The effect of the environment on the permeability and composition of Citrus leaf cuticles : I. Water permeability of isolated cuticular membranes

Cranberry fruit epicuticular wax benefits and identification of a wax-associated molecular marker

BACKGROUND

As the global climate changes, periods of abiotic stress throughout the North American cranberry growing regions will become more common. One consequence of high temperature extremes and drought conditions is sunscald. Scalding damages the developing berry and reduces yields through fruit tissue damage and/or secondary pathogen infection. Irrigation runs to cool the fruit is the primary approach to controlling sunscald. However, it is water intensive and can increase fungal-incited fruit rot. Epicuticular wax functions as a barrier to various environmental stresses in other fruit crops and may be a promising feature to mitigate sunscald in cranberry. In this study we assessed the function of epicuticular wax in cranberries to attenuate stresses associated with sunscald by subjecting high and low epicuticular wax cranberries to controlled desiccation and light/heat exposure. A cranberry population that segregates for epicuticular wax was phenotyped for epicuticular fruit wax levels and genotyped using GBS. Quantitative trait loci (QTL) analyses of these data identified a locus associated with epicuticular wax phenotype. A SNP marker was developed in the QTL region to be used for marker assisted selection.

RESULTS

Cranberries with high epicuticular wax lost less mass percent and maintained a lower surface temperature following heat/light and desiccation experiments as compared to fruit with low wax. QTL analysis identified a marker on chromosome 1 at position 38,782,094 bp associated with the epicuticular wax phenotype. Genotyping assays revealed that cranberry selections homozygous for a selected SNP have consistently high epicuticular wax scores. A candidate gene (GL1-9), associated with epicuticular wax synthesis, was also identified near this QTL region.

CONCLUSIONS

Our results suggest that high cranberry epicuticular wax load may help reduce the effects of heat/light and water stress: two primary contributors to sunscald. Further, the molecular marker identified in this study can be used in marker assisted selection to screen cranberry seedlings for the potential to have high fruit epicuticular wax. This work serves to advance the genetic improvement of cranberry crops in the face of global climate change.

Leaf wettability, anatomy and ultra-structure of Nothofagus antarctica and N. betuloides grown under a CO2 enriched atmosphere

Increasing CO₂ air concentration may affect wettability, anatomy and ultra-structure of leaves of Patagonian forest species, evergreen and deciduous plants potentially responding differently to such CO₂ increases. In this study, we analysed the wettability, anatomy and ultra-structure of leaves of Nothofagus antarctica (deciduous) and N. betuloides (evergreen) grown under high CO₂ concentrations. Leaf wettability was affected by increasing CO₂, in different directions depending on species and leaf side. In both species, soluble cuticular lipid concentrations per unit leaf area raised with higher CO₂ levels. Stomatal parameters (density, size of guard cells and pores) showed different responses to CO₂ increasing depending on the species examined. In both species, leaf tissues showed a general trend to diminish with higher CO₂ concentration. Cuticle thickness was modified with higher CO₂ concentration in N. betuloides, but not in N. antarctica leaves. In both species, chloroplasts were often damaged with the increase in CO₂ concentration. Our results show that several surface and internal leaf parameters can be modified in association with an increase in atmospheric CO₂ concentration which may very among plant species.

Surface moisture increases microcracking and water vapour permeance of apple fruit skin

