Modelling penetration of plant cuticles by crop protection agents and effects of adjuvants on their rates of penetration

PROTAC for agriculture: learning from human medicine to generate new biotechnological weed control solutions

Weed control has relied on the use of organic and inorganic molecules that interfere with druggable targets, especially enzymes, for almost a century. This approach, although effective, has resulted in multiple cases of herbicide resistance. Furthermore, the rate of discovery of new druggable targets that are selective and with favorable environmental profiles has slowed down, highlighting the need for innovative control tools. The arrival of the biotechnology and genomics era gave hope to many that all sorts of new control tools would be developed. However, the reality is that most efforts have been limited to the development of transgenic crops with resistance to a few existing herbicides, which in fact is just another form of selectivity. Proteolysis-targeting chimera (PROTAC) is a new technology developed to treat human diseases but that has potential for multiple applications in agriculture. This technology uses a small bait molecule linked to an E3 ligand. The 3-dimensional structure of the bait favors physical interaction with a binding site in the target protein in a manner that allows E3 recruitment, ubiquitination and then proteasome-mediated degradation. This system makes it possible to circumvent the need to find druggable targets because it can degrade structural proteins, transporters, transcription factors, and enzymes without the need to interact with the active site. PROTAC can help control herbicide-resistant weeds as well as expand the number of biochemical targets that can be used for weed control. In the present article, we provide an overview of how PROTAC works and describe the possible applications for weed control as well as the challenges that this technology might face during development and implementation for field uses. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Measuring the photostability of agrochemicals on leaves: understanding the balance between loss processes and foliar uptake

BACKGROUND

Photostability is an important property in agrochemicals, impacting their biological efficacy, environmental fate and registrability. As such, it is a property that is routinely measured during the development of new active ingredients and their formulations. To make these measurements, compounds are typically exposed to simulated sunlight after application to a glass substrate. While useful, these measurements neglect key factors that influence photostability under true field conditions. Most importantly, they neglect the fact that compounds are applied to living plant tissue, and that uptake and movement within this tissue provides a mechanism to protect compounds from photodegradation.

RESULTS

In this work, we introduce a new photostability assay incorporating leaf tissue as a substrate, designed to run at medium throughput under standardized laboratory conditions. Using three test cases, we demonstrate that our leaf-disc-based assays provides quantitatively different photochemical loss profiles to an assay employing a glass substrate. And we also demonstrate that these different loss profiles are intimately linked to the physical properties of the compounds, the effect that those properties have on foliar uptake and, thereby, the availability of the active ingredient on the leaf surface.

CONCLUSIONS

The method presented provides a quick and simple measure of the interplay between abiotic loss processes and foliar uptake, supplying additional information to facilitate the interpretation of biological efficacy data. The comparison of loss between glass slides and leaves also provides a better understanding of when intrinsic photodegradation is likely to be a good model for a compound's behaviour under field conditions. © 2023 Society of Chemical Industry.

A novel approach for measuring membrane permeability for organic compounds via surface plasmon resonance detection

Well-known methods for measuring permeability of membranes include static or flow diffusion chambers. When studying the effects of organic compounds on plants, the use of such model systems allows to investigate xenobiotic behavior at the cuticular barrier level and obtain an understanding of the initial penetration processes of these substances into plant leaves. However, the use of diffusion chambers has disadvantages, including being time-consuming, requiring sampling, or a sufficiently large membrane area, which cannot be obtained from all types of plants. Therefore, we propose a new method based on surface plasmon resonance imaging (SPRi) to enable rapid membrane permeability evaluation. This study presents the methodology for measuring permeability of isolated cuticles for organic compounds via surface plasmon resonance detection, where the selected model analyte was the widely used pesticide metazachlor. Experiments were performed on the cuticles of Ficus elastica, Citrus pyriformis, and an artificial PES membrane, which is used in passive samplers for the detection of xenobiotics in water and soils. The average permeability for metazachlor was 5.23 × 10^-14 m² s^-1 for C. pyriformis, 1.34 × 10^-13 m² s^-1 for F. elastica, and 7.74 × 10^-12 m² s^-1 for the PES membrane. We confirmed that the combination of a flow-through diffusion cell and real-time optical detection of transposed molecules represents a promising method for determining the permeability of membranes to xenobiotics occurring in the environment. This is necessary for determining a pesticide dosage in agriculture, selecting suitable membranes for passive samplers in analytics, testing membranes for water treatment, or studying material use of impregnated membranes.

Modeling Transcuticular Uptake from Particle-Based Formulations of Lipophilic Products

We report a mathematical model for the uptake of lipophilic agrochemicals from dispersed spherical particles within a formulation droplet across the leaf cuticle. Two potential uptake pathways are identified: direct uptake via physical contact between the cuticle and particle and indirect uptake via initial release of material into the formulation droplet followed by partition across the cuticle-formulation interface. Numerical simulation is performed to investigate the relevance of the particle-cuticle contact angle, the release kinetics of the particle, and the particle size relative to the cuticle thickness. Limiting cases for each pathway are identified and investigated. The input of typical physicochemical parameters suggests that the indirect pathway is generally dominant unless pesticide release is under strict kinetic control. Evidence is presented for a hitherto unrecognized “leaching effect” and the mutual exclusivity of the two pathways.

RNAi as a Foliar Spray: Efficiency and Challenges to Field Applications

RNA interference (RNAi) is a powerful tool that is being increasingly utilized for crop protection against viruses, fungal pathogens, and insect pests. The non-transgenic approach of spray-induced gene silencing (SIGS), which relies on spray application of double-stranded RNA (dsRNA) to induce RNAi, has come to prominence due to its safety and environmental benefits in addition to its wide host range and high target specificity. However, along with promising results in recent studies, several factors limiting SIGS RNAi efficiency have been recognized in insects and plants. While sprayed dsRNA on the plant surface can produce a robust RNAi response in some chewing insects, plant uptake and systemic movement of dsRNA is required for delivery to many other target organisms. For example, pests such as sucking insects require the presence of dsRNA in vascular tissues, while many fungal pathogens are predominately located in internal plant tissues. Investigating the mechanisms by which sprayed dsRNA enters and moves through plant tissues and understanding the barriers that may hinder this process are essential for developing efficient ways to deliver dsRNA into plant systems. In this review, we assess current knowledge of the plant foliar and cellular uptake of dsRNA molecules. We will also identify major barriers to uptake, including leaf morphological features as well as environmental factors, and address methods to overcome these barriers.

Diffusion of volatile organics and water in the epicuticular waxes of petunia petal epidermal cells

Plant cuticles are a mixture of crystalline and amorphous waxes that restrict the exchange of molecules between the plant and the atmosphere. The multicomponent nature of cuticular waxes complicates the study of the relationship between the physical and transport properties. Here, a model cuticle based on the epicuticular waxes of Petunia hybrida flower petals was formulated to test the effect of wax composition on diffusion of water and volatile organic compounds (VOCs). The model cuticle was composed of an n-tetracosane (C₂₄ H₅₀ ), 1-docosanol (C₂₂ H₄₅ OH), and 3-methylbutyl dodecanoate (C₁₇ H₃₄ O₂ ), reflecting the relative chain length, functional groups, molecular arrangements, and crystallinity of the natural waxes. Molecular dynamics simulations were performed to obtain diffusion coefficients for compounds moving through waxes of varying composition. Simulated VOC diffusivities of the model system were found to highly correlate with in vitro measurements in isolated petunia cuticles. VOC diffusivity increased up to 30-fold in completely amorphous waxes, indicating a significant effect of crystallinity on cuticular permeability. The crystallinity of the waxes was highly dependent on the elongation of the lattice length and decrease in gap width between crystalline unit cells. Diffusion of water and higher molecular weight VOCs were significantly affected by alterations in crystalline spacing and lengths, whereas the low molecular weight VOCs were less affected. Comparison of measured diffusion coefficients from atomistic simulations and emissions from petunia flowers indicates that the role of the plant cuticle in the VOC emission network is attributed to the differential control on mass transfer of individual VOCs by controlling the composition, amount, and dynamics of scent emission.

