Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge.

Hyperaccumulator plants are the material basis for phytoextraction research and for practical applications in decontaminating polluted soils and industrial wastes. China's high biodiversity and substantial mineral resources make it a global hotspot for hyperaccumulator plant species. Intensive screening efforts over the past 20 years by researchers working in China have led to the discovery of many different hyperaccumulators for a range of elements. In this review, we present the state of knowledge on all currently reported hyperaccumulator species from China, including Cardamine hupingshanensis (selenium, Se), Dicranopteris dichotoma (rare earth elements, REEs), Elsholtzia splendens (copper, Cu), Phytolacca americana (manganese, Mn), Pteris vittata (arsenic, As), Sedum alfredii, and Sedum plumbizincicola (cadmium/zinc, Cd/Zn). This review covers aspects of the ecophysiology and molecular biology of tolerance and hyperaccumulation for each element. The major scientific advances resulting from the study of hyperaccumulator plants in China are summarized and synthesized.

Mitigating cadmium exposure risk in rice with foliar nano-selenium: Investigations through Caco-2 human cell line in-vivo bioavailability assay.

The contamination of paddy fields by cadmium and lead is a major issue in China. The consumption of rice grown in heavy metals contaminated areas poses severe health risks to humans, where bioavailability and bioaccessibility remains the critical factor for risk determination. Selenium nanoparticles (Se-NPs) can mitigate the toxicity of heavy metals in plants. However, there exists limited information regarding the role of Se-NPs in dictating cadmium (Cd) toxicity in rice for human consumption. Moreover, the impact of Se-NPs under simultaneous field and laboratory controlled conditions is rarely documented. To address this knowledge gap, a field experiment was conducted followed by laboratory scale bioavailability assays. Foliar application of Se-NPs and selenite (at 5, 10 mg L-1) was performed to assess their efficiency in lowering Cd accumulation, promoting Se biofortification in rice grains, and evaluating Cd exposure risk from contaminated rice. Obtained results indicate that foliar treatments significantly reduced the heavy metal accumulation in rice grains. Specifically, Se-NP 10 mg L-1 demonstrated higher efficiency, reducing Cd and Pb by 56 and 32 % respectively. However, inconsistent trends for bioavailable Cd (0.03 mg kg-1) and bioaccessible (0.04 mg kg-1) were observed while simulated human rice intake. Furthermore, the foliage application of Se-NPs and selenite improved rice quality by elevating Se, Zn, Fe, and protein levels, while lowering phytic acid content in rice grains. In summary, this study suggests the promising potential of foliage spraying of Se-NPs in lowering the health risks associated with consuming Cd-contaminated rice.

Differential iron-bioavailability with relation to nutrient compositions in polished rice among selected Chinese genotypes using Caco-2 cell culture model.

Genotypic variation of iron bioavailability and the relationship between iron bioavailability and nutrient composition in polished rice among 11 rice genotypes were assessed using an in vitro digestion/Caco-2 cell model. The results indicated that significant differences in iron bioavailability were detected among tested rice genotypes, with a 3-fold range, suggesting a possibility of selecting high bioavailable iron by plant breeding. Although iron bioavailability was not significantly correlated with Fe concentration in polished rice among tested rice genotypes, the results also indicated that most of the iron dense genotypes showed relatively high ferritin formation in Caco-2 cell and transported iron. Additionally, iron bioavailability in polished rice was enhanced by addition of ascorbic acid, with a much wider range of Fe bioavailability variation in polished rice with addition of ascorbic acid than that without addition of ascorbic acid. The positive relationship between iron bioavailability in polished rice and cysteine concentration (R = 0.669) or sulfur (S) concentration (R = 0.744) among tested rice genotypes, suggests that cysteine and sulfur concentration in polished rice could be used as an indicator for high iron bioavailability.

Heavy metal phytoextraction by Sedum alfredii is affected by continual clipping and phosphorus fertilization amendment.