Surface moisture induces microcracking in the cuticle of fruit skins. Our objective was to study the effects of surface moisture on cuticular microcracking, the permeance to water vapour and russeting in developing 'Pinova' apple fruit. Surface moisture was applied by fixing to the fruit a plastic tube containing deionized water. Microcracking was quantified by fluorescence microscopy and image analysis following infiltration with acridine orange. Water vapour permeance was determined gravimetrically using skin segments (ES) mounted in diffusion cells. Cumulative water loss through the ES increased linearly with time. Throughout development, surface moisture significantly increased skin permeance. The effect was largest during early development and decreased towards maturity. Recovery time courses revealed that following moisture treatment of young fruit for 12 days, skin permeance continued to increase until about 14 days after terminating the moisture treatment. Thereafter, skin permeance decreased over the next 28 days, then approaching the control level. This behaviour indicates gradual healing of the impaired cuticular barrier. Nevertheless, permeance still remained significantly higher compared with the untreated control. Similar patterns of permeance change were observed following moisture treatments at later stages of development. The early moisture treatment beginning at 23 DAFB resulted in russeting of the exposed surfaces. There was no russet in control fruit without a tube or in control fruit with a tube mounted for 12 days without water. The data demonstrate that surface moisture increases microcracking and water vapour permeance. This may lead to the formation of a periderm and, hence, a russeted fruit surface.

Cuticular wax coverage and its transpiration barrier properties in Quercus coccifera L. leaves: does the environment matter?

Plants prevent uncontrolled water loss by synthesizing, depositing and maintaining a hydrophobic layer over their primary aerial organs-the plant cuticle. Quercus coccifera L. can plastically respond to environmental conditions at the cuticular level. When exposed to hot summer conditions with high vapour-pressure deficit (VPD) and intense solar radiation (Mediterranean atmospheric conditions; MED), this plant species accumulates leaf cuticular waxes even over the stomata, thereby decreasing transpirational water loss. However, under mild summer conditions with moderate VPD and regular solar radiation (temperate atmospheric conditions; TEM), this effect is sharply reduced. Despite the ecophysiological importance of the cuticular waxes of Q. coccifera, the wax composition and its contribution to avoiding uncontrolled dehydration remain unknown. Thus, we determined several leaf traits for plants exposed to both MED and TEM conditions. Further, we qualitatively and quantitatively investigated the cuticular lipid composition by gas chromatography. Finally, we measured the minimum leaf conductance (gmin) as an indicator of the efficacy of the cuticular transpiration barrier. The MED leaves were smaller, stiffer and contained a higher load of cuticular lipids than TEM leaves. The amounts of leaf cutin and cuticular waxes of MED plants were 1.4 times and 2.6 times higher than that found for TEM plants, respectively. In detail, MED plants produced higher amounts of all compound classes of cuticular waxes, except for the equivalence of alkanoic acids. Although MED leaves contained higher cutin and cuticular wax loads, the gmin was not different between the two habitats. Our findings suggest that the qualitative accumulation of equivalent cuticular waxes might compensate for the higher wax amount of MED plants, thereby contributing equally to the efficacy of the cuticular transpirational barrier of Q. coccifera. In conclusion, we showed that atmospheric conditions profoundly affect the cuticular lipid composition of Q. coccifera leaves, but do not alter its transpiration barrier properties.

Cuticular wax coverage and its transpiration barrier properties in Quercus coccifera L. leaves: does the environment matter?

Plants prevent uncontrolled water loss by synthesizing, depositing and maintaining a hydrophobic layer over their primary aerial organs-the plant cuticle. Quercus coccifera L. can plastically respond to environmental conditions at the cuticular level. When exposed to hot summer conditions with high vapour-pressure deficit (VPD) and intense solar radiation (Mediterranean atmospheric conditions; MED), this plant species accumulates leaf cuticular waxes even over the stomata, thereby decreasing transpirational water loss. However, under mild summer conditions with moderate VPD and regular solar radiation (temperate atmospheric conditions; TEM), this effect is sharply reduced. Despite the ecophysiological importance of the cuticular waxes of Q. coccifera, the wax composition and its contribution to avoiding uncontrolled dehydration remain unknown. Thus, we determined several leaf traits for plants exposed to both MED and TEM conditions. Further, we qualitatively and quantitatively investigated the cuticular lipid composition by gas chromatography. Finally, we measured the minimum leaf conductance (gmin) as an indicator of the efficacy of the cuticular transpiration barrier. The MED leaves were smaller, stiffer and contained a higher load of cuticular lipids than TEM leaves. The amounts of leaf cutin and cuticular waxes of MED plants were 1.4 times and 2.6 times higher than that found for TEM plants, respectively. In detail, MED plants produced higher amounts of all compound classes of cuticular waxes, except for the equivalence of alkanoic acids. Although MED leaves contained higher cutin and cuticular wax loads, the gmin was not different between the two habitats. Our findings suggest that the qualitative accumulation of equivalent cuticular waxes might compensate for the higher wax amount of MED plants, thereby contributing equally to the efficacy of the cuticular transpirational barrier of Q. coccifera. In conclusion, we showed that atmospheric conditions profoundly affect the cuticular lipid composition of Q. coccifera leaves, but do not alter its transpiration barrier properties.

