Comparative analysis of the low molecular weight and enzymatic antioxidants in response to the phototoxicity of accumulating uroporphyrin and protochlorophyllide in barley leaves treated with cesium chloride

Cesium chloride treatment of illuminated barley leaves leads to accumulatio n of uroporphyrinogen which is subsequently either oxidised to uroporphyrin in continuous light or converted to protochlorophyllide in darkness [Shaly go et al. (1998) J Photochem Photobiol 42: 151-158]. We were interested to elucidate the differences in the phototoxicity of uroporphyrin and protochl orophyllide in the CsCI-treated leaves. Photosensitization and the inductio n of oxidative stress responses in the barley leaves occurred much faster u pon protochlorophyllide than upon uroporphyrin accumulation. We compared th e time resolved changes in the pool sizes of low molecular weight antioxida nts, such as ascorbate, glutathione and tocopherol, as well as of the enzym atic activities of catalase, ascorbate peroxidase, glutathione reductase an d superoxide dismutase in illuminated barley leaves which accumulated uropo rphyrin or protochlorophyllide. A rapid loss of the antioxidant levels corr elated with the accumulation of reactive oxygen species. The contents of lo w molecular weight antioxidants and the activities of most of the antioxida tive enzymes declined more rapidly in the presence of protochlorophyllide t han of uroporphyrin. Due to its high lipophilicity, free protochlorophyllid e is associated with biomembranes. Therefore, it is assumed that it exerts its phototoxic effects to membranes more rapidly than uroporphyrin.