| In a pottted plant experiment, 3-year-old Viburnum odoratissimum and Viburnum tinus'seedlings were treated with 0, 25, 50, 100, 200, 400 mg/kg concentrations of Cd2+, and its growth responses, microstructure of leaves and changes of key bio?physiological indexes, such as Chlorophyll contents, rates of cell electrolyte leakage, MDA contents, POD activities, water contents, contents of soluble sugars and soluble proteins were investigated. Contents of Cd2+ in roots, stems and leaves in the two plants were also determined. The mean results as followed:(1) Viburnum tinus were hurt under cadmium stress and leaves showed harmful symptoms first. As Cd2+ stress concentrations rose, the amount of leaves reduced and leaves turned yellow. Plants were wilted and were led to leaf chlorosis, the growth potential of plants turned weak while Cd concentration reached 400 mg/kg. Cadmium stress caused the changes of microstructures, namely the thicknesses of upper epidermis, palisade tissue, spongy tissue and lower epidermis reduced to 79.61%,67.66%,85.37%,69.66% of control; cells were damaged, thus influenced the function of photosynthesis. However, Viburnum odoratissimum didn't show any harmful symptoms: the plants grew well and cells in leaves weren't damaged. Thicknesses of leaves were 93.18% of control and no obvious difference was found.(2) Bio?physiological indexes of two plants under cadmium stress changed: chlorophyll contents, chlorophyll a and b contents showed decreased trends as Cd2+ concentrations rose. While Cd2+ concentrations reached 200 and 400 mg/kg, chlorophyll contents were 34.4% and 12.3% respectively of control; chlorophyll a/b raised, which illuminates that chlorophyll b was hurt more easily by cadmium than cholorophyll a. However, no significant effects were detected in Viburnum odoratissimum on chlorophyll contents except 87.91% of control in 400 mg/kg Cd2+ stress. Water content of roots, stems and leaves in Viburnum tinus decreased as rising of cadmium stress. Minimum values occurred in 200, 400 and 400 mg/kg Cd2+ concentrations, and the values were respectively 51.53%,29.26% and 18.05% of control. Minimum values of water contents in Viburnum odoratissimum's roots, stems and leaves occurred in 400 mg/kg Cd2+ concentration, which were separately 67.95%,91.58% and 96.13% of control. Results show that Viburnum odoratissimum can endure cadmium stress, however Viburnum tinus had a moisture deficiency and its chloroplasts were hurt under cadmium stress, which was consistent with appearance and microstructure.(3) Rates of cell electrolyte leakage and MDA contents of the 2 plants raised as the rising concentrations of Cd2+. Electrolyte leakage rates of Viburnum tinus under 25,50,100,200,400 mg/kg Cd2+ stress were respectively 1.37,1.73,1.91,2.92,3.15 times of control. There were obvious differences of MDA contents when Cd2+ concentrations reach 100,200 and 400 mg/kg, and MDA contents were 1.32,1.46 and 1.73 times of control respectively. Significant effects in electrolyte leakage rates of Viburnum odoratissimum were only detected under 400 mg/kg, when the rate was 1.34 times of control. MDA contents showed significant differences when Cd2+ concentration reached 200 and 400 mg/kg, and they were 1.28 and 1.35 times of control.(4) POD activities, contents of soluble proteins and soluble sugars of Viburnum tinus increased at the beginnning and then decreased afterwards. Maximum values of POD activities and contents of soluble proteins were 274.02 U/(g·min) and 1035.18μg/g under 100 mg/kg Cd2+ concentration, while maximum value of soluble sugars was 34.5 mg/g when Cd2+ concentration reached 200 mg/kg. So it can be inferred that regulation of soluble sugars was lagging behind soluble proteins and POD. However, POD activities, contents of soluble proteins and soluble sugars increased as rising of Cd2+ concentrations, and maximum values were 256.81 U/(g·min), 625.31μg/g and 34.5 mg/g under 400 Cd2+ stress. Results show that POD, soluble proteins and sugars participated in resistance regulation, which is one of the rejection mechanisms to cadmium in Viburnum odoratissimum.(5) Accumulation of cadmium in roots, stems and leaves in Viburnum tinus increased as Cd2+ concentrations rose after 80 d treatment, and Cd2+ concentrations were 213.70 mg/kg, 79.17 mg/kg and 3.02 mg/kg seperately under 400 mg/kg cadmium stress, which showed that accumulation ability of root was stronger than stem and leaf. Viburnum tinus were not hyper-accumulator because of its poor tolerance to cadmium. Accumulation contents of cadmium in roots, stems and leaves in Viburnum odoratissimum increased as Cd2+ concentrations rose, and maximum values were 90.10 mg/kg, 3.04 mg/kg and 2.01 mg/kg respectively under 400 mg/kg cadmium stress. Cd2+ concentrations in root was much lower than Viburnum tinus. As Viburnum odoratissimum was not hurt under cadmium stress, we can inferred that it has stronger ability of tolerance to cadmium than Viburnum tinus. Roots can prevent cadmium from moving to shoots through some mechanisms of ammonium fixation (such as root exudates or chelating function, etc.) to reduce the damage of cadmium to shoots, so the plant maintains photosynthesis rate and grows well.In normal living condition, accumulation content of cadmium in Viburnum odoratissimum didn't reach the critical exponent ? 100 mg/kg of Cd2+ content in hyper-accumulator, but comparing with Cd hyper-accumulator, Viburnum odoratissimum, as an ornamental plant, has stronger adapTab.ility and higher biomass, and its root can absorb and fix cadmium which makes it not exhaust to the environment in short time. As a result, Viburnum odoratissimum can be planted in Cd contaminated area to absorb and fix cadmium, and it can reduce the harmfulness of cadmium to the health of human being by food chains.
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