Leaf Hairs of Olive (Olea europaea) Prevent Stomatal Closure by Ultraviolet-B Radiation

Abstract

In olive (Olea europaea L.), hair removal had no effect on the photosynthetic rate and the apparent leaf resistance to water vapour diffusion in leaves illuminated with white light (900 μmol m^-2 s^-1 photosynthetically active radiation) devoid of ultraviolet-B radiation. In addition, intact and dehaired leaves showed no significant differences in absorptance in the visible spectral region, while leaf temper- ature was independent of hair removal. These results indicate that leaf hairs of O. europaea may play only a marginal role in leaf energy balance and transpiration. When the white light was supplemented with ultraviolet-B radiation (5.89 W m^-2), however, there was a considerable decrease in the photo- synthetic rate, and a simultaneous increase in leaf resistance to water vapour in dehaired leaves. Photochemical efficiency of photosystem II, evaluated from chlorophyll fluorescence emitted from the illuminated side, was reduced in all cases, but the reduction in dehaired, ultraviolet-B treated leaves was more pronounced and irreversible, indicating that the reduction of the photosynthetic rate may result from both stomatal limitation and electron flow inhibition. Photosynthetic capacity of dehaired leaves, measured at 5% CO₂, however, was not influenced by ultraviolet-B radiation. We suggest, therefore, that ultraviolet-B radiation reduces photosynthetic rates by closing the stomata, while the observed reduction in photosystem II photochemical efficiency may concern only a superficial chloroplast population, contributing negligibly to whole leaf photosynthesis. Under the conditions of our experi- ments, the protective function of the indumentum against ultraviolet-B radiation predominates over the water conservation function.