Antioxidant Responses in Roots of Two Contrasting Sedum alfredii Hance Ecotypes under Elevated Zinc Concentrations

Effects of different zinc concentrations on antioxidant responses in the roots of the hyperaccumulating ecotype (HE) and nonhyperaccumulating ecotype (NHE) of Sedum alfredii Hance were investigated under hydroponic conditions. Growth of NHE was inhibited significantly when Zn concentration was >-50 mu M, whereas high Zn concentrations were beneficial for HE growth, and 500 mu M Zn induced a significant increase in the root biomass and reducing activity. Malondialdehyde content and electrical conductivity of the NHE roots increased significantly; however, no changes were observed in HE when the Zn concentration was > 10 mu M, suggesting a severe damage to the membrane of the NHE roots. Proline content in NHE roots increased rapidly, whereas it was low in HE roots even at high Zn concentrations, suggesting that proline may not play an important role in Zn hyperaccumulation. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) in NHE roots increased significantly when the Zn concentration was > 10 mu M and decreased sharply when the Zn concentration was >-500 mu M. For roots of HE, in contrast, no significant changes were observed in SOD, CAT, APX, and GPX activities at low Zn concentrations, whereas a high Zn concentration (>= 500 mu M) led to a marked enzyme activation, which was in accordance with Zn accumulation in shoots. The results suggest that antioxidant enzymes were important for Zn detoxification in NHE at low Zn concentrations (10-250 mu M) and were more critical for Zn detoxification and hyperaccumulation in HE under elevated Zn concentrations (500-1000 mu M).