Research On Accumulation Characteristics And Tolerance Mechanisms To Cadmium Of Salix Variegata Franch

Increasing anthropological practices have led to the addition of a highly toxic metal, cadmium, into the natural environment that affects the overall growth and metabolism of plants. Cadmium (Cd), a relatively rare transition metal with a density of8.6g/cm3, does not have any known biological functions. This metal is a significant pollutant due to its high toxicity and greater solubility, and extremely toxic even at low concentrations. Conventional methods for cleaning up contaminated sites include washing, excavation, and reburial of soild matrices such as soil, and pumping and treating systems for contaminated water. These approaches are expensive, generally inefficient if the contaminant is present at a low concentration,and often causes significant changes to the physicochemical characteristics of soils. A different method to reclaim polluted sites is to use plants that reduce the toxicity of metals in soil or water, a concept known as phytoremadiation. It is a novel, cost-effective, efficient, environment-and eco-friendly, in situ application, and solar-driven remediation strategy.Salix variegata Franch. is a fast-growing riparian plant which has a high biomass, potential flood tolerance, and eurytropic habitat. It is also a promising phytoremediation candidate for Cd contaminated area, for its relative considerable capability to extract Cd from soil and water, and survive from high Cd contaminant condition.To deterimine the accumulation characteristics and tolerance mechanisms of S. variegata to Cd, a microscale hydroponic system was employed in this study.4gradient concentration of Cd group, respectively valued2,10,20,50mg/L in caltivation solution, and a control group without Cd in solution experiment was conducted for research of2-5chapter. All treatments were done in quadruplicates. The Cd and4kinds of divalent metal (Ca, Mg, Mn, Zn) distribution in S1. variegata, cell viability of root, MDA contents,5kinds of antioxidant enzymes activities (SOD, POD, CAT, APX, GR), thiol-contaning peptides contents, and Cd concentration in subcellula fractionation of leaf were tested. The results of the experiments showed that:1. Cd distribution tendency in S. variegata was:root> xylem> phloem> leaf, Cd concentration reached up to801.82,48.59mg/kg DW respectively in root and xylem of T1group after18d’s treatment. Cd treatment concentration had a positive effect on Cd accumulation of all parts of S. variegata, whereas a negative effect on cell viability of root. There was no significant difference of cell viability between Tl group and CK group.2. After18d’s treatment all4kinds of divalent metal uptake in root decreased with the Cd treatment concentration increased. Zn, Ca concentration in root of T4group decreased respectively65.52%,25.73%compared to CK group. The concentration of Fe, Mg, Mn also decreased in all Cd treatment groups in phloem. There was no significant difference between Cd treatment groups and CK of metal conceatration in other parts of S. variegata.3. Increased Cd stress resulted in MDA accumulation in leaf, indicated increased lipid peroxidation and oxidative stress in leaf. Uder oxidative stress circumstance, the antioxidant enzymes system was induced to increase all the enzymes’activity to scave excess ROS, ROS and CAT were the most important antioxidant enzyme, which kept increasing activities during the test. APX, GR also had a quick and efficient response via the AsA-GSH cycle to against slight oxidative stress at6d of the experiment, however got unefficient activities with the prolonged treatment time. POD showed no significant function to Cd stress in S. variegata.4. S. variegata was induced to synthesis thiol-contaning peptides by using GSH as percursor under Cd stress. There were GSH, PC2, PC3determined in root and GSH, PC2, PC3, PC4, PC5in leaf. PC2and PC5were the main peptides form in leaf. The accumulation of thiol-containing peptides increased in leaf along with increased Cd in treatment solution and prolonged time. The concentration of peptides fluctuated with prolonged time also because of compounds translocation.5. The Cd concentration in subcellular fractionation results showed the same tendency:cell wall> cytoplasm> organelle. This indicated cell wall binding was the most effiencient mechanism to immolize Cd2+,. The Cd concentration binding to cell wall reached up to59.27%,64.54%,69.82%,55.42%of4treatment groups respectively after18d’s treatment. Compartmentalization of Cd-PC compounds and other form Cd compounds in wacuole was also a important tolerance mechanism in S. variegata. Cell wall binding and compartmentalization both could detoxify the oxidative stress caused byCd.From all the results stated above, we could know that there was a efficient net work of Cd resistance in S. variegata, which supported its tolerance to Cd. Antioxidant enzymes system could scave excess ROS, relieve oxidative stress caused by Cd, GR had a role in maining high GSH level, which was critical for PCs synthesis. PCs chelation with Cd was an important mechanism in heavy metal detoxification by chelating free Cd2+with thiol group. Cd-PC compounds were translocated to shoot throug xylem sequentially, sequestrated into vacuole as the tolerance mechanism called compartmentalization. Introduction or over-expression of selected genes is the promising way to generate S. variegata.