Accumulation and fractionation of rare earth elements (REEs) in wheat: controlled by phosphate precipitation, cell wall absorption and solution complexation

Previous studies on rare earth element (REE) bioaccumulation have focused on their accumulation rate and fractionation, but the processes involved remain unclear. In this study, the accumulation and fractionation of REEs in wheat (Triticum aestivum L.) were investigated using solution culture with exogenous mixed REEs. A decrease in REE contents was observed from the roots to the tops of wheat. Significant fractionations of REEs were found in wheat organs as compared to the exogenous mixed REEs. Middle REE (MREE, the elements from Sm to Gd) enrichment and an M-type tetrad effect (an effect that can cause a split of REE patterns into four consecutive convex segments) were observed in the roots, which were probably caused by phosphate precipitation of REEs in/on the roots and absorption of REEs to root cell walls. Light REE (LREE, the elements from La to Eu) and heavy REE (HREE, the elements from Gd to Lu) enrichments were observed in the stems and leaves, respectively, accompanied by conspicuous W-type tetrad effects (an opposite effect to the M-type tetrad effect) in the REE patterns. HREE enrichment decreased from the older to the younger leaves and increased upwards within a single leaf. It is suggested that the solution complexation that occurred in the xylem vessels plays an important role in REE fractionations in the above-ground parts of wheat.     

Fractionations of rare earth elements in plants and their conceptive model

Fractionations of rare earth elements (REEs) and their mechanisms in soybean were studied through application of exogenous mixed REEs under hydroponic conditions. Significant enrichment of middle REEs (MREEs) and heavy REEs (HREEs) was observed in plant roots and leaves respectively, with slight fractionation between light REEs (LREEs) and HREEs in stems. Moreover, the tetrad effect was observed in these organs. Investigations into REE speciation in roots and in the xylem sap using X-ray absorption spectroscopy (XAS) and nanometer-sized TiO2 adsorption techniques, associated with other controlled experiments, demonstrated that REE fractionations should be dominated by fixation mechanism in roots caused by cell wall absorption and phosphate precipitation, and by the combined effects of fixation mechanism and transport mechanism in aboveground parts caused by solution complexation by intrinsic organic ligands. A conceptive model was established for REE fractionations in plants based on the above studies.     

Fractionations of rare earth elements in plants and their conceptive model

Fractionations of rare earth elements (REEs) and their mechanisms in soybean were studied through application of exogenous mixed REEs under hydroponic conditions. Significant enrichment of middle REEs (MREEs) and heavy REEs (HREEs) was observed in plant roots and leaves respectively, with slight fractionation between light REEs (LREEs) and HREEs in stems. Moreover, the tetrad effect was observed in these organs. Investigations into REE speciation in roots and in the xylem sap using X-ray absorption spectroscopy (XAS) and nanometer-sized TiO₂ adsorption techniques, associated with other controlled experiments, demonstrated that REE fractionations should be dominated by fixation mechanism in roots caused by cell wall absorption and phosphate precipitation, and by the combined effects of fixation mechanism and transport mechanism in aboveground parts caused by solution complexation by intrinsic organic ligands. A conceptive model was established for REE fractionations in plants based on the above studies.     

Accumulation and fractionation of rare earth elements (REEs) in the naturally grown Phytolacca americana L. in southern China

The widespread use of rare earth elements (REEs) has resulted in problems for soil and human health. Phytolacca americana L. is a herbaceous plant widely distributed in Dingnan county of Jiangxi province, China, which is a REE mining region (ion absorption rare earth mine) and the soil has high levels of REEs. An investigation of REE content of P. americana growing naturally in Dingnan county was conducted. REE concentrations in the roots, stems, and leaves of P. americana and in their rhizospheric soils were determined. Results showed that plant REEs concentrations varied among the sampling sites and can reach 1040 mg/kg in the leaves. Plant REEs concentrations decreased in the order of leaf > root > stem and all tissues were characterized by a light REE enrichment and a heavy REE depletion. However, P. americana exhibited preferential accumulation of light REEs during the absorption process (from soil to root) and preferential accumulation of heavy REEs during the translocation process (from stem to leaf). The ability of P. americana to accumulate high REEs in the shoot makes it a potential candidate for understanding the absorption mechanisms of REEs and for the phytoremediation of REEs contaminated soil.     