Improving the efficacy of phytoextraction is critical for its successful application in metal contaminated soils. Mineral nutrition affects plant growth and metal absorption and subsequently the accumulation of heavy metal through hyper-accumulator plants. This study assessed the effects of di-hydrogen phosphates (KH2PO4, Ca(H2PO4)2, NaH2PO4 and NH4H2PO4) application at three levels (22, 88 and 352 mg P/kg soil) on Sedum alfredii growth and metal uptake by three consecutive harvests on aged and Zn/Cd combined contaminated paddy soil. The addition of phosphates (P) significantly increased the amount of Zn taken up by S. alfredii due to increased shoot Zn concentration and dry matter yield (DMY) (P < 0.05). The highest phytoextraction of Zn and Cd was observed in KH2PO4 and NH4H2PO4 treatment at 352 mg P/kg soil. The amount of Zn removed by phytoextraction increased in the order of 1st clipping < 2nd clipping < 3rd clipping, and for Cd extraction the order was 2nd clipping < 1st clipping < 3rd clipping. These results indicate that the application of P fertilizers coupled with multiple cuttings can enhance the removal of Zn and Cd from contaminated soils by S. alfredii, thus shortening the time needed for accomplishing remediation goals.

Combined remediation of DDT congeners and cadmium in soil by Sphingobacterium sp. D-6 and Sedum alfredii Hance.

Combined pollution of 1,1,1-trichloro-2,2-bis (4-chlorophenyl) ethane (DDT) and cadmium (Cd) in agricultural soils is of great concern because they present serious risk to food security and human health. In order to develop a cost-effective and safe method for the removal of DDTs and Cd in soil, combined remediation of DDTs and Cd in soil by Sphingobacterium sp. D-6 and the hyperaccumulator, Sedum alfredii Hance was investigated. After treatment for 210 days, the degradation half-lives of DDTs in soils treated by strain D-6 decreased by 8.1% to 68.0% compared with those in the controls. The inoculation of strain D-6 into soil decreased the uptake of DDTs by pak choi and S. alfredii. The shoots/roots ratios of S. alfredii for the Cd accumulation ranged from 12.32 to 21.75. The Cd concentration in soil decreased to 65.8%-71.8% for S. alfredii treatment and 14.1%-58.2% for S. alfredii and strain D-6 combined treatment, respectively, compared with that in the control. The population size of the DDTs-degrading strain, Simpson index (1/D) and soil respiratory rate decreased in the early stage of treatment and then gradually increased, ultimately recovering to or exceeding the initial level. The results indicated that synchronous incorporation of strain D-6 and S. alfredii into soil was found to significantly (p < or = 0.05) enhance the degradation of DDTs in soil and the hyperaccumulation of Cd in S. alfredii. It was concluded that strain D-6 and S. alfredii could be used successfully to control DDTs and Cd in contaminated soil.

Anti-Müllerian Hormone Regulates Stem Cell Factor via cAMP/PKA Signaling Pathway in Human Granulosa Cells by Inhibiting the Phosphorylation of CREB.

Anti-Müllerian hormone (AMH) downregulates the level of stem cell factor (SCF) via the cAMP/PKA signaling pathway in human granulosa cells (GCs). Little information is available on the molecular mechanism underlying the interaction. This study is aimed at determining whether AMH regulates expression of SCF via the cAMP-PKA-CREB signaling pathway in human GCs. In the present study, we verified the binding of cAMP-response element-binding protein (CREB) to promoter of SCF in human GCs. Furthermore, the effect of CREB was tested on the SCF promoter, and the site of CREB binding to SCF promoter was identified using truncations as well as assays of SCF-promoted mutation and CREB mutation. To investigate the correlation among AMH, SCF promoter, and CREB, pGL-Basic-SCF+CREB was transfected into overexpressed AMH GCs (AMH-high GCs), low expressed AMH GCs (AMH-low GCs), and normal GCs (GCs), respectively. Finally, immunofluorescence, double immunostaining, and Western blot were carried out in AMH-high and AMH-low GCs to confirm the AMH-mediated regulation of SCF expression by inhibiting the phosphorylation of CREB (pCREB) in GCs. Results indicated CREB interacted with SCF promoter and significantly enhanced the transcription level of SCF. The CREB binding site was localized at 318-321 bp of SCF gene promote. AMH inhibits the expression of SCF by phosphorylation of CREB via the PKA signaling pathway in GCs. These findings provide an in-depth understanding of the molecular mechanism underlying AMH suppressing the follicle growth, which would aid in the development of a novel therapy.

Stem and leaf sequestration of zinc at the cellular level in the hyperaccumulator Sedum alfredii.

