Abstract. Effective UV attenuation in the outer leaf layers may represent an important protective mechanism against potentially damaging solar UV-B radiation. Epidermal optical properties for Rumex patientia and Rumex obtusifolius were examined on field collected and greenhouse grown plants. Rumex patientia, a relatively UV-B sensitive plant, has substantially higher epidermal UV transmittance than Rumex obtusifolius, which indicated that the UV-B flux at the mesophyll layer for Rumex patientia would also be higher. Attenuation of UV-B radiation increased in Rumex obtusifolius by 27% after exposure to solar UV-B radiation. Flavonoid extract absorbance also increased in whole leaves of both species after solar UV-B irradiation. The epidermis is not only an effective filter for UV-B radiation, but is wavelength selective, and shows a degree of plasticity in this attenuation.

This research was supported by grants from the United States National Aeronautics and Space Administration (NAS-9-14871) and the Environmental Protection Agency.