Alcohol Ethoxylates Enhancing the Cuticular Uptake of Lipophilic Epoxiconazole Do Not Increase the Rates of Cuticular Transpiration of Leaf and Fruit Cuticles

Surfactants are known to enhance the foliar uptake of agrochemicals by plasticizing the transport-limiting barrier of plant cuticles. The effects of two different polydisperse alcohol ethoxylates with a low degree [mean ethoxylation of 5 ethylene oxide units (EOs)] and a high degree (mean ethoxylation of 10 EOs) of ethoxylation on cuticular barrier properties were investigated. The diffusion of the lipophilic organic molecule ¹⁴C-epoxiconazole and of polar ³H-water across cuticles isolated from six different plant species was investigated. At low surfactant coverages (10 μg cm^-2), the diffusion of water across the cuticles was not affected by the two surfactants. Only at very high surfactant coverages (100-1000 μg cm^-2) was the diffusion of water enhanced by the two surfactants between 5- and 50-fold. Unlike that of water, the diffusion of epoxiconazole was significantly enhanced 12-fold at surfactant coverages of 10 and 100 μg cm² by the surfactant with low ethoxylation (5 EOs), and it decreased to 6-fold at a surfactant coverage of 1000 μg cm^-2. The alcohol ethoxylate with a high degree of ethoxylation (10 EOs) only weakly increased the epoxiconazole diffusion. Our results clearly indicate that those surfactants that significantly enhance the uptake of the lipophilic agrochemicals (e.g., epoxiconazole) at a realistic leaf surface coverage of 10 μg cm^-2, as is applied in the field, do not interfere with cuticular transpiration as an unwanted negative side effect.

Cutin:cutin-acid endo-transacylase (CCT), a cuticle-remodelling enzyme activity in the plant epidermis

Cutin is a polyester matrix mainly composed of hydroxy-fatty acids that occurs in the cuticles of shoots and root-caps. The cuticle, of which cutin is a major component, protects the plant from biotic and abiotic stresses, and cutin has been postulated to constrain organ expansion. We propose that, to allow cutin restructuring, ester bonds in this net-like polymer can be transiently cleaved and then re-formed (transacylation). Here, using pea epicotyl epidermis as the main model, we first detected a cutin:cutin-fatty acid endo-transacylase (CCT) activity. In-situ assays used endogenous cutin as the donor substrate for endogenous enzymes; the exogenous acceptor substrate was a radiolabelled monomeric cutin-acid, 16-hydroxy-[3H]hexadecanoic acid (HHA). High-molecular-weight cutin became ester-bonded to intact [3H]HHA molecules, which thereby became unextractable except by ester-hydrolysing alkalis. In-situ CCT activity correlated with growth rate in Hylotelephium leaves and tomato fruits, suggesting a role in loosening the outer epidermal wall during organ growth. The only well-defined cutin transacylase in the apoplast, CUS1 (a tomato cutin synthase), when produced in transgenic tobacco, lacked CCT activity. This finding provides a reference for future CCT protein identification, which can adopt our sensitive enzyme assay to screen other CUS1-related enzymes.

A modified method for enzymatic isolation of and subsequent wax extraction from Arabidopsis thaliana leaf cuticle

BACKGROUND

The plant cuticle represents one of the major adaptations of vascular plants to terrestrial life. Cuticular permeability and chemical composition differ among species. Arabidopsis thaliana is a widely used model for biochemical and molecular genetic studies in plants. However, attempts to isolate the intact cuticle from fresh leaves of Arabidopsis have failed so far. The goal of this study was to optimise an enzymatic method for cuticle isolation of species with a thin cuticle and to test it on several A. thaliana wild types and mutants.

RESULTS

We developed a method for isolation of thin cuticles that allows reducing the isolation time, the separation of abaxial and adaxial cuticles, and avoids formation of wrinkles. Optical microscopy was used for studying cuticle intactness and scanning electron microscopy for visualisation of external and internal cuticle structures after isolation. Wax extracts were analysed by GC-MS. Isolation of intact cuticle was successful for all tested plants. The wax compositions (very-long-chained fatty acids, alcohols and alkanes) of intact leaves and isolated cuticles of wild type Col-0 were compared.

CONCLUSIONS

We conclude that the optimised enzymatic method is suitable for the isolation of A. thaliana adaxial and abaxial cuticles. The isolated cuticles are suitable for microscopic observation. Analysis of wax composition revealed some discrepancies between isolated cuticles and intact leaves with a higher yield of wax in isolated cuticles.

A revised model of fungicide translaminar activity

Translaminar redistribution is valuable for fungicide activity but difficult to measure and predict. The translaminar activity of 38 fungicides active against cucumber powdery mildew was measured experimentally and used to develop a QSAR (Quantitative structure-activity relationship) model of translaminar movement from calculated parameters. Over 300 physiochemical parameters generated from energy-minimized 3D structures were considered and one-parameter, two-parameter, and five-parameter models were developed. The one-parameter lipophilicity model explained 39% of variability in translaminar activity in the full dataset but none of the variability in the small succinate dehydrogenase inhibitor (SDHI) set. Adding a polar surface area parameter to the lipophilicity parameter improved predictability to 52% and explained over 70% of the variability in the SDHI class. The expanded model with five physiochemical parameters explained more than 80% of the variability in overall translaminar redistribution. The three additional parameters were correlated with molecular size and reactivity. The models were validated with a Leave-One-Out method that showed excellent robustness (r²adj = 0.83, q² = 0.79, p < .0001) for the five-parameter model. Because the models require only calculated parameters from a 3D chemical structure, they could enable the design or selection of compounds likely to be translaminar.

The permeation barrier of plant cuticles: uptake of active ingredients is limited by very long-chain aliphatic rather than cyclic wax compounds

BACKGROUND

The barrier to diffusion of organic solutes across the plant cuticle is composed of waxes consisting of very long-chain aliphatic (VLCA) and, to varying degrees, cyclic compounds like pentacyclic triterpenoids. The roles of both fractions in controlling cuticular penetration by organic solutes, e.g. the active ingredients (AI) of pesticides, are unknown to date. We studied the permeability of isolated leaf cuticular membranes from Garcinia xanthochymus and Prunus laurocerasus for lipophilic azoxystrobin and theobromine as model compounds for hydrophilic AIs.

RESULTS

The wax of P. laurocerasus consists of VLCA (12%) and cyclic compounds (88%), whereas VLCAs make up 97% of the wax of G. xanthochymus. We show that treating isolated cuticles with methanol almost quantitatively releases the cyclic fraction while leaving the VLCA fraction essentially intact. All VLCAs were subsequently removed using chloroform. In both species, the permeance of the two model compounds did not change significantly after methanol treatment, whereas chloroform extraction had a large effect on organic solute permeability.

