PROTECTION OF WHEAT CHLOROPLASTS FROM LIPID-PEROXIDATION AND LOSS OF PHOTOSYNTHETIC PIGMENTS BY QUERCETIN UNDER STRONG ILLUMINATION

Exposure of chloroplasts to strong visible light in the presence of DCMU and paraquat resulted in lipid peroxidation and pigment photobleaching, which could be because of increased rates of production of active oxygen species. Quercetin and its glycoside, rutin, protected the chloroplasts from thylakoid lipid peroxidation and pigment photo-oxidation. These flavonoids more effectively protected the chloroplasts when the light treatment was given in the presence of DCMU as compared with that in the presence of paraquat. This differential protection by the flavonoids was investigated by inhibition of crocin photo-bleaching by quercetin and rutin, which showed that both of the flavonoids effectively quenched singlet oxygen (O-1(2)). On the basis of our results, we suggest that singlet oxygen may be one of the predominant active species produced during strong illumination of chloroplasts, which may further be accelerated in the presence of DCMU. However, strong illumination in the presence of paraquat may lead to production of species such as superoxide and hydroxyl free radicals, which may damage the thylakoid membrane but may not be efficiently quenched by quercetin and rutin.