Implications of Heat Treatment and Systemic Delivery of Foliar-Applied Oxytetracycline on Citrus Physiological Management and Therapy Delivery

Huanglongbing is an economically devastating disease of citrus in Florida and around the world. This study was undertaken to assess two grower-used therapies, heat treatment, and foliar anti-bacterial application. Specifically, there was an industry claim that heat treatment improved subsequent systemic uptake of foliar-applied anti-bacterial compounds. We hypothesized that new vegetative growth induced by heat treatment could lead to increased foliar delivery because of a greater number of new leaves in which cuticles would be more permeable. The study included two factors (1) heat treatment (with or without) and (2) pruning, in which all new leaves, all mature leaves, or no leaves were removed. A commercial formulation of oxytetracycline (OTC) was applied to plants with a non-ionic penetrant surfactant, but one branch on each tree was covered to assess direct versus systemic delivery. The study was repeated twice, destructively assessing whole-plant leaf area and dry weights, as well as OTC content in directly applied and covered leaves. Heat treatment and defoliation treatments reduced growth, but did not affect systemic delivery of OTC. OTC was detected in nearly all covered leaf samples in both repetitions, though at lower concentrations than in directly applied leaves. We conclude that neither heat treatment nor leaf age strongly affect systemic OTC delivery. Implications of this study for leaf age effects on foliar delivery and for phloem delivery of foreign compounds through foliar application are discussed.

Postharvest changes in LIN5-down-regulated plants suggest a role for sugar deficiency in cuticle metabolism during ripening

The cell wall invertase gene (LIN5) was reported to be a key enzyme influencing sugar uptake of tomato (Solanum lycopersicum) fruit. It was additionally revealed to be a key regulator of total soluble solids content in fruit as well as for reproductive development, being mainly involved in flower development, early fruit and seed development but also in ripening. Here, we demonstrate that silencing of the LIN5 gene promotes changes affecting fruit cuticle development which has a direct effect on postharvest properties. Transformants were characterized by reduced transpirational water loss in mature fruits accompanied by several other changes in the cuticle. Quantitative chemical composition, coupled with microscopy of isolated cuticle fruits revealed that the cuticle of the transformants were characterized by an increase of the thickness as well as significant increase in the content of cuticle components (cutin, phenolic compounds, and waxes). Furthermore, detailed analysis of the waxes revealed that the transformants displayed changes in waxes composition, showing higher levels of n-alkanes and triterpenoids which can shift the proportion of crystalline and amorphous waxes and change the water flux through the cuticle. Expression of the genes involved in cuticle biosynthesis indicated that LIN5 influences the biosynthesis of components of the cuticle, indicating that this process is coupled to sugar uploading via a mechanism which links carbon supply with the capacity for fruit expansion.

Molecular and Evolutionary Mechanisms of Cuticular Wax for Plant Drought Tolerance

Cuticular wax, the first protective layer of above ground tissues of many plant species, is a key evolutionary innovation in plants. Cuticular wax safeguards the evolution from certain green algae to flowering plants and the diversification of plant taxa during the eras of dry and adverse terrestrial living conditions and global climate changes. Cuticular wax plays significant roles in plant abiotic and biotic stress tolerance and has been implicated in defense mechanisms against excessive ultraviolet radiation, high temperature, bacterial and fungal pathogens, insects, high salinity, and low temperature. Drought, a major type of abiotic stress, poses huge threats to global food security and health of terrestrial ecosystem by limiting plant growth and crop productivity. The composition, biochemistry, structure, biosynthesis, and transport of plant cuticular wax have been reviewed extensively. However, the molecular and evolutionary mechanisms of cuticular wax in plants in response to drought stress are still lacking. In this review, we focus on potential mechanisms, from evolutionary, molecular, and physiological aspects, that control cuticular wax and its roles in plant drought tolerance. We also raise key research questions and propose important directions to be resolved in the future, leading to potential applications of cuticular wax for water use efficiency in agricultural and environmental sustainability.

