Responses of Antioxidant Enzyme System to Copper Toxicity and Copper Detoxification in the Leaves of Elsholtzia splendens

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ABSTRACT Solution culture experiments were conducted to study physiological responses to copper (Cu) toxicity and Cu detoxification involved in the antioxidant enzyme system in the leaves of Elsholtzia splendens (E. splendens). Compared with the control, 500 μM Cu considerably reduced oxidizing force and Apase activity in roots, as well as net photosynthetic rate and transpiration rate in leaves, but greatly stimulated lipid peroxidation and proline accumulation. In contrast, 50 μM Cu elevated oxidizing force and Apase activity in roots, chlorophyll content, net photosynthetic rate, and transpiration rate in leaves, and elevated proline content but decreased lipid peroxidation in the plant compared to with the control. Furthermore, at 500 μM Cu, heavily damaged organelles were found in the leaf cell of E. splendens, while at 50 μM Cu organelles were not markedly affected. These results indicate that greater oxidative stresses were induced at 500 μM Cu than at 50 μM Cu. Plants responded to Cu-induced oxidative stresses by modulating antioxidant enzymes and substrates. Compared with the control, the activities of superoxide dismutases (SOD), catalase (CAT), glutathione peroxidase (GPX), and ascorbate peroxidase (APX) in the leaves of the plant increased pronouncedly at 500 μM Cu, while at 50 μM Cu, GPX activity decreased notably and the activities of APX, SOD, and CAT changed only slightly. Copper also altered glutathione (GSH) content and glutathione reductase (GR) activity. At 50 μM Cu, both GSH content and GR activity in the leaves of the plant increased dramatically; however, at 500 μM Cu, GR activity increased but GSH content changed only slightly compared with the control. These results suggest that some adaptive physiology responses to Cu toxicity and the antioxidant enzyme system are involved in Cu detoxification in leaves of E. splendens, but at 500 μM Cu the limited activities of the antioxidant enzyme system in leaves of E. splendens make it difficult to counteract the greater oxidation stresses in vivo.