| Heavy metal pollution in soils has become one of the important limiting factors for the development of sustainable agriculture in China. Phytoremediation based on the identification and application of heavy metal hyperaccumulating plants natively growing has regarded as an efficient measurement for the remediation of polluted soils. Bioavailabity of heavy metals in soils can be improved with the exudates such as organic acids by plant roots, which is relative to heavy metal uptake and accumulation in hyperaccumulating plants. With a field survey, concentrations of heavy metals in natively growing plants and soils in an ancient silver mining site were investigated. Cadmium accumulation in plant tisses and organic acids as well as soluble sugars in root exudates of the ancient silver mining ecotype of Sedum alferdii Hance were studied by hydroponics and ion chromatography. The important results are as follows:1. The results of the field survey indicated that copper concentrations in shoots of Sedum alferdii Hance, Deadnettle, Sweet-potato, and Samentosum were 12.92,10.9,9.66, 6.79 mg/kg in order; zinc concentrations in shoots were 1483.46,78.88,61.63,505.66 mg/kg in order; cadmium concentrations in shoots were 677.79,26.64,2.92,32.43 mg/kg in order; lead concentrations in shoots 376.12,8.41,2.25,26.62 mg/kg in order. Total copper concentrations in the soils collected with plant sampling were 150.52,30.60,26.95,34.08, 25.92 mg/kg in order; total zinc concentrations in the soils were 550.49,125.72,133.21, 210.39,459.89 mg/kg; total cadmium concentrations in the soils were 51.75,1.53,0.76, 2.01,0.54 mg/kg in order; total lead concentrations in the soils were 710.71,346.50,72.13, 123.86,95.64 mg/kg in order. Cadmium concentration in shoots of the ancient silver mining ecotype of Sedum alferdii Hance was as high as 677.79 mg/kg, higher than 100 mg/kg, and the bio-accumulating factor (ratio of shoot Cd concentration to total Cd concentration in soil) was as large as 13.1. The potential of cadmium accumulating of the ancient silver mining ecotype of Sedum alferdii Hance is presented.2. The results of the hydroponics indicated that the toxic symptoms as chlorosis on younger leaves and root necrosis were observed, and plants with other treatments grew normally. Differences on dry weights of shoots and roots among all treatments were not significant, implying that the ancient silver mining ecotype of Sedum alferdii Hance is tolerant to cadmium stress. Cadmium concentrations in shoots with Cd treatments (100,200, 400μmol/L Cd) were higher than 100 mg/kg, and the ratios of Cd concentration in shoots to Cd concentration in roots were larger than 1. Thus, the ancient silver mining ecotype of Sedum alferdii Hance could be regarded as a cadmium hyperaccumulator.3. The results of separation and determination by Ion Chromatography showed that the roots of the ancient silver mining ecotype of Sedum alferdii Hance could secrete acetic acid, tartaric acid, oxalic acid, lactic acid, malic acid, and critric acid. The detection of lactic acid in root exudates is not reported formerly. For organic acids, the secretion of tartaric acid and critric acid were largest, secondly as that of lactic acid and acetic acid, and that of oxalic acid and malic acid were trace or not detected.In nutrition solution, the amounts of acetic acid emitted by roots of the treatments (0, 100,200,400μmol/L Cd) were 0.1904,0.0609,0.1118,0.2530 mg/g (FW)·d in order; the amounts of tartaric acid emitted were 0.5107,0.4173,0.2966,0.4866μg/g (FW)·d in order; the amounts of oxalic acid emitted were 0.0136,0.0070, not detected,0.0083 mg/g(FW)·d in order; the amounts of lactic acid emmited were 1.62,2.67,57.15,37.61μg/g (FW)·d in order; the amounts of citric acid emitted were 219.36,369.65,688.97, 693.54μg/g(FW)·d. In calcium chloride collecting solution, the amounts of lactic acid emmited by roots of the treatments (0,100,200,400μmol/L Cd) were 1.08,10.66,25.24, 36.35μg/g(FW)·d in order, the amounts of citric acid emitted were 70.55,79.28,209.83, 130.47μg/g(FW)·d in order. The results of statistic analysis showed that the amounts of aectic acid and lactic acid emitted by roots with 200 and 400μmol/L Cd treatments were significantly (P<0.01) larger than that of the contral (0μmol/L Cd) under the nutrition solution conditions, implying that cadmium could promote the secrection of aectic acid and lactic acid by roots of the ancient silver mining ecotype of Sedum alferdii Hance.4. Under nutrient solution conditions, the amounts of soluble sugars secreted by roots of the ancient silver mining ecotype of Sedum alferdii Hance with the Cd addition treatments (100,200 and 400μmol/L Cd) were significantly (P<0.01) larger than that of the contral (0μmol/L Cd). And in calcium chloride collecting solution, the amounts of soluble sugars secreted by roots with the treatments with 200 and 400μmol/L Cd were significantly (P<0.01) lower than that of the contral (0μmol/L Cd). The impacts of substrate on the secretion of soluble sugars by plant roots need further research. |
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