Trichome layers versus dehaired lamina of Olea europaea leaves: differences in flavonoid distribution, UV-absorbing capacity, and wax yield

Leaf flavonoid compounds from six olive (Olea europaea L.) cultivars were analysed by HPLC. The composition of the soluble fractions of the dehaired lamina and the isolated trichome layers of the abaxial leaf surface were analysed in separate, as to study the distribution of flavonoids between the two leaf parts. Quercetin and quercetin 3-O-rhamnoside that have been reported to occur in the leaves seem to be located exclusively in the trichome layer. A greater variety of flavonoids was found in the lamina but the trichome layer was richer in terms of total flavonoids per unit mass. Trichome layer demonstrated an independent chemical character since its flavonoid concentration and composition was not coordinated with that of the lamina. The occurrence of flavonoid compounds in trichomes is related to the UV-filtering capacity of these cells. The results of the present study showed that apart from the soluble fraction, the cuticular waxes and cell walls of the trichomes also showed significant UV-absorbing capacity, indicating the occurrence of UV-absorbing compounds in these fractions as well. Moreover, the cuticular waxes of the trichome layer exhibited not only a higher investment of mass per unit of leaf area, but also a higher UV-absorbing capacity expressed per unit mass as compared to the cuticular waxes of the lamina surface. The importance for the separate chemical investigation of the phenolic composition of the leaf lamina and the trichome layers as well as the ecological significance of the findings is discussed.