Oxygen radical-generation in chilled leaves of cucumber (Cucumis sativus L.) cultivars with different tolerances to chilling temperatures

The possible involvement of oxygen radical generation in chilled leaves in the different chilling tolerances of cucumber (Cucumis sativus L.) cultivar s was investigated. Plants with their first : expanded leaves were exposed to 3 degrees C or 15 degrees C in the dark for 24 hr; they were warmed for another 24 hr at 28/22 degrees C: (day/night) with a 12-hr photoperiod. Chi lled leaves subsequently developed chilling injury symptoms (marginal and i nner necrosis) and accumulated malondialdehyde (MDA). Both chilling injury and rate of MDA accumulation were largest in 'Suyo', intermediate in 'Sharp I', and smallest in 'Jinchun No.3'. In leaves of chilling-sensitive 'Suyo'; NADPH-dependent superoxide and hydr ogen peroxide-generating activities increased markedly during chilling. Hyd roxyl radical-generating activities;as indicated by methane sulfonic acid f ormation increased rapidly upon warming, with a concomitant decrease in act ivities of superoxide and hydrogen peroxide generation. The NADPH: dependen t superoxide-generating activity increased again during the later warming p eriod. In contrast, chilled leaves of chilling-tolerant 'Jinchun No.3' show ed only a slight increase in the oxygen radical generation activities durin g both chilling and warming periods. In chilled leaves of 'Sharp I', activi ties of hydrogen peroxide and hydroxyl radical generation increased during the early warming period, whereas those of NADPK-dependent superoxide gener ation occurred later. The NADPH oxidase activity in chilled leaves changed with time similarly to the activity: of NADPH-dependent superoxide generati on in all cultivars examined. The results indicate that chilling injury of cucumber leaves is caused by t he increased oxygen radical generation under chilling and subsequent warmin g conditions. This increase in oxygen radical generation may be triggered p rimarily by the increased NADPH oxidase activity, the rate of which determi nes the degree of chilling tolerance in cucumber cultivars.