Surfactant effects on cuticular penetration of neutral polar compounds: dependence on humidity and temperature

Mechanistic Aspects and Effects of Selected Tank-Mix Partners on Herbicidal Activity of a Novel Fatty Acid Ester

Only a limited number of contact herbicides exist in agricultural production. While systemic herbicides are more efficient also at suboptimum spray coverage with long-lasting weed control, contact herbicides provide several advantages. There is no translocation to fruits or roots of plantation and other crop, low risk for resistance development, and minor risk for spray-drift damage. Besides, synthetic products that often have toxicological or residues issues, natural fatty acids, particularly pelargonic acid (PA), have contact activity and are safer for home and garden use. We recently described a methyl capped polyethylene glycol ester of pelargonic acid (PA-MPEG) that acts independent of acid formation. Both, PA-MPEG and PA are applied at high rates per hectare to achieve excellent weed control. Here, we report about potential additives to increase PA-MPEG efficacy. The herbicidal active, 1-decanol, and the non-phytotoxic alkylated seed oil-based adjuvant, Hasten^TM, improved performance and outperformed a commercial PA herbicide. Both, PA-MPEG and PA appear to mainly act by the disintegration of bio-membranes besides having effects on transpiration. The main suggested effect is desiccation due to cutting the water continuum at the site of evaporation in the intercellular spaces. The synergistic action of the adjuvant Hasten^TM and its practical uses are also discussed.

Capped polyethylene glycol esters of fatty acids as novel active principles for weed control

BACKGROUND

Ever since the beginning of agriculture, yields have been threatened by weeds. Chemical weed control is far more effective and economical than other methods. The frequent use of herbicides has led to environmental and human health concerns, resulting in the banning of several herbicides and challenges for the future of important active compounds such as glyphosate.

RESULTS

The herbicidal activity of sustainable alternatives based on certain esters of fatty acids (FA), the action of which is unrelated to the free acid, on common weeds is assessed and reported. The 13 derivatives of FA showed better physicochemical properties than pelargonic acid-based herbicides. All the reported compounds have phytotoxic activity, the highest efficacy being displayed by the methyl end-capped polyethylene glycol (mPEG) ester of pelargonic acid having 6EO (ethylene oxide). This mPEG ester showed equal or better phytotoxicity than the pelargonic acid benchmark at reduced application rate and spray volume. The active compound is a liquid at ambient temperatures, has no bad smell and is not volatile, in contrast to pelargonic acid. Notably, this active compound can be the final product, can be sprayed without adjuvants and is relatively easy to co-formulate.

CONCLUSION

A new lead substance is presented that is a sustainable alternative to current contact herbicides. In particular, it has potential application on railways, in precision agriculture and as a harvest aid. Its good performance and technical properties suggest this mPEG ester group may also overcome the volatility-related problems of other organic acids such as auxins. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Dynamics of moisture diffusion and adsorption in plant cuticles including the role of cellulose

Food production must increase significantly to sustain a growing global population. Reducing plant water loss may help achieve this goal and is especially relevant in a time of climate change. The plant cuticle defends leaves against drought, and so understanding water movement through the cuticle could help future proof our crops and better understand native ecology. Here, via mathematical modelling, we identify mechanistic properties of water movement in cuticles. We model water sorption in astomatous isolated cuticles, utilising three separate pathways of cellulose, aqueous pores and lipophilic. The model compares well to data both over time and humidity gradients. Sensitivity analysis shows that the grouping of parameters influencing plant species variations has the largest effect on sorption, those influencing cellulose are very influential, and aqueous pores less so but still relevant. Cellulose plays a significant role in diffusion and adsorption in the cuticle and the cuticle surfaces.

Tank-mixing adjuvants enhanced the efficacy of fludioxonil on cucumber anthracnose by ameliorating the penetration ability of active ingredients on target interface

In this study, pot and field experiments showed that S903, Hasten and Gemini-31511 can significantly enhanced the control efficacy of fludioxonil on cucumber anthracnose. Then by studying the deposition and penetration interaction between active ingredients and cucumber leaves to revealed how the adjuvants influence the interaction process between pesticide active ingredients and target plants to improve the control efficacy. By analysis the effect of fludioxonil deposition to synergism of adjuvants, indicated that fludioxonil active ingredient deposition caused by adjuvants was not the main factor for the adjuvants synergistic effect. Fludioxonil + S903 yielded the lowest surface tension and contact angle, which also implying the best wetting ability. The mean diameters in Hasten + fludioxonil group were much smaller than those in only fludioxonil group (5.39 μm-90 g a.i. ha^-1, 5.50 μm-180 g a.i. ha^-1), the average particle size only had 3.45 μm (90 g a.i. ha^-1) and 3.94 μm (180 g a.i. ha^-1). And the result of spray droplets was consistent with the particles of fludioxonil crystals observed on glass slides and cucumber leaves. Therefore, S903 improved the penetrability of fludioxonil in the target plants by improving the wetting and dispersion of active ingredients on the target interface. Meantime, Hasten improved the penetrability of fludioxonil in the target plants by decreasing the particle size of active ingredients.