Wax and cutin mutants of Arabidopsis: Quantitative characterization of the cuticular transport barrier in relation to chemical composition

Using (14)C-labeled epoxiconazole as a tracer, cuticular permeability of Arabidopsis thaliana leaves was quantitatively measured in order to compare different wax and cutin mutants (wax2, cut1, cer5, att1, bdg, shn3 and shn1) to the corresponding wild types (Col-0 and Ws). Mutants were characterized by decreases or increases in wax and/or cutin amounts. Permeances [ms(-1)] of Arabidopsis cuticles either increased in the mutants compared to wild type or were not affected. Thus, genetic changes in wax and cutin biosynthesis in some of the investigated Arabidopsis mutants obviously impaired the coordinated cutin and wax deposition at the outer leaf epidermal cell wall. As a consequence, barrier properties of cuticles were significantly decreased. However, increasing cutin and wax amounts by genetic modifications, did not automatically lead to improved cuticular barrier properties. As an alternative approach to the radioactive transport assay, changes in chlorophyll fluorescence were monitored after foliar application of metribuzine, an herbicide inhibiting electron transport in chloroplasts. Since both, half-times of photosynthesis inhibition as well as times of complete inhibition, in fact correlated with (14)C-epoxiconazole permeances, different rates of decline of photosynthetic yield between mutants and wild type must be a function of foliar uptake of the herbicide across the cuticle. Thus, monitoring changes in chlorophyll fluorescence, instead of conducting radioactive transport assays, represents an easy-to-handle and fast alternative evaluating cuticular barrier properties of different genotypes. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Intracuticular wax fixes and restricts strain in leaf and fruit cuticles

This paper investigates the effects of cuticular wax on the release of strain and on the tensile properties of enzymatically isolated cuticular membranes (CMs) taken from leaves of agave (Agave americana), bush lily (Clivia miniata), holly (Ilex aquifolium), and ivy (Hedera helix) and from fruit of apple (Malus × domestica), pear (Pyrus communis), and tomato (Lycopersicon esculentum). Biaxial strain release was quantified as the decrease in CM disc area following wax extraction. Stiffness, maximum strain and maximum force were determined in uniaxial tensile tests using strips of CM and dewaxed CMs (DCMs). Biaxial strain release, stiffness, and maximum strain, but not maximum force, were linearly related to the amount of wax extracted. Apple CM has the most wax and here the effect of wax extraction was substantially accounted for by the embedded cuticular wax. Heating apple CM to 80°C melted some wax constituents and produced an effect similar to, but smaller than, that resulting from wax extraction. Our results indicate that wax 'fixes' strain, effectively converting reversible elastic into irreversible plastic strain. A consequence of 'fixation' is increased cuticular stiffness.

A study of air pollution with heavy metals in Athens city and Attica basin using evergreen trees as biological indicators

