Heavy Metals Accumulation, Toxicity and Detoxification in Plants

In recent years, heavy metals have been widely used in agricultural, chemical, domestic, and technological applications, causing environmental and soil contaminations. Heavy metals enter the plant system through soil or via the atmosphere, and can accumulate, affecting physiological processes, plant growth, yield, and human health if heavy metals are stored in edible tissues. Understanding the regulation mechanisms of plant heavy metals accumulation and partitioning is important to improve the safety of the food chain. In this Special Issue book, a total of 19 articles were included; four reviews covering phytoremediation, manganese phytotoxicity in plants, the effect of cadmium on plant development, the genetic characteristics of Cd accumulation, and the research status of genes and QTLs in rice, respectively, as well as fifteen original research articles, mainly regarding the impact of cadmium on plants. Cadmium was therefore the predominant topic of this Special Issue, increasing the attention of the research community on the negative impacts determined by cadmium or cadmium associated with other heavy metals. The articles have highlighted a great genetic variability, suggesting different possibilities for accumulation, translocation and the reduction or control of heavy metal toxicity in plants

10th International Phytotechnologies Conference Proceedings

The International Phytotechnology Society (IPS) is a nonprofit, worldwide professional society comprised of individuals and institutions engaged in the science and application of using plants to deal with environmental problems. IPS’s mission is to promote research, education, training, and application of those technologies that use plants to deal with problems of environmental contamination, carbon sequestration, alternative fuels, and ecological restoration. IPS is open to all researchers, practitioners, regulators, site owners and interested and concerned individuals who want to promote a natural way to deal with environmental problems. Phytotechnologies are the use of plants to remedy environmental problems. Plants can be used to clean or contain contaminants from soil, sediments, or water. Planted systems can degrade organic pollutants and extract heavy metals. Plants can be used to restore impacted ecosystems, provide biofuel, sequester carbon, improve air quality, and beneficially impact our environment. Advances have been made in research to identify and optimize plant capability to reduce risk and enhance environmental benefits. This Society is devoted to bringing together the science, engineering, and applications of phytotechnologies worldwide.https://digitalcommons.esf.edu/phytocon/1000/thumbnail.jp

Plant associated soil mechanisms of cadmium uptake and translocation in chicory and plantain : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Environmental Science at Massey University, Palmerston North, New Zealand

