Reassessment of the role of cuticular waxes in the transfer of organic molecules through plant cuticles

The role of cuticular waxes in the penetration of a hydrophilic ([14C]glyphosate) and a lipophilic ([14C]isoproturon) compound through isolated plant cuticles was investigated using a finite-dose diffusion system (droplet/cuticle/agar), after partial or complete soluble wax extraction. Cuticle models were selected from leaves (Ilex aquifolium, Hedera helix, Ficus elastica) and fruit (Lycopersicon esculentum, Capsicum annuum). These cuticles represent two types of microstructure characterized by the presence or absence of a lamellar zone above a reticulate zone. The diffusion of glyphosate through the cuticle was very low for the five species. Wax extraction increased diffusion rate slightly, although it still remained very limited even after complete wax extraction. The diffusion of isoproturon depended to a large extent on the plant species: the highest values were obtained for pepper and tomato cuticles. Extraction of superficial waxes led to a considerable increase in the diffusion rate of this herbicide through the cuticle of these two species. More pronounced wax extraction increased the transfer through holly cuticles by a factor of 4, although cuticles still contained 70 % of cuticular waxes. After complete wax extraction, transcuticular penetration reached 93 % of the applied dose after 96 h for this species, but only 34.6 and 75.2 % for rubber and ivy cuticles, respectively. This lower transfer rate was observed for cuticles having a lamellar zone above the reticulate zone. Results show that waxes are not always the main barrier to the penetration of chemicals through plant cuticles, and that it is necessary to consider the complete picture of cuticle organization with the polymer matrix and waxes.