four lines assessed, Line 27 had a high photosynthetic capability and showed the best waterlogging tolerance. We screened many seedlings for long-term waterlogging tolerance and discovered that some seedlings can produce adventitious roots (AR) and survive after two weeks of waterlogging. Hence, AR could be a critical morphological adaptation to waterlogging in this crop. In summary, these results suggest that improvement in waterlogging tolerance has considerable potential for increasing the sustainable production of sacha inchi.

Large differences in leaf cuticle conductance and its temperature response among 24 tropical tree species from across a rainfall gradient

More frequent droughts and rising temperatures pose serious threats to tropical forests. When stomata are closed under dry and hot conditions, plants lose water through leaf cuticles, but little is known about cuticle conductance (g_min ) of tropical trees, how it varies among species and environments, and how it is affected by temperature. We determined g_min in relation to temperature for 24 tropical tree species across a steep rainfall gradient in Panama, by recording leaf drying curves at different temperatures in the laboratory. In contrast with our hypotheses, g_min did not differ systematically across the rainfall gradient; species differences did not reflect phylogenetic patterns; and in most species g_min did not significantly increase between 25 and 50°C. g_min was higher in deciduous than in evergreen species, in species with leaf trichomes than in species without, in sun leaves than in shade leaves, and tended to decrease with increasing leaf mass per area across species. There was no relationship between stomatal and cuticle conductance. Large species differences in g_min and its temperature response suggest that more frequent hot droughts may lead to differential survival among tropical tree species, regardless of species' position on the rainfall gradient.

Transpiration increases under high-temperature stress: Potential mechanisms, trade-offs and prospects for crop resilience in a warming world

The frequency and intensity of high-temperature stress events are expected to increase as climate change intensifies. Concomitantly, an increase in evaporative demand, driven in part by global warming, is also taking place worldwide. Despite this, studies examining high-temperature stress impacts on plant productivity seldom consider this interaction to identify traits enhancing yield resilience towards climate change. Further, new evidence documents substantial increases in plant transpiration rate in response to high-temperature stress even under arid environments, which raise a trade-off between the need for latent cooling dictated by excessive temperatures and the need for water conservation dictated by increasing evaporative demand. However, the mechanisms behind those responses, and the potential to design the next generation of crops successfully navigating this trade-off, remain poorly investigated. Here, we review potential mechanisms underlying reported increases in transpiration rate under high-temperature stress, within the broader context of their impact on water conservation needed for crop drought tolerance. We outline three main contributors to this phenomenon, namely stomatal, cuticular and water viscosity-based mechanisms, and we outline research directions aiming at designing new varieties optimized for specific temperature and evaporative demand regimes to enhance crop productivity under a warmer and dryer climate.

A co-opted steroid synthesis gene, maintained in sorghum but not maize, is associated with a divergence in leaf wax chemistry

Virtually all above-ground plant surfaces, such as leaf and stem exteriors, are covered in a cuticle: a wax-infused polyester. This waxy biocomposite is the largest interface between Earth’s biosphere and atmosphere. Its chemical composition is not only highly tuned to mediate nonstomatal water loss, but it also self-assembles to produce superhydrophobic surfaces, protects against UV radiation, and contains bioactive compounds that help resist microbial attack. Developing fundamental knowledge of waxy biocomposites, particularly those on crop species, is a prerequisite for an understanding of their structure–function relationships. Here, we uncover a likely genetic basis for the presence and absence, respectively, of triterpenoids in the leaf waxes of sorghum and maize—compounds previously associated with creating heat-tolerant cuticular water barriers.

