Effect of cold treatment on enzymic and nonenzymic antioxidant activities in leaves of chilling-tolerant and chilling-sensitive cucumber (Cucumis sativus L.) cultivars

Low activity of oxygen radical generation in leaves during and after chilling is implicated in the high chilling tolerance of cucumber (Cucumis sativus L.) cultivars (Shen et al., 1999). In this study, the activity or content of enzymic and nonenzymic antioxidants in leaves were compared between chilling- tolerant 'Jinchun No. 3' and chilling- sensitive 'Suyo', during a 24-hr chilling period at 3 degrees C in the dark and subsequent 24 hr at 28/22 degrees C (day/night) under a 12 hr- photoperiod (rewarming). During chilling, activities of catalase (CAT) decreased, while those of superoxide dismutase (SOD), guaiacol peroxidase (POX) and ascorbate peroxidase (APX) were equal to those in control leaves exposed to 15 degrees C in the dark for 24 hr in both cultivars. Ascorbic acid (AsA), dehydroascorbic acid (DHA), and reduced glutathione (GSH) increased during chilling in 'Jinchun No. 3' but not in 'Suyo'. During rewarming, however, most antioxidant enzymes increased in their activities in chilled leaves. SOD activity increased more greatly in 'Jinchun No. 3' than it did in 'Suyo'. APX activity also increased greatly in 'Jinchun No. 3' but it did not increase in 'Suyo'. The increases in CAT and POX activities were similar in the two cultivars. AsA, GSH, and oxidized glutathione (GSSG) also increased during rewarming in 'Jinchun No. 3' leaves, whereas in 'Suyo' leaves, DHA increased during early rewarming, but GSH and GSSG did so only toward the end of the rewarming period. The results are discussed in relation to the difference in chilling tolerance between the two cucumber cultivars.