Concentrations of five metals (cadmium, chromium, copper, nickel and lead) were determined in tree leaves collected from 13 areas of the Attica basin and Athens city, Greece. Geographical distribution patterns were investigated, and factors affecting toxic element accumulation in trees were discussed. The mean heavy metal content in the tree leaves is described in the descending order of copper>lead>nickel>chromium>cadmium. Generally, the most damaged areas have been proved to be those near the city center and in the vicinity of the Attica highway. The geomorphological relief of the area plays an important role in the dispersion of airborne particles from pollution sources to the surrounding area. Areas on the NE region are also polluted mainly due to wind directions. In Citrus aurantium leaves, with relatively impermeable cuticle, high chromium, copper and nickel concentration would be possibly caused only by significant stomatal uptake. The conifer tree Pinus brutia providing a rough leaf surface also showed elevated concentrations, especially of cadmium and lead. The thick waxy cuticle of the sclerophyllous broad-leaved Olea europaea forms a smooth sheet increasing the barrier properties of the leaf epidermis and causing a reduction in leaf permeability. The dense trichomes of the abaxial epidermis of Olea europaea also act as a pollution screen keeping away the air particles from the epidermis stomata. The presence of a certain metal within the leaf cells could reduce the uptake or toxicity of some others.

A radioactive assay allowing the quantitative measurement of cuticular permeability of intact Arabidopsis thaliana leaves

Cuticular penetration of five different ¹⁴C-labeled chemicals (benzoic acid, bitertanole, carbaryl, epoxiconazole and 4-nitrophenol) into Arabidopsis thaliana leaves was measured and permeances P (ms⁻¹) were calculated. Thus, cuticular barrier properties of A. thaliana leaves have been characterized quantitatively. Epoxiconazole permeance of A. thaliana was 2.79 × 10⁻⁸ ms⁻¹. When compared with cuticular permeances measured with intact stomatous and astomatous leaf sides of Prunus laurocerasus, frequently used in the past as a model species studying cuticular permeability, A. thaliana has a 48- to 66-fold higher permeance. When compared with epoxiconazole permeability of isolated cuticles of different species (Citrus aurantium, Hedera helix and P. laurocerasus) A. thaliana permeability is between 17- to 199-fold higher. Co-permeability experiments, simultaneously measuring ¹⁴C-epoxiconazole and ³H₂O permeability of isolated cuticles of three species (C. aurantium, H. helix and P. laurocerasus) showed that ³H₂O permeability was highly correlated with epoxiconazole permeability. The regression equation of this correlation can be used predicting cuticular transpiration of intact stomatous leaves of A. thaliana, where a direct measurement of cuticular permeation using ³H₂O is impossible. Water permeance estimated for A. thaliana was 4.55 × 10⁻⁸ m⁻¹, which is between 12- and 91-fold higher than water permeances measured with isolated cuticles of C. aurantium, H. helix and P. laurocerasus. This indicates that cuticular water permeability of the intact stomatous leaves of the annual species A. thaliana is fairly high and in the upper range compared with most P values of perennial species published in the past.

The positional sterile (ps) mutation affects cuticular transpiration and wax biosynthesis of tomato fruits

Cuticular waxes are known to play a pivotal role in limiting transpirational water loss across primary plant surfaces. The astomatous tomato fruit is an ideal model system that permits the functional characterization of intact cuticular membranes and therefore allows direct correlation of their permeance for water with their qualitative and quantitative composition. The recessive positional sterile (ps) mutation, which occurred spontaneously in tomato (Solanum lycopersicum L.), is characterized by floral organ fusion and positional sterility. Because of a striking phenotypical similarity with the lecer6 wax mutant of tomato, which is defective in very-long-chain fatty acid elongation, ps mutant fruits were analyzed for their cuticular wax and cutin composition. We also examined their cuticular permeance for water following the developmental course of fruit ripening. Wild type and ps mutant fruits showed considerable differences in their cuticular permeance for water, while exhibiting similar quantitative wax accumulation. The ps mutant fruits showed a five- to eightfold increase in water loss per unit time and surface area when compared to the corresponding wild type fruits. The cuticular waxes of ps mutant fruits were characterized by an almost complete absence of n-alkanes and aldehydes, with a concomitant increase in triterpenoids and sterol derivatives. We also noted the occurrence of alkyl esters not present in the wild type. Quantitative and qualitative cutin monomer composition remained largely unaffected. The significant differences in the cuticular wax composition of ps mutant fruits induced a distinct increase of cuticular water permeance. The fruit wax compositional phenotype indicates the ps mutation is responsible for effectively blocking the decarbonylation pathway of wax biosynthesis in epidermal cells of tomato fruits.