CONCLUSION

The VLCA wax fraction makes up the permeability barrier for organic solutes, whereas cyclic compounds even in high amounts have a negligible role. This is of significance when optimizing the foliar uptake of pesticides. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mathematical Modelling of Hydrophilic Ionic Fertiliser Diffusion in Plant Cuticles: Lipophilic Surfactant Effects

The global agricultural industry requires improved efficacy of sprays being applied to weeds and crops to increase financial returns and reduce environmental impact. Enhancing foliar penetration is one way to improve efficacy. Within the plant leaf, the cuticle is the most significant barrier to agrochemical diffusion. It has been noted that a comprehensive set of mechanisms for ionic active ingredient (AI) penetration through plant leaves with surfactants is not well defined, and oils that enhance penetration have been given little attention. The importance of a mechanistic mathematical model has been noted previously in the literature. Two mechanistic mathematical models have been previously developed by the authors, focusing on plant cuticle penetration of calcium chloride through tomato fruit cuticles. The models included ion binding and evaporation with hygroscopic water absorption, along with the ability to vary the AI concentration and type, relative humidity, and plant species. Here, we further develop these models to include lipophilic adjuvant effects, as well as the adsorption and desorption, of compounds on the cuticle surface with a novel Adaptive Competitive Langmuir model. These modifications to a penetration model provide a novel addition to the literature. We validate our theoretical model results against appropriate experimental data, discuss key sensitivities, and relate theoretical predictions to physical mechanisms. The results indicate the addition of the desorption mechanism may be one way to predict increased penetration at late times, and the sensitivity of model parameters compares well to those present in the literature.

Nanoscale Structural Organization of Plant Epicuticular Wax Probed by Atomic Force Microscope Infrared Spectroscopy

The cuticle covers external surfaces of plants, protecting them from biotic and abiotic stress factors. Epicuticular wax on the outer surface of the cuticle modifies reflectance and water loss from plant surfaces and has direct and indirect effects on photosynthesis. Variation in epicuticular wax accumulation, composition, and nanoscale structural organization impacts its biological function. Atomic force microscope infrared spectroscopy (AFM-IR) was utilized to investigate the internal and external surfaces of the cuticle of Sorghum bicolor, an important drought-tolerant cereal, forage, and high-biomass crop. AFM-IR revealed striking heterogeneity in chemical composition within and between the surfaces of the cuticle. The wax aggregate crystallinity and distribution of chemical functional groups across the surfaces was also probed and compared. These results, along with the noninvasive nondestructive nature of the method, suggest that AFM-IR can be used to investigate mechanisms of wax deposition and transport of charged molecules through the plant cuticle.

The Mode of Action of Adjuvants-Relevance of Physicochemical Properties for Effects on the Foliar Application, Cuticular Permeability, and Greenhouse Performance of Pinoxaden

We comprehensively studied the complexity of the mode of action of adjuvants by uncoupling the parameters contributing to the spray process during foliar application of agrochemicals. The ethoxylated sorbitan esters Tween 20 and Tween 80 improved the efficiency of pinoxaden (PXD) in controlling grass-weed species in greenhouse experiments by aiding retention, having humectant properties, maintaining the bioavailability, and increasing the cuticular penetration of PXD. The nonethoxylated sorbitan esters Span 20 and Span 80 showed minimal effects on retention, droplet hydration, or cuticular penetration, resulting in reduced PXD effects in the greenhouse. Tris(2-ethylhexyl)phosphate (TEHP) does not contribute much to retention and spreading but strongly enhances the diffusion of PXD across isolated P. laurocerasus cuticular membranes. As TEHP was most efficient in controlling the growth of grass-weed species, we propose that the direct effect of penetration aids on cuticular permeation plays a key role in the efficiency of foliar-applied agrochemicals.

Quantitative characterization of cuticular barrier properties: methods, requirements, and problems

The interface between the atmosphere and leaves and fruits is formed by the lipophilic plant cuticle, which seals the outer epidermal cell walls, thus significantly reducing water loss and uptake of dissolved solutes deposited on the cuticle surface. Different experimental and theoretical approaches for quantifying barrier properties of cutinized leaf and fruit surfaces are presented and discussed in this review. Quantitative characterization of cuticle barrier properties requires (i) the measurement of diffusion kinetics, namely the amount diffusing versus time, (ii) accurate knowledge of driving forces, namely concentration gradients, acting across the barrier, and (iii) the calculation of permeances, namely diffusion velocity. We suggest that on the basis of permeances, which are independent from experimental boundary conditions such as driving forces, the time period of measurement, and area, cuticle barrier properties of different plant organs, different plant species, and different lines, as well as barrier properties of suberized root tissue or synthetic membranes, can be directly compared. This review provides a short and easy to understand manual on what should be kept in mind when quantifying barrier properties of cutinized and suberized transport barriers. This could be helpful for scientists working on cuticle biosynthesis and its regulation.

Penetration Behavior of a Water Droplet into a Cylindrical Hydrophobic Pore

Understanding the dynamics with which a water droplet penetrates a pore is important because of its relationship with transfer phenomena in plants and animals. Using a high-speed camera, we observe the penetration processes of a water droplet into a cylindrical pore on a silicone substrate. The force on the water droplet is generated by dropping the substrate plus water droplet from a height of several centimeters onto an acrylic resin substrate. The penetration characteristics depend on pore size Dp, height of release of a drop h, and the viscosity of the droplet liquid and are classified into the following patterns: spreading, penetration, and breaking. During the process of relaxation to the steady state, various interesting deformation or oscillation phenomena occur. Based on high-speed images, we estimate the interfacial energy ΔG during the intermediate states and find an energy barrier ΔG = 1 × 10(-7) J when Dp = 1.0 mm and h = 15 mm for the spreading pattern and ΔG = 0.7 × 10(-7) J when Dp = 1.0 mm and h = 10 mm for the penetration pattern. Finally, based on a theoretical model considering the driving and suppression factors, we explain the experimentally obtained phase diagram including the separation, penetration, and breaking patterns.

Implementation of the effects of physicochemical properties on the foliar penetration of pesticides and its potential for estimating pesticide volatilization from plants

Volatilization from plant foliage is known to have a great contribution to pesticide emission to the atmosphere. However, its estimation is still difficult because of our poor understanding of processes occurring at the leaf surface. A compartmental approach for dissipation processes of pesticides applied on the leaf surface was developed on the base of experimental study performed under controlled conditions using laboratory volatilization chamber. This approach was combined with physicochemical properties of pesticides and was implemented in SURFATM-Pesticides model in order to predict pesticide volatilization from plants in a more mechanistic way. The new version of SURFATM-Pesticide model takes into account the effect of formulation on volatilization and leaf penetration. The model was evaluated in terms of 3 pesticides applied on plants at the field scale (chlorothalonil, fenpropidin and parathion) which display a wide range of volatilization rates. The comparison of modeled volatilization fluxes with measured ones shows an overall good agreement for the three tested compounds. Furthermore the model confirms the considerable effect of the formulation on the rate of the decline in volatilization fluxes especially for systemic products. However, due to the lack of published information on the substances in the formulations, factors accounting for the effect of formulation are described empirically. A sensitivity analysis shows that in addition to vapor pressure, the octanol-water partition coefficient represents important physicochemical properties of pesticides affecting pesticide volatilization from plants. Finally the new version of SURFATM-Pesticides is a prospecting tool for key processes involved in the description of pesticide volatilization from plants.

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.

Measuring Leaf Penetration and Volatilization of Chlorothalonil and Epoxiconazole Applied on Wheat Leaves in a Laboratory-Scale Experiment

Estimation of pesticide volatilization from plants is difficult because of our poor understanding of foliar penetration by pesticides, which governs the amount of pesticide available for volatilization from the leaf surface. The description of foliar penetration is still incomplete because experimental measurements of this complex process are difficult. In this study, the dynamics of leaf penetration of C-chlorothalonil and C-epoxiconazole applied to wheat leaves were measured in a volatilization chamber, which allowed us to simultaneously measure pesticide volatilization. Fungicide penetration into leaves was characterized using a well-defined sequential extraction procedure distinguishing pesticide fractions residing at different foliar compartments; this enabled us to accurately measure the penetration rate constant into the leaves. The effect of pesticide formulation was also examined by comparing formulated and pure epoxiconazole. We observed a strong effect of formulation on leaf penetration in the case of a systemic product. Furthermore, the penetration rate constant of formulated epoxiconazole was almost three times that of pure epoxiconazole (0.47 ± 0.20 and 0.17 ± 0.07, respectively). Our experimental results showed high recovery rates of the radioactivity applied within the range of 90.5 to 105.2%. Moreover, our results confirm that pesticide physicochemical properties are key factors in understanding leaf penetration of pesticide and its volatilization. This study provides important and useful parameters for mechanistic models describing volatilization of fungicides applied to plants, which are scarce in the literature.