Virtually all land plants are coated in a cuticle, a waxy polyester that prevents nonstomatal water loss and is important for heat and drought tolerance. Here, we describe a likely genetic basis for a divergence in cuticular wax chemistry between Sorghum bicolor, a drought tolerant crop widely cultivated in hot climates, and its close relative Zea mays (maize). Combining chemical analyses, heterologous expression, and comparative genomics, we reveal that: 1) sorghum and maize leaf waxes are similar at the juvenile stage but, after the juvenile-to-adult transition, sorghum leaf waxes are rich in triterpenoids that are absent from maize; 2) biosynthesis of the majority of sorghum leaf triterpenoids is mediated by a gene that maize and sorghum both inherited from a common ancestor but that is only functionally maintained in sorghum; and 3) sorghum leaf triterpenoids accumulate in a spatial pattern that was previously shown to strengthen the cuticle and decrease water loss at high temperatures. These findings uncover the possibility for resurrection of a cuticular triterpenoid-synthesizing gene in maize that could create a more heat-tolerant water barrier on the plant’s leaf surfaces. They also provide a fundamental understanding of sorghum leaf waxes that will inform efforts to divert surface carbon to intracellular storage for bioenergy and bioproduct innovations.

An improved theory for calculating leaf gas exchange more precisely accounting for small fluxes

The widely used theory for gas exchange proposed by von Caemmerer and Farquhar (vCF) integrates molar fluxes, mole fraction gradients and ternary effects but does not account for cuticular fluxes, for separation of the leaf surface conditions or for ternary effects within the boundary layer. The magnitude of cuticular conductance to water (g_cw) is a key factor for determining plant survival in drought but is difficult to measure and often neglected in routine gas exchange studies. The vCF ternary effect is applied to the total flux without the recognition of different pathways that are affected by it. These simplifications lead to errors in estimations of stomatal conductance, intercellular carbon dioxide concentration (C_i) and other gas exchange parameters. The theory presented here is a more precise physical approach to the electrical resistance analogy for gas exchange, resulting in a more accurate calculation of gas exchange parameters. Additionally, we extend our theory, using physiological concepts, to create a model that allows us to calculate cuticular conductance to water.

Where do leaf water leaks come from? Trade-offs underlying the variability in minimum conductance across tropical savanna species with contrasting growth strategies

Plants continue to lose water from their leaves even after complete stomatal closure. Although this minimum conductance (g_leaf-res ) has substantial impacts on strategies of water use and conservation, little is known about the potential drivers underlying the variability of this trait across species. We thus untangled the relative contribution of water leaks from the cuticle and stomata in order to investigate how the variability in leaf morphological and anatomical traits is related to the variation in g_leaf-res and carbon assimilation capacity across 30 diverse species from the Brazilian Cerrado. In addition to cuticle permeance, water leaks from stomata had a significant impact on g_leaf-res . The differential pattern of stomata distribution in the epidermis was a key factor driving this variation, suggesting the existence of a trade-off between carbon assimilation and water loss through g_leaf-res . For instance, higher g_leaf-res , observed in fast-growing species, was associated with the investment in small and numerous stomata, which allowed higher carbon assimilation rates but also increased water leaks, with negative impacts on leaf survival under drought. Variation in cuticle structural properties was not linked to g_leaf-res . Our results therefore suggest the existence of a trade-off between carbon assimilation efficiency and dehydration tolerance at foliar level.

Compositional, structural and functional cuticle analysis of Prunus laurocerasus L. sheds light on cuticular barrier plasticity

Barrier properties of the hydrophobic plant cuticle depend on its physicochemical composition. The cuticular compounds vary considerably among plant species but also among organs and tissues of the same plant and throughout developmental stages. As yet, these intraspecific modifications at the cuticular wax and cutin level are only rarely examined. Attempting to further elucidate cuticle profiles, we analysed the adaxial and abaxial surfaces of the sclerophyllous leaf and three developmental stages of the drupe fruit of Prunus laurocerasus, an evergreen model plant native to temperate regions. According to gas chromatographic analyses, the cuticular waxes contained primarily pentacyclic triterpenoids dominated by ursolic acid, whereas the cutin biopolyester mainly consisted of 9/10,ω-dihydroxy hexadecanoic acid. Distinct organ- and side-specific patterns were found for cuticular lipid loads, compositions and carbon chain length distributions. Compositional variations led to different structural and functional barrier properties of the plant cuticle, which were investigated further microscopically, infrared spectroscopically and gravimetrically. The minimum water conductance was highlighted at 1 × 10^-5 m s^-1 for the perennial, hypostomatous P. laurocerasus leaf and at 8 × 10^-5 m s^-1 for the few-month-living, stomatous fruit suggesting organ-specific cuticular barrier demands.