Does the microclimate under hail nets influence micromorphological characteristics of apple leaves and cuticles?

A higher frequency of hail storms, possibly due to climate change, has led to increased installation of hail nets worldwide. The objective of the present work was to investigate potential effects of the microclimate under these hail nets on micromorphological characteristics of the leaves and adaxial leaf cuticles. Leaves of apple cultivars 'Pinova' and 'Fuji' grown on trees under white (highly translucent) or red-black (low transmittance) hail nets or on uncovered (control) trees were evaluated in June, August, September and October. The microclimate under the colored hail nets had no impact on leaf micromorphology, amount of cuticular wax, or leaf thickness. Similarly, no differences in thickness and permeability for calcium could be established between cuticles of leaves grown on trees under the two types of hail nets or uncovered trees. For all evaluated parameters, significant differences were detected between the two cultivars examined. In both cultivars, leaf wax synthesis followed a characteristic curve, increasing from the first to the second evaluation, and then decreasing continuously without affecting cuticular penetration of calcium. Overall, our results show that a reduction of the hail nets by 6-10% in both light and humidity was insufficient to influence the surface properties of apple leaves and permeability of cuticles. This may suggest that pest management strategies, i.e. formulation of agrochemicals, their application and dose, do not need to be adapted when used under hail nets. Overall, the present results indicate that the microclimatic changes brought about by colored hail nets are sufficient to enhance the vegetative growth and induce the 'shade avoidance syndrome', but do not appear to affect the leaf cuticular properties.

Cutin deficiency in the tomato fruit cuticle consistently affects resistance to microbial infection and biomechanical properties, but not transpirational water loss

Plant cuticles are broadly composed of two major components: polymeric cutin and a mixture of waxes, which infiltrate the cutin matrix and also accumulate on the surface, forming an epicuticular layer. Although cuticles are thought to play a number of important physiological roles, with the most important being to restrict water loss from aerial plant organs, the relative contributions of cutin and waxes to cuticle function are still not well understood. Tomato (Solanum lycopersicum) fruits provide an attractive experimental system to address this question as, unlike other model plants such as Arabidopsis, they have a relatively thick astomatous cuticle, providing a poreless uniform material that is easy to isolate and handle. We identified three tomato mutants, cutin deficient 1 (cd1), cd2 and cd3, the fruit cuticles of which have a dramatic (95-98%) reduction in cutin content and substantially altered, but distinctly different, architectures. This cutin deficiency resulted in an increase in cuticle surface stiffness, and in the proportions of both hydrophilic and multiply bonded polymeric constituents. Furthermore, our data suggested that there is no correlation between the amount of cutin and the permeability of the cuticle to water, but that cutin plays an important role in protecting tissues from microbial infection. The three cd mutations were mapped to different loci, and the cloning of CD2 revealed it to encode a homeodomain protein, which we propose acts as a key regulator of cutin biosynthesis in tomato fruit.

Differences between water permeability of astomatous and stomatous cuticular membranes: effects of air humidity in two species of contrasting drought-resistance strategy

Cuticular water permeabilities of adaxial and abaxial leaf surfaces and their dependence on relative air humidity (RH) applied in long-term and short-term regimes have been analysed for Hedera helix, native in a temperate climate, and Zamioculcas zamiifolia, native in subtropical regions. The water permeability of cuticular membranes (CM) isolated from the adaxial (astomatous) and abaxial (stomatous) leaf sides was measured using a method which allowed the separation of water diffusion through the remnants of the original stomatal pores from water diffusion through the solid cuticle. The long-term effects of low (20-40%) or high (60-80%) RH applied during plant growth and leaf ontogeny ('growth RH') and the short-term effects of applying 2% or 100% RH while measuring permeability ('measurement RH') were investigated. With both species, water permeability of the solid stomatous CM was significantly higher than the permeability of the astomatous CM. Adaxial cuticles of plants grown in humid air were more permeable to water than those from dry air. The adaxial CM of the drought-tolerant H. helix was more permeable and more sensitive to growth RH than the adaxial CM of Z. zamiifolia, a species avoiding water stress. However, permeability of the solid abaxial CM was similar in both species and independent of growth RH. The lack of a humidity response in the abaxial CM is attributed to a higher degree of cuticular hydration resulting from stomatal transpiration. The ecophysiological significance of higher permeability of the solid stomatous CM compared to the astomatous CM is discussed.

