Tolerance of a field grown soybean cultivar to elevated ozone level is concurrent with higher leaflet ascorbic acid level, higher ascorbate-dehydroascorbate redox status, and long term photosynthetic productivity

We examined the characteristics of ascorbic acid (ASC) level, dehydroascorbate (DHA) level, and the ASC-DHA redox status in the leaflets of two soybean cultivars grown in a field environment and exposed to elevated ozone (O(3)) levels. These two cultivars, one that preliminary evidence indicated to be O(3)-tolerant (cv Essex), and one that was indicated to be O(3)-sensitive (cv Forrest), were grown in open-top chambers during the summer of 1997. The plants were exposed daily to a controlled, moderately high O(3) level ( approximately 58 nl l(-1) air) in the light, beginning at the seedling stage and continuing to bean maturity. Concurrently, control plants were exposed to carbon-filtered, ambient air containing a relatively low O(3) level ( approximately 24 nl l(-1) air) during the same period. Elevated O(3) did not affect biomass per plant, mature leaf area accretion, or bean yield per plant of cv Essex. In contrast, elevated O(3) level decreased the biomass and bean yield per plant of cv Forrest by approximately 20%. Daily leaflet photosynthesis rate and stomatal conductance per unit area did not decrease in either cultivar as a result of prolonged O(3) exposure. A 10% lower mature leaflet area in O(3)-treated cv Forrest plants contributed to an ultimate limitation in long-term photosynthetic productivity (vegetative and bean yield). Possible factors causing cv Essex to be more O(3) tolerant than cv Forrest were: 1) mature leaflets of control and O(3)-treated cv Essex plants consistently maintained a higher daily ASC level than leaflets of cv Forrest plants, and 2) mature leaflets of cv Essex plants maintained a higher daily ASC-DHA redox status than leaflets of cv Forrest plants.