Soil Cd and Zn contamination has become a serious environmental problem. This work explored the performance of wood vinegar (WV) in enhancing the phytoextraction of Cd/Zn by hyperaccumulator Sedum alfredii Hance. Rhizosphere chemical properties, enzyme activities and bacterial community were analyzed to determine the mechanisms of metal accumulation in this process. Results demonstrated that, after 120 days growth, different times dilution of WV increased the shoot biomass of S. alfredii by 85.2%–148%. In addition, WV application significantly increased soil available Cd and Zn by lowing soil pH, which facilitated plant uptake. The optimal Cd and Zn phytoextraction occurred from the 100 times diluted WV (D100), which increased the Cd and Zn extraction by 188% and 164%, compared to CK. The 100 and 50 times diluted WV significantly increased soil total and available carbon, nitrogen and phosphorus, and enhancing enzyme activities of urease, acid phosphatase, invertase and protease by 10.1–21.4%, 29.1–42.7%,12.2–38.3% and 26.8–85.7%, respectively, compared to CK. High-throughput sequencing revealed that the D 100 significantly increased the bacterial diversity compared to CK. Soil bacterial compositions at phylum, family and genera level were changed by WV addition. Compared to CK, WV application increased the relative abundances of genus with plant growth promotion and metal mobilization function such as, Bacillus, Gemmatimonas, Streptomyces, Sphingomonas and Polycyclovorans, which was positively correlated to biomass, Cd/Zn concentrations and extractions by S. alfredii. Structural equation modeling analysis showed that, soil chemical properties, enzyme activities and bacterial abundance directly or indirectly contributed to the biomass promotion, Cd, and Zn extraction by S. alfredii. To sum up, WV improved phytoextraction efficiency by enhancing plant growth, Cd and Zn extraction and increasing soil nutrients, enzyme activities, and modifying bacterial community.

Root cell wall (RCW) modification is a widespread important defense strategy of plant to cope with trace metals. However, mechanisms underlying its remolding in cadmium (Cd) accumulation are still lacking in hyperaccumulators. In this study, changes of RCW structures and components between nonhyperaccumulating ecotype (NHE) and hyperaccumulating ecotype (HE) of Sedum alfredii were investigated simultaneously. Under 25 μM Cd treatment, RCW thickness of NHE is nearly 2 folds than that of HE and the thickened cell wall of NHE was enriched in low-methylated pectin, leading to more Cd trapped in roots tightly. In the opposite, large amounts of high-methylated pectin were assembled around RCW of HE with Cd supply, in this way, HE S. alfredii decreased its root fixation of Cd and enhanced Cd migration into xylem. TEM and AFM results further confirmed that thickened cell wall was caused by the increased amounts of cellulose and lignin while root tip lignification was resulted from variations of sinapyl (S) and guaiacyl (G) monomers. Overall, thickened cell wall and methylated pectin have synchronicity in spatial location of roots, and their coordination contributed to Cd accumulation in S. alfredii.

Planting leafy vegetables, especially pakchoi, in cadmium (Cd) polluted farmland is easy to lead to excessive Cd content in edible parts, which results in high risk of food chain. In this study, a field experiment was carried out to study the effects of intercropping of pakchoi with Cd hyperaccumulator Sedum alfredii Hance, and the roles of endophytic bacterium SaMR12 was also investigated. When intercropping with Sedum, the growth of pakchoi was not affected but their Cd concentration and accumulation were significantly increased, while which were obviously decreased by SaMR12 inoculation. After intercropping, the biomass of Sedum was significantly reduced, but their Cd concentration increased. SaMR12 inoculation significantly increased Cd accumulation of Sedum, and which increased to 3 times in Sedum monoculture. Those results showed that although intercropping with hyperaccumulator could lead to higher risk of pakchoi in Cd polluted field, intercropping with SaMR12 inoculated Sedum can decrease Cd concentration of pakchoi and promote Cd absorption of Sedum, which indicated that this endophyte can be made into a microbial inoculum as a soil additive for the safe production of vegetables and the soil Cd pollution remediation.

