Plant cuticles are protective lipophilic membranes that cover leaves and many stems and fruits. They consist of a polymer matrix that is covered with epicuticular waxes and incorporates intracuticular waxes. Together, these constituents provide a good but imperfect barrier against the loss of water and solutes and ingression by pests and pathogens. The thickness, structure and chemical composition of cuticular matrices and epicuticular and intracuticular waxes vary very widely, and the potential functional consequences of such differences are poorly understood. Transport across the cuticle can occur by random diffusion within the lipophilic network (relevant for lipophilic and moderately polar substances) or along clusters of water molecules that form a continuous hydrophilic phase reaching across the membrane (relevant for polar and ionic substances).

Key concepts:

• All leaves and nonwoody stems are covered by a protective and resilient membrane called the cuticle.
  
• Its permeability is low but it can accumulate lipophilic xenobiotics. Thicker or waxier cuticles are generally not less permeable than thinner or less wax-rich ones.
  
• The trans-cuticular diffusion paths for ions and lipophilic compounds are different.