Too Much Carbon Dioxide Limits Plants

Earth’s atmospheric carbon dioxide concentration is expected to rise substantially from the current level of about 390 ppm to as much as 970 ppm by 2100, according to the Intergovernmental Panel on Climate Change. Global warming aside, the CO₂ increase is expected to benefit plant growth and food production by facilitating photosynthesis. But this positive effect has a limit—too much CO₂ over time inhibits assimilation of nitrate (NO₃ ^–) into proteins by plants. Arnold J. Bloom of the University of California, Davis, and coworkers are reporting results of experiments on wheat and Arabidopsis plants that provide better details of the biochemistry behind this limiting process (Science 2010, 328, 899). Most plants draw NO₃ ^– and ammonium (NH₄ ⁺) from soil and then convert NO₃ ^– into more NH₄ ⁺, which plants use to produce the amino acid glutamine and other organic nitrogen compounds. The researchers assessed NO₃ ^– assimilation via several methods, including changes in the relative rates of CO₂ consumption and O₂ evolution and the fate of ¹⁵N-enriched NO₃ ^–. They showed that additional CO₂ disrupts the conversion of NO₃ ^– to NH₄ ⁺. As CO₂ levels rise, adding NH₄ ⁺-based fertilizer could bypass the NO₃ ^– bottleneck and help maintain the yield and quality of food crops, Bloom and colleagues note.