Department of Plant Sciences

The responses of higher plants to rising carbon dioxide concentration in the atmosphere are strongly dependent on their ability to acquire mineral nitrogen, ammonium and nitrate. Elevated atmospheric carbon dioxide limits both sources and sinks of plant mineral nitrogen. With regard to sources, elevated carbon dioxide stimulates microbial immobilization and inhibits nitrogen fixation. With regard to sinks, elevated carbon dioxide inhibits nitrate assimilation into amino acids within the shoots of C3 plants, including gymnosperms, monocotyledons, and dicotyledons. The overall result is that the organic nitrogen content of plants tends to drop precipitously under exposure to elevated carbon dioxide. Key to these observations is the relationship between nitrate assimilation and photorespiration. Conditions that diminish photorespiration in C3 plants—namely, elevated carbon dioxide or low oxygen atmospheres—diminish nitrate assimilation as assessed by gas exchange, nitrogen isotopes, and accumulation of organic nitrogen. Photorespiration appears to be involved in the export of malate from the chloroplast that empowers nitrate reduction in the cytoplasm. C4 plants generate ample amounts of malate in the cytoplasm of their mesophyll cells, and nitrate assimilation in C4 plants does not show sensitivity to atmospheric carbon dioxide level. These results provide new insight into photorespiration, C4 carbon fixation, and the fate of higher plants under the conditions anticipated by the end of the century.