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 chilli ng is implicated in the high chilling tolerance of cucumber (Cucumis sativu s L.) cultivars (Shen et al., 1999). In this study, the activity or content of enzymic and nonenzymic antioxidants in leaves were compared between chi lling- 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 28/2 2 degrees C (day/night) under a 12 hr- photoperiod (rewarming). During chil ling, activities of catalase (CAT) decreased, while those of superoxide dis mutase (SOD), guaiacol peroxidase (POX) and ascorbate peroxidase (APX) were equal to those in control leaves exposed to 15 degrees C in the dark for 2 4 hr in both cultivars. Ascorbic acid (AsA), dehydroascorbic acid (DHA), an d reduced glutathione (GSH) increased during chilling in 'Jinchun No. 3' bu t not in 'Suyo'. During rewarming, however, most antioxidant enzymes increa sed in their activities in chilled leaves. SOD activity increased more grea tly in 'Jinchun No. 3' than it did in 'Suyo'. APX activity also increased g reatly 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 r esults are discussed in relation to the difference in chilling tolerance be tween the two cucumber cultivars.