Leaf-applied sodium chloride promotes cadmium accumulation in durum wheat grain

Cadmium (Cd) accumulation in durum wheat grain is a growing concern. Among the factors affecting Cd accumulation in plants, soil chloride (Cl) concentration plays a critical role. The effect of leaf NaCl application on grain Cd was studied in greenhouse-grown durum wheat (Triticum turgidum L. durum, cv. Balcali-2000) by immersing (10 s) intact flag leaves into Cd and/or NaCl-containing solutions for 14 times during heading and dough stages. Immersing flag leaves in solutions containing increasing amount of Cd resulted in substantial increases in grain Cd concentration. Adding NaCl alone or in combination with the Cd-containing immersion solution promoted accumulation of Cd in the grains, by up to 41%. In contrast, Zn concentrations of grains were not affected or even decreased by the NaCl treatments. This is likely due to the effect of Cl complexing Cd and reducing positive charge on the metal ion, an effect that is much smaller for Zn. Charge reduction or removal (CdCl₂⁰ species) would increase the diffusivity/lipophilicity of Cd and enhance its capability to penetrate the leaf epidermis and across membranes. Of even more significance to human health was the ability of Cl alone to penetrate leaf tissue and mobilize and enhance shoot Cd transfer to grains, yet reducing or not affecting Zn transfer.     

Production of organic acids and adsorption of Cd on roots of durum wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> L. var. <Emphasis Type="Italic">durum</Emphasis>)

A number of isolines of durum wheat (Triticum turgidum var. durum) differ in their translocation of Cd. In the field, the high isolines accumulate twice the Cd in leaves and grain when compared to the low isolines. The hypothesis that differential accumulation of Cd is associated with differential production of organic acids was tested by measuring Cd content in tissues, Cd partitioning within the root, and organic acids in tissues. In solution culture, the high and low isolines of W9261-BG did not differ in any of the variables measured. Within W9260-BC, the low isoline had half the Cd in its shoot, 30% more tightly bound Cd in the root and higher concentrations of fumaric, malic, and succinic acids in the root compared to the high isoline. Differential Cd accumulation may be linked to differential adsorption and retention of Cd in the roots of the low Cd-accumulating isolines, possibly via chelation with organic acids.     

Zinc effects on cadmium accumulation and partitioning in near-isogenic lines of durum wheat that differ in grain cadmium concentration

Here, we examined the effectiveness of two approaches for reducing cadmium (Cd) accumulation in durum wheat (Triticum turgidum L. var durum) grain: the application of supplemental zinc (Zn), and the use of cultivars exhibiting reduced grain Cd concentrations. Two durum wheat near-isogenic lines (NIL) that differ in grain Cd accumulation were grown to maturity in solution culture containing a chelating agent to buffer the free activities of Zn and Cd at levels approximating those of field conditions. The low Cd accumulating (L-Cd) isoline had Cd concentrations, in grains and shoot parts, which were 60-70% lower than those of the high Cd accumulating (H-Cd) isoline. Increasing the Zn activities in the nutrient solution from deficient to sufficient levels reduced the concentration of Cd in grains and vegetative shoot parts of both isolines. The results suggest that supplemental Zn reduces Cd tissue concentrations by inhibiting Cd uptake into roots. Cd partitioning patterns between roots and shoots and between spike components suggest that the physiological basis for the low Cd trait is related to the compartmentation or symplasmic translocation of Cd.     

Characterization of cadmium uptake, translocation and storage in near-isogenic lines of durum wheat that differ in grain cadmium concentration

Here we examined several physiological properties of two near-isogenic lines of durum wheat (Triticum turgidum var. durum) that differ in grain cadmium accumulation, to identify the function of a gene locus that confers differential grain Cd concentrations. Time- and concentration-dependent uptake and translocation studies using 109Cd were conducted on nutrient solution-grown seedlings. Root extracts were analysed by inductively coupled plasma emission spectrometry, gel filtration and capillary electrophoresis to determine the interaction between Cd and phytochelatins (PCs) in storage of Cd in roots. The two isolines did not differ in time- or concentration-dependent root Cd uptake, but the low grain-Cd-accumulating isoline showed decreased movement of Cd from roots to shoots. All buffer-soluble Cd extracted from roots of both isolines was in the form of a low-molecular-weight PC-containing complex. The data suggest that PC synthesis is not a limiting factor in the differential storage of Cd in roots, and that movement of Cd through the root and into the transpiration stream may be the cause of differential Cd partitioning in the two isolines.     