Cadmium (Cd) is a non-essential trace element that is extensively distributed in the environment. Cadmium is effectively absorbed by plant roots and transported to its aerial parts and plants growing in soils with high Cd concentration can accumulate Cd in their roots and shoots to levels which can threaten human and animal health. Elevated Cd concentrations in New Zealand agricultural soils are a function of the country’s long-term history of using Cd-contaminated phosphate fertiliser. Recent studies have identified that two forage species chicory (Cichorium intybus L.) and plantain (Plantago lanceolata L.), which are increasingly used in New Zealand agriculture, accumulate a significantly higher shoot Cd concentration than traditional pasture species. The variation in Cd accumulation between forage species suggests that different plants have different abilities to absorb Cd in roots and translocate this trace element from roots to shoots. Thus, Cd uptake and the potential translocation of Cd to aerial tissues deserves more research, particularly for forage species of economic importance to countries such as New Zealand, where agriculture is dependent on pastoral grazing systems. Information from such studies will be useful in mitigating the continuing risk of Cd transfer into the food chain. The overall aim of this thesis is to better understand Cd uptake and translocation mechanisms in chicory and plantain. Cadmium uptake by plant roots is a function of rhizosphere soil chemistry and the interaction between plant roots and soil solution. Plants exude Low Molecular Weight Organic Acids (LMWOA) into soil solution and these play a key role in regulating Cd bioavailability. A pot trial was conducted to evaluate the influence of increasing soil Cd concentration on the secretion of LMWOAs by chicory and plantain roots and to analyse their impact on plant Cd uptake. Chicory and plantain were grown under increasing Cd levels and showed variable secretion of oxalic, fumaric, malic and acetic acids as a function of Cd treatment. Results revealed that the primary cause for the significant increase of shoot and root Cd concentration in both chicory and plantain, as a function of treatment level, is the significantly greater bioavailable Cd concentration in soil solution with increasing Cd treatment level. The significantly higher shoot Cd accumulation in chicory (18.63 mg Cd/kg DW) than plantain (4.22 mg Cd/kg DW) at the highest tested soil Cd concentration (1.6 mg Cd/kg) can be explained by increased acetic acid and reduced fumaric acid excretion from chicory relative to plantain. Increased understanding of Cd translocation mechanisms in plants requires knowledge of the free Cd2+ ion concentration in xylem saps. However, the determination of low concentrations of free Cd2+ ions in a low volume of xylem sap poses an analytical challenge. To overcome this limitation, a thiosalicylic-acid-modified carbon-paste electrode was developed as an alternative and reliable measurement tool for the detection of free Cd2+ ions in environmental samples, including xylem saps. Compared to other Cd2+ ion ligands used to develop Cd2+-ion-specific electrodes in literature, thiosalicylic acid is a readily available solid, which is stable to air, making it a conveniently handled ligand. The developed electrode showed a lower detection limit of 11 μg Cd/L (0.1   10-6 mol Cd/L) with a linear range from 20 to 100 μg Cd/L (0.18   10-6 to 0.88   10-6 mol Cd/L). To the best of my knowledge, this is the first time a Cd2+ ion-specific electrode was developed to determine free Cd2+ ion concentration in plant xylem sap. The modified electrode has the ability to distinguish between total Cd and free Cd2+ in solution and measure only the free Cd2+ ions in environmental samples, including xylem sap, with high precision (RSD<5%). Subsequent analysis using the thiosalicylic acid modified electrode showed that Cd is mainly in a complex form in chicory and plantain xylem sap. Therefore, a glasshouse experiment was set up with six increasing Cd concentrations in hydroponic solution to assess the impact of LMWOA on xylem sap Cd translocation and shoot accumulation in chicory and plantain. Results revealed that both chicory and plantain showed variable production of oxalic, fumaric, citric, malic and acetic acids with increasing Cd concentration in the hydroponic media. The higher shoot Cd accumulation (by 28-208%) in chicory compared to plantain can be explained in terms of variations in LMWOA production between chicory and plantain. Functional relationship analysis showed that the primary cause for higher shoot Cd concentration in chicory relative to plantain is fumaric acid production in chicory xylem sap which may bind with Cd in chicory and translocate the metal towards shoots. To explore the specific role of fumaric and acetic acids on Cd uptake and translocation in chicory, a glasshouse experiment was conducted with the external addition of fumaric and acetic acid into the hydroponic solution. Increasing fumaric acid concentration in the hydroponic solution showed the ability to reduce Cd uptake and translocation in chicory with a maximum reduction achieved at 10 mg/L and 50 mg/L fumaric acid treatment for root and shoot Cd accumulation, (respectively) for a solution concentration of 1 mg/L Cd. The shoot Cd concentration significantly increased at lower acetic acid treatment levels (1 mg/L) and reduced with increasing acetic acid concentrations from 10 mg/L to 50 mg/L in the presence of 1 mg Cd/L solution concentration. However, the root Cd accumulation increased as a function of acetic acid concentration in the hydroponic solution up to 50 mg/L acetic acid treatment. The root: shoot Cd concentration ratio showed a significant positive correlation (R=0.729 P<0.05) with acetic acid treatments (up to 50 mg/L treatment). Chicory biomass significantly reduced at all LMWOA treatments compared to the control treatment in the presence of 1 mg Cd/L Cd level, showing that there was a limited potential ameliorative effect of LMWOA on Cd toxicity at any concentration for the experimental conditions used in this study. This study highlights that variations in plant root LMWOA secretion and xylem sap LMWOA production between chicory and plantain can explain the different shoot Cd accumulation characteristics of these two forage species. This work shows that fumaric acid plays a fundamental role in both Cd uptake and translocation in chicory, while such a role is not clear for plantain. Low secretion of fumaric acid by roots and production of fumaric acid in chicory xylem sap aid to increase shoot Cd accumulation in chicory compared to plantain while low acetic acid secretion by chicory roots supports the high shoot Cd accumulation in chicory compared to plantain. Future work is recommended to develop a new cultivar of chicory which express traits of variations in fumaric acid production and acetic acid production. Such work may yield new cultivars of chicory which restrict the translocation of Cd from roots to shoots in this important forage species. The future application of this work is to help develop strategies which could assist in mitigating high Cd accumulation in offal to maintain the standards of New Zealand’s food production