镧在草-菇-土系统中的循环与生物富集效应

利用稀土镧肥种植牧草南非马唐,采用含镧牧草栽培杏鲍菇和以菇渣作为有机肥种植牧草进行连续性试验,研究镧在草-菇-土系统中的分配与生物富集情况。结果表明：施镧处理的南非马唐和杏鲍菇各器官的镧元素含量均高于不施镧处理,其中镧在牧草南非马唐中的分布为根>叶>茎,镧在杏鲍菇中的分布为菌盖>菌柄;外源镧进入土壤以后,南非马唐不同器官的镧元素生物富集系数均随着镧施入量的增加而增大,其中以根的镧生物富集系数最大,介于0.443—0.580之间。除高剂量（M4）处理外,叶和茎的镧生物富集系数不同处理间无显著差异,但根出现明显变化;含镧牧草栽培杏鲍菇和菇渣种植南非马唐后,不同器官的镧含量无显著增加,说明镧残留在草-菇-土系统中迁移转化效率降低。     

有机配体对稀土元素在小麦体内积累和分异的影响

采用营养液培养和添加外源混合稀土等方法,研究了有机配体柠檬酸、EDTA和DTPA对稀土元素在小麦的根和叶中积累与分异的影响。结果表明,低浓度有机配体对小麦根和叶中的稀土元素,尤其是轻稀土元素的积累有轻微的促进作用,随浓度的升高则表现出显著的降低作用。有机配体对重稀土的作用比轻稀土强,使根和叶中稀土元素的分布曲线向重稀土相对亏缺的方向发展。3种配体对轻、重稀土分异的作用强度为:EDTA>DTPA>柠檬酸。通过VM INTEQ计算表明,在EDTA和DTPA作用下小麦叶中稀土元素的积累与轻、重稀土的分异主要由溶液中呈自由离子态稀土元素的含量和组成控制;柠檬酸作用下小麦叶中稀土元素的变化与自由离子态稀土的含量和组成关系较弱,推测REE-柠檬酸络合物可被小麦直接吸收并运转到小麦的叶中。     

The variation of REE (rare earth elements) patterns in soil-grown plants: a new proxy for the source of rare earth elements and silicon in plants

Rare earth elements (REEs) in five species of soil-grown plants (Taxodium japonicum, Populus sieboldii, Sasa nipponica, Thea sinensis and Vicia villosa) and in the soil on which each plant grew were determined with an inductively coupled plasma mass spectrometer (ICP-MS) in order to observe the variation in the distribution of REEs and to elucidate their source in soil-grown plants. The plant samples were divided into root (secondary root and main root), trunk (stem) and leaf; the soils into water soluble (soil_{soluble fraction}), HCl and HNO₃ soluble (soil_{non-silicate fraction}) and HF soluble (soil_{silicate fraction}). The REE abundances of samples were compared using REE patterns where the abundances were normalized to those of a chondrite and plotted on a logarithmic scale against the atomic number. All the plants showed similar REE patterns independent of species and location, and a W-shape variation (W-type tetrad effect) and abundance depletion of cerium (negative Ce anomaly) were found in each REE patterns of plants, more conspicuous tetrad effect being observed in HREE (heavier rare earth elements) region than in LREE (lighter rare earth elements) region. The overall variation of REE patterns of each secondary root was not similar to that of soil_{soluble fraction}, but similar to that of soil_{silicate fraction} except for the tetrad effect and Ce anomaly. The REE patterns can be interpreted by the idea that plants of different species take in REEs and Si from different parts in the soil. The results of this study seem to imply that Sasa nipponica and Vicia villosa take in free REEs and Si rather directly from silicate in the soil, and that a majority of REEs and Si in Taxodium japonicum and Thea sinensis are originated from the soluble fraction in the soil.     