* Sedum alfredii is a fast-growing, high-biomass zinc (Zn) hyperaccumulator native to China. Here, the characteristics of in vivo Zn distribution in stems and leaves of the hyperaccumulating (HE) and nonhyperaccumulating ecotypes (NHE) of S. alfredii were investigated by synchrotron radiation X-ray fluorescence (SRXRF) analysis, together with a Zn probe. * Preferential Zn accumulation in leaf and stem epidermis was observed in both ecotypes, but to a much greater extent for HE. Epidermal Zn increased largely in leaves and stems of HE as exposure time was prolonged, while Zn saturation occurred relatively early in HE leaf mesophyll cells and stem vascular bundles. A second peak of Zn enrichment in stem and leaf vascular systems was shown in both ecotypes. However, the proportion of Zn accumulated in stem vascular bundles relative to other tissues was much greater for HE than for NHE. * Leaf and stem distribution patterns of phosphorus (P) and sulphur (S) in the HE were very like that for Zn, while the calcium (Ca) distribution pattern was the reverse of that for Zn. No such relationship was observed in NHE. * Our study mainly suggested that epidermal layers serve as important storage sites for accumulated Zn in the S. alfredii HE.

Persistence of the herbicide butachlor in soil after repeated applications and its effects on soil microbial functional diversity.

Effects of repeated applications of the herbicide butachlor (N-(butoxymethyl)-2-chloro -N-2',6'-dimethyl acetanilide) in soil on its persistence and soil microbial functional diversity were investigated under laboratory conditions. The degradation half-lives of butachlor at the recommended dosage in soil were calculated to be 12.5, 4.5, and 3.2 days for the first, second, and third applications, respectively. Throughout this study, no significant inhibition of the Shannon-Wiener index H' was observed. However, the Simpson index 1/D and McIntosh index U were significantly reduced (P < or = 0.05) during the initial 3 days after the first application of butachlor, and thereafter gradually recovered to a similar level to that of the control soil. A similar variation but faster recovery in 1/D and U was observed after the second and third Butachlor applications. Therefore, repeated applications of butachlor led to more rapid degradation of the herbicide, and more rapid recovery of soil microorganisms. It is concluded that repeated butachlor applications in soil had a temporary or short-term inhibitory effect on soil microbial communities.

Vertical distribution of fluorine in farmland soil profiles around phosphorous chemical industry factories.

High concentration of fluorine (F) in agricultural soils has got significant attention considering its impacts on human health, but little information was available about F distribution in farmland soil profiles around phosphorous chemical industry factories. In present study, farmland soil profiles and relevant medium samples were collected from farmlands around a main phosphorous chemical base in southwest China. At 0-100-cm profiles, concentrations of soil total F (Ft, 400.9-1612.0 mg kg-1) and water soluble F (Fw, 3.4-26.0 mg kg-1) decreased with profile depth in industrial areas. Industrial activities enhanced F concentration in soil mainly at 0-40-cm profiles. No disparity for both Ft and Fw distributions in paddy-dry land rotation field and dry land indicates short-term land utilization could not affect the F distribution in soil profiles. Correlation analysis showed soil organic matter and wind direction were important factors influencing the distribution of F in soil profiles. The shutdown of factory and government control of industrial emissions effectively decreased the ambient air F (Fa) concentrations in industrial areas. In where Fa and dustfall F concentrations were high, high soil Ft, Fw, and crop edible part F concentrations were found.

Distribution, health risk assessment, and anthropogenic sources of fluoride in farmland soils in phosphate industrial area, southwest China.

The high concentration of fluoride (F) in soils has become a rising concern for its toxicity to microbes, plants, animals and human health. In the present study, the spatial and vertical distribution, health risk assessment and anthropogenic sources of F in farmland soils in an industrial area dominated by phosphate chemical plants were studied. Concentrations of total fluoride (TF) and water soluble fluoride (WSF) in the surface soils decreased with distance within the range of 2500 m at the prevailing downwind of the industrial area. The soil TF and WSF concentrations in 0-40 cm profiles were higher than those in 40-100 cm layers in the industrial area. At the prevailing downwind of the industrial area within 700 m, the hazard quotient values of human exposure to surface soils were higher than 1, indicating that a potential risk may exist for human health in this area. The main exposure pathway for children and adults was oral ingestion and particulate inhalation, respectively. The source apportionment model of soil F was modified based on years' historical data and experimental data. The results showed that the proportion of anthropogenic sources of soil F was dustfalls (69%) > irrigation water (23%) > air (5%) > chemical fertilizers (3%) in the industrial area. The high F concentration of dustfalls was mainly due to the phosphate rock, phosphogypsum, and surface soils with high F contents from the factories. In order to safeguard human health and alleviate hazards of F to surroundings, the control of pollutants emission from factories was a basic and vital step to reduce F in the soils in industrial areas.

Enhanced root-to-shoot translocation of cadmium in the hyperaccumulating ecotype of Sedum alfredii.

