Gas permeability of plant cuticles

Cuticles from the adaxial surface of Citrus aurantium L. leaves and from the pericarp of Lycopersicon esculentum L. and Capsicum annuum L. were isolated enzymatically and their oxygen permeability was determined. Isolated cuticles were mounted between a gaseous and an aqueous compartment with the physiological outer side of the membrane facing the gaseous compartment. Permeability for oxygen was characterized by permeability (P) and diffusion (D) coefficients. P and D were independent of the driving force (gradient of oxygen concentration) across the cuticle, thus, Henry's law was obeyed. P values for the diffusion of oxygen varied between 3·10^-7 (Citrus), 1.4·10^-6 (Capsicum), and 1.1·10^-6 (Lycopersicon) m·s^-1. Extraction of soluble lipids from the cuticles increased the permeability. By treating the cutin matrix and the soluble lipids as resistances in series, it could be demonstrated that the soluble lipids were the main resistance for oxygen permeability in Citrus cuticles. However, in Lycopersicon and Capsicum, both the cutin matrix and the soluble lipids determined the total resistance. P values were not affected by either the proton concentration (pH 3–9) or the cations (Na⁺, Ca²⁺) present at the morphological inner side of the cuticles. It is concluded that the water content of cuticles does not affect the permeability properties for oxygen. Partition coefficients indicated a high solubility of oxygen in the cuticle of Citrus. The data suggest a solubility process in the cuticle of Citrus with respect to oxygen permeation.Abbreviations CM cuticular membrane - MX cutin polymer matrix - SCL soluble cuticular lipids     

Water permeability of isolated cuticular membranes: The effect of cuticular waxes on diffusion of water

Summary The water permeability of astomatous cuticular membranes isolated from Citrus aurantium L. leaves, pear (Pyrus communis L.) leaves and onion (Allium cepa L.) bulb scales was determined before and after extraction of cuticular waxes with lipid solvents. In pear, the permeability coefficients for diffusion of tritiated water across cuticular membranes (CM) prior to extraction [P _d(CM)] decreased by a factor of four during leaf expansion. In all three species investigated P _d(CM) values of cuticular membranes from fully expanded leaves varied between 1 to 2×10^-7 cm^-3 s^-1·P _d(CM) values were not affected by pH. Extraction of cuticular waxes from the membranes increased their water permeability by a factor of 300 to 500. Permeability coefficients for diffusion of THO across the cutin matrix (MX) after extraction [P _d(MX)] increased with increasing pH. P _dvalues were not inversely proportional to the thickness of cuticular membranes. By treating the cutin matrix and cuticular waxes as two resistances acting in series it was shown that the water permeability of cuticles is completely determined by the waxes. The lack of the P _d(CM) values to respond to pH appeared to be due to structural effects of waxes in the cutin matrix. Cuticular membranes from the submerse leaves of the aquatic plant Potamogeton lucens L. were three orders of magnitude more permeable to water than the cuticular membranes of the terrestrial species investigated.Abbreviations CM cuticular membrane - MX cutin matrix - WAX waxes This study was supported by a grant from the Deutsche Forschungsgemeinschaft.     

Size selectivity of aqueous pores in astomatous cuticular membranes isolated from<Emphasis Type="Italic"> Populus canescens</Emphasis> (Aiton) Sm. leaves

Little is known about the permeability of plant cuticles to ionic molecules with hydration shells that render them lipid insoluble and limit their diffusion to narrow aqueous pores. Therefore, the permeation of cuticular membranes to ionised calcium salts with anhydrous molecular weights ranging from 111 to 755 g mol^–1 was studied. Penetration was a first-order process and rate constants (k) (proportional to permeability) decreased exponentially with molecular weight. Plots of log k vs. molecular weight had slopes of –2.11×10^–3 and –2.80×10^–3, respectively, depending on the year in which the cuticular membranes were isolated. This corresponds to decreases in permeability by factors of about 7 to 13 when molecular weight increased from 100 to 500 g mol^–1. This size selectivity is small compared to the dependence on molecular weight of solute mobility in Populus cuticles. A decrease in mobility of neutral molecules by more than 3 orders of magnitude has been reported [A. Buchholz et al. (1998) Planta 206:322–328] for the same range of molecular weights. Hence, discrimination of large ionic species diffusing in aqueous pores (polar pathway) is much smaller than that for neutral solutes diffusing in cutin and waxes (lipophilic pathway). This indicates that formulating large solutes as ionic species would be advantageous.Abbreviation CM Cuticular membrane     