High water use in desert plants exposed to extreme heat

Many plant water use models predict leaves maximize carbon assimilation while minimizing water loss via transpiration. Alternate scenarios may occur at high temperature, including heat avoidance, where leaves increase water loss to evaporatively cool regardless of carbon uptake; or heat failure, where leaves non-adaptively lose water also regardless of carbon uptake. We hypothesized that these alternative scenarios are common in species exposed to hot environments, with heat avoidance more common in species with high construction cost leaves. Diurnal measurements of leaf temperature and gas exchange for 11 Sonoran Desert species revealed that 37% of these species increased transpiration in the absence of increased carbon uptake. High leaf mass per area partially predicted this behaviour (r²  = 0.39). These data are consistent with heat avoidance and heat failure, but failure is less likely given the ecological dominance of the focal species. These behaviours are not yet captured in any extant plant water use model.

Comparative studies on structure of the floral nectaries and the abundance of nectar production of Prunus laurocerasus L

There is very scanty information concerning the floral nectary structure and nectar secretion in Prunus laurocerasus L. Therefore, the aim of the study was to determine the micromorphology, anatomy and ultrastructure of nectaries; the abundance of nectar production; and the quantitative and qualitative composition of sugars contained in the nectar of two P. laurocerasus cultivars: 'Schipkaensis' and 'Zabeliana'. The nectary structure was studied using light, fluorescence, scanning and transmission electron microscopy techniques. The nectar sugars were analysed with HPLC. The 'Schipkaensis' had longer inflorescences with a larger number of flowers and a longer perianth than 'Zabeliana'. The micromorphological structure of the nectaries in 'Schipkaensis' exhibited denser (approx. 39%) and larger (approx. 50%) stomata and thicker (approx. 13%) cuticular striae forming wider bands (approx. 26%) than in 'Zabeliana'. The results provide new data on the micromorphology, anatomy and ultrastructure of these floral nectaries. Nectary cuticle ornamentation as well as the size, type and density of stomata and stomatal complex topography can have a diagnostic value in Prunus. The nectar sugar weight indicates a significant apicultural value of the cherry laurel, especially in the case of 'Schipkaensis'. Cherry laurel is an entomophilous species recommended for cultivation in nectariferous zones and insect pollinator refuges; however, climatic conditions eliminating the invasiveness of these plants should be considered.

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

Anthropogenic activities such as uncontrolled deforestation and increasing greenhouse gas emissions are responsible for triggering a series of environmental imbalances that affect the Earth's complex climate dynamics. As a consequence of these changes, several climate models forecast an intensification of extreme weather events over the upcoming decades, including heat waves and increasingly severe drought and flood episodes. The occurrence of such extreme weather will prompt profound changes in several plant communities, resulting in massive forest dieback events that can trigger a massive loss of biodiversity in several biomes worldwide. Despite the gravity of the situation, our knowledge regarding how extreme weather events can undermine the performance, survival, and distribution of forest species remains very fragmented. Therefore, the present review aimed to provide a broad and integrated perspective of the main biochemical, physiological, and morpho-anatomical disorders that may compromise the performance and survival of forest species exposed to climate change factors, particularly drought, flooding, and global warming. In addition, we also discuss the controversial effects of high CO2 concentrations in enhancing plant growth and reducing the deleterious effects of some extreme climatic events. We conclude with a discussion about the possible effects that the factors associated with the climate change might have on species distribution and forest composition.

Effects of temperature on the cuticular transpiration barrier of two desert plants with water-spender and water-saver strategies

The efficacy of the cuticular transpiration barrier and its resistance to elevated temperatures are significantly higher in a typical water-saver than in a water-spender plant growing in hot desert.