Comparative Effectiveness of Potential Elicitors of Plant Resistance against Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) in Four Crop Plants

Feeding by insect herbivores activates plant signaling pathways, resulting in the enhanced production of secondary metabolites and other resistance-related traits by injured plants. These traits can reduce insect fitness, deter feeding, and attract beneficial insects. Organic and inorganic chemicals applied as a foliar spray, seed treatment, or soil drench can activate these plant responses. Azelaic acid (AA), benzothiadiazole (BTH), gibberellic acid (GA), harpin, and jasmonic acid (JA) are thought to directly mediate plant responses to pathogens and herbivores or to mimic compounds that do. The effects of these potential elicitors on the induction of plant defenses were determined by measuring the weight gains of fall armyworm, Spodoptera frugiperda (J. E. Smith) (FAW) (Lepidoptera: Noctuidae) larvae on four crop plants, cotton, corn, rice, and soybean, treated with the compounds under greenhouse conditions. Treatment with JA consistently reduced growth of FAW reared on treated cotton and soybean. In contrast, FAW fed BTH- and harpin-treated cotton and soybean tissue gained more weight than those fed control leaf tissue, consistent with negative crosstalk between the salicylic acid and JA signaling pathways. No induction or inconsistent induction of resistance was observed in corn and rice. Follow-up experiments showed that the co-application of adjuvants with JA failed to increase the effectiveness of induction by JA and that soybean looper [Chrysodeixis includens (Walker)], a relative specialist on legumes, was less affected by JA-induced responses in soybean than was the polyphagous FAW. Overall, the results of these experiments demonstrate that the effectiveness of elicitors as a management tactic will depend strongly on the identities of the crop, the pest, and the elicitor involved.

The contribution of spray formulation component variables to foliar uptake of agrichemicals

BACKGROUND

The objective of the present study was to determine the contribution of the active ingredient (AI) and surfactant, and their concentrations, to the foliar uptake of agrichemicals, and to examine the physical properties that would need to be included in a model for foliar uptake.

RESULTS

All spray formulation component variables significantly affected uptake, explaining 73% of the deviance. The deviance explained by each factor ranged from 43% (AI concentration nested within AI) to 5.6% (surfactant). The only significant interaction was between AI and surfactant, explaining 15.8% of the deviance. Overall, 90% of the deviance could be explained by the variables and their first-order interactions.

CONCLUSIONS

Uptake increased with increasing lipophilicity of the AI at concentrations below those causing precipitation on the leaf surface. AI concentration had a far greater (negative) effect on the uptake of the lipophilic molecule epoxiconazole. The uptake of 2-deoxy-D-glucose (DOG) and 2,4-dichlorophenoxyacetic acid (2,4-D) increased with increasing hydrophile-lipophile balance (HLB) of the surfactant, the effect of HLB being far greater on the hydrophilic molecule DOG. However, the differences observed in epoxiconazole uptake owing to the surfactant were strongly positively related to the spread area of the formulation on the leaf surface. For all AIs, uptake increased in a similar manner with increasing molar surfactant concentration.

Transport properties of the mung bean (Vigna radiata) non-aerial hypocotyl membrane: permselectivity to hydrophilic compounds

BACKGROUND

Aerial plant surfaces are covered by a lipophilic cuticular membrane (CM) that restricts the transport of water and small solutes. Non-aerial tissues do not exhibit such a barrier. Recent data have shown that large relative to CM hydrophilic agrochemicals were able to pass at high rates through the non-aerial coleoptile.

RESULTS

A moderately large hydrophilic solute like PEG 1000 with a mean molar volume of 782 cm(3) mol(-1) was rejected by the non-aerial hypocotyl. Uptake of smaller solutes like urea (46.5 cm(3) mol(-1) ) was fast and with 99% after 1 day. Cut-off size estimations suggest a pore size diameter below 1.5 nm.

CONCLUSION

Aerial and non-aerial CM differ largely in their absolute barrier properties. This difference is related to the absence of embedded cuticular waxes in the non-aerial hypocotyl membrane, which make the CM physically dense and cause low solubility of hydrophilic solutes. The free volume for diffusion at the interface of the non-aerial hypocotyl cuticle to the environment is much larger resulting in higher penetration rates. It is suggested that diffusion through the non-aerial hypocotyl does not proceed in a real channel system with continuous aqueous phase but is more like transport through a filter with restricted diffusion in the pore openings.

Effects of physical properties on the translaminar activity of fungicides

Translaminar redistribution is a key component of activity for many fungicides. The influence of physical properties (including water solubility, lipophilicity, melting point, and molar volume) on translaminar activity, however, is not well understood. Cucumber powdery mildew was used as a biological indicator to examine the influence of physical properties on translaminar activity of 61 fungicides in simple, uniform formulations, including three modes-of-action and a range of physical properties. Results were modeled using multiple regression and ordinal logistic fit. We confirmed that translaminar activity is a frequent attribute of fungicides and that lipophilicity and water solubility are important predictors of translaminar activity. The hypothesis that melting point drives translaminar movement and translaminar activity was not supported. Translaminar movement (driven only by physical properties) could be differentiated with the models from fungitoxicity-influenced translaminar control. Translaminar activity is a complex attribute and differences in inherent activity as well as physical properties and formulations must be considered when comparing compounds for relative translaminar activity.

Effects of poly- and monodisperse surfactants on 14C-epoxiconazole diffusion in isolated cuticles of Prunus laurocerasus

BACKGROUND

Surfactants are known to enhance the foliar uptake of agrochemicals. It was the aim of this study to compare the enhancing effect of three polydisperse surfactants (Brij 30, Plurafac LF300 and Wettol LF700) and five monodisperse alcohol ethoxylates (C12 E3, C12 E4, C12 E5, C12 E6 and C12 E8) on (14)C-epoxiconazole diffusion in cuticles isolated from cherry laurel (Prunus laurocerasus L.).

RESULTS

Rate constants (k*) of (14) C-epoxiconazole diffusion were measured in the presence and in the absence of the surfactants. Polydisperse surfactants increased the rates of foliar penetration of (14) C-epoxiconazole by factors of between 8 and 16. With monodisperse surfactants, enhancing effects on cuticular penetration were 2-16-fold. Effects were highest with alcohol ethoxylates of intermediate size, whereas they were lower for the smaller, more lipophilic and the larger, more polar monomers. In addition, diffusion of four monodisperse alcohol ethoxylates (C12 E3, C12 E4, C12 E5 and C12 E6 ) across cuticles was measured. Rate constants of alcohol ethoxylates decreased with decreasing lipophility and increasing molecular weight.

CONCLUSION

The results indicate that enhancement of foliar penetration across cuticles by surfactants was most efficient when both (14)C-epoxiconazole and surfactants had similar mobilities in the transport-limiting barrier of the cuticles. This observation should be of interest in future strategies to optimise foliar uptake of agrochemicals using surfactants.

Factors influencing the association between active ingredient and adjuvant in the leaf deposit of adjuvant-containing suspoemulsion formulations

BACKGROUND

For an oil adjuvant to enhance uptake of a particulate active ingredient (AI), it is hypothesised that closer association between the two should result in higher uptake. Accordingly, factors important for the spray deposit size on grapevine leaves have been investigated for a series of model suspoemulsion formulations containing colloidal crystalline AI or fluorescent pigment particles and an emulsion of an oil adjuvant with different degrees of wetting and different spray volumes.