Size selectivity of aqueous pores in stomatous cuticles of <Emphasis Type="Italic">Vicia faba</Emphasis> leaves

Size selectivity of aqueous pores in Vicia leaf cuticles was investigated by measuring the penetration of calcium salts into the abaxial surface of detached leaves. Molecular weights of salts ranged from 111 g mol^–1 to 755 g mol^–1. Penetration in light at 20°C and 100% humidity was a first order process and rate constants of penetration ranged from 0.39 h^–1 (CaCl₂) to 0.058 h^–1 (Ca-lactobionate). Penetration was a first order process in the dark as well, but the rate constants were smaller by a factor of 1.82. Plotting logarithmatised rate constants versus anhydrous molecular weights resulted in straight lines both in light and in the dark. The slopes per hour were very similar and the average slope was –1.2×10^–3 mol g^–1. Hence, size selectivity was not affected by stomatal opening, and in light or darkness permeability of Vicia cuticles decreased by a factor of 2.9 when molecular weight increased from 100 g mol^–1 to 500 g mol^–1. Silver nitrate was preferentially precipitated as silver chloride in guard cells, glandular trichomes and at the base of trichomes. It was concluded that these precipitates mark the location of aqueous pores in Vicia leaf cuticles. The size selectivity of aqueous pores in Vicia leaf cuticles is small compared to that observed in poplar leaf cuticles, in which permeability decreased by a factor of 7–13 for the same range of molecular weights. It is also much smaller than size selectivity of the lipophilic pathway in cuticles. These findings suggest that active ingredients of pesticides, growth regulators and chemical inducers with high molecular weights penetrate leaves at higher rates when formulated as ions.     

OZONE-INDUCED MEMBRANE PERMEABILITY CHANGES

The effect of 0.5 ppm ozone for 0.5-1 hr on plant cell membrane permeability was ascertained. Permeabilities to both water and solutes were estimated by measuring leaf disc weight changes and following tritiated water and ⁸⁶Rb fluxes. Measurements were made immediately after ozone exposure and 24 hr after exposure. The reflection coefficient, σ, an index of solute permeability, decreased in ozone-treated primary leaves of pinto bean (Phaseolus vulgaris). The latter indicates an increase in membrane solute permeability or internal solute leakage. Water and THO flux estimates both indicated a decrease in membrane permeability to water; both the hydraulic conductivity (L_p) and the water diffusional coefficient (L_D) apparently decreased, an anomaly which is discussed. These data indicate that ozone has a direct effect on membrane function by altering permeability characteristics. We assume from these data that cell membranes are primary target sites for ozone injury.     

Diffusion in plant cuticles as affected by temperature and size of organic solutes: similarity and diversity among species

Mobilities of lipophilic organic solutes in cuticular membranes (CM) isolated from mature leaves of Citrus aurantium L., Citrus grandis L., Hedera helix L., IIex aquifolium L., Ilex paraguariensis St.-Hil., Mains domestica Borkh., Prunus armeniaca L., Primus laurocerasus L., Pyrus communis L., Pyrus pyrifolia (Burm. f.) Nakai, Stephanotis florihunda Brongn. and Strophantus gratus Baill. were measured over a temperature range of 15–78°C. In this range, solute mobilities increased up to 1000-fold, which corresponds to temperature coefficients Q₁₀ of 3 (IAA in P. armeniaca) to 14 (cholesterol in H. helix). For most species, Arrhenius graphs showed good linearity up to 40°C, and up to 78°C for some species, while for others activation energies declined with increasing temperature. However, no distinct phase transitions caused by sudden structural changes in the CM were observed. In three species we examined whether heating to 70°C changed solute mobility irreversibly by comparing Arrhenius graphs for two successive experiments with the same CM. The two graphs were very similar for P. laurocerasus, while mobilities in the second graph were somewhat reduced for C. aurantium and greatly increased (at 25 and 35°C) for H. helix. This indicates rearrangements of at least some wax constituents when heated to high temperatures. The activation energies of diffusion (E_D) ranged from 75 to 189 KJ mol^?11 depending on species and solute size. Size selectivity and variability between cuticles decreased with increasing temperature, and this is caused by differences in (E_D). An excellent correlation between the pre-exponential factor of the Arrhenius equation and E_D was observed, which is evidence that organic solutes differing greatly in molecular size (130–349 cm³ mol^?1) and cuticle/water partition coefficient (25–10⁸) use similar diffusion paths in the CM of all 12 plant species tested. Diffusion occurs in regions with identical physicochemical properties and differs only in magnitude.     