On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls

Contents Summary 693 I. Introduction 693 II. Comparison of various definitions and measurement techniques of minimum conductance 694 III. Cuticular conductance 695 IV. Contribution of stomata 696 V. Environmental and ecological variation in minimum conductance 696 VI. Use of minimum conductance in models 698 VII. Conclusions 703 Acknowledgements 703 References 703 SUMMARY: When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (g_min ). In this review, we critically evaluate the sources of variation in g_min , quantitatively compare various methods for its estimation, and illustrate the role of g_min in models of leaf gas exchange. A literature compilation of g_min as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that g_min is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity - as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball-Berry model of stomatal conductance, and caution against the usual regression-based method for its estimation. The synthesis presented here provides guidelines for the use of g_min in ecosystem models, and points to clear research gaps for this drought tolerance trait.

Effects of changes in leaf properties mediated by methyl jasmonate (MeJA) on foliar absorption of Zn, Mn and Fe

Background and Aims

Foliar fertilization to overcome nutritional deficiencies is becoming increasingly widespread. However, the processes of foliar nutrient absorption and translocation are poorly understood. The present study aimed to investigate how cuticular leaf properties affect the absorption of foliar-applied nutrients in leaf tissues.

Methods

Given that methyl jasmonate (MeJA) can cause alterations in leaf properties, we applied 1 mm MeJA to sunflower (Helianthus annuus), tomato (Solanum lycopersicum) and soybean (Glycine max) to assess changes in leaf properties. Using traditionally analytical approaches and synchrotron-based X-ray fluorescence microscopy, the effects of these changes on the absorption and translocation of foliar-applied Zn, Mn and Fe were examined.

Key Results

The changes in leaf properties caused by the application of MeJA increased foliar absorption of Zn, Mn and Fe up to 3- to 5-fold in sunflower but decreased it by 0·5- to 0·9-fold in tomato, with no effect in soybean. These changes in the foliar absorption of nutrients could not be explained by changes in overall trichome density, which increased in both sunflower (86%) and tomato (76%) (with no change in soybean). Similarly, the changes could be not attributed to changes in stomatal density or cuticle composition, given that these properties remained constant. Rather, the changes in the foliar absorption of Zn, Mn and Fe were related to the thickness of the cuticle and epidermal cell wall. Finally, the subsequent translocation of the absorbed nutrients within the leaf tissues was limited (<1·3mm) irrespective of treatment.

Conclusions

The present study highlights the potential importance of the combined thickness of the cuticle and epidermal cell wall in the absorption of foliar-applied nutrients. This information will assist in increasing the efficacy of foliar fertilization.

Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir

The frequency and intensity of climatic extremes, such as heat waves, are predicted to increase globally, with severe implications for terrestrial carbon and water cycling. Temperatures may rise above critical thresholds that allow trees to function optimally, with unknown long-term consequences for forest ecosystems. In this context, we investigated how photosynthetic traits and the water balance in Douglas-fir are affected by exposure to three heat waves with temperatures about 12°C above ambient. Photosynthetic carboxylation efficiency (V_cmax) was mostly unaffected, but electron transport (J_max) and photosynthetic rates under saturating light (A_sat) were strongly influenced by the heat waves, with lagging limitations on photosynthesis still being observed six weeks after the last heat wave. We also observed lingering heat-induced inhibitions on transpiration, minimum stomatal conductance, and night-time stomatal conductance (g_s-night). Results from the stomatal models used to calculate minimum stomatal conductance were similar to g_s-night and indicated changes in leaf morphology, e.g. stomatal occlusions and alterations in epicuticular wax. Our results show Douglas-fir's ability to restrict water loss following heat stress, but at the price of reduced photosynthetic performance. Such limitations indicate potential long-term restrictions that heat waves can impose on tree development and functioning under extreme climatic conditions.