Comparative transcriptomic analysis reveals the coordinated mechanisms of Populus × canadensis ‘Neva’ leaves in response to cadmium stress

Cadmium (Cd), a heavy metal element has strong toxicity to living organisms. Excessive Cd accumulation directly affects the absorption of mineral elements, inhibits plant tissue development, and even induces mortality. Populus × canadensis ‘Neva’, the main afforestation variety planted widely in northern China, was a candidate variety for phytoremediation. However, the genes relieving Cd toxicity and increasing Cd tolerance of this species were still unclear. In this study, we employed transcriptome sequencing on two Cd?treated cuttings to identify the key genes involved in Cd stress responses of P. × canadensis ‘Neva’ l induced by 0 (CK), 10 (C10), and 20 (C20) mg/L Cd(NO3)2 4H2O. We discovered a total of 2,656 (1,488 up-regulated and 1,168 downregulated) and 2,816 DEGs (1,470 up-regulated and 1,346 down-regulated) differentially expressed genes (DEGs) between the CK vs C10 and CK vs C20, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses in response to the Cd stress indicated that many DEGs identified were involved in the catalytic activity, the oxidoreductase activity, the transferase activity, and the biosynthesis of secondary metabolites. Based on the enrichment results, potential candidate genes were identified related to the calcium ion signal transduction, transcription factors, the antioxidant defense system, and transporters and showed divergent expression patterns under the Cd stress. We also validated the reliability of transcriptome data with the real-time PCR. Our findings deeper the understanding of the molecular responsive mechanisms of P. × canadensis ‘Neva’ lon Cd tolerance and further provide critical resources for phytoremediation applications

The use of organic materials as amendments in the remediation of soils contaminated by lead, copper and zinc

The effectiveness of using amendments in metal stabilization was determined, and their potential for remediation of contaminated soil was evaluated. A combined approach of evaluating soil and plant metal availability as well as microbial respiration was used as an indication of effectiveness of metal immobilization. This was done by sorption studies, incubation experiments in contaminated media, and greenhouse pot experiments. Batch sorption studies were conducted to evaluate the ability of bone meal, composts, peat, coir and wood bark to sorb Pb, Cu and Zn. Single sorption carried out over a metal concentration range of 0.1 mmol 1-1 using 0.001M and 0.1M Ca(NO3)2 as background electrolytes showed very high sorption of Pb, Cu and Zn by the amendments, but high background salt led to a reduction in the amount of metal sorbed by all amendments. Of the amendments tested, coir, compost and wood bark were most effective. Sorption was evaluated by applying the Langmuir equation, and maximum sorption values were calculated for all amendments. Theoretical maximum sorption of Pb was 87 mg g-1 (0.42 mmol g-1) by coir and green waste compost; Cu was 30 mg g-1 (0.47 mmol g-1) by green waste and general compost, while maximum Zn sorption was 14 mg g-1 (0.21 mmol g-1) in composts, closely followed by 13 mg g-1 (0.19 mmol g-1) in coir. Sorption mixed metal solutions of Pb, Cu and Zn was evaluated in a background salt of 0.001M Ca(NO3)2 only, either at equimolar concentrations of 0.1 mmol 1-1, equimolar concentrations of 1 mmol 1-1, or combinations of metals at either concentration. Metal sorption was reduced in the presence of other metals when compared with sorption in single metal solution. Pb sorption in equimolar solution of 0.1 mmol 1-1 was approximately 50% of that in single solution, Cu was 35%, while Zn was 40% if wood bark was not considered (wood bark sorption of Zn in low equimolar metal was not different from that in single Zn solution). The effect of metals on microbial respiration was evaluated in metal spiked amendments over a ten week period using metal solutions of Pb, Cu and Zn as contaminants. Pb, Cu and Zn toxicity led to an inhibition in CO2 evolved in all amendments, as addition of any amount of Pb, Cu or Zn led to a decrease I amount of CO2 evolved when compared with the non-contaminated amendments. The effect of increasing metal toxicity on CO2 evolution was reflected best in coir, where inhibition increased with an increase I metal concentration. In other contaminated amendments however, the inhibition was highly pronounced once there was metal contamination regardless of the metal concentration

The Transfer and Fate of Cadmium and Zinc from Sewage Sludge Amended Agricultural Soil in an Arthropod Food Chain

