Elevated atmospheric CO 2 concentration ([CO 2 ]) stimulates seed mass production in many species, but the extent of stimulation shows large variation among species. We examined (1) whether seed production is enhanced more in species with lower seed nitrogen concentrations, and (2) whether seed production is enhanced by elevated [CO 2 ] when the plant uses more N for seed production. We grew 11 annuals in open top chambers that have different [CO 2 ] conditions (ambient: 370 lmol mol À1 , elevated: 700 lmol mol À1 ). Elevated [CO 2 ] significantly increased seed production in six out of 11 species with a large interspecific variation (0.84-2.12, elevated/ambient [CO 2 ]). Seed nitrogen concentration was not correlated with the enhancement of seed production by elevated [CO 2 ]. The enhancement of seed production was strongly correlated with the enhancement of seed nitrogen per plant caused by increased N acquisition during the reproductive period. In particular, legume species tended to acquire more N and produced more seeds at elevated [CO 2 ] than non-nitrogen fixing species. Elevated [CO 2 ] little affected seed [N] in all species. We conclude that seed production is limited primarily by nitrogen availability and will be enhanced by elevated [CO 2 ] only when the plant is able to increase nitrogen acquisition.

Elevated CO 2 increases seed production more in plant species that form a symbiotic association with N 2 -fixing bacteria than in species without such association. We studied the mechanism of the increase of seed production with elevated CO 2 using nodulated soybean (Glycine max cv. Enrei) and its non-nodulated isogenic line (cv. En1282). Increase in seed production with elevated CO 2 was observed in nodulated Enrei but was not in non-nodulated En1282. The increase in seed production in Enrei was explained by the increase in the rate of dry mass production during the reproductive period. This increase was associated with the increase in N assimilation in the reproductive period and the seed N concentration that remained the same as that at ambient CO 2 . Dry mass production and nitrogen assimilation did not increase in the vegetative phase in both lines. These results accorded with the amount of nodules in Enrei that increased at elevated CO 2 especially after flowering. We conclude that the increase in N assimilation in the reproductive period would be the key for increasing soybean yield in the future high-CO 2 world.