Heterologous expression of two Medicago truncatula putative ERF transcription factor genes, WXP1 and WXP2, in Arabidopsis led to increased leaf wax accumulation and improved drought tolerance, but differential response in freezing tolerance

Cuticular waxes are the major components of plant cuticle and play an important role in protecting aerial organs from damage caused by biotic and abiotic stresses. Here we report the functional characterization of two putative ERF transcription factor genes WXP1 and its paralog WXP2 from Medicago truncatula. Transgenic expression of WXP1 and WXP2 in Arabidopsis (ecotype Columbia) led to significantly increased cuticular wax deposition on leaves of 4-week-old and 6-week-old transgenic plants, assessed based on fresh weight or based on surface area. Differences in the accumulation of various wax components as well as their chain length distributions were found in the WXP1 and WXP2 plants. The major wax component in Arabidopsis, n-alkanes, increased substantially in both WXP1 and WXP2 transgenics, however, another wax component, primary alcohols, increased in WXP1 plants but decreased in WXP2 plants. Cuticle properties of the transgenic leaves were analyzed by chlorophyll leaching assay; while the WXP1 plants had no change, the WXP2 plants showed more chlorophyll leaching. Analysis of fresh weight loss from detached leaves revealed that the transgenic leaves tend to retain more water than the control. Both WXP1 and WXP2 transgenic plants showed significantly enhanced whole plant drought tolerance. Analysis of freezing tolerance at the whole plant level and measurement of electrolyte leakage from detached leaves revealed that the WXP1 plants had increased freezing tolerance while the WXP2 plants were more sensitive to low temperature when compared to the control. Transgenic expression of WXP1 had no obvious effects on plant growth and development, however, the expression of WXP2 led to slower plant growth. These results indicate that WXP1 is a useful candidate gene for improving plant drought and freezing tolerance by genetic transformation.

Effect of temperature on enzymatic and physiological factors related to chilling injury in carambola fruit (Averrhoa carambola L.)

Three groups of carambola fruits (Averrhoa carambola L.) were stored at 2 and 10 degrees C (85-90% relative humidity). The major physicochemical, physiological, and enzymatic responses of fruit were measured in each group over a 30-day period: chilling injury index (CII), decay (%), intracuticular waxes, cuticle permeability, pulp firmness, weight loss, sucrose, fructose and glucose contents, ion electrolyte leakage in pulp (%), ethylene and carbon dioxide production rates, and the activities of peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) enzymes. CII values were statistically different at 2 and 10 degrees C, showing high significance with respect to sucrose content and weight loss (P < 0.05). Chilling injury included darkened ribs and skin desiccation. According to the CI symptom development, a possible relationship of POD and PPO activities was found at 2 degrees C. A significant sucrose content increase was observed at 10 degrees C. CI symptoms were associated with POD and PAL activities.

Protecting against water loss: analysis of the barrier properties of plant cuticles

The cuticle is the major barrier against uncontrolled water loss from leaves, fruits and other primary parts of higher plants. More than 100 mean values for water permeabilities determined with isolated leaf and fruit cuticles from 61 plant species are compiled and discussed in relation to plant organ, natural habitat and morphology. The maximum barrier properties of plant cuticles exceed that of synthetic polymeric films of equal thickness. Cuticular water permeability is not correlated to the thickness of the cuticle or to wax coverage. Relationships between cuticular permeability, wax composition and physical properties of the cuticle are evaluated. Cuticular permeability to water increases on the average by a factor of 2 when leaf surface temperature is raised from 15 degrees C to 35 degrees C. Organic compounds of anthropogenic and biogenic origin may enhance cuticular permeability. The pathway taken by water across the cuticular transport barrier is reviewed. The conclusion from this discussion is that the bulk of water diffuses as single molecules across a lipophilic barrier while a minor fraction travels along polar pores. Open questions concerning the mechanistic understanding of the plant cuticular transport barrier and the role the plant cuticle plays in ensuring the survival and reproductive success of an individual plant are indicated.

