Global Change and Nitrate Assimilation

University Of California-Davis, Davis CA

Investigators

Abstract

Global Change and Nitrate Assimilation Carbon dioxide concentrations in the atmosphere that are associated with global warming have risen by more than 30 percent during the past two centuries and are expected to double during the next. Some scientists believe these rising levels of carbon dioxide will benefit plants because carbon dioxide is one of the essential ingredients in photosynthesis, the process by which green plants use sunlight to manufacture the chemical energy they need. Indeed, many plants initially respond to a doubling of atmospheric carbon dioxide levels by assimilating 30 percent more carbon dioxide into carbohydrate. Further study, however, reveals that the accelerated rate of carbon dioxide assimilation is not sustained: within a few days or weeks of exposure to elevated carbon dioxide, carbon dioxide assimilation drops back to just 12 percent greater than normal, a phenomenon known a carbon dioxide acclimation. Bloom and colleagues discovered that carbon dioxide acclimation derives from nitrogen deprivation because elevated carbon dioxide inhibits the assimilation of nitrate into amino acids in leaves and nitrate is a major form of nitrogen that plants obtain from the soil. Three mechanisms appear to be responsible. First, plants place a higher priority on assimilating carbon dioxide than they do nitrogen, so when carbon dioxide levels rise, some of the high energy compounds needed to assimilate nitrate are already tied up in assimilating carbon dioxide. Second, to make use of nitrate, the plants initially convert nitrate into nitrite in the cytoplasm and move the nitrite into the chloroplast for conversion into ammonium and then amino acids. Bloom's research indicated that elevated levels of carbon dioxide blocked this vital transfer of nitrite into the chloroplasts. Third, under current levels of carbon dioxide and oxygen, most plants lose about one-quarter of the carbohydrate that they could accumulate from a process known as photorespiration. This process was thought to be wasteful, unavoidable consequence of the method that most plants use to generate carbohydrate. Bloom's latest results show that these plants must photorespire in order to convert nitrate into amino acids. These studies suggest that plant and tree species in natural ecosystems that depend on nitrate conversion into amino acids in their leaves are likely to be at a competitive disadvantage to those species that are either able to convert nitrate into amino acids in their roots or use ammonium as their predominant nitrogen source. As a result the distribution of plants in the wild may change significantly as atmospheric carbon dioxide levels continue to rise. Nitrous oxide is another major greenhouse gas that contributes to global warming. Bloom and colleagues showed that wheat plants emit a significant amount of nitrous oxide as part of nitrate assimilation. The form of nitrogen that a plant uses, therefore, may influence its nitrous oxide emissions. The proposed research will determine the interdependence among photorespiration, carbon dioxide assimilation, nitrous oxide production, and nitrate assimilation. It will employ Arabidopsis genotypes with altered capacities to assimilate nitrate and a series of Flaveria species that vary in their extent of photorespiration. Laboratory and field studies will also examine the relative importance of ammonium and nitrate as plant nitrogen sources. Lastly, at a national facility for monitoring the influence of elevated carbon dioxide on desert flora, experiments will assess in situ the extent to which carbon dioxide inhibits plant nitrate assimilation.