Analysis of Epicuticular Phenolics of Prunus persica and Olea europaea Leaves: Evidence for the Chemical Origin of the UV-induced Blue Fluorescence of Stomata

Epicuticular waxes were removed from the leaf surfaces of Olea europaea and Prunus persica by washing with chloroform and the resulting rinses were analysed by high performance liquid chromatography (HPLC) for the presence of fluorescing compounds. Removal of epicuticular waxes from leaves of some representative plants by the same treatment resulted in a significant reduction in the intensity of the blue fluorescence emitted from guard cells (Karabourniotis et al., 2001: Annals of Botany. doi:10.1006/anbo.2001.1386). Ferulic acid and p-coumaric acid, as well as a number of unidentified compounds, were constituents of the rinses of both plants examined but only after alkaline hydrolysis of the samples. This indicates that both phenolic acids are tightly bound to the epicuticular waxes of the leaves of these plants. HPLC chromatograms of rinses derived either from the abaxial or adaxial surfaces of the hypostomatic leaves of O. europaea did not show significant qualitative differences. Nevertheless, ferulic acid (the main blue fluorescent component) was much more abundant in the abaxial than the adaxial surface. In P. persica, the composition of the sample derived from the abaxial surface was far more complex, and all constituents were present in much higher concentrations than in the sample derived from the adaxial surface. Given the particular fluorescence characteristics of ferulic acid, the differences in its concentration between abaxial and adaxial surfaces, and between the two species, and the fluorescence images of these surfaces under the microscope, we propose that this compound is probably the main epicuticular constituent responsible for the blue fluorescence emitted by guard cells of the species examined. The functional significance of the findings is discussed.