Pre-industrial to 2000 ppm; soybean response to increasing CO2

Many studies addressing the effects of rising carbon dioxide concentrations ([CO2]) on agricultural crops have demonstrated the critical role that this gas has on plant physiology. It is now essential to assess the responses of our major crop systems to even higher [CO2] as these photosynthetic and physiological responses may determine our food security in the next century. The majority of previous studies have focused on scenarios published in the Intergovernmental Panel on Climate Change (IPCC) reports. None of these scenarios have accurately predicted the rise in recent years of [CO2]; in fact, measured [CO2] has exceeded the worst-case IPCC emissions scenario (A1F1). In this study, I tested soybean (Glycine max L. cv 93B15) responses to eight different [CO2] levels in growth chambers at the University of Illinois Plant Sciences Laboratory. Five individual plants were grown for five weeks in each of eight chambers with [CO2] ranging from pre-industrial (250ppm) to a level much higher than any predicted for the next century (2000ppm). The objective of this experiment is to assess the physiological and photosynthetic responses to [CO2] exceeding levels predicted by current models. Measurements included plant developmental stages, photosynthesis rates and underlying biochemistry, respiration, as well as plant growth and yields. I predict that soybean biomass accumulation and photosynthesis will increase linearly with increases in [CO2] due to the decrease in photorespiration; however, above a certain threshold, the benefits of continued increases in [CO2] will diminish. The data show that physiological development was delayed as [CO2] increased. Plant height and total leaf area increased with higher [CO2]. Photosynthesis increased with increasing [CO2] up until the 1000ppm treatment, after which it plateaued. Stomatal conductance showed a decreasing trend with increasing [CO2]. These results indicate soybean productivity will increase as [CO2] continues to rise, but as the concentrations exceed the “worst-case” scenarios, physiology, growth, and yields will begin to plateau