The effects of enhanced UV-B radiation on the Mediterranean evergreen sclerophyll Nerium oleander depend on the extent of summer precipitation

Seedlings of Nerium oleander L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras), and received natural precipitation or additional irrigation during the summer dry period. Two experiments, in 1994 and 1995, were performed. Natural precipitation during summer in 1994 was near the mean for the area but summer 1995 was exceptionally wet. During 1994, supplemental UV-B radiation reduced by ca 25% the above-ground biomass under natural summer precipitation but had no effect on the plants receiving additional irrigation. During the wet 1995, UV-B radiation caused a 5%, non-significant reduction in biomass under natural summer precipitation, and a 16%, non-significant increase under ample water availability. No effects of W-B radiation were observed on PSII photochemical efficiency (assessed from in viva Chi fluorescence), photosynthetic pigments, W-B absorbing compounds, leaf relative water content, leaf specific mass or leaf thickness. However, cuticle thickness and cuticle mass (on a unit area basis) were increased by supplemental UV-B radiation, on both leaf surfaces, independent of water availability We conclude that UV-B radiation effects on growth depend on the extent of summer precipitation. The increase in cuticle thickness may counteract UV-B radiation damage, through attenuation of UV-B radiation and/or reduction of cuticular transpiration during the dry summer.