GROWTH AND PHYSIOLOGICAL-CHARACTERISTICS OF SOYBEAN IN OPEN-TOP CHAMBERS IN RESPONSE TO OZONE AND INCREASED ATMOSPHERIC CO2

Open-top chamber studies were conducted during 1989 at USDA-BARC to determine interactive effects of increased atomospheric CO2 and O3 air pollution on physiological and growth responses in soybeans. The plants were grown full-season in chambers supplied with charcoal-filtered air (CF), non-filtered (NF) air or NF + 40 nl l-1 O3 having CO2 concentrations of ambient, + 50 or + 150-mu-l l-1 CO2. The resultant seasonal 7 h O3 concentrations for the three air quality regimes were 23.0 nl l-1, 40.3 nl l-1 and 66.4 nl l-1 O3, respectively. Photosynthesis rates (PS) and stomata conductance (SC) values were measured on fully expanded leaves on 11 dates throughout the growing season and vegetative samples were collected for carbohydrate analyses and for plant biomass determinations. Seed weights, grain yields and quality measurements were made following harvest at physiological maturity. Photosynthesis rates were stimulated by the + 50 and + 150-mu-l l-1 CO2 treatments and reduced by the NF + 40 nl l-1 O3 treatment. Reductions in PS rates observed for the NF + O3 treatment during preflowering were counteracted by the + 150-mu-l l-1 CO2 treatment. Wide variation in stomata conductance was observed throughout the study; however, the combined effects of CO2 and O3 on stomata conductance appeared additive. Leaf glucose and fructose concentrations were lower for the high O3 exposures but were unchanged by CO2 concentrations. Sucrose and starch concentrations were significantly increased by + 150-mu-l l-1 CO2. Sucrose was unaffected by O3 treatments, but starch was significantly increased by the NF + O3 treatment at ambient and + 50-mu-l l-1 CO2 thus suggesting that O3 injury reduced the flux of carbohydrates from leaves, especially at low CO2 levels. Leaf N concentrations were significantly reduced by the NF + O3 treatment. Significant changes in plant biomass, leaf area, specific leaf weight, pods and seeds per plant, and grain yields were observed among treatments. Maximum increases in plant growth and productivity were observed in response to CO2 increases in the absence of O3 air pollution. Changes observed in plant biomass, pods and seeds per plant and grain yields were consistent with the results for the PS and leaf carbohydrate data which suggest that the + 150-mu-l l-1 CO2 treatment had a protective role against adverse effects of O3 exposure. Grain oil contents were increased and protein contents significantly reduced in response to CO2; however, both quality features were unaffected by the O3 treatments.