Decomposition and sorption characterization of plant cuticles in soil

The sorption of organic compounds by plant cuticular matter has been extensively investigated; however, little has been studied regarding the effect of plant cuticle degradation on their role in the sorption of organic compounds in soils. The sorption of phenanthrene was studied in soil samples which had been incubated for up to 9 months with three different types of plant cuticle isolated from tomato fruits, pepper fruits and citrus leaves. The main change in the diffuse reflectance Fourier-transform infrared (DRIFT) spectra during incubation of the cuticles was related to cutin decomposition. The peaks assigned to methyl and ethyl vibration and C=O vibration in ester links decreased with decomposition. In general, with all samples, the phenanthrene sorption coefficients calculated for the whole incubated soils (K _d) decreased with incubation time. In contrast, the carbon-normalized K _d values (K _oc) did not exhibit a similar trend for the different cuticles during incubation. The origin of the cuticle also affected the linearity of the sorption isotherms. With the tomato and citrus cuticle samples, the Freundlich N values were close to unity and were stable throughout incubation. However with the green pepper cuticle, the N values exhibited a significant decrease (from 0.98 to 0.70). This study demonstrates that the structural composition of the plant cuticle affects its biodegradability and therefore its ability to sorb organic compounds in soils. Of the residues originating from plant cuticular matter in soils, the cutan biopolymer and lignin-derived structures appear to play a dominant role in sorption as decomposition progresses. Responsible Editor: Alfonso Escudero.     

Determination of water sorption by cuticles isolated from fir tree needles

Summary The water sorption by isolated cuticles from needles of Abies alba was measured between 4% and 80% relative humidity using a magnetic suspension microbalance. The sorption isotherms were not linear and sorption increased more rapidly at the lowest and highest values of relative humidity. The mean values, calculated for 1- to 5-year-old adaxial cuticles developed from 1984 to 1988, increased from 2.9% to 17.1% of the dehydrated weight, and from 4% to 80% relative humidity. Results did not depend on the age of needles and the comparison between healthy and declining trees revealed no major difference in water sorption by cuticles isolated from these two types of trees. Data are discussed in relation to cuticular permeability and one determinant of water permeability, the partition coefficient relating the equilibrium water concentration of the cuticle to that of the surrounding atmosphere, was calculated.     

A radioactive assay allowing the quantitative measurement of cuticular permeability of intact <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> 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 ms⁻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.     

Thermodynamic analysis of diffusion of non-electrolytes across plant cuticles in the presence and absence of the plasticiser tributyl phosphate

 Solute mobility in cuticular membranes (CMs) of 14 plant species (Citrus aurantium L., Citrus grandis L., Hedera helix L., Ilex aquifolium L., Ilex paraguariensis St.-Hil., Malus domestica Borkh. cv. Golden Delicious, Populus alba L., Prunus laurocerasus L., Pyrus communis L. cv. Bartlett, Conference and Gellerts Butterbirne, Pyrus pyrifolia (Burm. f.) Nakai, Schefflera actinophylla (Endl.) Harms and Strophanthus gratus Baill.) was measured over the temperature range 25–55 °C. The five organic model compounds differed in size (130–349 cm³ mol⁻¹) and cuticle/water partition coefficient (18–10⁸). For all individual CMs (n = 297), the data were plotted according to the thermodynamic relationship between the preexponential factor (which is proportional to entropy) of the Arrhenius equation and the activation energy (enthalpy) of diffusion (E _D ). A strict linear correlation was obtained, providing evidence that the five compounds diffused along the same lipophilic diffusion path in all plant species tested. Extracting cuticular waxes from CMs of four plant species (Hedera, Pyrus, Schefflera and Strophanthus) had no effect on the slope of the plot but a parallel displacement towards higher entropy was observed with these polymer matrix (MX) membranes. This displacement is interpreted as a temperature-independent tortuosity factor directly related to entropy. The influence of the plasticiser tributyl phosphate on solute mobility at various temperatures was measured for CM and MX membranes. The plasticiser increased solute mobility and E _D was reduced drastically for both membrane types. This plasticiser effect was almost completely reversible, when tributyl phosphate was desorbed from the membranes. For both, plasticised CM and MX, the thermodynamic correlation exists whereby all data points lie on the same line. The data are used to characterise the lipophilic pathway across plant cuticles in terms of the free-volume theory. Received: 14 December 1999 / Accepted: 31 March 2000     

