Abstract. The activities of five active‐oxygen scavenging enzymes were compared for cold‐lability and three were compared for chilling induction in two Zea genotypes of contrasting susceptibility to photoinhibition during chilling. Activities of superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GTR, EC 1.6.4.2) in leaf extracts from plants grown without chilling stress were assayed at 19°C and 5°C. Enzymes from the chilling‐susceptible Z. Mays cv. LG11 had lower specific activities at 5°C than did enzymes from the chilling‐tolerant Z. diploperennis, except for MDHAR where no significant differences were observed. The activities of SOD and APX from Z. diploperennis were double those of Z. mays at both assay temperatures. Monodehydroa‐scrobate reductase and glutathione reductase activities in both species were reduced by 63–78% at a 5°C assay temperature. The dehydroascorbate reductase (DHAR) showed the greatest low‐temperature lability losing 96% (Z. diploperennis) and 100% (Z. mays) of its activity at 5°C. To examine possible chilling‐induced changes in levels of enzyme activity, plants of both Zea genotypes were transferred to growth chambers at 10°C at moderate light intensities. Glutathione reductase activity was found to increase within 24h in Z. diploperennis, but it decreased slightly in Z. mays. MDHAR activity decreased by 50% in Z. diploperennis but showed only a transient increase in activity in Z. mays.

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When Zea mays cv. LG11 plants were grown 14˚C(close to the lower thermal limit for leaf expansion), three of the five enzymes of the active oxygen scavenging cycle (Halliwell-Asada Pathway) showed changes in total leaf activity compared with growth at 25˚C. Two of theseenzymes, ascorbate peroxidase (APX) and glutathione reductase (GTR), were selected for further investigation. The effects of assay temperature on threekinetic parameters (Vmax,Km,Vmax/Km) were determined inextracts from Z. mays and compared with extracts fromits low temperature tolerant relative, Z. diploperennisIltis, Doebley & Guzman. The kinetic power(Vmax/Km) was determinedbecause the Km alone may not be a useful predictor of anenzyme’s effectiveness in situ. The decrease inthe kinetic power of APX on lowering the temperature to 5ºC was muchsmaller in Z. diploperennis thaninZ. mays. This suggests that theZ. diploperennis APX is better able to removeH2O2 at severely reducedtemperatures than is APX from Z. mays.Z. diploperennis had a twofold greater ascorbate poolthan the chilling-susceptible Z. mays. Only minordifferences were seen in the kinetic properties of GTR and the size of theglutathione pool between the genotypes.

Actively growing sugarbeet is treated with the post-emergent herbicide phenmedipham at times when ozone pollution episodes are likely to occur. There is a possibility of an interaction occurring between ozone and phenmedipham as both treatments produce similar effects in susceptible plants, such as a reduction in growth and photosynthesis and an increase in the activities of endogenous antioxidant enzymes. To investigate this likelihood, laboratory experiments were conducted in which two-to three-leaf sugarbeet plants (Beta vulgaris L. cv. Saxon) were exposed to a simulated two-day ozone episode (100 nl litre-'. 7 h day-I ) followed three days later by treatment with field rate phenmedipham (1.14 kg A1 ha-'). Growth analysis indicated that an interaction was occurring in which plants treated with ozone and phenmedipham had less reduction in shoot fresh weight than expected. Exposure to phenmedipham alone or ozone followed by phenmedipham reduced net photosynthesis by over 50% and transpiration rate by 30%. The activities of antioxidant enzymes such as catalase, guaiacol peroxidase and superoxide dismutase were stimulated by both treatments individually, but to a greater extent when ozone and phenmedipham were combined. For example, three days after herbicide treatment, the activity of superoxide dismutase increased by 20% in plants treated with ozone alone, 20% in plants treated with phenmedipham alone and 85% in plants that were treated with ozone followed by phenmedipham. We conclude that ozone pollution may predispose sugarbeet to tolerate the herbicide phenmedipham by enhancing the activity of the endogenous antioxidant detoxification enzyme system.

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