Remediation of cadmium- and lead-contaminated agricultural soil by composite washing with chlorides and citric acid.

Composite washing of cadmium (Cd)- and lead (Pb)-contaminated agricultural soil from Hunan province in China using mixtures of chlorides (FeCl3, CaCl2) and citric acid (CA) was investigated. The concentrations of composite washing agents for metal removal were optimized. Sequential extraction was conducted to study the changes in metal fractions after soil washing. The removal of two metals at optimum concentration was reached. Using FeCl3 mixed with CA, 44% of Cd and 23% of Pb were removed, and 49 and 32% by CaCl2 mixed with CA, respectively. The mechanism of composite washing was postulated. A mixture of chlorides and CA enhanced metal extraction from soil through the formation of metal-chloride and metal-citrate complexes. CA in extract solutions promoted the formation of metal-chloride complexes and reduced the solution pH. Composite washing reduced Cd and Pb in Fe-Mn oxide forms significantly. Chlorides and CA exerted a synergistic effect on metal extraction during composite washing.

How phytohormone IAA and chelator EDTA affect lead uptake by Zn/Cd hyperaccumulator Picris divaricata.

In this paper, the effects of indole-3-acetic acid (IAA) and/or ethylenediaminetetraacetic acid (EDTA) on lead uptake by a Zn/Cd hyperaccumulator Picris divaricata were studied. P. divaricata responded to Pb by better root system and increased biomass in presence of phytohormone IAA, which was able to reduce the inhibiting effects of Pb on transpiration without reducing the uptake of Pb The application of 100 microM IAA increased plant transpiration rate by about 20% and Pb concentration in leaves by about 37.3% as compared to treatment exposed to Pb alone. The enhanced phytoextraction efficiency could be attributed to the mechanisms played by IAA through alleviating Pb toxicity, creating better root system and plant biomass, promoting a higher transpiration rate as well as regulating the level of nutrient elements. On the contrary, inefficiency of phytoextraction was found with EDTA or the combination of IAA and EDTA probably because most Pb was in the form of Pb-EDTA complex which blocked the uptake by P. divaricata. The present study demonstrated that IAA was able to enhance the phytoextraction of Pb by Zn/Cd hyperaccumulator P. divaricata, providing a feasible method for the phytoremediation of polymetallic contaminated soils.

Interaction of cadmium and zinc on accumulation and sub-cellular distribution in leaves of hyperaccumulator Potentilla griffithii.

Potentilla griffithii Hook is a newly found hyperaccumulator plant capable of high tolerance and accumulation of Zn and Cd. We investigated the interactive effects between Cd and Zn on accumulation and vacuolar sequestration in P. griffithii. Stimulatory effect of growth was noted at 0.2 mM Cd and 1.25 and 2.5 mM Zn tested. Accumulation of Zn and Cd in roots, petioles and leaves were increased significantly with addition of these metals individually. However, the Zn supplement decreased root Cd accumulation but increased the concentration of Cd in petioles and leaves. The results from sub-cellular distribution showed that up to 94% and 70% of the total Zn and Cd in the leaves were present in the protoplasts, and more than 90% Cd and Zn in the protoplasts were localized in the vacuoles. Nearly, 88% and 85% of total Cd and Zn were extracted in the cell sap of the leaves suggesting that most of the Cd and Zn in the leaves were available in soluble form. The present results indicate that Zn supplement significantly enhanced the petiole accumulation of Cd and further vacuolar sequestration plays an important role in tolerance, detoxification and hyperaccumulation of these metals in P. griffithii.

Cadmium tolerance of carbon assimilation enzymes and chloroplast in Zn/Cd hyperaccumulator Picris divaricata.

To better understand the photosynthesis under stress, the effect of cadmium on carbon assimilation and chloroplast ultrastructure of a newly found Zn/Cd hyperaccumulator Picris divaricata in China was investigated in solution culture. The shoot and root Cd concentrations increased with increase in Cd supply, reaching maxima of 1109 and 5604mgkg(-1) dry weight at 75microM Cd, respectively. As Cd supply to P. divaricata increased, the shoot and root dry weight, leaf water content (except 75microM Cd), concentrations of chlorophyll a and b, chlorophyll a/b ratio and the concentration of carotenoids were not depressed at high Cd. However, the stomatal conductance, transpiration rate, net photosynthetic rate and intercellular CO(2) concentration were significantly affected when the Cd concentration reached 10, 10, 25 and 75microM, respectively. Meanwhile, carbonic anhydrase (CA; EC 4.2.1.1) activity and Rubisco (EC 4.1.1.39) content reached maxima in the presence of 50 and 5microM Cd, respectively. In addition, CA activity correlated positively with shoot Cd in plants treated with Cd at a range of 0-50microM. Moreover, the activities of NADP(+)-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13), Rubisco and fructose-1, 6-bisphosphatase (EC 3.1.3.11) were not significantly suppressed by increased Cd supply. Although the mesophyll cell size was reduced, chloroplast ultrastructure remained intact at the highest Cd treatment. Our finding revealed that P. divaricata chloroplast and the enzymes of carbon assimilation tolerate high levels of Cd, demonstrating its potential in possible application in phytoremediation.

Antioxidative response to Cd in a newly discovered cadmium hyperaccumulator, Arabis paniculata F.