Characterization of the diffusion of non-electrolytes across plant cuticles: properties of the lipophilic pathway

Systemic crop protection products are commonly sprayed onto foliage, whereupon the active substances must penetrate into the leaves in order to become biologically active. Penetration of the plant cuticle is the rate-limiting step. The diffusion of organic non-electrolytes within cuticles is a purely physical process that can be described and analysed in the same way as is done for diffusion in synthetic polymer membranes. Solute mobilities in cuticles vary considerably between plant species. For a given species they decrease with increasing solute size, and this size selectivity holds for all of the plant species investigated so far. Wax extraction from leaf cuticles increases the mobility of solutes tremendously, but size selectivity is not affected. Furthermore, diffusion within plant cuticles is extremely temperature dependent. An analogous increase in solute mobility can be achieved by using accelerators, which enhance the fluidity of the polymer matrix and of the waxes. The effects of temperature and plasticizers on the diffusion of non-electrolytes in wax and the cutin matrix have been used to characterize the nature of the lipophilic pathway. The 'free volume' theory can be used to explain the influence of the size and shape of the solute, and its dependence on temperature. The physico-chemical nature of the diffusion pathway has been shown, by thermodynamic analysis, to be identical for a wide range of solute lipophilicities. This approach also explains the mode of action and the intrinsic activity of plasticizers.     

Water permeability of plant cuticles: The effect of temperature on diffusion of water

The effect of temperature on water permeability of plant cuticles (astomatous Citrus leaf cuticles) has been investigated. The Arrhenius plot (logarithm of the permeability coefficient vs. 1/temperature) has two linear portions that intersect at 44° C. Evidence is presented to show that this intersection represents the solid/liquid phase transition of cuticular lipids. As the Arrhenius plot has only one phase transition in the temperature range of 5 to 80° C, it appears that all soluble cuticular lipids in the cuticle are present as a homogeneous mixture rather than as individual layers differing in composition. This view is supported by electron spin resonance evidence showing homogenous distribution of spin label fatty acids. The original distribution of soluble cuticular lipids is irreversibly altered by heating cuticular membranes above the transition temperature. This is accompanied by an irreversible increase in water peremeability, demonstrating the importance of the structure of cuticular lipids with regard to cuticular permeability.Abbreviations CM cuticular membranes - MX polymer matrix - SCL soluble cuticular lipids - MES morpholinoethane sulphonic acid - J flux - ESR electron spin resonance - THO tritiated water     

Cation Penetration through Isolated Leaf Cuticles

The rates of penetration of various cations through isolated apricot Prunus armeniaca L. leaf cuticles were determined. Steady state rates were measured by using a specially constructed flow-through diffusion cell. The penetration rates of the monovalent cations in group IA followed a normal lyotropic series, i.e., CS⁺ ≥ Rb⁺ > K⁺ > Na⁺ > Li⁺. The divalent cations all penetrated through the cuticle more slowly than the monovalent cations. Comparison of the relative values of k (permeability coefficient) and D (diffusion coefficient) indicates that the penetration of ions through isolated cuticles took place by diffusion and was impeded by charge interactions between the solute and charge sites in the penetration pathway. Cuticular penetration rates of K⁺ and H₂O at pH above 9 were of similar magnitude. At pH 5.5 H₂O penetration was not affected but that of K⁺ was greatly reduced. From this observation and from data on cuticle titration and ion adsorption studies, we hypothesize that cuticular pores are lined with a substance (perhaps a protein) which has exposed positively charged sites.     

Patterns of Effective Permeability of Leaf Cuticles to Acids

Plants in the field are frequently exposed to anthropogenic acid precipitation with pH values of 4 and below. For the acid to directly affect leaf tissues, it must pass through the leaf cuticle, but little is known about the permeability of cuticles to protons, or about the effect of different anions on this permeability. We investigated the movement of protons through isolated astomatous leaf cuticles of grapefruit (Citrus X paradisi Macfady.), rough lemon (Citrus limon [L.] Burm. fils cv Ponderosa), and pear (Pyrus communis L.) using hydrochloric, sulfuric, and nitric acids. Cuticles were enzymically isolated from leaves and placed in a diffusion apparatus with pH 4 acid on the morphological outer surface of the cuticle and degassed distilled water on the inner surface. Changes in pH of the solution on the inner surface were used to determine rates of effective permeability of the cuticles to the protons of these acids. Most cuticles exhibited an initial low permeability, lasting hours to days, then after a short transition displayed a significantly higher permeability, which persisted until equilibrium was approached. The change in effective permeability appears to be reversible. Effective permeabilities were higher for sulfuric acid than for the others. A model of the movement of protons through the cuticle is presented, proposing that dissociated acid groups in channels within the cutin are first protonated by the acid, accounting for the low initial effective permeability; then protons pass freely through the channels, resulting in a higher effective permeability.     