RESULTS

Low spray volumes (<100 L ha(-1)) produced small deposits with high particle-adjuvant association. Complementary uptake studies showed increased uptake with decreasing deposit size, in agreement with the above hypothesis. Higher spray volumes produced larger deposits that consisted of annuli formed by pinning of the contact line by particles. Low surfactant concentrations favoured particles in the annulus and adjuvant separated in the centre. Intermediate surfactant concentrations produced annuli containing both particles and adjuvant, while with high surfactant concentrations the deposits were large with few annuli.

CONCLUSIONS

Small deposits result in high AI-adjuvant association. With larger deposits, annulus structures allow for enhanced AI-adjuvant association (5-20 times greater). The formation of annuli appears to be important in enhancing the biodelivery of particulate AIs in adjuvant-containing suspoemulsion formulations at intermediate spray volumes.

Cuticular waxes from ivy leaves (Hedera helix L.): analysis of high-molecular-weight esters

Soluble waxes were extracted from the cuticle of ivy (Hedera helix L.) leaves with dichloromethane in a yield of ca. 13%. The cuticular waxes were directly analysed by GC-MS, high-temperature GC-MS and ESI-MS/MS. The GC-MS analysis showed mostly n-alkanols (45.3%), monoacids (18.8%), triterpenes (9.7%), n-aldehydes (8.7%) and n-alkanes (7.7%). The high-temperature GC-MS and the ESI-MS/MS analyses showed the presence of ester waxes, namely alkyl alkanoates and alkyl coumarates. Alkyl alkanoates comprised esters of the hexadecanoic acid with n-alkanols ranging from C16 to C34. Alkyl coumarates included esters of coumaric acid with n-alkanols ranging from C16 to C32. The cuticular waxes were hydrolysed and the resulting organic and aqueous phases analysed by GC-MS. The hydrolysate showed a major increase in the quantities of n-alkanols, hexadecanoic acid and coumaric acid derived from the alkyl and acyl moieties from the ester waxes. A content of ester waxes of 38% was estimated based on the results from the GC-MS analysis of the non-hydrolysed and hydrolysed cuticular waxes. Alkyl alkanoates were analysed by ESI-MS/MS as [M + Li]+ adduct ions and the alkyl coumarates as [M - H]- deprotonated ions. The ESI-MS/MS analysis allowed the detection of a wider range of ester waxes than high-temperature GC-MS, and was shown to be a useful technique for the qualitative analysis of ester waxes from plant cuticles.

Effect of surface waxes on the persistence of chlorpyrifos-methyl in apples, strawberries and grapefruits

The effects of the cuticle and epicuticular waxes of grapefruit, strawberry and apple on the photodegradation and penetration of chlorpyrifos-methyl were studied. Photodegradation experiments were conducted by exposing the insecticide to the light of a xenon lamp in the presence of a film of wax extracted from the fruit surface. The half-life of chlorpyrifos-methyl irradiated in absence of waxes was 9.6 min. The half-lives of pesticide irradiated in the presence of wax extracts of apple, grapefruit and strawberry were 83, 34 and 26 min, respectively. In penetration studies, fruit with and without wax layers were treated with an aqueous suspension of pesticide. The penetration of the pesticide from the cuticle to the pulp was measured after 24 h. Samples without wax contained a higher total amount of insecticide than those with wax. No pesticide was detected in samples of apple and grapefruit pulp. Residues were detected in all fractions of strawberry. The waxes and cuticle appear to have some effect on the photodegradation and penetration of chlorpyrifos-methyl in fruit samples.

Relationship between physicochemical properties and maximum residue levels and tolerances of crop-protection products for crops set by the USA, European Union and Codex

Residues on foodstuffs resulting from the use of crop-protection products are a function of many factors, e.g. environmental conditions, dissipation and application rate, some of which are linked to the physicochemical properties of the active ingredients. Residue limits (maximum residue levels (MRLs) and tolerances) of fungicides, herbicides and insecticides set by different regulatory authorities are compared, and the relationship between physicochemical properties of the active ingredients and residue limits are explored. This was carried out using simple summary statistics and artificial neural networks. US tolerances tended to be higher than European Union MRLs. Generally, fungicides had the highest residue limits followed by insecticides and herbicides. Physicochemical properties (e.g. aromatic proportion, non-carbon proportion and water solubility) and crop type explained up to 50% of the variation in residue limits. This suggests that physicochemical properties of the active ingredients may control important aspects of the processes leading to residues.

Cuticular uptake of xenobiotics into living plants. Part 2: influence of the xenobiotic dose on the uptake of bentazone, epoxiconazole and pyraclostrobin, applied in the presence of various surfactants, into Chenopodium album, Sinapis alba and Triticum aestivum leaves

This study has determined the uptake of three pesticides, applied as commercial or model formulations in the presence of a wide range of surfactants, into the leaves of three plant species (bentazone into Chenopodium album L. and Sinapis alba L., epoxiconazole and pyraclostrobin into Triticum aestivum L.). The results have confirmed previous findings that the initial dose (nmol mm(-2)) of xenobiotic applied to plant foliage is a strong, positive determinant of uptake. This held true for all the pesticide formulations studied, although surfactant concentration was found to have an effect. The lower surfactant concentrations studied showed an inferior relationship between the amount of xenobiotic applied and uptake. High molecular mass surfactants also produced much lower uptake than expected from the dose uptake equations in specific situations.

Mechanisms of cuticular uptake of xenobiotics into living plants: evaluation of a logistic-kinetic penetration model

The objective of this study was to determine whether a logistic-kinetic penetration model could be applied to whole plant uptake. Uptake over 24 h was determined for three model compounds, applied in the presence and absence of surfactants, into the leaves of two plant species. Data for two time intervals were used in the model to predict uptake at intermediate intervals and compared with experimental results. Overall, the model fit the whole plant uptake data well. The study confirmed that an increase (or decrease) in active ingredient (ai) concentration or an increase in contact area will have no effect on the penetration rate factor, q, within the normal working concentration range. This enabled uptake to be predicted at different times for concentrations of ai not already studied, having first derived q for one concentration of the formulation of interest and having 24 h (maximum) uptake results for all formulations and concentrations of interest. The advantages of the models and equations described are that few variables are required, and they are simple to measure.

Modelling the effects of alcohol ethoxylates on diffusion of pesticides in the cuticular wax of Chenopodium album leaves

Cuticular waxes represent the first and, in most cases, the limiting barrier for foliar uptake of pesticides from solution. Sorption of pesticides in reconstituted cuticular wax (wax/water partition coefficients) of Chenopodium album L. and in isolated cuticular membranes (cuticle/water partition coefficients) of Prunus laurocerasus L. was determined. Diffusion coefficients of pesticides in reconstituted cuticular wax of C. album leaves were size-dependent, increasing with increasing molar volume. In the presence of alcohol ethoxylates, diffusion coefficients were enhanced by up to two orders of magnitude, and size selectivity was significantly decreased. The accelerating effect and the decrease in size selectivity were attributed to plasticisation of the cuticular wax by the alcohol ethoxylates increasing the fluidity in the wax. A free volume model adopted from polymer science was successfully applied to predict diffusion coefficients of pesticides on the basis of the transport properties of the wax (size selectivity and crystallinity), the molar volume of the diffusing compound and the accelerator concentration in the wax.

