Seed germination of plants grown in coal mine wastes in response to Copper, Lead, and Cadmium stress

Mining activities causes substantial damage to the environment worldwide. Abandoned mine wastes from mining activities limit the growth of plants due to unfavorable conditions and the concentration of a wide range of heavy metals. Identifying plant species growing on mine waste and assessing their seed germinations are important for vegetation restoration on mining areas. This study aimed at assessing the germination characteristics of Artemisia absinthium, Lepidium draba, and Silybum marianum naturally growing on coal mine wastes in Mazandaran province under different concentrations of copper (Cu), lead (Pb), and cadmium (Cd).

Seeds were collected from plants growing in coal wastes during 2 years. Completely randomized design was conducted with three replicates. Treatments were CuSO4 and Pb(NO3)2 at 0 (control), 50, 100, 200, and 300 mg/L, and Cd(NO3)2 at 0 (control), 5, 10, 20, and 30 mg/L. In each replicate 20 seeds were placed in a Petri dish containing a layer of filter paper. Then treatments were applied and Petri dishes were taken to a germinator under controlled temperature, moisture, and light. Germinated seeds were counted daily and then germination percentage, germination rate, allometric coefficient, seed vigor index, phytotoxicity percentage and seed tolerance index were calculated.

The results showed that increasing the concentration of lead, copper, and cadmium significantly affected the seed germination of studied plants. S. marianum had better germination percentage and rate in lead, and copper while germination of A. absinthium, L. draba, was better in cadmium, and copper, respectively. The greatest reduction in germination percentage was found for A. absinthium in 20, and 30 mg/L Cd. As the concentration of metals increased, significant reduction in germination percentage and rate for A. absinthium started in lower concentrations than the other species. Root and shoot length of all species significantly reduced even in low concentration of metals and S. marianum had greater root and shoot length under all treatments. Reduction in seed vigor index was greater under Cu than that in Pb and Cd. High tolerance index was found for L. draba in response to cadmium and for S. marianum under the stress of lead and copper. In all treatments increase in metal concentration significantly increased phytotoxicity index. Under Cu and Pb S. marianum showed least phytotoxicity index while the lowest phytotoxicity index was found for L. draba under Cd stress. Copper and cadmium in high concentration were more toxic for A. absinthium than that for the other two species. Under similar concentrations of copper and lead, all species showed reduced germination in copper stress. Cadmium in lower concentrations were tolerable for plants but generally it had the greatest inhibitory especially on A. absinthium.

Overall the responses were different regarding to the plant species, metal and concentrations. All three species were able to germinate at high concentrations of heavy metals. The presence of these species in coal wastes and their ability to germinate in high concentrations of heavy metals makes it possible to use these plants in future restoration and phytoremediation programs.