Soybean photosynthetic and biomass responses to carbon dioxide concentrations ranging from pre-industrial to the distant future

Increasing atmospheric carbon dioxide concentration ([CO(2)]) directly impacts C(3) plant photosynthesis and productivity, and the rate at which [CO(2)] is increasing is greater than initially predicted by worst-case scenario climate models. Thus, it is increasingly important to assess the physiological responses of C(3) plants, especially those that serve as important crops, to [CO(2)] beyond the mid-range levels used in traditional experiments. Here, we grew the C(3) crop soybean (Glycine max) at eight different [CO(2)] levels spanning subambient (340 ppm) to the highest level thought plausible (~2000 ppm) in chambers for 5 weeks. Physiological development was delayed and plant height and total leaf area increased at [CO(2)] levels higher than ambient conditions, with very little difference in these parameters among the elevated [CO(2)] treatments >900 ppm. Daily photosynthesis initially increased with rising [CO(2)] but began to level off at ~1000 ppm CO(2). Similar results occurred in biomass accumulation. Thus, as [CO(2)] continues to match or exceed the worst-case emission scenarios, these results indicate that carbon gain, growth, and potentially yield increases will diminish, thereby ultimately constraining the positive impact that continuing increases in atmospheric [CO(2)] could have on crop productivity and global terrestrial carbon sinks.