The complexation of Zn, Cd and Pb with dissolved organic matter (DOM) in the rhizosphere of hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of Sedum alfredii was measured using resin equilibration method. After the growth of HE S. alfredii, the rhizosphere soil pH was reduced by 0.27–0.33 units, due to enhanced DOM derived from root exudation. For both ecotypes of S. alfredii, the fraction of free metal as a percentage of soluble metal varied from 22.1 to 42.5% for Zn2+, from 8.1 to 15.5% for Cd2+, and from 4.5 to 10.4% for Pb2+. Resin equilibration experiment results indicated that HE–DOM had greater ability to form complexes with Zn, Cd and Pb than NHE–DOM, Visual MINTEQ model gave excellent predictions of the complexation of Zn and Cd by DOM (R2 > 0.97). DOM in the rhizosphere of HE S. alfredii could significantly increase metal mobility through the formation of soluble DOM-metal complexes.

Nitric oxide (NO) and ethylene are both important signaling molecules which participate in numerous plant development processes and environmental stress resistance. Here, we investigate whether and how NO interacts with ethylene during the development of endodermal barriers that have major consequences for the apoplastic uptake of cadmium (Cd) in the hyperaccumulator Sedum alfredii. In response to Cd, an increased NO accumulation, while a decrease in ethylene production was observed in the roots of S. alfredii. Exogenous supplementation of NO donor SNP (sodium nitroprusside) decreased the ethylene production in roots, while NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) had the opposite effect. The exogenous addition of NO affected the ethylene production through regulating the expression of genes related to ethylene synthesis. However, upon exogenous ethylene addition, roots retained their NO accumulation. The abovementioned results suggest that ethylene is downstream of the NO signaling pathway in S. alfredii. Regardless of Cd, addition of SNP promoted the deposition of endodermal barriers via regulating the genes related to Casparian strips deposition and suberization. Correlation analyses indicate that NO positively modifies the formation of endodermal barriers via the NO-ethylene signaling pathway, Cd-induced NO accumulation interferes with the synthesis of ethylene, leading to a deposition of endodermal barriers in S. alfredii.

Sedum alfredii is a Cd/Zn hyperaccumulator native to China, which was collected from a mined area where Mn content in soil was extremely high, together with Zn and Cd content. We investigated the tolerance and accumulation ability of Mn and its possible association with Cd hyperaccumulation in this plant species by using MP-AES, SR-μ-XRF, and RT-PCR. The results showed that the hyperaccumulating ecotype (HE) S. alfredii exhibited high tolerance to Mn and accumulating around 10,000 and 12,000 mg kg⁻¹ Mn in roots and shoots, respectively, without exhibiting toxicity under 5000 mg kg⁻¹ Mn treatment for 4 weeks. Exposure to Cd significantly reduced plant uptake of Mn. In contrast, exogenous Mn application significantly improved root uptake and root-to-shoot translocation of Cd, resulting in the increased Cd accumulation in the shoots of HE S. alfredii. SR-μ-XRF analysis demonstrated that high Mn (20 μM) exposure resulted in higher intensities of Cd localized in both stem vascular bundles and cortex, as well as leaf mesophyll cells, than in those treated with low Mn levels (0.2 μM or 2.0 μM). RT-PCR analysis of several genes possibly involved in Mn/Cd transportation showed that expression of SaNramp3 in roots was significantly reduced under high Mn exposure. These results suggested a significant interaction between Cd and Mn in the HE S. alfredii plants, possibly through their competition for transporters and theoretically provided a strategy to improve the efficiency of Cd extraction from polluted soils by this plant species, after using appropriate nutrient management of Mn.

High sensitivity towards Cu toxicity is problematic when using some hyperaccumulator plants for phytoremediation of soils with mixed contamination of Cu. Sedum alfredii, a Cd/Zn co-hyperaccumulator and Pb accumulator, is widely used for remediation of Cd, Zn, and Pb co-contaminated soils in China. In this paper, the tolerance and accumulation ability of S. alfredii towards Cu stress and its potential for phytoremediation of multi-metal polluted soils have been studied. Both the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii accumulated high Cu in the roots and translocated minimal Cu to the shoots, and Cu in the stems and leaves mostly restricted in the vascular tissues (phloem zone). The HE plants, however, exhibited high Cu resistance with stimulated lateral root growth and increased chlorophyll content under 10 μM Cu treatment. XANES analysis showed that Cu in HE roots comprised Cu²⁺ (46.7%), Cu-histidine (35.2%) and Cu-cell wall (18.1%). The NHE under Cu stress showed decreased biomass, reduced leaf chlorophyll content, altered root architecture, and higher Cu localized to root cell wall as compared with the HEs. Potted HE plants thrived six months in multi-metal contaminated soils including 3897 mg kg⁻¹ available Cu. In conclusion, HE S alfredii is highly tolerant toward Cu due to metal homeostasis in root cells. Therefore, this plant has great potential to remediate Zn, Cd, and Pb contaminated soils those also contain high levels of Cu.

