Reduced chilling tolerance in elongating cucumber seedling radicles is related to their reduced antioxidant enzyme and DPPH-radical scavenging activity

Cucumber seedling radicles become more chilling sensitive as they elongate. Chilling seedlings with radicles 20 mm long for 48 h at 2.5degreesC inhibited subsequent growth by 36%, while it reduced the growth of 70 mm-long radicles by 63%. Although the growth rate of non-chilled cucumber radicles at 25degreesC is constant from 20 to 80 nun, tissue viability [i.e. reduction of TTC (2,3,5-triphenyltetrazolium chloride) to formazan] and DPPH (alpha,alpha-diphenyl-beta-picrylhydrazyl) radical scavenging activity of apical tissue declines as radicles elongate from 20 to 80 mm in length. TTC reduction, DPPH-radical scavenging activity and protein content of apical tissue were higher in 20 than in 70 mm radicles immediately after chilling and after an additional 48 h of growth at 25degreesC. Catalase (CAT; EC 1.11.1.6) and ascorbate peroxidase (APX; EC activity was higher in the apical tissue of 20 than in 70 mm radicles before chilling. Immediately after chilling and after an additional 48 h at 25degreesC, superoxide dismutase (SOD; EC 1.15.1.1), glutathione reductase (GR; EC 1.6.4.2), and guaiacol peroxidase (GPX; EC 1.11.1.7) activity increased more rapidly in 70 mm radicles than in 20 mm radicles (SOD, GR, and GPX activity in 70 mm radicles was 1.5-, 1.9- and 8.6-fold higher, respectively, than in 20 mm radicles). However, APX and CAT activity in 20 mm radicles were always higher than in 70 mm radicles. Growth after chilling enhanced the activity of all antioxidant enzymes compared to that found in non-chilled tissue; however, CAT activity in 70 mm radicles did not recover to levels found in non-chilled tissue. Higher levels of CAT, APX and DPPH-radical scavenging activity are correlated with higher chilling tolerance of 20 mm-long cucumber radicles compared to 70 mm-long radicles.