Fig. 5: Schematics of the photoprotection processes observed in plant cuticles.

Of the UV light reaching the surface of the cuticle, a small fraction can be reflected or scattered due to the presence of trichomes, crystallised wax structures, etc. A variable fraction of UV light is transmitted to the epidermal cell. Within the UV-B range, transmittance varies among species from a negligible 0.3% to almost 50% in species with a very little amount of cuticle phenolics, such as Brassica oleracea and Beta vulgaris. Most of the UV light reaching the surface of the plant is thus absorbed by the phenolic compounds present in the cuticle. Part of this absorbed energy can be released with the emission of blueish fluorescence. However, most of the absorbed UV light seems to be dissipated via non-radiative mechanisms, being an ultrafast change in molecular geometry within the trans conformation of the phenolic acid and return to the ground state considered in the present work.