Studies On The Responses Of Three Species Of Ground Cover Plants To Cd And Pb Stress

In this paper, the effects of heavy metal stress to three species of ground cover plants (Orychophragmus violaceus, Duchesnea indica, Achillea millefolium) in Peiking were studied by sand cultivation experiments. The plantes wer used to study how the lead(Pb) or the cadmium(Cd) contamination impacted on the growth, photosynthesis, physiological and biochemical characteristics.The main results were as follows:1.The effect of heavy metal stress to growth of plants.The growth was obviously inhibited.Within the increasing of concentration of heavy metals, the shoot fresh weight, root fresh weight, length of leaf, length of root and leaf area decreased.Espeicil ly within high concentration of Cd(10 mg·L^-1) and Pb(800 mg·L^-1), the growth characteristics decreased significantly, and some palnts were dead. On the contrary, the variation of root fresh weight was greater than that of shoot fresh weight, and the variation of length of leaf was greater than that of root. Capared with the characteristics, the effect of Cd and Pb toxicity on growth of three species followed the order of Achillea millefolium＞Orychophragmus violaceus＞Duchesnea indica.2. The effect of heavy metal stress to ecological benefits of plants.There was an obvious negative effect on the photosynthesis. With increasing Cd and Pb concentration, all the species had a significant decrease of Pn, so the benefits of cooling and humidification were inhibited obviously. Tr of Achillea millefolium decreased significantly, but the others decreased slightly. So the benefit of carbon fixation and oxygen release of Achillea millefolium was inhibited.3. The inhibiting Mechanism of heavy metals stress against photosynthesis.Under low concentration of Cd(1 mg·L^-1) and Pb(50 mg·L^-1), the contents of chlorophyll increased slightly, chlorophyll fluorescence parameters decreased slightly, and Cond decreased.Then the decrease of Pn was due to stomatal limitation. However, under high concentration ofCd(4mg·L^-1, 10 mg·L^-1) and Pb(200 mg·L^-1, 800 mg·L^-1), the contents of chlorophyll,ΦPSâ…¡, ETR, qP, Fv/Fm, Pn decreased significantly, F₀ increased significantly, PSâ…¡in the leaves was obviously damaged and the nonstomatal limitation dominated.4. The molecular mechanisms of heavy metal toxicity and the resistance mechanisms of plant.The CMP and the MDA contents increased obviously with increasing Cd and Pb concentration. It showed that lipid peroxidation, which was considered an indication of oxidative stress in plants, could be induced via free radicals that were generated as a result of heavy metal toxicity in plants. Lipid peroxidation can degrade biological membranes making them susceptible to oxidative damage. The DNA and protein were also damaged by Lipid peroxidation. Under high concentration, the activities of POD and NR decreased significantly. It showed that other mechanisms of toxicity: blocking of essential functional groups in biomolecules, and displacement of essential metal ions from biomolecules. But the activities of POD and the contents of Pro increased obviously under low concentration. It showed the ability of plants to increase antioxidative protection to combat negative consequences of heavy metal stress.Compositively, compared with the variation of all the characteristics, the tolerance of three species to heavy metals was in order of Duchesnea indica＞Orychophragmus violaceus＞Achillea millefolium. As a result, Duchesnea indica and Orychophragmus violaceus could be widely used in urban green space of Peiking. And Duchesnea indica should be studied to identify whether it be considered as hyperaccumlator and used in remediation of heavy metal contaminated soil.