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The Battle for Soybean Seed Production: CO2 vs. Ozone
Reference
Booker, F.L., Miller, J.E., Fiscus, E.L., Pursley, W.A. and Stefanski, L.A.  2005.  Comparative responses of container- versus ground-grown soybean to elevated carbon dioxide and ozone.  Crop Science 45: 883-895.

What was done
The authors grew well watered and fertilized soybean (Glycine max (L.) Merr. cv Essex) plants from seeds that were sown either directly in the ground or in 15-liter pots out-of-doors in open-top chambers maintained at all combinations of low (24 nmol O3 per mol air) or high (75 nmol O3 per mol air) ozone concentration and ambient (373 ppm) or elevated (699 ppm) CO2 concentration in 1999, and in 21-liter pots maintained at all combinations of low (24 nmol O3 per mol air) or high (75 nmol O3 per mol air) ozone concentration and ambient (369 ppm) or elevated (717 ppm) CO2 concentration in 2000.  Throughout the first of the growing seasons they measured net photosynthesis of upper-canopy leaves on a regular basis; and at the ends of both seasons they measured seed yields.

What was learned
In 1999 in the pot-grown plants, the 200% increase in atmospheric O3 concentration decreased net photosynthesis by approximately 21%; but when the air's CO2 concentration was simultaneously boosted by 87%, the negative impact of the O3 increase was more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases exhibited net photosynthesis rates that were 26% greater than those exhibited by the plants growing in low O3 and CO2 air.  Likewise, in the ground-grown plants, the 200% increase in atmospheric O3 concentration decreased net photosynthesis by approximately 14%; but when the air's CO2 concentration was simultaneously boosted by 87%, the negative impact of the O3 increase was again more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases exhibited net photosynthesis rates that were 40% greater than those exhibited by the plants growing in low O3 and CO2 air.

With respect to seed yield in 1999, in the pot-grown plants the 200% increase in atmospheric O3 concentration decreased total seed biomass by approximately 27%; but when the air's CO2 concentration was simultaneously boosted by 87%, the negative impact of the O3 increase was also more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases produced 15% more total seed biomass than the plants growing in low O3 and CO2 air.  Likewise, in the ground-grown plants, the 200% increase in atmospheric O3 concentration decreased total seed biomass by approximately 24%; but when the air's CO2 concentration was simultaneously boosted by 87%, the negative impact of the O3 increase was once again more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases produced 15% more total seed biomass than the plants growing in low O3 and CO2 air.

Last of all, with respect to seed yield in 2000, in the pot-grown plants the 200% increase in atmospheric O3 concentration decreased total seed biomass by approximately 41%; but when the air's CO2 concentration was simultaneously boosted by 94%, the negative impact of the O3 increase was yet again more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases produced 18% more total seed biomass than the plants growing in low O3 and CO2 air.  Likewise, in the ground-grown plants, the 200% increase in atmospheric O3 concentration decreased total seed biomass by approximately 39%; but when the air's CO2 concentration was simultaneously boosted by 94%, the negative impact of the O3 increase was also yet again more than ameliorated, with the result that the plants exposed to elevated concentrations of both trace gases produced 9% more total seed biomass than the plants growing in low O3 and CO2 air.

What it means
In all of the many situations investigated in this study, slightly less than a doubling of the air's CO2 concentration more than compensated for the deleterious effects of a tripling of the air's O3 concentration on both leaf net photosynthesis and total seed biomass production in soybeans.  Even with one hand tied behind its back, CO2 was victorious.

Reviewed 8 June 2005