Sedum alfredii (Crasulaceae) is the only known Cd-hyperaccumulating species that are not in the Brassica family; the mechanism of Cd hyperaccumulation in this plant is, however, little understood. Here, a combination of radioactive techniques, metabolic inhibitors, and fluorescence imaging was used to contrast Cd uptake and translocation between a hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of S. alfredii. The K(m) of (109)Cd influx into roots was similar in both ecotypes, while the V(max) was 2-fold higher in the HE. Significant inhibition of Cd uptake by low temperature or metabolic inhibitors was observed in the HE, whereas the effect was less pronounced in the NHE. (109)Cd influx into roots was also significantly decreased by high Ca in both ecotypes. The rate of root-to-shoot translocation of (109)Cd in the HE was >10 times higher when compared with the NHE, and shoots of the HE accumulated dramatically higher (109)Cd concentrations those of the NHE. The addition of the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP) resulted in a significant reduction in Cd contents in the shoots of the HE, and in the roots of the NHE. Cd was distributed preferentially to the root cylinder of the HE but not the NHE, and there was a 3-5 times higher Cd concentration in xylem sap of the HE in contrast to the NHE. These results illustrate that a greatly enhanced rate of root-to-shoot translocation, possibly as a result of enhanced xylem loading, rather than differences in the rate of root uptake, was the pivotal process expressed in the Cd hyperaccumulator HE S. alfredii.

Ultrastructural changes, zinc hyperaccumulation and its relation with antioxidants in two ecotypes of Sedum alfredii Hance.

Zn phytotoxicity and its possible detoxifying responses in two ecotypes of Sedum alfredii Hance, i.e. hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) were investigated. HE grew better with high Zn concentrations of 29.11gkg(-1) DW in shoots when exposed to 500microM Zn2+. Toxicity symptoms caused by Zn in root cells of both ecotypes mainly included plasmolysis, disruption of plasma membranes and increased cell vacuolation. At high supplied Zn concentration, chloroplasts suffered from structural disorganization in both ecotypes. Zn-induced hydrogen peroxide (H2O2) and superoxide radical (O(2)-) productions in leaves were determined by a histochemical method, which revealed that Zn stress may have involved NADPH oxidase, protein phosphatases and intracellular Ca2+ to activate the reactive oxygen species production. Inhibition of glutathione synthesis may have led to increased H2O2 and O(2)- accumulations in leaves of HE. In response to higher Zn concentrations, ascorbic acid significantly increased in both ecotypes and levels of glutathione increased in both leaves and roots of HE and in roots of NHE without any change in the leaves of NHE. The enzymatic activities like those of superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), guaiacol peroxidase (GPX, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GR, EC 1.6.4.2) in leaves of HE were all enhanced at supplied Zn concentration of 500microM, which may account for its better growth.

Comparison of iron uptake from reduced iron powder and FeSO4 using the Caco-2 cell model: effects of ascorbic acid, phytic acid, and pH.

The reduced iron powder has considerable potential for use as an iron fortificant because it does not change organoleptically during storage or food preparation for cereal flour, and its bioavailability is scarcely influenced by iron absorption inhibitors in foods. The objective of this article is to study the effects of ascorbic acid, phytic acid, and pH on iron uptake from reduced iron powder (43 microm) and FeSO 4, and to compare iron bioavailability of reduced iron powders among four selected granularity levels. The cell ferritin formation is used as a marker of iron uptake. Obviously, iron uptake of reduced iron powder is increased with decreasing of powder granularity and is much lower than FeSO 4 when the size is above 43 microm, but significantly higher at 40-60 nm. In the presence of ascorbic acid or phytic acid, Caco-2 cell iron absorption from reduced iron powder (43 microm) is significantly higher than that from FeSO 4. And iron uptake of Caco-2 cells is decreased with increasing of pH from 5.5 to 7.5. Moreover, the decrease trend is more obvious for reduced iron powder than for FeSO 4. Our results indicated that iron bioavailability of reduced iron powder by intestinal enterocytes is similar to that of iron salts, and reduced iron powder is more excellent than FeSO 4 as food fortificant, especially at ultramicroscopic granularity.

Role of EDTA in alleviating lead toxicity in accumulator species of Sedum alfredii H.

