Differences in the ROS-generating efficacy of various ultraviolet wavelengths in detached spinach leaves

The capacity of ultraviolet (UV) radiation to generate reactive oxygen species (ROS) in spinach leaves was studied with fluorescent sensors. Leaf segments were exposed to short-term (15-30 min), narrow-band UV irradiation of various wavelengths between 290-390 nm corresponding to equal numbers of photons and, depending on the wavelength, 18-36 μmol m^-2 s^-1 quantum flux. This caused 5-30% loss of photosynthesis measured as loss of variable chlorophyll fluorescence. In accordance with previous results, photosynthesis was less affected by longer than by shorter wavelength UV. UV-irradiated leaves were probed for two major representatives of ROS, namely the superoxide radical anion, a product of electron transfer to oxygen, and singlet oxygen, which is usually formed in photosensitised reactions between oxygen and triplet dyes. Both ROS were detected, but the efficacy of UV in promoting their production was different. Formation of superoxide radicals was positively correlated with loss of photosynthesis; more radicals were observed in leaves exposed to shorter UV wavelengths than in leaves irradiated at 370-390 nm, suggesting a functionally damaged electron transport as a possible site of oxygen reduction. Singlet oxygen was, however, mainly inducible in leaves exposed to longer UV wavelengths and was barely induced by 290-300-nm irradiation. The mechanism of singlet oxygen generation by UV radiation in the leaf, reported for the first time, is not clear, but the process does not appear to be linked to the UV-induced damage of photosynthesis.