Cadmium uptake and translocation in seedlings of near isogenic lines of durum wheat that differ in grain cadmium accumulation

Background  

Cadmium (Cd) concentrations in durum wheat (Triticum turgidum L. var durum) grain grown in North American prairie soils often exceed proposed international trade standards. To understand the physiological processes responsible for elevated Cd accumulation in shoots and grain, Cd uptake and translocation were studied in seedlings of a pair of near-isogenic durum wheat lines, high and low for Cd accumulation in grain.     

Phytochelatin and cadmium accumulation in wheat

Cadmium (Cd) is a nonessential heavy metal that can be harmful at low concentrations in organisms. Therefore, it is necessary to decrease Cd accumulation in the grains of wheats aimed for human consumption. In response to Cd, higher plants synthesize sulphur-rich peptides, phytochelatins (PCs). PC–heavy metal complexes have been reported to accumulate in the vacuole. Retention of Cd in the root cell vacuoles might influence the symplastic radial Cd transport to the xylem and further transport to the shoot, resulting in genotypic differences in grain Cd accumulation. We have studied PC accumulation in 12-day-old seedlings of two cultivars of spring bread wheat (Triticum aestivum), and two spring durum wheat cultivars (Triticum turgidum var. durum) with different degrees of Cd accumulation in the grains. Shoots and roots were analysed for dry weight, Cd and PC accumulation. There were no significant differences between the species or the varieties in the growth response to Cd, nor in the distributions of PC chain lengths or PC isoforms. At 1 μM external Cd, durum wheat had a higher total Cd uptake than bread wheat, however, the shoot-to-root Cd concentration ratio was higher in bread wheat. When comparing varieties within a species, the high grain Cd accumulators exhibited lower rates of root Cd accumulation, shoot Cd accumulation, and root PC accumulation, but higher shoot-to-root Cd concentration ratios. Intraspecific variation in grain Cd accumulation is apparently not only explained by differential Cd accumulation as such, but rather by a differential plant-internal Cd allocation pattern. However, the higher average grain Cd accumulation in the durum wheats, as compared to the bread wheats, is associated with a higher total Cd accumulation in the plant, rather than with differential plant-internal Cd allocation. The root-internal PC chain length distributions and PC–thiol-to-Cd molar ratios did not significantly differ between species or varieties, suggesting that differential grain Cd accumulation is not due to differential PC-based Cd sequestration in the roots.     

The OsNRAMP1 iron transporter is involved in Cd accumulation in rice

Cadmium (Cd) is a heavy metal toxic to humans and the accumulation of Cd in the rice grain is a major agricultural problem, particularly in Asia. The role of the iron transporter OsNRAMP1 in Cd uptake and transport in rice was investigated here. An OsNRAMP1:GFP fusion protein was localized to the plasma membrane in onion epidermal cells. The growth of yeast expressing OsNRAMP1 was impaired in the presence of Cd compared with yeast transformed with an empty vector. Moreover, the Cd content of OsNRAMP1-expressing yeast exceeded that of the vector control. The expression of OsNRAMP1 in the roots was higher in a high Cd-accumulating cultivar (Habataki) than a low Cd-accumulating cultivar (Sasanishiki) regardless of the presence of Cd, and the amino acid sequence of OsNRAMP1 showed 100% identity between Sasanishiki and Habataki. Over-expression of OsNRAMP1 in rice increased Cd accumulation in the leaves. These results suggest that OsNRAMP1 participates in cellular Cd uptake and Cd transport within plants, and the higher expression of OsNRAMP1 in the roots could lead to an increase in Cd accumulation in the shoots. Our results indicated that OsNRAMP1 is an important protein in high-level Cd accumulation in rice.     

