Plants grown under elevated atmospheric [CO 2 ] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO 2 ]. 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 CO 2 . 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 CO 2 , although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO 2 , plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.Key words: carbon dioxide; dilution; elevated CO 2 ; graphical vector analysis; nitrogen; plants; root uptake; tissue concentrations. Growth of plants at atmospheric concentrations of carbon dioxide (CO 2 ) greater than the current ambient can greatly affect plant tissue chemistry (Poorter et al. 1997;Loladze 2002). One of the most commonly seen effects is a decrease in the dry mass concentration of N (N m ). Cotrufo et al. (1998), synthesizing data from a broad range of studies, found mean decreases in N m of 14% in aboveground tissues and 9% in roots. This compares closely with the findings of other data syntheses, which have found elevated [CO 2 ] mediated decreases in N m Received 25 Feb. 2008 Accepted 11 Jun. 2008 Supported by the Cullen Fund of Southwestern University to D. R. Taub.* Author for correspondence. Yin (2002) found that the effect of elevated [CO 2 ] was greatest for woody deciduous species and that decreases in N m under elevated CO 2 were most pronounced at high light levels, high temperatures and large pot sizes. Yin (2002) and Taub et al. (2008) both found that the effect of [CO 2 ] on N m was reduced by N fertilization. Several studies have reported that the effect of elevated [CO 2 ] on N m is less for nitrogen-fixing species than for other types of plants (Cotrufo et al. 1998; Jablonski et al. 2002;Taub et al. 2008). Ainsworth and Long (2005) and Taub et al. (2008) found that the effect of elevated CO 2 on N m increased under ozone stress (although see Taub et al. 2008 for divergent results for soybean).Although it has been well established that elevated [CO 2 ] typically decreases N m , the mechanisms by which this occurs are n...