Effectiveness of cuticular transpiration barriers in a desert plant at controlling water loss at high temperatures

Maintaining the integrity of the cuticular transpiration barrier even at elevated temperatures is of vital importance especially for hot-desert plants. Currently, the temperature dependence of the leaf cuticular water permeability and its relationship with the chemistry of the cuticles are not known for a single desert plant. This study investigates whether (i) the cuticular permeability of a desert plant is lower than that of species from non-desert habitats, (ii) the temperature-dependent increase of permeability is less pronounced than in those species and (iii) whether the susceptibility of the cuticular permeability barrier to high temperatures is related to the amounts or properties of the cutin or the cuticular waxes. We test these questions with Rhazya stricta using the minimum leaf water vapour conductance (gmin) as a proxy for cuticular water permeability. gmin of R. stricta (5.41 × 10(-5) m s(-1) at 25 °C) is in the upper range of all existing data for woody species from various non-desert habitats. At the same time, in R. stricta, the effect of temperature (15-50 °C) on gmin (2.4-fold) is lower than in all other species (up to 12-fold). Rhazya stricta is also special since the temperature dependence of gmin does not become steeper above a certain transition temperature. For identifying the chemical and physical foundation of this phenomenon, the amounts and the compositions of cuticular waxes and cutin were determined. The leaf cuticular wax (251.4 μg cm(-2)) is mainly composed of pentacyclic triterpenoids (85.2% of total wax) while long-chain aliphatics contribute only 3.4%. In comparison with many other species, the triterpenoid-to-cutin ratio of R. stricta (0.63) is high. We propose that the triterpenoids deposited within the cutin matrix restrict the thermal expansion of the polymer and, thus, prevent thermal damage to the highly ordered aliphatic wax barrier even at high temperatures.

Proteome analysis of Pueraria mirifica tubers collected in different seasons

Pueraria mirifica-derived tuberous powder has been long-term consumed in Thailand as female hormone-replacement traditional remedies. The protein profiles of tubers collected in different seasons were evaluated. Phenol extraction, 2D-PAGE, and mass spectrometry were employed for tuberous proteome analysis. Out of the 322 proteins detected, over 59% were functionally classified as being involved in metabolism. The rest proteins were involved in defense, protein synthesis, cell structure, transportation, stress, storage, and also unidentified function. The proteins were found to be differentially expressed with respect to harvest season. Importantly, chalcone isomerase, isoflavone synthase, cytochrome p450, UDP-glycosyltransferase, and isoflavone reductase, which are all involved in the biosynthesis pathway of bioactive isoflavonoids, were most abundantly expressed in the summer-collected tubers. This is the first report on the proteomic patterns in P. mirifica tubers in relevant with seasonal variation. The study enlights the understanding of variance isoflavonoid production in P. mirifica tubers.

Epicuticular wax on cherry laurel (Prunus laurocerasus) leaves does not constitute the cuticular transpiration barrier

Epicuticular wax of cherry laurel does not contribute to the formation of the cuticular transpiration barrier, which must be established by intracuticular wax. Barrier properties of cuticles are established by cuticular wax deposited on the outer surface of the cuticle (epicuticular wax) and in the cutin polymer (intracuticular wax). It is still an open question to what extent epi- and/or intracuticular waxes contribute to the formation of the transpiration barrier. Epicuticular wax was mechanically removed from the surfaces of isolated cuticles and intact leaf disks of cherry laurel (Prunus laurocerasus L.) by stripping with different polymers (collodion, cellulose acetate and gum arabic). Scanning electron microscopy showed that two consecutive treatments with all three polymers were sufficient to completely remove epicuticular wax since wax platelets disappeared and cuticle surfaces appeared smooth. Waxes in consecutive polymer strips and wax remaining in the cuticle after treatment with the polymers were determined by gas chromatography. This confirmed that two treatments of the polymers were sufficient for selectively removing epicuticular wax. Water permeability of isolated cuticles and cuticles covering intact leaf disks was measured using (3)H-labelled water before and after selectively removing epicuticular wax. Cellulose acetate and its solvent acetone led to a significant increase of cuticular permeability, indicating that the organic solvent acetone affected the cuticular transpiration barrier. However, permeability did not change after two subsequent treatments with collodion and gum arabic or after treatment with the corresponding solvents (diethyl ether:ethanol or water). Thus, in the case of P. laurocerasus the epicuticular wax does not significantly contribute to the formation of the cuticular transpiration barrier, which evidently must be established by the intracuticular wax.

