title: Why are nitrogen concentrations in plant tissues lower under elevated CO2? A critical examination of the hypotheses.
abstract: Plants grown under elevated atmospheric [CO2] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO2]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO2. This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO2, although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO2, plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.
authors.affiliation: Biology Department and Environmental Studies Program, Southwestern University, Georgetown, Texas 78626, USA. taubd@southwestern.edu
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Taub
authors.forename: Xianzhong
authors.initials: X
authors.lastname: Wang
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002245:Carbon Dioxide / Q000633:toxicity*; D009584:Nitrogen / Q000378:metabolism*; D018526:Plant Transpiration / Q000187:drug effects; D010944:Plants / Q000187:drug effects* / Q000378:metabolism*
chemical_list: D002245:Carbon Dioxide; D009584:Nitrogen