Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 1, 2000
Publication Date: N/A
Interpretive Summary: A problem of increasing severity in North America and Canada is the quality of the air that limits crop growth and yield. Ozone levels in the troposphere again are rising due to increased daily total exhaust emissions resulting from a rapidly increasing population of various types of motor vehicles. Coupled with increased gaseous emissions from motor vehicles are eperiods of drought, high light, and elevated temperatures which have also affected agriculture productivity, especially during the 1999 growing season. During the summer of 1999, daily ozone levels in the troposphere of the mid-Atlantic states have often been in the range of 80-100 nL/L air, and these levels have probably reduced soybean yield as much as 40% in some areas of the mid-Atlantic states. Thus, it is extremely important to identify factors in crop plants that can help them withstand prolonged periods of exposure to elevated tropospheric ozone. Beltsville scientists are showing that a cultivar of soybean that exhibits normal vegetative and reproductive activity during prolonged periods of exposure to elevated ozone level, also has a has 20 to 30% higher foliar ascorbic acid level than does another cultivar which was more ozone sensitive. The ozone sensitive cultivar exhibited an approximately 20% reduction in biomass and bean yield per plant. This study suggests that genetic manipulation of soybean cultivars in order to increase ascorbic acid synthesis and accumulation in their leaves would result in more ozone tolerant cultivars. Thus, this study is important to soybean geneticists and breeders. It is also important to other plant physiologists studying the enzymatic pathway of ascorbic acid biosynthesis.
Evidence is presented that foliar ascorbate (ASC) level and ASC and dehydroascorbate (DHA) redox status are important factors in sustaining soybean plant tolerance to elevated ozone levels in polluted air. In this study, done in the summer of 1997, two soybean cultivars, one that preliminary evidence indicated to be O3-tolerant (cv Essex), and one that was indicated to be O3-sensitive (cv Forrest), were grown in open-top chambers in a field environment. The experimental plants were exposed daily to a controlled, moderately high O3 level (58 nL/L air) beginning at the seedling stage and continuing to bean maturity. Elevated O3 did not affect biomass per plant, mature leaf area accretion, or bean yield per plant of cv Essex. In contrast, elevated O3 level decreased the biomass and bean yield per plant of cv Forrest by approximately 20%. Daily leaflet photosynthesis rate and conductance per unit area was not decreased in either cultivar by O3 exposure, but a 10% lower mature leaflet area in O3-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 O3 tolerant than cv Forrest were: 1) mature leaflets of control and O3-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 to DHA redox status than leaflets of cv Forrest plants.