Impedance of the grape berry cuticle as a novel phenotypic trait to estimate resistance to Botrytis cinerea

Warm and moist weather conditions during berry ripening provoke Botrytis cinerea (B. cinerea) causing notable bunch rot on susceptible grapevines with the effect of reduced yield and wine quality. Resistance donors of genetic loci to increase B. cinerea resistance are widely unknown. Promising traits of resistance are represented by physical features like the thickness and permeability of the grape berry cuticle. Sensor-based phenotyping methods or genetic markers are rare for such traits. In the present study, the simple-to-handle I-sensor was developed. The sensor enables the fast and reliable measurement of electrical impedance of the grape berry cuticles and its epicuticular waxes (CW). Statistical experiments revealed highly significant correlations between relative impedance of CW and the resistance of grapevines to B. cinerea. Thus, the relative impedance Z_rel of CW was identified as the most important phenotypic factor with regard to the prediction of grapevine resistance to B. cinerea. An ordinal logistic regression analysis revealed a R²_McFadden of 0.37 and confirmed the application of Z_rel of CW for the prediction of bunch infection and in this way as novel phenotyping trait. Applying the I-sensor, a preliminary QTL region was identified indicating that the novel phenotypic trait is as well a valuable tool for genetic analyses.

The hydroclimatic and ecophysiological basis of cloud forest distributions under current and projected climates

BACKGROUND

Tropical montane cloud forests (TMCFs) are characterized by a unique set of biological and hydroclimatic features, including frequent and/or persistent fog, cool temperatures, and high biodiversity and endemism. These forests are one of the most vulnerable ecosystems to climate change given their small geographic range, high endemism and dependence on a rare microclimatic envelope. The frequency of atmospheric water deficits for some TMCFs is likely to increase in the future, but the consequences for the integrity and distribution of these ecosystems are uncertain. In order to investigate plant and ecosystem responses to climate change, we need to know how TMCF species function in response to current climate, which factors shape function and ecology most and how these will change into the future.

SCOPE

This review focuses on recent advances in ecophysiological research of TMCF plants to establish a link between TMCF hydrometeorological conditions and vegetation distribution, functioning and survival. The hydraulic characteristics of TMCF trees are discussed, together with the prevalence and ecological consequences of foliar uptake of fog water (FWU) in TMCFs, a key process that allows efficient acquisition of water during cloud immersion periods, minimizing water deficits and favouring survival of species prone to drought-induced hydraulic failure.

CONCLUSIONS

Fog occurrence is the single most important microclimatic feature affecting the distribution and function of TMCF plants. Plants in TMCFs are very vulnerable to drought (possessing a small hydraulic safety margin), and the presence of fog and FWU minimizes the occurrence of tree water deficits and thus favours the survival of TMCF trees where such deficits may occur. Characterizing the interplay between microclimatic dynamics and plant water relations is key to foster more realistic projections about climate change effects on TMCF functioning and distribution.

Solute permeation across the apoplastic barrier in the perisperm-endosperm envelope in cucumber seeds

An apoplastic barrier consisting of callose and lipid layers in the perisperm-endosperm (PE) envelope is known to restrict inward and outward transport of solutes in cucurbit seeds. The present work examines permeability properties of the barrier using cucumber seed as a model system. Osmometrically determined osmotic potential of the apoplastic fluid was used as a basis for osmotic studies aimed at examining solute exclusion from the apoplastic barrier in the PE envelope. The assessment of apoplastic permeability involved measuring the amount of anionic and cationic organic dyes diffused into agarose gel discs through the PE envelope. Ionic/non-ionic solutes including polyethylene glycols having Stokes radii <or= 0.6 nm showed considerable permeation through the apoplastic barrier in the PE envelope as indicated by greater seed thickness/breadth ratios. Permeances of dyes across the PE envelope were in the order: 2,6-dichlorophenolindophenol (DCPIP) > 2,3,5-triphenyltetrazolium chloride (TTC) approximately methyl orange approximately methylene blue > Eosin Y >> Janus green approximately crystal violet approximately Evans Blue. Permeation time(0.5) for DCPIP and TTC was 9.71 and 9.96 h, respectively. Dyes having Stokes radii < 0.5 nm showed significant inward as well as outward diffusion across the PE envelope in contrast to restricted diffusion of dyes having Stokes radii > 0.5 nm. Size exclusion limit for apoplastic barrier in cucumber PE envelope was resolved to be about 0.5 nm by dye permeation and around 0.8 nm by osmotic studies. Dye permeances depended primarily on particle size as described by a quadratic polynomial function rather than on charge or log D.