A hydroponic experiment was carried out to study the effect of cadmium (Cd) on growth, Cd accumulation, lipid peroxidation, reactive oxygen species (ROS) content and antioxidative enzymes in leaves and roots of Arabis paniculata F., a new Cd hyperaccumuator found in China. The results showed that 22-89 microM Cd in solution enhanced the growth of A. paniculata after three weeks, with 21-27% biomass increase compared to the control. Cd concentrations in shoots and roots increased with increasing Cd supply levels, and reached a maximum of 1662 and 8670 mg kg(-1) Cd dry weight at 178 microM Cd treatment, respectively. In roots, 22-89 microM Cd reduced the content of malondialdehyde (MDA), superoxide (O(2)(-1)) and H(2)O(2) as well as the activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR). In leaves, the contents of MDA, O(2)(-1) and H(2)O(2) remained unaffected by 22-89 microM Cd, while 178 microM Cd treatment significantly increased the MDA content, 69.5% higher than that of the control; generally, the activities of SOD, catalase (CAT), GPX and APX showed an increasing pattern with increasing Cd supply levels. Our present work concluded that A. paniculata has a great capability of Cd tolerance and accumulation. Moderate Cd treatment (22-89 microM Cd) alleviated the oxidative stress in roots, while higher level of Cd addition (178 microM) could cause an increasing generation of ROS, which was effectively scavenged by the antioxidative system.

[Effects of cadmium and lead on subcellular distribution and chemical form of zinc in Potentilla griffithii var. velutina].

Using the differential centrifugation technique and sequential chemical extraction method, effects of Cd, Pb and different Zn salts on subcellular distribution and chemical form of Zn in Zn hyperaccumulator Potentilla griffithii var. velutina under nutrient solution culture were analyzed. Under all treatments except for the control, 46%-74% and 16%-33% of total Zn in the plants are distributed in cell wall and in soluble fraction, respectively. Further, 74%-95% of total Zn are localized in these two parts under all treatments, which suggest that cell wall and soluble fraction in the plant are major storage sites for Zn. Compared with the control, Zn percentage significantly increases by 9%-38% in the cell wall and decreases by 6%-40% in the soluble fraction with addition of Zn, Cd and Pb treatment (p < 0.05). Although the addition of Cd and Pb has no influence on the pattern of Zn subcellular distribution presenting cell wall > soluble fraction > karyon and chloroplast > mitochondrion, it generally reduces Zn percentage in the chloroplast, karyon and mitochondrion and increases that in the cell wall or soluble fraction, suggesting that Cd and Pb promote the transferring processes of Zn from organelle to either cell wall or vacuole. As to the chemical forms, 61%-87% of total Zn exist as ethanol- and water-extractable forms in plants under control and only leaves under Zn addition treatment; while 62%-73% of total Zn exist as NaCl- and ethanol-extractable forms in leafstalks and roots under Zn addition treatment. NaCl-, ethanol- and water-extractable forms are also the main chemical forms in the plants, occupied almost 70%-89% of total Zn under Zn/Cd and Zn/Pb compound treatments. The addition of Zn, Cd and Pb generally increases the percentage of NaCl-extractable Zn forms, but decreases that of ethanol-extractable Zn, which facilitates Zn chemical form transferring from relatively higher active forms to less active ones. These results mentioned above indicate that cell wall binding, vacuolar compartmentalization and reduction of total percentage in higher active chemical forms are main tolerance mechanisms for Zn in Potentilla griffithii var. velutina in response to Zn, Zn/Cd and Zn/Pb treatments. Additionally, different Zinc salts have no obvious influence on Zn subcellular distribution in the plant, whereas the treatment of Zinc nitrate turns Zn ethanol-extraction to a dominant chemical form.

[Kinetic characteristics of Zn uptake by Potentilla griffithii Hook. f. var. velutina Card].

The kinetic characteristics of Zn uptake by Potentilla griffithii Hook. f. var. velutina Card. were studied with hydroponic culture under different Zn supply and its exposure time. When the Zn supply was 10 mg x L(-1), the Zn concentration in shoot and root reached the peak on the 8th day, being 2.49 x 10(3) mg x kg(-1) and 2.21 x 10(3) mg x kg(-1), respectively; while when the supply was 100 mg x L(-1), the Zn concentration in shoot reached the maximum (1.23 x 10(3) mg x kg(-1)) on the 16th day, but that in root did not exhibit saturation. The Zn concentration in leaf and leafstalk increased with increasing Zn supply from 0 to 160 mg x L(-1), but had no increase or even decreased when the Zn supply was higher than 160 mg x L(-1). The Zn concentration in root exhibited a positive correlation with Zn supply and its exposure time, and the kinetic curve of Zn uptake fitted linear-quadratic (LQ) model, i. e., fast linear uptake first and slow saturation uptake later, with the dividing point at about 1-2 hours' exposure, which could be related with the Zn adsorption on root cell wall and the Zn transportation across the cell membrane.

[Effects of cadmium on the growth and nitrogen metabolism in Brassica chinensis].

Hydroponic culture was conducted to study the effect of Cd on the growth, metal accumulation and nitrogen metabolism in Brassica chinensis. The enzymatic activities of nitrogen metabolism including nitrate reductase (NR), glutamine synthetase (GS) and GS-transferase as well as the concentrations of chlorophyll, free proline, soluble protein, NO3(-) -N, NH4+ -N and nutrients in Brassica chinensis were determined. Results indicated that the addition of Cd reduced the content of the soluble protein and the accumulation of Cu, Ca, Fe and Mg, but promoted the P uptake. Low level of Cd (1 mg x L(-1)) could significantly increase the biomass and the content of chlorophyll of Brassica chinensis and the activities of NR, GS and GS-transferase when compared to control plants. However, when the Cd levels were above 2.5 mg x L(-1) in the culture medium, the activities of these enzymes were inhibited. Accordingly, the contents of NO3(-) -N, NH4+ -N, free proline and the activity of protease in the leaf of Brassica chinensis increased significantly. These results suggested that Cd addition could interfere with the assimilation of N in Brassica chinensis. The increase of free proline might alleviate the toxicity of ammonium in Brassica chinensis.