How do elevated CO2 and O3 affect the interception and utilization of radiation by a soybean canopy?

Net productivity of vegetation is determined by the product of the efficiencies with which it intercepts light («i) and converts that intercepted energy into biomass («c). Elevated carbon dioxide (CO2) increases photosynthesis and leaf area index (LAI) of soybeans and thus may increase «i and «c; elevated O3 may have the opposite effect. Knowing if elevated CO2 and O3 differentially affect physiological more than structural components of the ecosystem may reveal how these elements of global change will ultimately alter productivity. The effects of elevated CO2 and O3 on an intact soybean ecosystem were examined with Soybean Free Air Concentration Enrichment (SoyFACE) technology where large field plots (20-m diameter) were exposed to elevated CO2 (550lmol mol1) and elevated O3 (1.2 ambient) in a factorial design. Aboveground biomass, LAI and light interception were measured during the growing seasons of 2002, 2003 and 2004 to calculate «i and «c. A 15 % increase in yield (averaged over 3 years) under elevated CO2 was caused primarily by a 12 % stimulation in «c, as «i increased by only 3%. Though accelerated canopy senescence under elevated O3 caused a 3 % decrease in «i