Activity of enzymatic antioxidant defense systems in chilled and heat shocked cucumber seedling radicles

Chilling whole cucumber seedlings that had 10-mm long radicles for 4 days at 2.5 degreesC significantly inhibited subsequent radicle growth both by increasing the time it took the seedlings to recover from chilling and attain a linear rate of radicle growth, and by decreasing the subsequent rate of linear growth. Exposing cucumber seedlings to 45 degreesC for up to 20 min had no effect on subsequent radicle growth, while longer exposures produced reductions in growth. A heat shock at 45 degreesC for 10 min induced the optimal protection to 4 days of chilling at 2.5 degreesC by reducing chilling inhibition from 60 to 42%. Two hours after being chilled, heat shocked or heat shocked and then chilled, there was no difference in protein content of the apical I cru of the seedling radicle among these treatments and the non-beat shocked, non-chilled control. Two days after treatment, the protein content was still similar in tissue that had been heat shocked or heat shocked and chilled, while it was significantly reduced in tissue that had been chilled. In general, 2 h after treatment, the activity of the 5 antioxidant enzymes examined in this study [superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC guaiacol peroxidase (GPX; EC 1.11.1.7) and glutathione reductase (GR; EC 1.6.4.2)] were reduced by chilling and unaffected or increased by heat shock. When heat shock was followed by chilling, there was a consistent effect of the beat shock treatment on preventing the loss of enzyme activity following chilling. This protective effect of the heat shock treatment was even more pronounced after 2 days of recovery at 25 degreesC for SOD, CAT and APX. In contrast, the activity of GR and GPX was substantially higher in chilled tissue than in tissue that had been heat shocked before being chilled. Elevated levels of GR and GPX therefore appear to be correlated with the development of chilling injury, while elevated levels of SOD, CAT and APX appear to be correlated with the development of heat shock-induced chilling tolerance.