Coming to Order: Adsorption and Structure of Nonionic Polymer at the Oil/Water Interface as Influenced by Cationic and Anionic Surfactants

Polymer-surfactant mixtures are versatile chemical systems because of their ability to form a variety of complexes both in bulk solution and at surfaces. The adsorption and structure of polymer-surfactant complexes at the oil/water interface define their use surface chemistry applications. Previous studies have investigated the interactions between charged polyelectrolytes and surfactants; however, a similar level of insight into the interfacial behavior of nonionic polymers in mixed systems is lacking. The study herein uses vibrational sum frequency (VSF) spectroscopy to elucidate the molecular details of nonionic polyacrylamide (PAM) adsorption to the oil/water interface in the presence of surfactant. The polymer's adsorption and conformational structure at the interface is investigated as it interacts with cationic and anionic surfactants. Where the polymer will not adsorb to the interface on its own in solution, the presence of either cationic or anionic surfactant causes favorable adsorption of the polymer to the oil/water interface. VSF spectra indicate that the cationic surfactant interacts with PAM at the interface through charge-dipole interactions to induce conformational ordering of the polymer backbone. However, conformational ordering of polymer is not induced at the interface when anionic surfactant is present.

Fungicide Formulations Influence Their Control Efficacy by Mediating Physicochemical Properties of Spray Dilutions and Their Interaction with Target Leaves

In this study, three types of pyraclostrobin formulations (including emulsifiable concentrate (EC), suspension concentrate (SC), and microcapsules (MCs)) were used to control cucumber anthracnose. Pyraclostrobin EC had the highest inhibitory activity against Colletotrichum orbiculare in vitro. Much different from the bioactivity in vitro, pyraclostrobin MCs exhibited the highest control efficacy on cucumber anthracnose both in pot and field experiments. The physicochemical properties (particle size, surface tension) of the spray dilution, their interaction with target leaves (contact angle, adhesional tension, work of adhesion, retention, crystallization) and dissipation dynamic of the active ingredient were found to be highly potential factors that would significantly influence the control efficacy of pesticide formulations. Results showed that the control efficacies of different formulations of pyraclostrobin were determined mainly by the final behavior of the pesticides at the target interface, namely, the retention, crystallization, and dissipation dynamics of active ingredients. This study had revealed crucial factors that would influence the efficacy of different formulations of pyraclostrobin and thus could guide the rational and efficient use of different formulations of pesticides on target crops.

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.

Optoregulated Drug Release from an Engineered Living Material: Self-Replenishing Drug Depots for Long-Term, Light-Regulated Delivery

On-demand and long-term delivery of drugs are common requirements in many therapeutic applications, not easy to be solved with available smart polymers for drug encapsulation. This work presents a fundamentally different concept to address such scenarios using a self-replenishing and optogenetically controlled living material. It consists of a hydrogel containing an active endotoxin-free Escherichia coli strain. The bacteria are metabolically and optogenetically engineered to secrete the antimicrobial and antitumoral drug deoxyviolacein in a light-regulated manner. The permeable hydrogel matrix sustains a viable and functional bacterial population and permits diffusion and delivery of the synthesized drug to the surrounding medium at quantities regulated by light dose. Using a focused light beam, the site for synthesis and delivery of the drug can be freely defined. The living material is shown to maintain considerable levels of drug production and release for at least 42 days. These results prove the potential and flexibility that living materials containing engineered bacteria can offer for advanced therapeutic applications.

Mathematical Modeling of Diffusion of a Hydrophilic Ionic Fertilizer in Plant Cuticles: Surfactant and Hygroscopic Effects

The agricultural industry requires improved efficacy of sprays being applied to crops and weeds to reduce their environmental impact and increase financial returns. One way to improve efficacy is by enhancing foliar penetration. The plant leaf cuticle is the most significant barrier to agrochemical diffusion within the leaf. The importance of a mechanistic mathematical model has been noted previously in the literature, as each penetration experiment is dictated by its specific parameters, namely plant species, environmental conditions such as relative humidity and spray formulation including adjuvant addition. A mechanistic mathematical model has been previously developed by the authors, focusing on plant cuticle diffusion of calcium chloride through tomato fruit cuticles including pore swelling, ion binding and evaporation, along with the ability to vary the active ingredient concentration and type, relative humidity and plant species. Here we further develop this model to include adjuvant effects as well as the hygroscopic nature of deliquescent ionic solutions with evaporation on the cuticle surface. These modifications to a penetration and evaporation model provide a novel addition to the literature and allow the model to be applied to many types of evaporating ionic hygroscopic solutions on many types of substrates, not just plant cuticles. We validate our theoretical model results against appropriate experimental data, discuss key sensitivities and relate theoretical predictions to physical mechanisms. The important governing mechanisms influencing surfactant enhanced penetration of ionic active through plant cuticles were found to be aqueous pore radius, pore density, cuticle thickness and initial contact angle of the applied droplet; ion binding, relative humidity and evaporation including hygroscopic water absorption parameters for point of deliquescence. The sensitivity analysis indicated surfactants increase penetration by changing the point of deliquescence of a solution, which alters the water absorption and the initial contact angle, which alters the number of pores under the droplet. The results of the validation and sensitivity analysis imply that this model accounts for many of the mechanisms governing penetration in plant cuticles.