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

Cuticular transpiration was measured in the temperature range between 10 degrees C and 55 degrees C using tritiated water and five species (Vinca major L., Prunus laurocerasus L., Forsythia intermedia L., Citrus aurantium L., and Hedera helix L.). Cuticular water permeabilities measured with isolated cuticular membranes were not different from cuticular water permeabilities measured with leaf discs. Depending on the species cuticular water permeabilities increased by factors between 12 (V. major) to 264 (H. helix) when temperature was increased from 10 degrees C to 55 degrees C. Arrhenius plots (lnP versus 1/T) of all investigated species were characterized by phase transitions occurring in the temperature range of 30-39 degrees C. Activation energies for water permeability across plant cuticles below and above the midpoint of phase transition were calculated from Arrhenius plots. Depending on the species they varied between 26 (F. intermedia) to 61 kJ mol(-1) (H. helix) below the phase transition and from 67 (V. major) to 122 kJ mol(-1) (F. intermedia) above the phase transition. Since the occurrence of phase transitions always lead to significantly increased rates of cuticular transpiration it is argued that temperatures higher than 35 degrees C caused structural defects to the transport-limiting barrier of the plant cuticles of all species investigated.

The internal cuticle of Cirsium horridulum (Asteraceae) leaves

Leaf internal cuticle has not previously been studied in detail, and yet its existence has profound implications for the path of water movement. The internal cuticle forms a uniform layer on the inner periclinal epidermal walls that border substomatal cavities. This cuticle is continuous with the external cuticle through the stomatal pores. The thickness of the internal cuticle on nonstomatal epidermal cells is approximately one-third that of the external cuticle on the same cells. On both the abaxial and adaxial sides of the leaf the internal cuticle forms irregularly shaped islands bordered by mesophyll cells. The size of the islands coincides with the epidermal area of the substomatal cavity. The internal cuticle remains intact and connected to the external cuticle after incubation in cellulytic enzymes. After treatment with sulfuric acid or chloroform, both cuticles remain intact. The autofluorescence of both cuticles is increased by staining with auramine O. These results indicate that large portions of the leaf epidermis are covered by both an internal and an external cuticle.

Ecophysiology of cuticular transpiration: comparative investigation of cuticular water permeability of plant species from different habitats

Water permeabilities of astomatous, isolated cuticular membranes (CM) of 24 different plants species were measured. Permeances varied from 1.7×10^-11 m·s^-1 (Vanilla planifolia leaf) up to 2.1×10^-9 m·s^-1 (Malus cf. domestica fruit) among different plant species, thus covering a range of over 2 orders of magnitude. Ranking of species according to permeances resulted in four distinct groups. The first group, of species with the lowest cuticular transpiration rates, included evergreen species growing in warm dry tropical climates (e.g. Vanilla planifolia and Monstera deliciosa leaves). The second class, with slightly higher water permeabilities, included evergreen species with typical scleromorphic leaf properties, adapted to a typical mediterranean type of climate with a dry period during the year (e.g. Citrus limon and Olea europaea leaves). The third group of species, where the highest leaf cuticular transpiration rates were observed, included deciduous species normally growing in a tempeate climate (e.g. Juglans regia and Forsythia suspensa leaves). Fruit cuticular membranes (CM) made up the fourth group (e.g. Capsicum annuum and Malus cf. domestica fruits), with even higher permeances than leaves of species from group 3. Thus, it appears that the plant species investigated show ecophysiological adaptations to the climatic demands of their natural habitats in cuticular water permeability.