Rare earth elements in forest-floor herbs as related to soil conditions and mineral nutrition

Mixtures of rare earth elements (REEs) in fertilizers are widely used in Chinese agriculture to improve crop nutrition. REE concentrations in wild-growing plants, especially herbs, are little known. This study describes differences in the concentrations and proportions of REEs in eight forest-floor herbaceous plants and relates these differences to soil and mineral nutrient conditions. REEs studied were yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Leaf concentrations of sum REEs differed more than one order of magnitude between species, being highest in Anemone nemorosa (10.1 nmol/g dry mass) and lowest in Convallaria majalis (0.66 nmol/g) from the same site. Leaf concentrations of all REEs correlated positively (p<0.001), as did sum REE with calcium (Ca) and strontium (Sr) concentrations (p<0.001). A negative relationship (r=−0.83, (p<0.001) was measured between phosphorus (P) concentrations and sum REE concentrations in leaves. However, the proportions of the single REEs in the REE sum differed among species. In A. nemorosa, 57% of the molar REE sum was taken by Y+La, and only 21% by Ce. The other extreme was Maianthemum bifolium, with 37% La+Y and 41% Ce. These two species had 2.7–3.0% of the REE sum as heavier lanthanides, compared to 4.1–5.2% in the six other species. No clear relationship between soil properties or REE contents and leaf REE concentrations was detected. For La, however, an overrepresentation in leaves prevailed throughout all species compared to soils, whereas particularly Nd, Sm, and Tb had a lower proportion in the leaves of all species than in their soils. Possible uptake mechanisms of REEs in plants are discussed.     

Interactive effects of inorganic phosphate nutrition and carbon dioxide enrichment on assimilate partitioning in barley roots

The combined effects of inorganic phosphate (Pi) insufficiency and CO₂ enrichment on metabolite levels and carbon partitioning were studied using roots of 9-, 13- and 17-day-old barley seedlings (Hordeum vulgare L. cv. Brant). Plants were grown from seed in controlled environment chambers providing 36 ± 1 Pa (ambient) or 100 ± 2 Pa (elevated) CO₂ and either 1.0 mM (Pi sufficient) or 0.05 mM (Pi insufficient) Pi. When values were combined for both Pi treatments, plants grown under enhanced CO₂ showed increased root dry matter, adenylates (ATP + ADP), glutamine and non- structural carbohydrates other than starch. In contrast with shoots, enhanced CO₂ partially reversed the inhibition of root dry matter formation imposed by Pi insufficiency. The Pi-insufficient treatment also increased sucrose, glucose and fructose levels in barley roots. The Pi and CO₂ treatments were additive, so that the highest soluble carbohydrate levels were observed in roots of Pi-insufficient plants from the elevated CO₂ treatment. Pi limitation decreased dry matter formation, acid-extractable Pi, nitrate, hexose-phosphates, glutamate, glutamine and acid invertase activity of barley roots in plants grown in both ambient and elevated CO₂. Adenylate levels in roots were unaffected by the moderate Pi insufficiency described here. Thus, the reduced hexose-phosphate levels of Pi-insufficient roots were not likely to be the result of low adenylate concentrations. The above results suggest that the capacity of barley roots to utilize carbohydrates from the shoot is inadequate under both Pi-insufficient and CO₂-enriched treatments. In addition, the Pi and CO₂ treatments used here alter the nitrogen metabolism of barley roots. These findings further emphasize the importance of avoiding nutrient stress during CO₂ enrichment experiments.     

Rare earth elements (REEs) in PM10 and associated health risk from the polymetallic mining region of Nandan County,China

A total of 26 samples were collected from Dachang, Chehe and Liuzhai in Nandan County, China, in order to investigate the concentrations of rare earth elements (REEs) and associated health risk in particulate matter. The concentrations of REEs in Dachang (23.54 ng/m³), Chehe (20.29 ng/m³) were significantly higher than those in Liuzhai (8.1 ng/m³). The light rare earth elements (LREEs) account for 87.08%, 87.09%, and 86.17% of the total REE burden in PM10 at Dachang, Chehe, and Liuzhai, respectively, indicating that the distribution pattern of REEs in PM10 was characterized by the obvious fractionation of LREEs. Enrichment factor calculation indicated that EFs for La, Ce, Nd, Pr, and Nd in PM10 from Dachang and Chehe were greater than 2, indicating moderate enrichment. Source identification indicated that REEs in PM10 from Dacheng and Chehe originated from anthropogenic activities such as mining and smelting activities, while REEs in PM10 from Liuzhai were associated with natural sources, like soil erosion. Noncarcinogenic and carcinogenic risks associated with the exposure of REEs in PM10 were negligible based on the health risk assessment models. However, greater noncarcinogenic risk for children was found in studied areas compared with the adults.     

