Growth Response And Metal Accumulation Of Houttuynia Cordata Thunb To Combined Contamination Of Lead And Zinc

Combination contamination of heavy metals is a common but important form of soil contamination. Phytoremediation has become an effective approach for remediation of heavy metal contaminated soils. Hyperaccumulator plants are potential tools to remove excessive metals from contaminated soils by continuous phytoextraction. The overall objective of this study is to evaluate Houttuynia cordata Thunb as a candidate plant to be used in the phytoremediation of heavy metal contaminated soils. More specifically, pot experiments and hydroponic cultivation of Houttuynia cordata Thunb were conducted to determine the effect of Pb, Zn and their interactions on the growth of the Houttuynia cordata Thunb, accumulation and allocation, transportation and enrichment in plant; the relationship between various forms of Pb, Zn in soil and the Pb, Zn uptake and accumulation by Houttuynia cordata Thunb, and the effects of Pb and Zn, and their interactions on the physiological and biochemical characteristics of the plant. The major results are summarized as follows:1. At low concentrations, Pb promoted the growth of Houttuynia cordata Thunb, especially the procreation growth; while at the high concentration, it restrained the growth of Houttuynia cordata Thunb. The retardation effect, however, was noe significant since Houttuynia cordata Thunb showed no apparent symptoms of Pb toxicity, indicated that Houttuynia cordata Thunb have a high tolerance to Pb. Similarly, Zn can promoted the growth of Houttuynia cordata Thunb at a Zn concentration below 500mg/kg, but restrained the plant growth significantly as the concentration increased beyond 500mg/kg. Differently, the damage of the Zn toxicity to the plant tissues was severe. The effect of Pb-Zn interactions on the growth of Houttuynia cordata Thunb can be either antagonism or synergism, depending on the concentration of Pb. At low concentrations, Pb can reduce the toxic effects of Zn on the plant growth while it aggravated the toxicity of Zn at high Pb concentrations. The significance of Pb, Zn and their interactions on the growth of the Houttuynia cordata Thunb was in the order of Zn>Pb>Pb-Zn.2. In pot experiments with the concentration variations of single heavy metal, Pb accumulation increased significantly in different parts of Houttuynia cordata Thunb plant such as roots, stems and leaves as the Pb treatment concentrations increased. Similarly, Zn accumulation increased significantly in roots of Houttuynia cordata Thunb as the Zn treatment concentration increased. In stems and leaves, however, Zn accumulation increased initially and then decreased sharply as Zn concentration increased to 1000mg/kg. These results indicated that the high Zn concentration restrained the Zn transportation from roots. Numerically, the trends of Pb accumulation of Houttuynia cordata Thunb could be best described with a quadratic equation of concentration, while the trends of Zn accumulation could be best described with a logarithm equation in the roots, but with the quadratic equations in the stems and leaves of Houttuynia cordata Thunb. These different mathematical relationships indicated that Houttuynia cordata Thunb has different accumulation mechanisms for Pb and Zn. Zn showed antagonistic effect on the Pb accumulation as the Zn concentration increased. Pb promoted the Zn accumulation of Houttuynia cordata Thunb initially and then restrained Zn accumulation as Pb treatment concentration increased beyond 1000mg/kg. In summary, different concentration compound of Pb and Zn have different forms of interantions to the growth and heavy metal accumulation of Houttuynia cordata Thunb.3. The increase of Pb treatment concentration promoted transportation of Pb from roots to shoots. The ratio of Pb accumulation in shoots to Pb accumulation in roots was more than 1, indicating that Houttuynia cordata Thunb has better transportation ability for Pb. Under Zn stress, the ratio of Zn accumulation in shoots to Zn accumulation in roots increased initially and then decreased sharply with the increase of Zn treatment concentration, suggesting that the Zn uptake by Houttuynia cordata Thunb be active mechanism and Houttuynia cordata Thunb had better transportation ability for Zn when Zn concentration was in a fitting range. However, the high Zn concentration resulted in obvious toxicity to Houttuynia cordata Thunb and the Zn uptake by Houttuynia cordata Thunb might become passive at this time. Therefore, the transportation ability to Zn of Houttuynia cordata Thunb weakened sharply. Houttuynia cordata Thunb had strong enrichment ability for Pb as shown by the high enrichment coefficient in the entire tested concentration range. The enrichment coefficient of Zn increased significantly initially but then decreased sharply as Zn treatment concentration increased beyond 500mg/kg. This result indicated that Houttuynia cordata Thunb had enrichment ability for Zn only at a low concentration range.4. In pot experiments, total contents and concentrations of various fractions of Pb and Zn in soil increased significantly as the amount of Pb and Zn applied to the soil increased. The resultant concentrations of various fractions of Pb in soil followed the order: EXC>CAB>RES>OXI>ORG while that for Zn followed a slightly different order: EXC>CAB>OXI>RES>ORG. Exchangeable fraction of Pb in soil was the major contributor to Pb uptake by Houttuynia cordata Thunb. Other fractions of Pb such as ORG, CAB and OXI could be utilized by plants under suitable conditions. The contribution of various forms of Pb in soil to the Pb accumulation in Houttuynia cordata Thunb was in the order: EXC>ORG.>OXI>CAB. Similary, exchangeable and organic fraction of Zn in soil were the major contributors to Zn uptake by Houttuynia cordata Thunb. The contribution of various forms of Zn to the Zn accumulation by Houttuynia cordata Thunb was: EXC>ORG.>OXI>CAB. Residual fractions of both Zn and Pb played negligible role in the plant uptake for both elements.5. In single-heavy-metal systems, as Pb concentration varied from 50 to 400 mg/L in the nutrient solution, total chlorophyll content in leaves decreased gradually. As Zn concentration varied from 0 to 50 mg/L in the similar systems, the total chlorophyll content in leaves increased initially and then decreased. The activities of SOD, POD and CAT in those single heavy metal systems increased with Pb concentration initially to pass a maximum and then declined. For the effects of Zn concentrations on the enzyme activities in similar systems, the same trends were found for SOD and CAT, but not for POD. Instead, the activity of POD increased with the Zn concentration in the entire tested range. In the dual heavy metal systems, Pb-Zn interactions increased chlorophyll content of Houttuynia cordata Thunb, restrained the activities of SOD and POD, but has no significant effect on the activity of CAT. The effects of single Pb and single Zn treatment on chlorophyll content and antioxidant enzyme systems were more severe than the effects of Pb-Zn interactions.