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.

Nonlinear Porous Diffusion Modeling of Hydrophilic Ionic Agrochemicals in Astomatous Plant Cuticle Aqueous Pores: A Mechanistic Approach

The agricultural industry requires improved efficacy of sprays being applied to crops and weeds in order to reduce their environmental impact and deliver improved financial returns. Enhanced foliar uptake is one means of improving efficacy. The plant leaf cuticle is known to be the main barrier to diffusion of agrochemicals within the leaf. The usefulness of a mathematical model to simulate uptake of agrochemicals in plant cuticles has been noted previously in the literature, as the results of each uptake experiment are specific to each formulation of active ingredient, plant species and environmental conditions. In this work we develop a mathematical model and numerical simulation for the uptake of hydrophilic ionic agrochemicals through aqueous pores in plant cuticles. We propose a novel, nonlinear, porous diffusion model for ionic agrochemicals in isolated cuticles, which extends simple diffusion through the incorporation of parameters capable of simulating: plant species variations, evaporation of surface droplet solutions, ion binding effects on the cuticle surface and swelling of the aqueous pores with water. We validate our theoretical results against appropriate experimental data, discuss the key sensitivities in the model and relate theoretical predictions to appropriate physical mechanisms. Major influencing factors have been found to be cuticle structure, including tortuosity and density of the aqueous pores, and to a lesser extent humidity and cuticle surface ion binding effects.

Foliar application of microdoses of sucrose to reduce codling moth Cydia pomonella L. (Lepidoptera: Tortricidae) damage to apple trees

BACKGROUND

The effects of foliar applications of microdoses of sucrose to reduce the damage by the codling moth have been reported from nine trials carried in France and Algeria from 2009 to 2014. The activity of sucrose alone was assessed by comparison with an untreated control and some treatments with the Cydia pomonella granulovirus or a chemical insecticide. The addition of sucrose to these different treatments was also investigated.

RESULTS

The application of sucrose at 0.01% reduced the means of infested fruits with a value of Abbott's efficacy of 41.0 ± 10.0%. This involved the induction of resistance by antixenosis to insect egg laying. Indeed, it seems that acceptance of egg laying on leaves treated with sucrose was reduced. The addition of sucrose to thiacloprid improved its efficacy (59.5% ± 12.8) by 18.4%. However, the sucrose had no added value when associated with C. pomonella granulovirus treatments.

CONCLUSION

Foliar applications of microdoses of sucrose every 20 days in commercial orchards can partially protect against the codling moth. Its addition to thiacloprid increases the efficacy in integrated control strategies, contrary to C. pomonella granulovirus treatments. This work opens a route for the development of new biocontrol strategies. © 2016 Society of Chemical Industry.

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.

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.

Phyto-saponins as a natural adjuvant for delivery of agromaterials through plant cuticle membranes

With growing use of synthetic adjuvants in modern agriculture, their impacts on the environment are being questioned. In a search for an environmentally safe phyto-adjuvant, we have investigated natural glycosidic saponin for delivery of agromaterials through plant cuticle membranes. Four saponin preparations from Quillaja saponaria bark (QE), obtained from Sigma-Aldrich, and Balanites aegyptiaca fruit mesocarp (ME), kernel (KE), and root (RE), isolated and characterized in our laboratory, were used for testing the delivery of [14C]-2,4-dichlorophenoxyacetic acid (2,4-D) across isolated intact astomatous adaxial Citrus grandis leaf cuticle membranes (CMs). The results showed that both Q. saponaria and B. aegyptiaca saponin preparations enhanced delivery of 2,4-D through CMs. Among the saponin preparations, ME exhibited a significantly higher level of delivery of 2,4-D with a concentration effect (2% being the highest). Transmission electron microscope (TEM) and dynamic light scattering (DLS) characterization of these saponin preparations in aqueous solution clearly demonstrated the formation of nanoscale vesicles. Various possibilities for a natural amphiphatic phyto-saponin as a delivery adjuvant through CMs are discussed.

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.

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.

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

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