The effects of Pb chelater (EDTA-Pb) and ionic Pb (Pb(NO(3))(2)) on root cell death, Pb accumulation, changes of ROS, activities of antioxidant enzymes and uptake of mineral elements in response to Pb toxicity in Sedum alfredii H. were compared. Loss of plasma membrane integrity became serious by increasing Pb concentration in the medium, 200 microM Pb + 200 microM EDTA has alleviated the root cell death. The biomass was significantly affected by high concentration of Pb, and root growth was also affected by EDTA-Pb compared with ionic Pb. Lead accumulation was higher in the samples treated with ionic lead than that of the control. The concentration of reactive oxygen species (ROS) was determined by fluorescence microscopy, which indicates that the Pb stress increased the content of ROS significantly, whereas the EDTA-Pb decreased the burst of H(2)O(2). High Pb concentrations increase the activity of SOD and LOX. The Cu concentration in root increased significantly under Pb and EDTA-Pb treatment, and 200 microM Pb markedly increased the Fe content in roots. Under ionic Pb condition, the contents of Mg, Ca and K in shoots decreased, whereas they were significantly increased in case of EDTA-Pb. These results suggested that accumulating ecotype of S. alfredii roots were inefficient in uptake of higher concentration of EDTA-chelated Pb for long treatment duration, and that lead toxicity could be alleviated by EDTA.

Lead induced changes in the growth and antioxidant metabolism of the lead accumulating and non-accumulating ecotypes of Sedum alfredii.

The phytotoxicity and antioxidative adaptations of lead (Pb) accumulating ecotype (AE) and non-accumulating ecotype (NAE) of Sedum alfredii Hance were investigated under different Pb treatments involving 0, 0.02 mmol/L Pb, 0.1 mmol/L Pb and 0.1 mmol/L Pb/0.1 mmol/L ethylenediaminetetraacetic acid (EDTA) for 6 days. With the increasing Pb level, the Pb concentration in the shoots of AE plants enhanced accordingly, and EDTA supply helped 51% of Pb translocation to shoots of AE compared with those treated with 0.1 mmol/L Pb alone. Moreover, the presence of EDTA alleviated Pb phytotoxicity through changes in plant biomass, root morphology and chlorophyll contents. Lead toxicity induced hydrogen peroxide (H2O2) accumulation and lipid peroxidation in both ecotypes of S. alfredii. The activities of superoxide dismutase (SOD), guaiacol peroxidase (G-POD), ascorbate peroxidase, and dehydroascorbate reductase elevated in both leaves and roots of AE as well as in leaves of NAE with the increasing Pb levels, but SOD and G-POD declined in roots of NAE. Enhancement in glutathione reductase activity was only detected in roots of NAE while a depression in catalase activity was recorded in the leaves of NAE. A significant enhancement in glutathione and ascorbic acid (AsA)levels occurred in both ecotypes exposed to Pb and Pb/EDTA treatment compared with the control, however, the differences between these two treatments were insignificant. The dehydroascorbate (DHA) contents in roots of both ecotypes were 1.41 to 11.22-fold higher than those in leaves, whereas the ratios of AsA to DHA (1.38 to 6.84) in leaves altering more to the reduced AsA form were much higher than those in roots. These results suggested that antioxidative enzymes and antioxidants play an important role in counteracting Pb stress in S. alfredii.

Phytoremediation of heavy metal polluted soils and water: progresses and perspectives.

Environmental pollution affects the quality of pedosphere, hydrosphere, atmosphere, lithosphere and biosphere. Great efforts have been made in the last two decades to reduce pollution sources and remedy the polluted soil and water resources. Phytoremediation, being more cost-effective and fewer side effects than physical and chemical approaches, has gained increasing popularity in both academic and practical circles. More than 400 plant species have been identified to have potential for soil and water remediation. Among them, Thlaspi, Brassica, Sedum alfredii H., and Arabidopsis species have been mostly studied. It is also expected that recent advances in biotechnology will play a promising role in the development of new hyperaccumulators by transferring metal hyperaccumulating genes from low biomass wild species to the higher biomass producing cultivated species in the times to come. This paper attempted to provide a brief review on recent progresses in research and practical applications of phytoremediation for soil and water resources.

Mechanisms and assessment of water eutrophication.

Water eutrophication has become a worldwide environmental problem in recent years, and understanding the mechanisms of water eutrophication will help for prevention and remediation of water eutrophication. In this paper, recent advances in current status and major mechanisms of water eutrophication, assessment and evaluation criteria, and the influencing factors were reviewed. Water eutrophication in lakes, reservoirs, estuaries and rivers is widespread all over the world and the severity is increasing, especially in the developing countries like China. The assessment of water eutrophication has been advanced from simple individual parameters like total phosphorus, total nitrogen, etc., to comprehensive indexes like total nutrient status index. The major influencing factors on water eutrophication include nutrient enrichment, hydrodynamics, environmental factors such as temperature, salinity, carbon dioxide, element balance, etc., and microbial and biodiversity. The occurrence of water eutrophication is actually a complex function of all the possible influencing factors. The mechanisms of algal blooming are not fully understood and need to be further investigated.

