The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background

This review first summarizes the numerous studies that have described the i nteraction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO2]. The initial stimulation of photosy nthesis in elevated [CO2] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate c arboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These ch anges are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emergin g evidence is discussed that signals derived from nitrate and nitrogen meta bolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation? organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expr ession of genes involved in photosynthesis, and to modulate whole plant eve nts including shoot-root allocation, root architecture and flowering. Third , increased rates of growth in elevated [CO2] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed th at an increased supply of sugars can increase the rates of nitrate and ammo nium uptake and assimilation, the synthesis of organic acid accepters, and the synthesis of amino acids. Fourth, interpretation of experiments in elev ated [CO2] requires that the nitrogen status of the plants is monitored, Th e suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevat ed [CO2] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO2], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar-mediated repression of gene expression, end-product fee dback of photosynthesis, nitrogen-induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do ch anges in sugar production and inorganic nitrogen assimilation interact in d ifferent conditions and at different stages of the life history to determin e the response of whole plant growth and allocation to elevated [CO2]?