Leaf defense system of Robinia pseudoacacia L. seedlings exposed to 3years of elevated atmospheric CO and Cd-contaminated soils.

Short-term exposure to elevated CO increases cadmium (Cd) uptake in some plant species (wheat, poplars, and willows), which triggers an increase in antioxidative system activity to deal with additional reactive oxygen species that are generated. Here, we examined leaf defenses in Robinia pseudoacacia L. seedlings exposed to elevated CO+Cd for 3years. Three years of elevated CO decreased Cd uptake into leaves and the Cd content in soils and increased the pH of rhizosphere soil relative to ambient CO. In plants exposed to Cd stress, leaf chlorophyll content was greater under elevated CO than under ambient CO. Superoxide dismutase, peroxidase, and catalase activity increased, glutathione content increased, and malondialdehyde and phytochelatins contents decreased under elevated CO+Cd relative to Cd alone. Proline, soluble sugars, flavonoids, saponins, and phenolic acids contents were greater under elevated CO+Cd than under Cd alone, and condensed tannin content was lower. Overall, long-term elevation of CO enhanced the leaf defense system of R. pseudoacacia exposed to Cd by stimulating antioxidant enzyme activity, osmotic adjustment, and the production of glutathione, flavonoids and phenolic acids. Future research should focus on understanding the mechanisms involved in the decrease in Cd uptake into leaves and Cd content in soils and the increase in rhizosphere soil pH under long-term exposure to elevated CO.