Microbial activity and community diversity in a variable charge soil as affected by cadmium exposure levels and time.

Effects of cadmium (Cd) on microbial biomass, activity and community diversity were assessed in a representative variable charge soil (Typic Aquult) using an incubation study. Cadmium was added as Cd(NO3)(2) to reach a concentration range of 0-16 mg Cd/kg soil. Soil extractable Cd generally increased with Cd loading rate, but decreased with incubation time. Soil microbial biomass was enhanced at low Cd levels (0.5-1 mg/kg), but was inhibited consistently with increasing Cd rate. The ratio of microbial biomass C/N varied with Cd treatment levels, decreasing at low Cd rate (<0.7 mg/kg available Cd), but increasing progressively with Cd loading. Soil respiration was restrained at low Cd loading (<1 mg/kg), and enhanced at higher Cd levels. Soil microbial metabolic quotient (MMQ) was generally greater at high Cd loading (1-16 mg/kg). However, the MMQ is also affected by other factors. Cd contamination reduces species diversity of soil microbial communities and their ability to metabolize different C substrates. Soils with higher levels of Cd contamination showed decreases in indicator phospholipids fatty acids (PLFAs) for Gram-negative bacteria and actinomycetes, while the indicator PLFAs for Gram-positive bacteria and fungi increased with increasing levels of Cd contamination.

Differential response of root morphology to potassium deficient stress among rice genotypes varying in potassium efficiency.

Disparity in the root morphology of six rice (Oryza sativa L.) genotypes varying in potassium (K) efficiency was studied with three K levels: 5 mg/L (low), 10 mg/L (moderate) and 40 mg/L (adequate) in hydroponic culture. Morphological parameters included root length, surface area, volume and count of lateral roots, as well as fine (diameter<0.2 mm) and thick (diameter>0.2 mm) roots. The results indicate that the root growth of all genotypes was reduced under low K, but moderate K deficiency increased the root length of the efficient genotypes. At deficient and moderate K levels, all the efficient rice genotypes developed more fine roots (diameter<0.2 mm) than the inefficient ones. Both fine root count and root surface area were found to be the best parameters to portray K stress in rice. In accordance with the root morphology, higher K concentrations were noted in shoots of the efficient genotypes when grown at moderate and deficient K levels, indicating that root morphology parameters are involved in root uptake for K and in the translocation of K up to shoots. K deficiency affected not only the root morphology, but also the root ultra-structure. The roots of high-efficient genotypes had stronger tolerance to K deficient stress for root membrane damage, and could maintain the developed root architecture to adapt to the low K growth medium.

Differential generation of hydrogen peroxide upon exposure to zinc and cadmium in the hyperaccumulating plant species (Sedum alfredii Hance).

Sedum alfredii Hance has been identified as zinc (Zn) and cadmium (Cd) co-hyperaccumulator. In this paper the relationships of Zn or Cd hyperaccumulation to the generation and the role of H2O2 in Sedum alfredii H. were examined. The results show that Zn and Cd contents in the shoots of Sedum alfredii H. treated with 1000 micromol/L Zn2+ and/or 200 micromol/L Cd2+ increased linearly within 15 d. Contents of total S, glutathione (GSH) and H2O2 in shoots also increased within 15 d, and then decreased. Total S and GSH contents in shoots were higher under Cd2+ treatment than under Zn2+ treatment. However, reverse trends of H2O2 content in shoots were obtained, in which much higher H2O2 content was observed in Zn2+-treated shoots than in Cd2+-treated shoots. Similarly, the microscopic imaging of H2O2 accumulation in leaves using H2O2 probe technique showed that much higher H2O2 accumulation was observed in the Zn2+-treated leaf than in the Cd2+-treated one. These results suggest that there are different responses in the generation of H2O2 upon exposure to Zn2+ and Cd2+ for the hyperaccumulator Sedum alfredii H. And this is the first report that the generation of H2O2 may play an important role in Zn hyperaccumulation in the leaves. Our results also imply that GSH may play an important role in the detoxification of dissociated Zn/Cd and the generation of H2O2.