Phytoremediation coupled with agro-production is considered a sustainable strategy for remediation of trace element contaminated fields without interrupting crop production. In this study hyperaccumulator Sedum alfredii was intercropped with a leguminous plant fava bean (Vicia fava) in cadmium (Cd) and lead (Pb) co-contaminated field to evaluate the effects of intercropping on growth performance and accumulations of trace elements in plants with plant growth promoting endophyte (PGPE) consortium application. The results showed, compared with monoculture, intercropping coupled with inoculation application promoted biomass as well as Cd and Pb concentrations in individual parts of both plants, thus increasing the removal efficiencies of trace elements (4.49-folds for Cd and 5.41-folds for Pb). Meanwhile, this superposition biofortification measure maintained normal yield and nutrient content, and limited the concentration of Cd and Pb within the permissible limit (<0.2 mg kg⁻¹ FW) in fava bean during the grain production. These results demonstrated a feasible technical system for phytoremediation coupled with agro-production in slightly or moderately Cd and Pb co-contaminated field, and also provided useful information for further investigation of interaction mechanisms between intercropping and PGPEs inoculation.

Numbers of greenroofs in urban areas continue to grow internationally; so designing greenroof soil to reduce the amount of nutrients in the stormwater runoff from these roofs is becoming essential. This study evaluated changes in extensive greenroof water discharge quality and quantity after adding biochar, a soil amendment promoted for its ability to retain nutrients in soils and increase soil fertility. Prototype greenroof trays with and without biochar were planted with sedum or ryegrass, with barren soil trays used as controls. The greenroof trays were subjected to two sequential 7.4cm/h rainfall events using a rain simulator. Runoff from the rain events was collected and evaluated. Trays containing 7% biochar showed increased water retention and significant decreases in discharge of total nitrogen, total phosphorus, nitrate, phosphate, and organic carbon. The addition of biochar to greenroof soil improves both runoff water quality and retention.

Plant growth regulators (PGRs) assisted phytoextraction was investigated as a viable phytoremediation technology to increase the phytoextraction efficiency in contaminated soils. This study aimed to evaluate the cadimum (Cd)/lead (Pb)/zinc (Zn) phytoextraction efficiency by a hyperaccumulator Sedum alfredii Hance (S. alfredii) treated with 9 PGRs, including indole-3-acetic acid (IAA), gibberellin (GA₃), cytokinin (CKs), abscisic acid (ABA), ethylene (ETH), brassinosteroid (BR), salicylic acid (SA), strigolactones (SL) and jasmonic acid (JA), in slightly or heavily contaminated (SC and HC, respectively) soil. Results demonstrated that PGRs were able to improve S. alfredii biomass, the most significant increases were observed in GA₃ and SL for HC soil, while for SC soil, IAA and BR exhibited positive effects. The levels of Cd, Pb and Zn in the shoots of S. alfredii treated with ABA and SL were noticeably greater than in the CK treatment in HC soil, while the uptake of metals were increased by IAA and CKs in SC soil. Combined with the results of biomass and metal contents in shoots, we found that ABA showed the highest Cd removal efficiency and the maximum Pb and Zn removal efficiency was observed with GA₃, which was 62.99%, 269.23%, and 41.18%, respectively higher than the control in HC soil. Meanwhile, compared to control, the maximum removal efficiency of Cd by IAA treatment (52.80%), Pb by JA treatment (165.1%), and Zn by BR treatment (44.97%) in the SC soil. Overall, our results suggested that these PGRs, especially, ABA, SL, IAA, BR and GA₃ had great potential in improving phytoremediation efficiency of S. alfredii grown in contaminated soils.