The combined effects of elevated atmospheric CO and cadmium exposure on flavonoids in the leaves of Robinia pseudoacacia L. seedlings.

Flavonoids participate in several plant processes such as growth and physiological protection in adverse environments. In this study, we investigated the combined effects of eCO and cadmium (Cd)-contaminated soils on the total flavonoid and monomer contents in the leaves of Robinia pseudoacacia L. seedlings. Elevated CO, Cd, and eCO+ Cd increased the total flavonoids in the leaves relative to the control, and eCO mostly increased (p < 0.05) the total flavonoid content under Cd exposure. Elevated CO increased (p < 0.05) robinin, rutin, and acacetin contents in the leaves of 45-day seedlings and decreased (p < 0.05) the content of robinin and acacetin at 90 and 135 d under Cd exposure except for robinin at day 45 under Cd1 and acacetin on day 135 under Cd1. Quercetin content decreased (p < 0.05) under the combined conditions relative to Cd alone. Kaempferol in the leaves was only detected under eCO on day 135. The responses of total chlorophyll, total soluble sugars, starch, C, N, S, and the C/N ratio in the leaves to eCO significantly affected the synthesis of total flavonoids and monomers under Cd exposure. Overall, rutin was more sensitive to eCO+ Cd than the other flavonoids. Cadmium, CO and time had significant interactive effects on the synthesis of flavonoids in the leaves of R. pseudoacacia L. seedlings. Elevated CO may improve the protection and defense system of seedlings grown in Cd-contaminated soils by promoting the synthesis of total flavonoids, although robinin, rutin, quercetin, and acacetin yields may reduce with time. Additionally, increased Cd in the leaves suggested that eCO could improve the phytoremediation of Cd-contaminated soils.