Effect of citric acid on phytoextraction and antioxidative defense in Solanum nigrum L. as a hyperaccumulator under Cd and Pb combined pollution

The impact of citric acid on the ability of Solanum nigrum L. to accumulate heavy metals and on its antioxidant enzyme activity were investigated to further elucidate the effect of chelation on the heavy metal uptake and antioxidative defense in plants under conditions of heavy metal pollution. In the presence of multiple metal contaminants (Cd and Pb), citric acid treatment can significantly enhance the Cd (10–30%) and Pb (10–20%) accumulation in S. nigrum when compared to non-citric acid-treated controls; with this enhanced accumulation, S. nigrum becomes a hyperaccumulator. However, citric acid treatment only slightly influences on the increase of S. nigrum biomass. Furthermore, the combined stress of Pb and Cd resulted in a decrease in S. nigrum photorespiration; the rate of carbon dioxide fixation was restored with the application of citric acid. In addition, the presence of Cd and Pb in the soil led to disturbances in the antioxidative responses of S. nigrum; endogenous superoxide dismutase activity was approximately 3–4 times higher in the leaves than in the roots, and guaiacol peroxidase activity in the leaves was only slightly affected. It was also observed that CA increased the malondialdehyde content and total acid soluble thiol content in the presence of elevated Cd and Pb concentrations in the soil. Moreover, the combined stress of Cd and Pb resulted in progressive decrease in total glutathione and significant increase in root phytochelatins as the heavy metal concentration increased. The application of citric acid improved the mechanisms of antioxidative response and phytoextraction in S. nigrum; this improvement resulted from an increase in the solubility of heavy metals in the soil, which alleviated the metal toxicity. Therefore, citric acid could be involved in the expression of specific proteins or defense-related enzymes.