137Cs penetration by contact exchange through isolated plant cuticles: cuticles as asymmetric transport membranes

The penetration of ¹³⁷Cs by contact exchange through cuticular membranes from the adaxial surface of leaves of Pyrus communis and Prunus cerasus has been investigated. The resistance of the cuticles to the caesium penetration was dependent on the counter-ions associated with the fixed negative ion exchange sites in the membrane. The mobility of hydrated potassium ions and their tenuous connection to -COOH^?-groups in the membrane encouraged caesium permeation in contrast to cuticular membranes with predominantly protonized ion exchange sites. Divalent calcium ions caused a strong reduction (4–20 times) of the caesium permeability which is decisive for the calculation of the caesium uptake by the intact leaf. Under these conditions, a penetration rate of the deposited caesium of 0·11±0·05% h^?1 for pear cuticles and of 0·036±0·025% h^?1 for cherry cuticles was measured after the adjustment to steady state conditions. Approximately 12–24% and 4·5–7·5% of initially retained caesium could be absorbed by the leaves of pear and cherry, respectively, in a rain-free period of 7 d in the area of Munich after wet deposition of fallout from the Chernobyl reactor accident. Furthermore, the caesium penetration from the physiological inside to the outside of the membrane was found to be smaller by a factor of 100–150 compared with that of the opposite direction.     

Solute dynamics in a North Brazilian mangrove: the influence of sediment permeability and freshwater input

Although water in mangrove sediments influences nutrient cycling in both, mangrove forest and estuary, little information exists on seasonal and vertical distribution of dissolved organic and inorganic compounds in the sediment column. We studied the influence of sediment texture and chemistry, permeability (K), tides, and rainfall on dissolved organic carbon (DOC) and nitrogen (DON), dissolved inorganic phosphate (DIP) and salinity in creek and sediment waters of a mangrove in Pará, Brazil. Water samples were taken from boreholes and piezometers in the mangrove forest and from an adjacent tidal creek at neap and spring tides, during the dry and rainy season. Forest sediment was analysed for carbon (C), nitrogen (N), salinity and permeability. Clay, C and N decreased with depth. Sediment permeability (K) was lowest (<0.1 m day⁻¹) in the upper, clay-rich and crab-burrow-free mud layer. In the deeper, fine sand strata, K ranged from 0.7 to 1.8 m day⁻¹. Tidal range in the creek was 3.5 and 5.5 m for neap and spring tides, respectively. Salinity, DOC, DON and DIP in creek water were inversely related to tidal height. Piezometer data revealed significant water level changes in deeper, sandy sediment layer, which followed, time-lagged, the tidal fluctuations. In contrast, tide did not affect the water level in the upper sediment due to low permeability. Compared with creek water, sediment water was enriched in DOC, DON and DIP because of organic matter input and mineralization. In deeper layers, solute concentration was most likely affected by sorption processes (DOC and DIP) and reduction reactions (DIP). During the rainy season, DOC and DON in creek and sediment water were higher than in the dry season. DIP appeared invariant to seasonal changes. In the rainy season, salt flushing from surface sediments resulted in higher salinities at intermediate sediment depths, while in the deeper layers salinity was lower due to exchange with water from the tidal creek.     

Leaf cuticles behave as asymmetric membranes : evidence from the measurement of diffusion potentials

Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCl were perfused over the physiological inner and outer surfaces while the electrical potential (E¹⁰) that developed across the cuticles and was caused by ion diffusion was measured. E¹⁰ was always positive, indicating that the permeability of K⁺ was always greater than that of Cl^-. Measured E¹⁰ in cuticles did not fit the Goldman equation, whereas, E¹⁰ measured during KCl diffusion across selected artificial membranes fit the equation. The magnitude of E¹⁰ in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution.     

