OXIDATIVE DAMAGE TO CHLOROPLASTS FROM CHLORELLA-VULGARIS EXPOSED TO ULTRAVIOLET-B RADIATION

Upon UV-B irradiation, Chlorella vulgaris cells and isolated chloropla sts increased in size and starch accumulation. Photosynthetic capacity and chlorophyll content of chloroplasts isolated from irradiated alga e decreased by 72 and 66%, as compared to chloroplasts isolated from c ontrol cells. Dihydrorhodamine 123 conversion to rhodamine 123 was use d as a sensitive method for detection of peroxide (presumably hydrogen peroxide) formation in isolated chloroplasts. The accumulation of rho damine 123 is higher in irradiated than in nonirradiated chloroplasts and the increased accumulation of rhodamine 123 depended on the UV-B d ose. Quantitation of alkyl radical-EPR signals in chloroplasts indicat ed that UV-B exposure significantly increased radical content in the m embranes. The content of an oxidized DNA base (8-hydroxy-2'-deoxyguano sine) in chloroplasts was increased by 72 and 175% after irradiation o f the algal culture with 17.3 and 42.6 kJ m(-2), respectively. The chl oroplastic activity of superoxide dismutase decreased by 50% as compar ed with control values after irradiation with 42.6 kJ m(-2) and no cha nges in ascorbate peroxidase activity and ascorbic acid content were d etected at the irradiation doses tested. The beta-carotene content in chloroplasts was not affected by the irradiation, but the alpha-tocoph erol content increased approximately 4-fold after UV-B irradiation. Th e results suggest that oxidative damage related to UV-B exposure is re sponsible for alterations in chloroplasts function and integrity, and that an antioxidant response is triggered in chloroplasts through an i ncrease in alpha-tocopherol content.