Characterization of hydrophilic and lipophilic pathways of Hedera helix L. cuticular membranes: permeation of water and uncharged organic compounds

The permeability of astomatous leaf cuticular membranes of Hedera helix L. was measured for uncharged hydrophilic (octanol/water partition coefficient log K(O/W) < or =0) and lipophilic compounds (log K(O/W) >0). The set of compounds included lipophilic plant protection agents, hydrophilic carbohydrates, and the volatile compounds water and ethanol. Plotting the mobility of the model compounds versus the molar volume resulted in a clear differentiation between a lipophilic and a hydrophilic pathway. The size selectivity of the lipophilic pathway was described by the free volume theory. The pronounced tortuosity of the diffusional path was caused by cuticular waxes, leading to an increase in permeance for the lipophilic compounds after wax extraction. The size selectivity of the hydrophilic pathway was described by hindered diffusion in narrow pores of molecular dimensions. A distinct increase in size selectivity was observed for hydrophilic compounds with a molar volume higher than 110 cm3 mol(-1). Correspondingly, the size distribution of passable hydrophilic pathways was interpreted as a normal distribution with a mean pore radius of 0.3 nm and a standard deviation of 0.02 nm. The increased permeance of the hydrophilic compounds by the removal of cuticular waxes was attributed to an increase in the porosity, a decrease in the tortuosity, and a widening of the pore size distribution. Cuticular transpiration resulted from the permeation of water across the hydrophilic pathway. The far-reaching implications of two parallel pathways for the establishment of correlations between cuticular structure, chemistry, and function are discussed.

Comparison of sorption and diffusion by pyridate and its polar metabolite in isolated cuticular wax of Chenopodium album and Hordeum vulgare

Sorption and diffusion of the herbicide pyridate and its metabolite CL9673 were measured in reconstituted cuticular waxes isolated from Chenopodium album L. and Hordeum vulgare L. (cultivar Igri) leaves. The compounds have the same basic chemical structure, except that pyridate is characterized by a C8-alkyl chain bound via a thioester to the ionizable hydroxyl group of CL9673. Sorption of the weak acid CL9673 from aqueous solutions into cuticular waxes was pH-dependent, and the apparent wax/water partition coefficients decreased with increasing pH. Wax/water partition coefficients of pyridate were not dependent on pH, and they were about 4 orders of magnitude higher as compared to the nondissociated species of CL9673. Diffusion coefficients measured in reconstituted cuticular wax for CL9673 fitted established models predicting diffusion coefficients in relation to molar volumes. However, this was not the case with pyridate, which was characterized by a self-accelerating effect leading to diffusion coefficients, which were up to 2 orders of magnitude higher than predicted from the molar volume. This is a remarkable result since pyridate represents a compound combining the properties of an active ingredient and of a plasticizer.

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.

Accelerators increase permeability of cuticles for the lipophilic solutes metribuzin and iprovalicarb but not for hydrophilic methyl glucose

Effects of diethylsuberate (DESU), tributyl phosphate (TBP), and monodisperse ethoxylated alcohols (EAs) on rate constants of penetration (k) of model solutes across astomatous cuticular membranes isolated from Madagascar ivy (Stephanotis floribunda) and pear (Pyrus communis) leaves were studied. Model solutes (selected on the basis of their octanol/water partition coefficients, K(ow)) were iprovalicarb (log K(ow) = 3.18), metribuzin (log K(ow) = 1.60), and methyl glucose (MG) (log K(ow) = -3.0). K(ow) varied by more than 6 orders of magnitude. Accelerators had wax/water partition coefficients (log K(ww)) ranging from 1.75 (DESU) to 4.32 (C(12)E(2)), and their equilibrium concentrations in Stephanotis wax varied from 0 to about 160 g kg(-)(1). Accelerators increase solute mobility in cuticles by increasing fluidity of cutin and waxes. This effect was quantified by plotting log k versus the accelerator concentration in wax. With the lipophilic solutes metribuzin and iprovalicarb, these plots were linear. Slopes of these plots characterize the intrinsic activities of the accelerators, and they decreased in the order DESU (0.029) > TBP (0.015) > EAs (0.01). Using these intrinsic activities, the effects of accelerators on rate constants of penetration can be calculated for any accelerator concentration in wax. For instance, at 50 g kg(-)(1), rate constants for lipophilic solutes increased by factors of 28 (DESU), 5.6 (TBP), and 3.2 (C(12)E(n)()), respectively. Permeability of cuticles for the hydrophilic MG was not increased by DESU, TBP, C(12)E(2), and C(12)E(4), while C(12)E(6) and C(12)E(8) increased it. Small hydrophilic solutes such as MG can access aqueous pores in cuticles, and this pathway is not affected by changes in fluidity of amorphous waxes. After rate constants of penetration of ionic CaCl(2) were compared with those for nonionic MG, it was concluded that 60% of the MG diffused across aqueous pores, while 40% used an alternative pathway. Because the solubility of MG in wax is extremely low, it is unlikely that MG diffused along the lipophilic pathway used by metribuzin and iprovalicarb. This agrees with the observation that DESU and TBP had no effect on rate constants for MG. An alternative pathway of unknown properties is suggested. It is speculated that C(12)E(6) and C(12)E(8) sorbed in cuticles might have generated a polar pathway for MG.

Sorption in reconstituted waxes of homologous series of alcohol ethoxylates and n-alkyl esters and their effects on the mobility of 2,4-dichlorophenoxybutyric acid

Equilibrium sorption of n-alkyl esters (dimethyl suberate, diethyl suberate, diethyl sebacate, dibutyl suberate and dibutyl sebacate) and monodisperse alcohol ethoxylates (diethylene glycol, tetraethylene glycol, pentaethylene glycol, hexaethylene glycol and octaethylene glycol monododecyl ether) between the reconstituted cuticular waxes of Stephanotis floribunda Brongn (Madagascar jasmine) or Hordeum vulgare L (barley) leaves and an external aqueous receptor solution was determined. Logarithms of the wax/receptor partition coefficient (K(wax/rec)) of the n-alkyl esters increased linearly with the number of C-atoms. With alcohol ethoxylates, log K(wax/rec) decreased linearly with the number of ethylene oxide units. For both groups of compounds, K(wax/rec) increased with increasing lipophilicity. The values of K(wax/rec) in Stephanotis wax were between 5 and 16 times higher than in barley wax. It is argued that this difference was due to different chemical composition and crystallinity of the waxes. Mobility of [14C]2,4-dichlorophenoxybutyric acid (2,4-DB) in reconstituted Stephanotis and barley wax was increased by a factor of 2-8 by both n-alkyl esters and alcohol ethoxylates. Effects on the mobility of 2,4-DB were linearly related to the internal concentrations of n-alkyl esters and alcohol ethoxylates in reconstituted Stephanotis or barley wax. At the same internal concentrations the effect of n-alkyl esters on the mobility of 2,4-DB in wax exceeded that of alcohol ethoxylates by between 1 and 2 orders of magnitude. Results are discussed in relation to formulating systemic pesticides.

