Trichome density and its protective potential against ultraviolet-B radiation damage during leaf development

Trichome density and the capacity of leaf hairs to protect underlying tissues against ultraviolet-B radiation damage were assessed during leaf development in three tree species. In all cases, trichome density and the relative quantities of ultraviolet radiation absorbing phenolic constituents (expressed on a leaf area basis) declined considerably with leaf age. In addition, the percent leaf dry mass invested in trichome decreased significantly. Reductions were greater on the adaxial leaf surface, leading to an almost glabrous upper epidermis in mature leaves. Internal (i.e., leaf total minus trichome) phenolics showed no significant changes with leaf age in Eriobotrya japonica and Cydonia oblonga. In Olea europaea, however, the concentration of internal phenolics was doubled in mature leaves. Ultraviolet-B radiation resulted in a reduction of photosystem II photochemical efficiency and extensive epidermal browning only in young, dehaired leaves. No such effects were observed in young, dehaired leaves in the absence of ultraviolet-B radiation or in normal young or mature ultraviolet-B-irradiated leaves. It is suggested that the dense trichomes often covering young leaves may, in addition to other functions, protect transiently the underlying cells against ultraviolet-B radiation damage during the time period required for the maturation of internal avoidance and (or) repairing mechanisms. Key words: Cydonia oblonga, Eriobotrya japonica, Olea europaea, leaf development, leaf hairs, phenolics, ultraviolet-B radiation damage.