Evaluation of heavy metal tolerance at different clover plant growth stages

In order to study heavy metal tolerance at different clover (Trifolium resupinatum L.) plant growth stages, a pot experiment contaminated by cadmium, copper, nickel, or zinc at 500 mg/l was conducted in three replications. Applied seven days after germination, all heavy metals except zinc had lethal effects on the clover seedlings. Furthermore, when they were applied 14 days after germination, nickel still had a lethal effect on the seedling, and cadmium and copper killed more than half of the plant population. While approximately 90% of the plants exposed to cadmium, copper and zinc were able to grow without apparent negative effects, 21 days after germination nickel still showed lethal effects. In this study, the concentration of heavy metals in shoot dry tissues were 940 mg/kg for cadmium, 781 mg/kg for copper and 670 mg/kg for zinc. These results suggest that the tolerance of clover to cadmium, copper and zinc positively correlates with the age of the plant.