Separate and combined effects of Cu and Cd on seedling growth and active oxygen metabolism system of Trifolium repens L

Pot-culture experiments were used to examine the individual and combined effects of Cu and Cd pollutants on Trifolium repens L. seedlings, both on their growth and their active oxygen metabolism system, mainly superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities. The results showed that the negative action took place at low concentrations of Cu (less than 500 ppm) and Cd (less than 0.5 ppm), which had no obvious effects on the seedlings' growth. However, as the concentrations of Cu and Cd increased (500-3000 ppm and 0.5-50 ppm respectively), synergistic activities was observed, showing obvious negative effects (P less than 0.05). Compared with the control samples, the seedlings affected by Cu and Cd pollutants were shorter and smaller, their fresh/dry weight and content of soluble protein decreased drastically, their leaf electric conductivity increased, and the contents of their leaf pigments decreased. Chlorophyll a was more sensitive than chlorophyll b to Cu and Cd pollutants, and chlorophyll b was more sensitive than carotenoid. It was also shown that the active oxygen metabolism of T. repens seedlings was destroyed by high amounts of Cu and Cd, the balance of the anti-oxidase system was broken, and the CAT and SOD activities noticeably decreased while POD activity evidently increased. Cd had a more noticeable effect on seedling growth than Cu.