Concentrations of secondary metabolites in tissues and root exudates of wheat seedlings changed under elevated atmospheric CO2 and cadmium-contaminated soils

• Elevated CO2 improved the accumulation of root exudates of wheat under Cd stress.
• Elevated CO2 inhibited secondary metabolite production in tissues under Cd stress.
• This study implies elevated CO2 contributes to remediation of Cd-polluted soil.

We investigated the short-term effects of elevated atmospheric CO2 and cadmium (Cd)-contaminated soils on wheat seedlings root exudates and secondary metabolites in tissues. On average, total soluble sugar, total free amino acid, total phenolic acid, and total organic acid in root exudates were 45.7%, 63.2%, 40.0%, and 89.5% higher (p < 0.01), respectively, under elevated CO2 + Cd when compared to ambient CO2 chambers + Cd, which were greater than those under either elevated CO2 or Cd stress. Total phenolic acid, condensed tannin, and total flavonoid in tissues of wheat seedlings were 18.1%, 15.7%, and 21.9% (p < 0.01) lower, respectively, on average under elevated CO2 combined with Cd stress, which were lower than those under either elevated CO2 or Cd stress. Furthermore, the content of indole alkaloids in tissues at 1.0, 5.0, and 10.0 Cd mg Cd kg soil−1 levels differed by −11.8%, −4.2%, and 37.3% (p < 0.01), respectively, under elevated CO2. The combination of elevated CO2 and Cd stress significantly influenced free amino acid, total phenolic acid, and organic acid in root exudates and condensed tannin, total flavonoid, and indole alkloids in tissues; however, the effect on total soluble sugar in root exudates and total phenolic acid in tissues was insignificant. Overall, elevated CO2 improved the concentration of root exudates of wheat seedlings under Cd-stressed conditions. However, elevated CO2 inhibited the accumulation of total phenolic acid, total flavonoid, and condensed tannin in tissues under Cd-stressed conditions. In addition, there might be a threshold in the response of indole alkaloids in tissues to elevated CO2 with increasing Cd levels.