| Water flooding and Cd pollution had caused serious environmental problems in the area of the Three Gorges Reservoir. Species for revegetation not only needed to have the ability to tolerate flooding, but also had strong tolerance to heavy metal stress and better heavy metal accumulation ability. Salix variegate was a native riparian plant species, which had a wide distribution in the water-level-fluctuation zone. In the study of revegetation in the area of the Three Gorges Reservoir,many scholars had suggested that S. variegate is one of the recommended species for revegetation in the region of 170 m water level above area. In order to reveal the phytoremediation ability of S.variegate under flooding, the growth adaptation of S. variegate to flooding-and-Cd stress, the influence of flooding on Cd accumulation characteristic and the distribution of Cd in subcellular fractions were researched. Also, the role of the main polysaccharide components of the cell wall in the Cd accumulation were further explored.Cutting plants of S. variegate were treated by four different concentrations of cadmium,interacted with two different water conditions(Control group, CK; Flooding group, FL) were investigated, to determine the biomass and cadmium concentrations in various parts of S. variegate plant. The cadmium treatments were 0 mg?kg-1,0.5 mg?kg-1,2 mg?kg-1 and 10 mg?kg-1 individually,achieved by adding the Cd Cl2?2.5H2 O to the soil.The plant height, branch number, branch length,total biomass, aboveground biomass, root biomass, stem biomass and leaf biomass were tested. The Cd concentrations in various parts of S. variegate plant, such as root, stem, leaf, xylem and bark of cutting, were determined. The Cd concentrations in subcellular fractionation of root, stem and leaf,and the Cd concentrations in cell walls which removed pectin and the cell walls removed pectin and hemicellulose were tested. The main results were as follows:(1) The survival rate of S. variegate in all the treatments were 100%. The root biomass were decreased significantly by flooding treatments(P<0.05). With increasing in Cd stress, the root biomass decreased gradually under non-flooding condition. The root biomass were not influenced significantly by Cd treatments under flooding(P>0.05). The results indicated that the total biomass,aboveground biomass, stem biomass, leaf biomass, plant height, branch number and stem length ofS. variegate were not influenced significantly by flooding-and-Cd stress treatments(P>0.05). S.variegate had a high capacity of growth adaptation under flooding-and-Cd stress. Flooding could relieve the inhibitory effects of cadmium stress on root biomass of S. variegate.(2)The Cd contents in the aboveground parts of S. variegate were higher in the control groups.In the 2 mg?kg-1 Cd treatment group of 60 d, the Cd contents in branch, leaf, bark and xylem were12.98,10.08,7.47 and 17.2 mg?kg-1, respectively. The cadmium contents of S. variegate were influenced significantly by flooding treatment(P<0.05). Under flooding condition, the Cd contents in branch, leaf, bark and xylem decreased significantly(P<0.05). Compared with the control treatment, the branch Cd contents under flooding accounted for 39.0%, 23.7%, 34.0% and 51.8%orderly with increasing Cd stress. The Cd transfer capability from root to branch in flooding soils reduced significantly compared to control groups(P<0.05). Flooding reduced the Cd accumulations of S. variegate obviously. The Cd accumulations in total plant and aboveground part decreased obviously under flooding. In the group of 2 mg?kg-1 Cd treatment, the total plant Cd accumulation and aboveground part Cd accumulation without flooding were 146.40 μg?plant-1 and 125.35μg?plant-1 respectively, while the corresponding accumulations under flooding were accounted for only 56.7% and 35.2% of the control treatments. The Cd contents of the aboveground parts and the Cd accumulations of the total plant of S. variegate reduced obviously under flooding.(3) Under flooding and non-flooding conditions, the cell wall of roots, branches and leaves of S. variegate were the main enrichment of Cd and the Cd content of nucleus and mitochondria were always at a low level. The Cd contents in cell wall of roots, branches and leaves were influenced significantly by experimental treatments(P<0.05). Under non-flooding condition, in the group of 2mg?kg-1 and 10 mg?kg-1 Cd treatments, the Cd contents in cell wall of roots, branches and leaves of S. variegate were significantly higher than other subcellular fractions(P<0.05). In the group of 10mg?kg-1 Cd treatments, the Cd content of cell wall of root, stem and leaf accounted for the proportion of 46.4%, 66.2% and 73.3%, respectively. Flooding had little effect on the Cd fixation ability of the cell wall of root. The percentage of Cd contents in leaf and stem cell wall decreased in general under flooding. In the group of 0.5 mg?kg-1, 2 mg?kg-1 and 10 mg?kg-1 Cd treatments, the percentages of cell wall Cd content of stem decreased from 44.8%, 46.1%, 66.2% to 41.3%, 38.9%,50.5%, and the leaf from 57.5%, 49.7%, 73.3% to 42.9%, 45.9%, 58.2%, orderly.(4) In non-flooding condition, the 2 mg?kg-1 and 10 mg?kg-1 Cd treatment groups of 60 d samples and all the Cd treatment groups of 120 d samples showed the Cd contents in cell wall of root decreased significantly after removing pectin(P<0.05), the highest decline reached to74.9%(10mg?kg-1Cd2+, 120d). The Cd contents of cell wall of stem decreased significantly after removing pectin in the middle and high Cd treatments, the highest decline reached to 36.4%(10mg?kg-1Cd2+, 120d). The Cd contents of removing-pectin cell wall of stem and root decreased after removing hemicellulose in all the groups, but there were no significant difference(P>0.05).The pectin and hemicellulose in cell wall had a certain ability to combine Cd ions and the bindingcapacity of pectin to Cd was much greater than that of hemicellulose. Under flooding condition,with the decreasing of cell wall composition, the Cd contents of cell wall in 60 d groups decrease gradually, but there were no significant difference(P>0.05). The Cd contents of cell wall in 120 d groups decrease significantly after removing pectin and hemicellulose. There was a negative effect of flooding on the Cd binding capacity of pectin and hemicellulose. But pectin still played an important role in Cd binding under flooding.In summary, S. variegate had a strong ability of cadmium absorption and accumulation. The cell wall played an important role in the accumulation of Cd in S. variegate. Pectin was an important Cd binding polysaccharide in cell wall and played an important role in the accumulation of Cd. S. variegate had a high capacity of growth adaptation under flooding-and-Cd stress. But the Cd contents of the aboveground parts, the Cd accumulations of the aboveground parts and the total plant of S. variegate reduced obviously under flooding. There was a negative effect of flooding on the Cd binding capacity of cell wall, pectin and hemicellulose. Flooding reduced the Cd uptake capacity of S. variegate obviously. The application potential of S. variegate in phytoremediation was inhibited seriously under flooding. In the region of 170 m water level above area, the flooding period was short and the growth of plant was slow in winter. Considering the growth and Cd accumulation, S. variegate still has a strong potential for phytoremediation in water-level-fluctuation zone. At the same time, in order to improve the efficiency of phytoremediation in the Cd contaminated area of the Three Gorges Reservoir Region, and to prevent the secondary pollution caused by the decayed shoots, it is necessary to harvest the shoots of S. variegate before flooding every year. |
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