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.

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.