Abstract:The combined effects of osmotic stress and light on the generation of singlet oxygen (102) and its relation to the breakdown of photosynthetic pigments in leaves of hybrid rice (Oryza sativa L. subsp, indica cv. Shanyou 63) seedlings were studied under the condition of incubating the leaves with –0.8 MPa polyethylene glycol (PEG) solution. Under osmotic stress and increasing light intensity, the production of ¹O2 monitored as p-nitrosodimethylaniline (NDA) bleaching were increased in chloroplasts, degradation of chloro- phyll (Chl) and carotenoid (Car) were accelerated and Car loss preceded Chl causing a significant increase of Chl/Car ratio. A close correlation was observed between ¹O2 production and the contents of Chl, Car and malondialdehyde (MDA). Pretreatment with scavengers for ¹O2. such as β-carotene (β-Car) and histidine (His) reduced MDA content and retarded the degradation of photosynthetic pigments in rice leaves exposed to osmotic stress of -0.8 MPa and light intensity of 250 μmol · m- 2 · s-1, in contrast to that with photosensitizer riboflavin (RF). These results indicate that ¹O2 which generated in chloroplast from photosensitized reactions involving triplet Chl may play a significant role in the breakdown of photo- synthetic pigments and the preferential destruction of Car in the leaves under combined osmotic stress with light.