The re-cycling of sewage sludgeis the most significant entry point for trace metalsin to the agroecosystem. However, the investigation of the transfer and fate of trace metals in soil-plant-arthropod systems in an agricultural context has received little attention, despite the potential threat that secondary toxicity to predatory arthropodsposes to the biological control of crop pests.In this study, an agricultural soil was amended with sewage sludgeat rates up to an equivalent of 100 t (dry solids) ha-1.The subsequenttransfer of Zn and Cd through an ecologically relevant soil-crop-aphid-arthropod predator systemwasinvestigatedin a series of pot trials. Results show that Zn was transferred to a greater extent than Cd between all components of the system, except between the roots and shoots. Cadmium was only biomagnified in roots and was biominimised in shoots, aphids and ladybirds. Zinc was biomagnified in roots, shoots, andaphids compared to the soil, but concentrationsin ladybirds were similar to those in the aphids they consumed.Differences between winter and spring wheat were found to have a larger influence on the transfer of Cd and Zn in the systemthan differences between winter wheat and spring barley. It was also shown that the rose grain aphid (Metopolophium dirhodum) accumulateshigher concentrationsof Cd than the grain aphid (Sitobion avenae).Whilst concentrationsof Zn did not differ between the two species of aphid, concentrations in M. dirhodum appearedto be more closely regulated than in S.avenae. Consumptionof S.avenaeby the fourth instar larvae of the seven-spottedladybird (Coccinella septempunctata)did not result in significant differences between treatmentsin newly emergedadults.This was partly the result of the sequestrationof the two metalsin the pupal exuviae. This mechanism had a greater effect on the Cd concentrations in newly emerged adult ladybirds than on Zn concentrations.In a further experiment,there appeared to be no pathway for the transfer of Cd from aphids to adult ladybirds, but a pathway was indicatedfor Zn. The implications of the results are discussed in relation to the agroecosystem andthe wider environment

Phytoremediation potential for co-contaminated soils

Phytoremediation is a plant-based remediation process for treating contaminated soils. The overall aim of this thesis was to determine whether phytoremediation could be applied to co-contaminated soils. Copper (Cu) and pyrene, and Chromium (Cr) and Benzo[a]pyrene (B[a]P) were used as contaminants. The first study involved the joint effect of Cu and pyrene or Cr and B[a]P on the early seedling growth of Lolium perenne. Results suggest that co-contamination showed several types of interactions for seedling growth with different combinations of the pollutants. The second study involved the role Brassica juncea and Zea mays during the remediation of Cu and/or pyrene, and Cr and/or B[a]P co-contaminated soils respectively. Brassica juncea and Z. mays showed contrasting results for metal and polycyclic aromatic hydrocarbon (PAH) remediation. The third study compared freshly spiked soils and aged soils. Ageing affected the plant biomass, metal phytoextraction and PAH dissipation in different ways when compared to fresh soils. Finally, the efficiency of ethylenediaminetetraacetic acid-EDTA and/or citric acid as chelators in co-contaminated soils was studied. The combined application of EDTA and citric acid was more effective in co-contaminated soils. The overall findings from the four studies suggest that phytoremediation could be applied to co-contaminated soils

Fitorremediación de cadmio con especies herbáceas en diferentes tipos de suelo en condiciones de invernadero, Amazonas