Ab initio calculation of the Zn isotope effect in phosphates, citrates, and malates and applications to plants and soil

Stable Zn isotopes are fractionated in roots and leaves of plants. Analyses demonstrate that the heavy Zn isotopes are enriched in the root system of plants with respect to shoots and leaves as well as the host soil, but the fractionation mechanisms remain unclear. Here we show that the origin of this isotope fractionation is due to a chemical isotope effect upon complexation by Zn malates and citrates in the aerial parts and by phosphates in the roots. We calculated the Zn isotope effect in aqueous citrates, malates, and phosphates by ab initio methods. For pH<5, the Zn isotopic compositions of the various parts of the plants are expected to be similar to those of groundwater. In the neutral to alkaline region, the calculations correctly predict that (66)Zn is enriched over (64)Zn in roots, which concentrate phosphates, with respect to leaves, which concentrate malates and citrates, by about one permil. It is proposed that Zn isotope fractionation represents a useful tracer of Zn availability and mobility in soils.     

氮磷亏缺对小麦TaIPS基因表达的影响

为了解小麦高效利用土壤磷的分子机理和实现对小麦缺磷的分子诊断,以普通小麦(Triticum aestivum L.)小偃54为材料,克隆了5个受缺磷诱导的IPS基因,同源比较结果显示,小麦IPS基因属于典型的受缺磷条件特异诱导的TPSI1/MT4小基因家族.对小麦根系和地上部的半定量RT-PCR研究结果表明,与全营养处理对照相比,3叶期小麦幼苗经过缺氮、缺磷和氮磷同时缺乏处理8d后,缺磷显著增加了根系中3个TaIPS1(TaIPS1.1、TaIPS1.2和TaIPS1.3)基因和地上部TaIPS1.1基因的表达,中度上调了根系中2个TaIPS2基因(TaIPS2.1和TaIPS2.2)的表达,轻度上调了地上部TaIPS1.2和2个TaIPS2基因的表达.通过比较5个基因在根系和地上部对缺磷的响应,认为TaIPS1.1是一个较理想的用于诊断小麦植株磷素丰缺的基因.缺氮不仅降低了3个TaIPS1基因在根系中的表达,并抑制了IPS基因对缺磷的响应.这一研究结果预示了TaIPS基因对低磷胁迫的响应依赖于植株体内的氮素营养状况.     

Zinc isotopic fractionation in Phragmites australis in response to toxic levels of zinc

Stable isotope signatures of Zn have shown great promise in elucidating changes in uptake and translocation mechanisms of this metal in plants during environmental changes. Here this potential was tested by investigating the effect of high Zn concentrations on the isotopic fractionation patterns of Phragmites australis (Cav.) Trin. ex Steud. Plants were grown for 40?d in a nutritive solution containing 3.2?μM (sufficient) or 2?mM (toxic) Zn. The Zn isotopic composition of roots, rhizomes, shoots, and leaves was analysed. Stems and leaves were sampled at different heights to evaluate the effect of long-distance transport on Zn fractionation. During Zn sufficiency, roots, rhizomes, and shoots were isotopically heavy (δ(66)Zn(JMC Lyon)=0.2‰) while the youngest leaves were isotopically light (-0.5‰). During Zn excess, roots were still isotopically heavier (δ(66)Zn=0.5‰) and the rest of the plant was isotopically light (up to -0.5‰). The enrichment of heavy isotopes at the roots was attributed to Zn uptake mediated by transporter proteins under Zn-sufficient conditions and to chelation and compartmentation in Zn excess. The isotopically lighter Zn in shoots and leaves is consistent with long-distance root to shoot transport. The tolerance response of P. australis increased the range of Zn fractionation within the plant and with respect to the environment.     

