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 (approximate to 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 (approximate to 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.