El cadmio es un metal que afectan los recursos naturales, plantas y seres humanos. Ante ello, se ha buscado diferentes métodos para mitigar el problema, uno de ellos es la fitorremediación que hace uso de especies que tienen el potencial de acumular el metal pesado en sus tejidos vegetales, además de ello es alternativa amigable con el ambiente. Esta investigación tuvo por objetivo evaluar la capacidad de fitorremediación de cadmio en dos especies herbáceas con diferentes sustratos en condiciones de invernadero. Se utilizó dos especies herbáceas, el cadillo (Bidens pilosa L.) y la hierba mora (Solanum sp.), que fueron sembradas en dos tipos de sustrato con pH 6.27 y 5.53, agregando diferentes concentraciones de cloruro de cadmio (0, 5 y 10 ppm) dentro de un invernadero; se tuvo 12 tratamientos con 5 repeticiones, teniendo así 60 unidades experimentales. Se evaluó parámetros morfológicos, fisiológicas y concentraciones de cadmio en las especies herbáceas y el sustrato. En los resultados obtenidos el cadillo sembrado en el sustrato con pH 5.53 con 10 ppm CdCl2 tuvo mayor altura con 27.18 cm, con 19 brotes en peso fresco foliar 26.70 g y peso seco foliar 5.77 g; mientras que en las variables evaluadas de raíz la hierba mora tuvo mayor peso fresco con 20 g, peso seco 2.57, el longitud con 35.80 y en número de raíces con 23.80; en las variables fisiológicas, el índice de clorofila y conductancia estomática el cadillo tuvo el mayor resultado con 62.79 y 488.02 respectivamente, en el sustrato donde no se aplicó CdCl2; en la acumulación de cd en la planta el cadillo acumuló 4.12 ppm en la parte foliar y la hierba mora 2.12 en la parte radicular. El factor de bioacumulación y de transferencia indican que ambas especies son plantas hiperacumuladoras y que en pH mayores tienden a acumular más Cd. En conclusión, la fitorremediación con estas 2 especies herbáceas es una alternativa óptima, es una técnica sostenible

Carbonaceous soil amendments to reduce plant uptake of Cd in NZ’s agricultural systems

Cadmium (Cd) is a non‐essential trace element that accumulates in agricultural soils through the application of Cd-rich phosphate fertiliser and industrial activity. Plants can accumulate Cd to concentrations that sometimes exceed food safety standards presenting a human health risk. Cadmium is readily taken up by plants and can be transferred to grazing animals. In many agricultural systems, Cd concentrations in leafy vegetables and the offal products of grazing animals are at or above food safety standards. There is no practical means of removing Cd from contaminated agricultural soils. Various soil amendments have been used to reduce plant Cd-uptake, but these have mostly focused on heavily contaminated soils and mine tailings. This work aimed to determine whether low cost carbonaceous amendments could effectively reduce Cd uptake by crop plants in agricultural soils with moderate levels of Cd contamination. We used two contrasting market garden soils (a silt loam and a brown granular allophanic soil) for these experiments, where Cd concentrations in selected vegetables were occasionally above food safety standards (0.1 mg/kg fresh weight). Batch sorption experiments were used to determine the ability of the soils and potential soil amendments to bind Cd from a solution of 0.05M Ca(NO3)2. The sorption experiments revealed that composts and lignite bound an order of magnitude more Cd than soils and other potential soil amendments. For all materials, sorption increased with increasing pH of the ambient solution up to a pH of 7.5. Pot trials were used to determine the effect of various composts, lignite and lime on the uptake of Cd by spinach (Spinacia oleracea L.), lettuce (Lactuca sativa L.), onion (Allium cepa L.) and potato (Solanum tuberosum L.). All composts, added at a rate of 2.5% or 5% (w/w) reduced plant Cd uptake by up to 60%. The composts did not induce a deficiency in the uptake of plant nutrients, including essential trace elements such as zinc, and copper. Composts invariably increased plant biomass. An incubation experiment, including treatments with increased temperature and nitrogen as well as regular disturbance, was used to determine the likely longevity of the Cd-immobilising properties of the composts. After one year of incubation, there was no significant release of Cd from the compost-amended soils, despite a significant reduction in soil carbon. A pot trial using incubated soil also revealed that the beneficial effects of compost for reducing plant Cd uptake persisted for at least one year. The results indicated that mechanical disturbance of the soil may have resulted in the dissolution / suspension of iron moieties that subsequently occluded Cd on the surfaces of soil colloids. Lignite generally reduced plant biomass and its effect on plant Cd-uptake was variable. In some cases, lignite caused a significant increase in plant Cd uptake. This was attributed to acidification, probably caused by oxidation of sulphide compounds in the lignite. Potentially, lignite-lime mixtures may be effective, however, the costs of lignite are significantly greater than composts. Using lime to increase the soil pH from 6.0 to 6.5 generally reduced the Cd concentration in soil solution and in vegetables. However, this effect was not consistent, with some treatments causing an increase in plant uptake. Moreover, liming significantly reduced the uptake of essential micronutrients, especially zinc, which offsets its usefulness as a tool to reduce Cd uptake. I conclude that biological wastes, especially composts, are an underutilised resource that can not only reduce plant Cd-uptake but also improve plant production. Future research should include field trials to determine the performance of Cd in field conditions as well as agronomic practicalities