Self-healing of voids in the wax coating on plant surfaces

The cuticles of plants provide a multifunctional interface between the plants and their environments. The cuticle, with its associated waxes, is a protective layer that minimizes water loss by transpiration and provides several functions, such as hydrophobicity, light reflection and absorption of harmful radiation. The self-healing of voids in the epicuticular wax layer has been studied in 17 living plants by atomic force microscopy (AFM), and the process of wax film formation is described. Two modes of wax film formation, a concentric layer formation and striped layer formation, were found, and the process of multilayer wax film formation is discussed. A new method for the preparation of small pieces of fresh, water-containing plant specimens for AFM investigations is introduced. The technique allows AFM investigations of several hours duration without significant shrinkage or lateral drift of the specimen. This research shows how plants refill voids in their surface wax layers by wax self-assembly and should be useful for the design of self-healing materials.

Fate of deoxynivalenol in contaminated wheat grain during preparation of Egyptian 'balila'

Soaking and boiling whole wheat kernels in water are the key steps in the preparation of an Egyptian dish called 'balila'. The effects of washing, soaking and boiling wheat kernels in tap water or in 0.1 M Na2CO3 solution on the deoxynivalenol (DON) content of the wheat kernels were studied. Boiling contaminated wheat kernels in water reduced the DON content of the grain by 70%. The mechanism of decontamination due to boiling is probably a leaching of DON out of the grain into the boiling medium. A combined treatment of soaking in 0.1 M Na2CO3 solution (pH 11) with subsequent boiling reduced the DON content of the grain by 93%. Data suggest that apart from leaching DON out of the kernels into the boiling medium, a degradation of DON occurred in alkaline medium. Modifying the traditional process of 'balila' preparation by using Na2CO3 solution may be useful to reduce the risk of mycotoxin exposure via 'balila'.

Polar paths of diffusion across plant cuticles: new evidence for an old hypothesis

BACKGROUND

The plant cuticle is an extracellular lipophilic biopolymer covering leaf and fruit surfaces. Its main function is the protection of land-living plants from uncontrolled water loss. In the past, the permeability of the cuticle to water and to non-ionic lipophilic molecules (pesticides, herbicides and other xenobiotics) was studied intensively, whereas cuticular penetration of polar ionic compounds was rarely investigated.

RECENT PROGRESS

Recent work measuring cuticular penetration of inorganic and organic ions is presented; the effects of molecular size of ions, temperature, wax extraction, humidity and plasticizers strongly support the conclusion that ions penetrate cuticles via water-filled pores. The cuticle covering stomata and trichomes forms the preferential site of ion penetration. This indicates that cuticles possess a pronounced lateral heterogeneity: the largest fraction of the cuticle surface is covered by the lipophilic domains of cutin and wax, but to a certain extent polar domains are also present in the cuticle, which form preferential sites of penetration for polar compounds.

THE FUTURE

The chemical nature of these polar domains awaits detailed characterization, which will be of major importance in agriculture and green biotechnology, since polar paths of diffusion represent the most important transport routes for foliar-applied nutrients. Furthermore, many compounds acting as inducers of gene expression in transgenic plants are ionic and need to penetrate the cuticle via polar paths in order to be active.

