Abstract: Thirty cultivars of pak-choi (Brassica campestris ssp. chinensis Makino) were divided into five grades according to their photooxidation tolerance by the residual content of total chlorophylls under artificial photooxidation condition, where the tolerant ones included Qingyou 4, Shulü, Kangre 605, Lüyang Qing, Aikang 6 and Huawang, and the suscep tible oneswere Lüxing and Aijiao Huang. In order to study the characteristics of reactive oxygen metabolism between pak-chio varietieswith different photooxidation tolerance, Kangre 605
and Lüxingwere used to compare the differences in the paper. The results showed that the activities of antioxidant enzymes including superoxide dismutase (SOD) , peroxidase (POD) , catalase (CAT) and ascorbate peroxidase (APX) in the two cultivarswere induced at the beginning of photooxidation stress, and then decreased in the sensitive Lüxing, but that of the tolerant Kangre 605 continued to keep at a higher levels. Additionally, the O₂·production rate and the content of H₂O₂ and malondialdehyde (MDA) in Lüxing were higher than that in Kangre 605, whichmight be the cause leading to a lower level of chlorophylls. These results suggested that higher antioxidant enzyme activities p layed an important role in Kangre 605 against photooxidation stress.

Key words: Pak-choi, Photooxidation, Reactive oxygen species, Antioxidant enzymes