Antioxidant metabolite levels in ozone-sensitive and tolerant genotypes of snap bean

Ozone-sensitive and tolerant genotypes of snap bean (Phaseolus vulgarius L.) were compared for differences in leaf ascorbic acid (vitamin C), glutathione and alpha-tocopherol (vitamin E) content to determine whether antioxidant levels were related to ozone tolerance. Seven genotypes were grown in pots under field conditions during the months of June and July. Open top chambers were used to establish either a charcoal filtered (CP) air control (36 nmol mol(-1) ozone) or a treatment where CF air was supplemented with ozone from 8:00 to 20:00 h with a daily 12 h mean of 77 nmol mol(-1). Fully expanded leaves were analyzed for ascorbic acid, chlorophyll, glutathione, guaiacol peroxidase (EC 1.11.1.7) and alpha-tocopherol. Leaf ascorbic acid was the only variable identified as at potential factor in ozone tolerance. Tolerant genotypes contained more ascorbic acid than sensitive lines, but the differences were not always statistically significant. Genetic differences in glutathione and alpha-tocopherol were also observed, but no relationship with ozone tolerance was found. Guaiacol peroxidase activity and leaf alpha-tocopherol content increased in all genotypes following a one week ozone exposure, indicative of a general ozone stress response. Ozone had little effect on the other variables tested. Overall, ozone sensitive and tolerant plants were not clearly distinguished by differences in leaf antioxidant content. The evidence suggests that screening for ozone tolerance based on antioxidant content is not a reliable approach.