Influences of heavy metal cadmium alone and in combination with zinc on the growth and activities of antioxidant enzymes of Cucumis sativus hairy roots

Abstract:

The effects of heavy metal cadmium (Cd), alone and in combination with zinc (Zn), on the root growth and activity of antioxidant enzymes superoxide dismutase(SOD) and peroxidase(POD) in Cucumis sativus L. hairy roots were studied. The purpose was to study the possibilities on using C. sativus hairy roots for phytoremediation of cadmium contamination. The results showed that less than 10 mg/L Cd enhanced the growth of C. sativus hairy roots and increased root diameter only in 5–15 days of root culture. At Cd concentrations above 15 mg/L hairy root growth was gradually inhibited with increasing Cd concentration. The roots formed were shorter with smaller lateral roots. Among all the Cd concentrations tested, except with 10 mg/L Cd, the soluble protein contents in the C. sativus hairy roots cultured with the other Cd concentrations decreased, but the POD and SOD activities increased gradually with time during the culture process. Further tests were conducted using a control culture containing 25 mg/L Zn alone. The addition of 1mg/L Cd to the 25 mg/L Zn culture stimulated the growth of C. sativus hairy roots after 7–15 days of growth, compared with the control. At all other Cd concentrations the growth of C. sativus hairy roots was inhibited compared to the control. Growth inhibition increased with increasing Cd concentration, and the hairy roots formed fewer, shorter and smaller lateral roots, the tips of which became swollen. After 5 days culture with different concentrations of Cd + 25 mg/L Zn, the root biomass and the activity of POD and SOD were lower than in C. sativus hairy roots cultured without the addition of Zn. However, the soluble protein content was significantly higher when the culture contained 25 mg/L Zn. Our results suggested that C. sativus hairy roots have higher tolerance to heavy metal Cd but higher concentration of Cd inhibited the growth. Cd in combination with Zn would result in more serious Cd-induced growth inhibition.