Penetration of UV-A, UV-B and blue light through the leaf trichome layers of two xeromorphic plants, olive and oak, measured by optical fibre microprobes

Quartz fibre-optic microprobes were used to monitor the Light microenvironm ent beneath trichome layers of the xeromorphic lea, es of two Mediterranean evergreen sclerophylls, Olea europaea and Quercus ilex. Young developing l eaves of both plants were densely pubescent on both surfaces of the lamina, whereas the mature leaves were pubescent only on the abaxial side, Trichom e layers of young as well as of mature leaves of both plants attenuated alm ost all incident ultraviolet (UV)-B (310 nm) and UV-A (360 nm) radiation an d a considerable portion of blue light (430 nm), Abaxial trichome layers of young leaves were more effective in screening out the incident radiation c ompared to the adaxial ones of the same leaves and also compared to the aba xial layer of the mature leaves. The abaxial epidermis of dehaired mature l eaves of O. europaea was ineffective in absorbing most of the incident UV-B and UV-A radiation. UV and visible spectra beneath trichome layers of O. e uropaea in mature leaves confirmed that the light microenvironment on the e pidermis was deprived in the UV-B, UV-A and partly in the blue spectral reg ions, It is proposed that the occurrence of a dense trichome Layer, especia lly in young leaves, may play a protective role against not only UV-B radia tion damage, but also against high visible irradiance. This function is per formed irrespective of the differing anatomy of individual hairs of both pl ants. The protection provided by the trichomes could afford advantages unde r stress conditions, especially during leaf development.