Effects of accelerators on mobility of 14C-2,4-dichlorophenoxy butyric acid in plant cuticles depends on type and concentration of accelerator

Effects of diethyl suberate (DESU), diethyl sebacate (DES), dibutyl suberate (DBSU), dibutyl sebacate (DBS), and tributyl phosphate (TBP) on diffusion of 14C-2,4-dichlorophenoxy butyric acid (2,4-DB) across cuticular membranes (CM) was studied. Astomatous CM were isolated enzymatically from Stephanotis floribunda Brongn. leaves, and diffusion was measured at 20 degrees C. The alkyl-substituted dicarboxylic acids constitute a homologous series with carbon numbers increasing from C12 to C18. Molecular weights increased only moderately from 230.0 (DESU) to 314.5 (DBS), while partition coefficients varied over orders of magnitude from 92 (DESU), to 1213 (DES), to 15,988 (DBSU), to 210,762 (DBS). All the above compounds turned out to be accelerators as they increased 2,4-DB mobility by up to 40-fold with accelerator concentrations in the CM ranging from only 9.2 to 105 g kg(-1). Efficacy (2,4-DB mobility in the presence/mobility in the absence of accelerators) increased with increasing concentrations of accelerators in CM or in reconstituted cuticular waxes. Plotting efficacy vs accelerator concentration in the CM resulted in straight lines, and their slopes increased in the order DBS (0.14), DBSU (0.31), DES (0.51), and DESU (0.85). Hence, DESU was the most powerful accelerator in this series as it increased 2,4-DB mobility in the CM about 6 times more than DBSU. Waxes constitute the major barrier in plant cuticles, and plots of efficacy vs accelerator concentration in Stephanotis wax were also linear, but compared to CM slopes were steeper by factors of 3.20 (DBS), 2.97 (DBSU), 2.70 (DES), and 1.62 (DESU). TBP was similarly effective as DESU, but plots of efficacy vs concentration were not linear, and curves approached a plateau at 60-80 g kg(-1). These data are discussed with regard to suitability of these accelerators for formulating systemic pesticides.

Mechanisms of cuticular uptake of xenobiotics into living plants: 1. Influence of xenobiotic dose on the uptake of three model compounds applied in the absence and presence of surfactants into Chenopodium album, Hedera helix and Stephanotis floribunda leaves

This study determined the uptake of three model compounds, applied in the presence and absence of surfactants, into the leaves of three plant species (Chenopodium album L, Hedera helix L and Stephanotis floribunda Brongn). The results with 2-deoxy-D-glucose, 2,4-dichlorophenoxyacetic acid and epoxiconazole in the presence ofsurfactants (the polyethylene glycol monododecyl ethers C12EO3, C12EO6, C12EO10 and a trisiloxane ethoxylate with mean EO of 7.5, all used at one equimolar concentration and therefore different percentage concentrations) illustrate that the initial dose (nmol mm(-2)) of xenobiotic applied to plant foliage is a strong positive determinant of uptake. This held true for all the xenobiotics studied over a wide concentration range in the presence of these surfactants. Uptake on a unit area basis (nmol mm(-2)) could be related to the initial dose of xenobiotic applied per unit area (ID) by an equation of the form: Uptake = a [ID]b at time t = 24h. ID is given by the mass of xenobiotic applied, M divided by the droplet spread area, A. Total mass uptake is then calculated from an equation of the form: Total Uptake = a [ID]b x A.

Effects of surfactants on foliar uptake of herbicides - a complex scenario

Surfactants are almost always present in herbicide treatment solutions with the aim to improve spray droplet retention on and penetration of active ingredients (a.i.s) into plant foliage. The effects of surfactants on the foliar uptake of herbicides are complex and only partially understood. The influence of a range of non-ionic polyoxyethylene surfactants on the uptake of two herbicides, glyphosate and 2,4-dichlorophenoxyacetic acid (2,4-D), was compared using three plant species, wheat, broad bean and common lambsquarters. Surfactants of higher ethylene oxide (EO) content provided greater uptake enhancement for glyphosate, whereas those of lower EO content were more beneficial for 2,4-D uptake. Among surfactants of the same EO content, those containing a C(13)/C(15) linear alkane hydrophobe moiety appeared to be more efficient for promoting the uptake of both herbicides. When a suitable surfactant was used, glyphosate uptake into both bean and wheat foliage increased steadily with increasing surfactant concentration and reached a maximum at 0.5%. In the presence of a constant surfactant, higher percentage uptake of herbicide was obtained with higher a.i. concentrations for glyphosate, but with lower a.i. concentrations in the case of 2,4-D. In the presence of an organosilicone surfactant, the stomatal uptake of glyphosate varied with both surfactant concentration and plant species. The effect of non-silicone surfactants on the cuticular uptake of glyphosate also varied with plant species. It can be concluded that the effects of surfactants on foliar uptake of herbicides depend not only on their chemical structures (hydrophobe and hydrophile moieties) and concentration, but also on the physicochemical properties and concentration of a.i.s and the leaf surface character of the plant species.

Rotenone and rotenoids in cubè resins, formulations, and residues on olives

Rotenone and rotenoids (deguelin, beta-rotenolone (12a beta-hydroxyrotenone), tephrosin (12a beta-hydroxydeguelin), 12a alpha-hydroxyrotenone, and dehydrorotenone) were determined in cubè resins and formulations. Cubè resins from Lonchocarpus contain large quantities of deguelin (ca. 21.2%) and smaller quantities of tephrosin (ca. 3.5%) and beta-rotenolone (ca. 3.0%). The composition of commercial formulations may present very different rotenoid contents depending on the extracts used to prepare them. Because these rotenoids also present insecticide activity, the efficacy of these formulations may be very different. The storage stability and photodegradation of some rotenone formulations were studied. Rotenone and rotenoids are very sensitive to solar radiation, which degrades them rapidly, with half-lives in the order of a few tens of minutes. Some formulations show greater disappearance rates than that of cubè resin, indicating that not much attention has been paid to protecting the active ingredients from photodegradation in the formulation. A study on the residues on olives was also carried out to assess not only the rotenone content, but also that of the main rotenoids. At harvest, the residues of deguelin, tephrosin, and beta-rotenolone were 0.10, 0.06, and 0.10 mg/kg, respectively, very similar to rotenone (0.08 mg/kg), and though a few data indicate similar acute toxicity values for deguelin, only rotenone is taken into consideration in the legal determination of the residue.

Confocal laser scanning microscopy - an attractive tool for studying the uptake of xenobiotics into plant foliage

The application of confocal laser scanning microscopy (CLSM) to the study of xenobiotic uptake into plant foliage is explored in this paper. Three fluorescent dyes of low molecular weight and contrasting polarities (hydrophilic, moderately lipophilic and lipophilic) were selected to represent foliage-applied pesticides. These model compounds were applied as droplets to the surfaces of various leaves and/or fruits according to the particular experiment. The transcuticular diffusion behaviour, the compartmentation into epidermal cells and the influence of a surfactant on the uptake of these fluorescent compounds were visualized by CLSM. Distinct differences in diffusion speed across the cuticle and distribution in cell compartments were found between different fluorescent compounds. The presence of a surfactant significantly accelerated the uptake of the moderately lipophilic dye into both thin- and thick-cuticled leaves. The results are discussed in relation to the current knowledge on pesticide uptake and translocation. The advantages and limitations of this technique are highlighted.

Bentazone uptake into plant foliage as influenced by surfactants and carrier pH

The influence of surfactants and carrier pH on the foliar uptake of bentazone, a representative weak acid herbicide, was studied using bean (Vicia faba) and mustard (Sinapis alba) plants. The promoting effect of surfactants on bentazone uptake varied with surfactant hydrophobe structure, ethylene oxide (EO) content, and surfactant and bentazone concentrations. Among the 5 hydrophobe moieties tested, the efficiency order was C13 /C15 linear alcohols > C10 linear alcohol > C16 /C18 linear alcohols > nonylphenol > octylphenol. For C13 /C15 linear alcohol surfactants of differing EO content, the enhancing effect on bentazone uptake was 5 EO > 10 EO > 14 EO. All surfactants improved uptake more when bentazone was applied at low concentration. The effect of carrier pH (5, 7, and 9) on the uptake of bentazone, applied both as unformulated acid and as a sodium salt, was also investigated in this work. Lower carrier pH did not provide greater uptake for bentazone sodium salt and was only beneficial for the uptake of bentazone acid applied at very low concentration. The results are discussed in relation to the current knowledge on pesticide uptake as influenced by surfactants and the change in lipophilicity and solubility of weak acid chemicals at different pH.

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.