Plant-microbe interactions: identification of epiphytic bacteria and their ability to alter leaf surface permeability

Bacteria were either isolated from leaf surfaces of Hedera helix or obtained from a culture collection in order to analyse their effect on barrier properties of isolated Hedera and Prunus laurocerasus cuticles. On the basis of the 16S rDNA sequences the genera of the six bacterial isolates from Hedera were identified as Pseudomonas sp., Stenotrophomonas sp. and Achromobacter. Water permeability of cuticles isolated from H. helix was measured before and after inoculation with the six bacterial strains. In addition water permeability of cuticles isolated from P. laurocerasus was measured before and after inoculation with the three bacterial strains Pseudomonas aeruginosa, Xanthomonas campestris and Corynebacterium fascians. Rates of water diffusing across isolated cuticles of both species significantly increased by up to 50% after inoculation with all bacterial strains. Obtained results show that epiphytic bacteria have the ability of increasing water permeability of Hedera and Prunus cuticles, which in turn should increase the availability of water and dissolved compounds in the phyllopshere. Consequently, living conditions in the habitat phyllosphere are improved. It can be concluded that the ability to change leaf surface properties will improve epiphytic fitness of leaf surface bacteria.

Current issues and uncertainties in the measurement and modelling of air-vegetation exchange and within-plant processing of POPs

Air-vegetation exchange of POPs is an important process controlling the entry of POPs into terrestrial food chains, and may also have a significant effect on the global movement of these compounds. Many factors affect the air-vegetation transfer including: the physicochemical properties of the compounds of interest; environmental factors such as temperature, wind speed, humidity and light conditions; and plant characteristics such as functional type, leaf surface area, cuticular structure, and leaf longevity. The purpose of this review is to quantify the effects these differences might have on air/plant exchange of POPs, and to point out the major gaps in the knowledge of this subject that require further research. Uptake mechanisms are complicated, with the role of each factor in controlling partitioning, fate and behaviour process still not fully understood. Consequently, current models of air-vegetation exchange do not incorporate variability in these factors, with the exception of temperature. These models instead rely on using average values for a number of environmental factors (e.g. plant lipid content, surface area), ignoring the large variations in these values. The available models suggest that boundary layer conductance is of key importance in the uptake of POPs, although large uncertainties in the cuticular pathway prevents confirmation of this with any degree of certainty, and experimental data seems to show plant-side resistance to be important. Models are usually based on the assumption that POP uptake occurs through the lipophilic cuticle which covers aerial surfaces of plants. However, some authors have recently attached greater importance to the stomatal route of entry into the leaf for gas phase compounds. There is a need for greater mechanistic understanding of air-plant exchange and the 'scaling' of factors affecting it. The review also suggests a number of key variables that researchers should measure in their experiments to allow comparisons to be made between studies in order to improve our understanding of what causes any differences in measured data between sites.

Formation of plant cuticle: evidence for the occurrence of the peroxygenase pathway

Cuticle plays a major role as a protective barrier in plants. Despite its physiological importance, the mode of formation of this complex structure remains poorly understood. In particular, none of the putative enzymes involved in the biosynthesis of the cutin, the matrix of cuticle, have been cloned. We have shown previously that peroxygenase is able to catalyze in vitro the epoxidation step required for the biosynthesis of C18 cutin monomers. In the present work, we have confirmed in planta that this oxidase is indeed a key enzyme involved in the formation of cutin. Thus, in maize leaves, the specific inactivation of peroxygenase by organophosphorothioates resulted in a dramatic decrease of cuticular epoxide content, as visualized by a specific histochemical technique that was accompanied by a reduced thickness of the cuticle. A strict correlation could also be established between the extent of inhibition of the peroxygenase and the modification of the cuticle triggered by a family of structurally related inhibitors. Importantly, these effects were restricted to plants that contain a cutin originating from C18 monomers. The altered cuticle of maize, treated with the peroxygenase inhibitor, was characterized by an increased permeability to pesticides. In addition, such plants became largely susceptible to infection by fungi, implying that the cuticle represents a crucial target for the modulation of the response in plant-pathogen interactions.