Increases in soybean photosynthesis and biomass diminish at growth CO 2 concentrations >1000 ppm, constraining the benefits of increasing atmospheric CO 2 concentration on soybean productivity.
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.