Studies on a new group of biodegradable surfactants for glyphosate

The effectiveness of a homologous series of biodegradable rapeseed oil derivatives (triglyceride ethoxylates; Agnique RSO series containing an average of 5, 10, 30 and 60 units of ethylene oxide (EO) as adjuvants for foliage-applied, water-soluble, systemic active ingredients was evaluated employing glyphosate as an example. Previous experiments had revealed that the surfactants used are not phytotoxic at concentrations ranging from 1 to 10 g litre-1. The experiments were performed using Phaseolus vulgaris L and nine selected weed species, grown in a growth chamber at 25/20 (+/- 2) degrees C day/night temperature and 40/70 (+/- 10)% relative humidity. The surfactants were evaluated for enhancement of spray retention, and foliar penetration biological efficacy of glyphosate. Glyphosate was applied at a concentration of 43 mM. The surfactants were added at concentrations of 1 g litre-1. The commercial glyphosate 360 g AE litre-1 SL Roundup Ultra and unformulated glyphosate served as references. The surfactants used improved spray retention, foliar penetration and biological efficacy. Some of the formulations were comparable to the performance of Roundup Ultra in the aspects evaluated; some were even more effective in enhancing spray liquid retention and promoting glyphosate phytotoxicity in several plant species. In these studies Agnique RSO 60 generally was most effective.

A mechanistic analysis of penetration of glyphosate salts across astomatous cuticular membranes

Penetration of glyphosate salts across isolated poplar (Populus canescens (Aiton) Sm) cuticular membranes (CM) was studied using Na+, K+, NH4+, trimethylsulfonium+ (TMS) and isopropylamine+ (IPA) as cations. After droplet drying, humidity over the salt residues on the outer surfaces of the CM was kept constant, and cuticular penetration was monitored by sampling the receiver solution facing the inner surfaces of the CM. Glyphosate salts disappeared exponentially with time from the surfaces of the CM. This first-order process could be quantitatively described using rate constants (k) or half-times (time for 50% penetration; t1/2). Humidity strongly affected the velocity of penetration, as k increased by factors of 5.3 (K-glyphosate), 6.9 (TMS-glyphosate), 7.1 (NH4-glyphosate), 8.5 (Na-glyphosate) and 10.5 (IPA-glyphosate) when humidity was increased from 70 to 100%. Depending on the type of cation and humidity, t1/2 varied between 4 and 70h, but the humidity effect was statistically significant only at 100% humidity, when half-times were highest with IPA-glyphosate and lowest with TMS-glyphosate. Glyphosate acid penetration was measured only at 90% humidity and found to be extremely slow (t1/2 = 866 h). Adding 0.2 g litre-1 of a wetter (alkylpolyglucoside) to the donor increased IPA-glyphosate rate constants by about four times, but increasing concentration produced no further increase in k. When donors contained 0.2 g litre-1 wetter, further additions of 4 g litre-1 Ethomeen T25 did not change rate constants measured with IPA-glyphosate at 90% humidity, while Genapol C-100 and diethyl suberate increased k by only 35%. Concentration of IPA-glyphosate (1, 2 and 4 g litre-1) did not influence k at 90% humidity, and pH of donor solutions (4.0, 7.7, 9.5) had no effect on k of K-glyphosate at 90% humidity. Temperature (10 to 25 degrees C) had only a small influence on velocity of penetration of IPA-glyphosate and K-glyphosate, as energies of activation amounted to only 4.26 and 2.92 kJ mole-1, respectively. These results are interpreted as evidence for penetration of glyphosate salts in aqueous pores.

Finite dose diffusion studies: III. Effects of temperature, humidity and deposit manipulation on NAA penetration through isolated tomato fruit cuticles

Effects of temperature, humidity, rewetting and removal of deposits on penetration of NAA [2-(1-naphthyl)acetic acid] through isolated tomato (Lycopersicon esculentum Mill) fruit cuticles were studied using a finite dose diffusion system. In this system, an aqueous 5-microliter droplet (0.1 mM NAA in 20 mM citric acid buffer) is applied to the outer surface of a cuticle, which is mounted in a glass diffusion half-cell. The cell wall surface is in contact with a receiver solution (20 mM citrate). Penetration is monitored by repeated sampling of the receiver solution. Droplets appeared dry on visual inspection within 1 h of application, but significant NAA penetration continued after droplet drying. Maximum rates of NAA penetration increased exponentially as temperature was increased (from 5 degrees to 35 degrees C), the energy of activation averaging 153 (+/- 11.6)kJ mol-1. At 35 degrees C, penetration reached a plateau within 10 h of application (at 91.1 (+/- 1.0)% of dose applied) while at 5 degrees C penetration after 800 h reached only 30.2 (+/- 7.5)%. Increasing relative humidity from 20 to 80% increased maximum rates [from 1.0 (+/- 0.21) to 2.7 (+/- 0.80)% h-1] and penetration at 120 h after application [from 36.8 (+/- 2.1) to 64.3 (+/- 3.7)%]. Rewetting deposits at 120, 240 and 360 h after application resulted in increased NAA penetration. However, amounts and rates of NAA penetration progressively decreased with each subsequent rewetting. Removal of deposits by cellulose acetate stripping at various times after droplet application resulted in a rapid decrease in NAA penetration. NAA penetration following deposit removal was always less than 6.1% of the amount of NAA applied and averaged 0.5 (+/- 0.2)% when deposits were removed immediately after droplet drying.

Seasonal and species differences in the air--pasture transfer of PAHs

A field plot was established at a semirural site in the U.K. to investigate the atmospheric transfer of PAHs to different pasture species over the whole growing season. The PAHs displayed a range of partitioning behaviors in the atmosphere from exclusively gas phase to exclusively particle bound, resulting in different modes of deposition to the plant surface. The different pasture species had different plant and sward characteristics, e.g., leaf morphologies, yields, etc. For the majority of PAHs, the plant species displayed a seasonality in concentrations, with concentrations being higher in the winter than in the summer. For the lighter PAHs, this seasonality was absent with soil outgassing and/or summer sources of PAHs being implicated. Air-plant transfer factors (scavenging coefficients, with units m3/g dw) typically ranged between 4 and 52 during the summer, increasing to 8-88 during winter. Despite different plant and sward characteristics, the mixtures and concentrations of PAHs were similar for all the plant species. This indicates that there was little difference in the interception and retention behavior of the gas- and particle-phase PAHs. The implications of this for food chain transfer and air-vegetation modeling are discussed.

Leaf surfaces and the bioavailability of pesticide residues

Laboratory bioassays were carried out to determine the toxicity to Folsomia candida Willem (Collembola: Isotomidae) of residues of a pyrethroid insecticide, deltamethrin, and an organophosphorus insecticide, dimethoate, on different leaf surfaces. The test leaves included a range of species and leaves of different ages. Dose-response relationships were estimated for F candida walking over the various treated leaf substrates. Probit analysis was used to estimate the means and standard deviations of the associated tolerance distributions expressed as gAIha-1. Parallelism tests were undertaken to compare the susceptibilities of F candida to the two compounds applied to the different leaf surfaces. On deltamethrin-treated leaf surfaces, the LD50 values for F candida varied from 6.36 to 77.14 gAIha-1. F candida was least susceptible to deltamethrin residues when applied to leaves of dwarf bean (Phaseolus vulgarus L) and the highest susceptibility was observed following application to leaves of seedlings of barley (Hordeum vulgare L). In contrast, the LD50 values observed for dimethoate treatments did not differ significantly between leaf types, ranging from 1.35 to 8.69 gAIha-1. The laboratory data on susceptibility of F candida on different leaf types for different pesticides can be used to investigate the role of leaf surface properties in modifying the toxicity of applied pesticides to exposed invertebrates.