Chilling stress-induced changes of antioxidant enzymes in the leaves of cucumber: in gel enzyme activity assays.
To investigate the antioxidant defense system, chilling stress-induced changes of antioxidant enzymes were examined in the leaves of cucumber (Cucumis sativus L.). Chilling stress preferentially enhanced the activities of the superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and peroxidase specific to guaiacol, whereas it induced the decrease of catalase activity. In order to analyze the changes of antioxidant enzyme isoforms against chilling stress, foliar extracts were subjected to native PAGE. Leaves of cucumber had four isoforms of Mn-SOD and two isoforms of Cu/Zn-SOD. Fe-SOD isoform was not observed in this plant. Expression of Cu/Zn-SOD and Mn-SOD was preferentially enhanced by chilling stress. Expression of Mn-SOD-2 and -4 was enhanced after 48 h of the poststress period. Five APX isoforms were presented in the leaves of cucumber. The intensities of APX-4 and -5 were enhanced by chilling stress, whereas that of APX-3 was significantly increased in the poststress periods after chilling stress. Gel stained for GR activity revealed six isoforms in the plant. Activation levels for most of GR isoforms were higher in the stressed-plants than the control and poststressed-plants, but that of GR-1 isoform was significantly higher in the poststressed-plants than chilling stressed-plants. These results collectively suggest that chilling stress activates the enzymes of an SOD/ascorbate-glutathione cycle under catalase deactivation in the leaves of cucumber, but the response timing of enzyme isoforms against various environmental stresses is not the same for all isoforms of antioxidant enzymes.