Carbohydrate and Proline Contents in Leaves, Roots, and Apices of Salt-Tolerant and Salt-Sensitive Wheat Cultivars1

Intra-specific variations in nonstructural carbohydrates and free proline were determined in leaves, apices, roots, and maturing seeds of two salt-tolerant cultivars (CR and Kharchia-65) and one salt-sensitive cv. Ghods of spring wheat (Triticum aestivum L.) grown in sand culture at various levels of salinity (0, 100, 200, and 300 mM NaCl and CaCl₂ at 5 : 1 molar ratio) under controlled environmental conditions. The levels of leaf, apex, and root ethanol-soluble carbohydrates, fructans, starch, and proline increased in line with elevating level of salinity in all three cultivars under investigation. The contents of proline, soluble and insoluble carbohydrates in the apex increased to levels exceeding those in the leaves and roots. Soluble carbohydrate content of salt-sensitive cv. Ghods was higher in the leaves, apices, and roots and lower in the maturing seeds than in the other cultivars at all levels of salinity except at 300 mM. The results show considerable variation in the amount of soluble, insoluble sugars, and proline among plant tissues and wheat genotypes in response to salinity. Higher soluble carbohydrates and fructan in leaves, roots and maturing seeds of stressed plants indicate that their accumulation may help plant to tolerate salinity. Salt-sensitive cv. Ghods accumulated less soluble sugars in the maturing seeds and higher soluble sugars in the apices, which might be used as an indicator in screening wheat genotypes for salinity tolerance.     

Recovery and Separation of Rare Earth Elements Using Salmon Milt

Recycling rare earth elements (REEs) used in advanced materials such as Nd magnets is important for the efficient use of REE resources when the supply of several REEs is limited. In this work, the feasibility of using salmon milt for REE recovery and separation was examined, along with the identification of the binding site of REEs in salmon milt. Results showed that (i) salmon milt has a sufficiently high affinity to adsorb REEs and (ii) the adsorption capacity of the milt is 1.04 mEq/g, which is comparable with that of commercial cation exchange resin. Heavier REEs have higher affinity for milt. A comparison of stability constants and adsorption patterns of REEs discussed in the literature suggests that the phosphate is responsible for the adsorption of REE in milt. The results were supported by dysprosium (Dy) and lutetium (Lu) L_III-edge extended x-ray absorption fine structure (EXAFS) spectroscopy. The REE-P shell was identified for the second neighboring atom, which shows the importance of the phosphate site as REE binding sites. The comparison of REE adsorption pattern and EXAFS results between the milt system and other adsorbent systems (cellulose phosphate, Ln-resin, bacteria, and DNA-filter hybrid) revealed that the coordination number of phosphate is correlated with the slope of the REE pattern. The separation column loaded with milt was tested to separate REE for the practical use of salmon milt for the recovery and separation of REE. However, water did not flow through the column possibly because of the hydrophobicity of the milt. Thus, sequential adsorption–desorption approach using a batch-type method was applied for the separation of REE. As an example of the practical applications of REE separation, Nd and Fe(III) were successfully separated from a synthetic solution of Nd magnet waste by a batch-type method using salmon milt.     

Root release and metabolism of organic acids in tea plants in response to phosphorus supply

Self-rooted, 10-month-old, uniform tea [Camellia sinensis (L.) O. Kuntze cv. Huangguanyin] plants were supplied for 17 weeks with 0, 40, 80, 160, 400, or 1000μM phosphorus (P) to investigate the effects of P supply on root citrate and malate release, the concentrations of malate and citrate and the activities of acid-metabolizing enzymes in leaves and roots. Root malate release and accumulation was induced by both 0 and 40μM P, while root citrate release and accumulation was induced only by 0μM P. Phosphorus-deficiency-induced malate and citrate release coincided with higher concentrations of root malate and citrate. The higher concentrations of malate and citrate were accompanied by increased activities of phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate phosphatase (PEPP), citrate synthase (CS) and NAD-malic enzyme (NAD-ME) and decreased activities of pyruvate kinase (PK), NADP-ME and NADP-isocitrate dehydrogenase (NADP-IDH) in roots. In contrast to roots, malate accumulated in the leaves only in response to 0μM P, and no change was observed in citrate levels. The P-deficiency-induced leaf malate accumulation coincided with increased activities of NADP-ME, NAD-ME and PK. Overall, the P-deficiency-induced changes in organic acid (OA) metabolism differed between roots and leaves. The high tolerance of tea plants to P-deficiency might be involved in two major processes: (a) increasing the availability of P by inducing root release of OA anions; and (b) improving the ability to use P efficiently by inducing bypass enzymes involved in tissue P economy.