The electrical response ofAvena coleoptile cortex to auxins

Solute mobilities of 28 compounds in isolated cuticular membranes (CM) from Capsicum annuum L. fruit, Citrus aurantium L. and Pyrus communis L. leaves were studied using unilateral desorption from the outer surface. First-order rate constants of desorption (k^*), which are directly proportional to the diffusion coefficient in the waxy outer limiting skins of cuticles were measured. When log k^* was plotted vs. molar volumes of test compounds linear graphs were obtained. The y-intercepts of these graphs (k^*) represent the mobility of a hypothetical molecule having zero molar volume and the slopes of the graphs () represent the size selectivity of the barrier and are related to the free volume available for diffusion. Thus, solute mobilities in cuticles are composed of two independent terms which are subtractive. If k^* and are known, k^* can be estimated for any solute from its molar volume (V_x) using the equation log k^*=log k^* –V_x. These parameters were used to analyse the effects of plant species, extraction of cuticular waxes and molecular structure of solutes on solute mobilities in plant cuticles. For aliphatic solutes, k^* was a factor of 10 smaller than for cyclic compounds, while was 0.011 and 0.012, respectively. The k^*-values for CM of the three species were very similar, but was higher for bitter-orange CM (0.012) than for those of pepper fruits and pear leaves (0.009). This has the consequence that differences in solute mobilities (k^*) among cuticles from different plan species increase with increasing molar volumes of solutes. Our data and our analysis provide evidence that constituents of cuticular waxes are mobile, at least in the solid amorphous wax fraction, but mobility decreases rapidly with increasing molar volume. For instance, if amounts to 0.01, mobilities of wax monomers decrease by a factor of 10 for every increase in molar volume of 100 cm³ · mol^–1. Thus, hexadecanoic acid is quite mobile in the amorphous wax fraction of Citrus (k^*=1.5×10^–6·s^–1), but for dotriacontane having twice the molar volume, k^* was only 2.5×10^–9·s^–1, which is almost three orders of magnitude smaller. Wax esters have even higher molar volumes and their mobilities will be even smaller (about 4×10^–12·s^–1 for a C₄₈-ester). Since low chain mobilities are a prerequisite for low mobilities and permeabilities, the selective advantage of high-molecular-weight wax monomers in plant cuticular waxes becomes obvious. Extracting cuticular waxes from pear leaf CM increased solute mobilities by a factor of 182, but it had no effect on size selectivity. We interpret this result as evidence to the effect that cuticular waxes reduce mobility by increasing tortuosity of the diffusion path, rather than by decreasing the mean free path of diffusional jumps and jump frequencies of diffusants.Abbreviations CM cuticular membrane(s) - 2,4-D 2,4-dichloro-phenoxyacetic acid - LAB lactic acid buffer - MX polymer matrix membranes - UDOS unilateral desorption from the outer surface     

Co-permeability of 3H-labelled water and 14C-labelled organic acids across isolated Prunus laurocerasus cuticles: effect of temperature on cuticular paths of diffusion

Co‐permeability of ³H‐labelled water and ¹⁴C‐labelled benzoic acid or 2,4‐dichlorophenoxyacetic acid across isolated cuticular membranes of Prunus laurocerasus L. was measured at temperatures ranging from 15 to 50 °C. The water and benzoic acid permeances were highly correlated over the whole temperature range investigated, whereas water and 2,4‐dichlorophenoxyacetic acid permeances were only correlated between 15 and 30 °C. The activation energies of cuticular permeability calculated from Arrhenius plots were 40 kJ mol^?1 for water and benzoic acid and 115 kJ mol^?1 for 2,4‐dichlorophenoxyacetic acid. The slopes of the Arrhenius plots of 2,4‐dichlorophenoxyacetic acid were linear between 15 and 50 °C, whereas pronounced phase transitions around 30 °C were observed for water and benzoic acid permeability. However, with isolated polymer matrix membranes, where cuticular waxes forming the transport‐limiting barrier of cuticles have been extracted, phase transitions were not observed for water and benzoic acid. It is concluded that temperatures above 30 °C caused structural changes in the transport‐limiting barrier of the cuticles leading to additional paths of diffusion for water and benzoic acid but not for 2,4‐dichlorophenoxyacetic acid.     

The interpretation of leaf-drying curves

A theoretical expression for the rate of water loss through the surface of excised leaves has been derived so that curves fitted to experimental data can be used to infer physical properties of the leaves such as the solute concentration and the conductance for water vapour loss. The use of the equations is illustrated by reference to data from an experiment in which red spruce (Picea rubens Sarg.) and Norway spruce (P. abies(L.) Karst.) seedlings were grown under five different regimes: outdoors (±50% shade), in a greenhouse (±50% shade) and in controlled-environment chambers.