UV-B induced changes in antioxidant enzymes and their isoforms in cucumber (Cucumis sativus L.) cotyledons.

To assess the role of antioxidant defense system on exposure to ultra-violet-B (UV-B) radiation, the activities of antioxidant enzymes superoxide dismutase (SOD), ascorbic acid peroxidase (APX), glutathione reductase (GR) and guaiacol peroxidase (GPX), as well as the level of antioxidants ascorbic acid (AA) and alpha-tocopherol were monitored in cucumber (Cucumis sativus L. var long green) cotyledons. UV-B enhanced the activity of antioxidant enzymes as well as AA content, but decreased the level of alpha-tocopherol. Significant increase was observed in the activities of SOD and GPX. Analysis of isoforms of antioxidant enzymes by native-PAGE and activity staining revealed three isoforms of GPX in unexposed dark-grown cotyledons (control), and their intensity was enhanced by UV-B exposure. In addition, four new isoforms of GPX were observed in cotyledons after UV-B exposure. Although no new isoforms were observed for the other antioxidant enzymes, the activities of their existing isoforms were enhanced by UV-B.

Involvement of oxyradicals in promotion/inhibition of expansion growth in cucumber cotyledons.

Cytokinin-induced expansion growth of cucumber cotyledons and its interaction with UV-B (280-320 nm) was studied with reference to oxyradicals. UV-B radiation enhanced the level of oxyradicals in the cotyledons measured by EPR spectroscopy. Cytokinin promoted expansion growth was inhibited by UV-B radiation. Cytokinins reduced the level of oxyradicals in dark grown cotyledons, while promoting growth. Overproduction of oxyradicals by UV-B could not be fully accounted for the inhibition of growth of cotyledons, since quenching of radicals by cytokinins did not fully restore inhibition of growth.

Oxyradicals under UV-b stress and their quenching by antioxidants.

Formation of oxyradicals under UV-B stress was investigated using cucumber cotyledons. UV-B radiation induced production of free radicals which were analyzed by ESR spectroscopy. Evidence was obtained for the formation of superoxide and hydroxyl radicals in the tissues by comparing PBN-adducts formed with radicals obtained by chemical autooxidation of KO2 and Fenton's reaction. Addition of superoxide dismutase (SOD) to the reaction mixture partially reduced the intensity of signals confirming the production of superoxide radical as well as hydroxyl radicals. These radicals were quenched in vitro by the natural antioxidants alpha-tocopherol, ascorbic acid and benzoquinone. Changes in the level of antioxidants were also monitored under UV-B stress. The endogenous level of ascorbic acid was enhanced and alpha-tocopherol level was reduced in the tissue after exposure to UV-B radiation. The present report happens to be the first direct evidence obtained for the formation of superoxide and hydroxyl radicals in plant tissues exposed to UV-B radiation.