Earlier studies with Arabidopsis thaliana exposed to ultraviolet 6 (UV-6) and ozone (O,) have indicated the differential responses of superoxide dismutase and glutathione reductase. In this study, we have investigated whether A. thaliana genotype Landsberg erecfa and its flavonoid-deficient mutant fransparent testa ( t f 5 ) is capable of metabolizing UV-B-and O,-induced activated oxygen species by invoking similar antioxidant enzymes. UV-6 exposure preferentially enhanced guaiacol-peroxidases, ascorbate peroxidase, and peroxidases specific to coniferyl alcohol and modified the substrate affinity of ascorbate peroxidase. O, exposure enhanced superoxide dismutase, peroxidases, glutathione reductase, and ascorbate peroxidase to a similar degree and modified the substrate affinity of both glutathione reductase and ascorbate peroxidase. Both UV-B and O, exposure enhanced similar Cu,Zn-superoxide dismutase isoforms. New isoforms of peroxidases and ascorbate peroxidase were synthesized in tt5 plants irradiated with UV-B. UV-B radiation, in contrast to O,, enhanced the activated oxygen species by increasing membrane-localized NADPH-oxidase activity and decreasing catalase activities. These results collectively suggest that (a) UV-B exposure preferentially induces peroxidase-related enzymes, whereas O, exposure invokes the enzymes of superoxide dismutase/ ascorbate-glutathione cycle, and (b) in contrast to O,, UV-B exposure generated activated oxygen species by increasing NADPHoxidase activity.

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The potential role of antioxidant enzymes in protecting maize (Zea mays 1.) seedlings from chilling injury was examined by analyzing enzyme activities and isozyme profiles of chilling-susceptible (CO 316) and chilling-tolerant (CO 328) inbreds. Leaf superoxide dismutase (SOD) activity in CO 316 was nearly one-half that of CO 328, in which the high activity was maintained during the chilling and postchilling periods. Activity of glutathione reductase (CR) was much higher in roots than in leaves. CO 328 also possessed a new CR isozyme that was absent in roots of CO 316. Ascorbate peroxidase (APX) activity was considerably lower in leaves of CO 328 than in CO 316, and nearly similar in roots. Paclobutrazol treatment of CO 316 induced several changes in the antioxidant enzyme profiles and enhanced their activities, especially those of SOD and APX, along with the induction of chilling tolerance. These results suggest that increased activities of SOD in leaves and CR in roots of CO 328, as well as SOD and APX in leaves and roots of paclobutrazol-treated CO 31 6, contribute to their enhanced chilling tolerance.

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