Antioxidative response in leaves and allelochemical changes in root exudates of Ricinus communis under Cu, Zn, and Cd stress

We have previously reported that Ricinus communis is a good candidate for the phytoremediation of Cd- and Zn-contaminated soil and for fuel production. In this study, changes in the activity of antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; and guaiacol peroxidase, POD) and the contents of chlorophyll and malondialdehyde (MDA) in R. communis leaves under Cu, Zn, and Cd stress were examined. Compounds from the exudate of R. communis roots were collected and analyzed using GC-MS chromatograms. The results of enzyme activity showed that Cd treatment significantly increased the SOD content of R. communis leaves and slightly elevated the CAT content, whereas the POD content increased markedly at low Cd treatment concentrations and decreased as Cd concentrations increased. Zn treatment distinctly elevated SOD and POD content in R. communis leaves but had no great influence on CAT content. Cu treatment slightly increased CAT activity, while Cu did not evidently change SOD and POD activity. We found 17, 29, 18, 18, and 33 different compounds in the R. communis root exudates from the control group and Cd, Cu, Zn, and Cd+Cu+Zn treatment groups, respectively. The root exudates mainly included ester, alcohol, ether, amide, acid, phenol, alkanes, ketone, aromatic hydrocarbon, and nitrile compounds. However, the root exudates of R. communis grown in uncontaminated soils were dominated by esters, alcohols, and ethers. Single Cu or Zn treatment slightly changed the root exudates, which were dominated by esters, alcohols, and amides. In the Cd and Cd+Cu+Zn treatment groups, the compositions of root exudates apparently increased, with alkanes as the major species (> 88%).