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

AGPase, ADP glucose pyrophosphorylase GS, glutamine synthetase GOGAT, glutamate : oxoglutarate amino transferase NADP‐ICDH, NADP‐dependent isocitrate dehydrogenase NR, nitrate reductase OPPP, oxidative pentose phosphate pathway 3PGA, glycerate‐3‐phosphate PEPCase, phosphoenolpyruvate carboxylase Rubisco, ribulose‐1,5‐bisphosphate carboxylase/oxygenase SPS, sucrose phosphate‐synthase This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated CO 2 . The initial stimulation of photosynthesis in elevated CO 2 is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes 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, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites 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 expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated CO 2 will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated CO 2 requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated CO 2 and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated CO 2 , and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar‐mediated repression of gene expression, end‐product feedback 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 changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated CO 2 ?