The Role Of Antioxidant Systems In Excess Copper Detoxification In Indian Mustard (Brassica Juncea L.)

The differential responses of Indian mustard leaves and roots to the oxidative stress induced by excess copper were investigated. It was observed that treatments of seedlings with 4-16 mol/L copper for 4d led to a remarkable inhibition of both shoots and roots, and about 90 percent of copper uptaken accumulated in roots. The degree of oxidative stress mediated by excess copper was greater in the roots than that in the leaves. The antioxidative capacity of plants treated with 0-8 mol/L copper increased with the increasing Cu concentrations, while plants treated with 16 mol/L copper failed to cope with oxidative damages. CAT was showed to be the key enzyme for the removal of H2O2 in the leaves, while APX and POD functioned mainly in the roots. Non-protein thiols (NPT) played an important role in binding the heavy metal in the roots.The induction of antioxidative reactions in the roots of Indian mustard was investigated in a time-dependent manner. The rapid uptake of Cu was observed immediately after the start of treatment. Application of Cu at 8 mol/L resulted in a 50 percent reduction of biomass of Cu-treated roots compared with controls. It was observed that the degree of Cu-induced root growth inhibition paralleled the level of lipid peroxidation. Treatment with Cu at 8 mol/L induced a 2-fold increase in H2O2 content during the first 4 d, but it declined to the control level thereafter. The increased H2O2 level might serve as signal molecule, triggering the antioxidative response. We observed that treatment with 8 mol/L Cu resulted in nearly doubling superoxide dismutase (SOD) activities during the first 2 d. The double stimulation remained up to day 4 and then gradually declined. Activities of ascorbate peroxidase (APX), glutathione S-transferase (GST) and peroxidase(POD) in roots were found to be low during the first 4 d after exposure of seedlings to 8 mol/L Cu, but significantly increased after that, suggesting that the increased enzyme activities were most likely responsible for the removal of reactiveoxygen species at late period of Cu treatment. In contrast to activities of SOD, APX and POD, activities of catalase (CAT) and glutathione reductase (GR) were always suppressed under Cu stress. We measured the total non-protein thiol (NPT) content in the roots of seedlings exposed to Cu and observed an increase in its levels during first several days. This indicated that Cu-induced increase in the levels of thiol compounds might represent another defensive mechanism against oxidative stress. The content of AsA in the roots treated with 8 mol/L Cu increased by 20% initial two days, and then declined to the level below th controls. The content of GSH decreased during first 6 d and recovered after that, and on day 8, a complete recovery was observed. The POD and SOD isozymes in Indian mustard roots were studied. It was found that treatment with 4 and 8 umol/L copper increased the number of POD and SOD isozymes.