UV-B and UV-A radiation effects on photosynthesis at the molecular level

Ultraviolet radiation is a well known damaging factor of plant photosynthesis. Here we studied the mechanism of damage induced by the UV-B and UV-A spectral regions to the light energy converting Photosystem II (PSII) complex, which is the origin of electron flow for the whole photosynthetic process. Our results show that the primary UV damage occurs at the catalytic Mn cluster of water oxidation, which is most likely sensitized by the UV absorption of Mn(III) and Mn(IV) ions ligated by organic residues. The presence of visible light enhances the photodamage of PSII, but has no synergistic interaction with UV radiation. UV-induced damage of PSII can be repaired via de novo synthesis of the D1 and D2 reaction center protein subunits. This process is facilitated by low intensity visible light, which thereby can protect against UV-induced damage. However, the photodamage induced by visible light at high intensity (above 1000 mu Em(-2)s(-1)) cancels the protective effect. The protein repair of PSII is also retarded by the lack of DNA repair as shown in a photolyase deficient cyanobacterial mutant.