不同水分管理下优质稻花后植株碳氮流转与籽粒生长及品质的相关性

以桂华占、八桂香为材料,在干湿交替灌溉、亏缺灌溉、淹水灌溉3种水分条件下,研究优质稻花后植株碳氮流转与籽粒生长及品质的相关性。结果表明:不同水分管理下,桂华占和八桂香花后碳氮流转与籽粒的生长间存在密切相关。主要表现在:(1)茎鞘和叶片干物质转运对籽粒干物质积累的贡献率为16.86%～25.68%,花后茎叶干物质运转速度和运转率与籽粒起始灌浆势呈显著甚至极显著正相关;籽粒最大灌浆速率、活跃灌浆期、持续灌浆时间与叶片干物质运转速度和运转率呈极显著正相关,与茎鞘干物质运转速度和运转率呈极显著负相关;(2)茎鞘碳同化物转运对籽粒的产量和淀粉产量的贡献率则为干湿交替灌溉>亏缺灌溉>淹水灌溉;但叶片碳同化物转运对籽粒的产量和淀粉产量的贡献率则为淹水灌溉>亏缺灌溉>干湿交替灌溉;茎叶可溶性糖积累量的减少和籽粒直链淀粉含量和积累量增加是同步的,且茎叶可溶性糖积累量快速递减期(花后3～12d)与直链淀粉含量和积累量快速递增期(花后6～12d)同步;(3)茎鞘和叶片氮素转运对籽粒氮素积累的贡献率为44.05%～117.66%,叶片总氮转运对籽粒氮素积累的贡献率大于茎鞘,茎鞘和叶片氮同化物对籽粒氮素的贡献率以淹水灌溉处理的最大,亏缺灌溉处理的次之,干湿交替灌溉处理的最小。     

Uptake and retranslocation of leaf-applied cadmium (109Cd) in diploid, tetraploid and hexaploid wheats

Uptake and retranslocation of leaf-applied radiolabeled cadmium (109Cd) was studied in three diploid (Triticum monococcum, AA), four tetraploid (Triticum turgidum, BBAA) and two hexaploid (Triticum aestivum, BBAADD) wheat genotypes grown for 9 d under controlled environmental conditions in nutrient solution. Among the tetraploid wheats, two genotypes were primitive (ssp. dicoccum) and two genotypes modern wheats (ssp. durum). Radiolabelled Cd was applied by immersing the tips (3 cm) of mature leaf into a 109Cd radiolabelled solution. There was a substantial variation in the uptake and export of 109Cd among and within wheat species. On average, diploid wheats (AA) absorbed and translocated more 109Cd than other wheats. The largest variation in 109Cd uptake was found within tetraploid wheats (BBAA). Primitive tetraploid wheats (ssp. dicoccum) had a greater uptake capacity for 109Cd than modern tetraploid wheats (ssp. durum). In all wheats studied, the amount of the 109Cd exported from the treated leaf into the roots and the remainder of the shoots was poorly related to the total absorption. For example, bread wheat cultivars were more or less similar in total absorption, but differed greatly in the amount of 109Cd retranslocated. The diploid wheat genotype 'FAL-43' absorbed the lowest amount of 109Cd, but retranslocated the greatest amount of 109Cd in roots and remainder of shoots. The results indicate the existence of substantial genotypic variation in the uptake and retranslocation of leaf-applied 109Cd. This variation is discussed in terms of potential genotypic differences in binding of Cd to cell walls and the composition of phloem sap ligands possibly affecting Cd transport into sink organs.     

Sulphur accumulation and redistribution in wheat (Triticum aestivum): a study using stable sulphur isotope ratios as a tracer system

Wheat plants were grown hydroponically and fed with two sulphate sources differing in stable isotope composition, one having a δ ³⁴S of 13·7‰ and the other 4·1‰. Plant sulphur (S) isotope ratios were determined using an on-line continuous flow-isotope ratio mass spectrometer. This method greatly simplified the procedure for the measurement of S isotope ratios, and was found to be precise for samples containing > 1 mg S g^–1 dry weight. The δ ³⁴S values of plant shoots, which had been grown on a single sulphate source, were very close to the source values, suggesting little isotope fractionation during sulphate uptake and transport from roots to shoots. By changing the sulphate sources at different growth stages, it was possible to estimate S accumulation and redistribution within different plant parts. At maturity, wheat grain derived 14, 30, 6 and 50% of its S from the accumulation during the following successive growth stages: between emergence and early stem extension, between stem extension and flag leaf emergence, between flag leaf emergence and anthesis, and after anthesis, respectively. It was estimated that 39, 32 and 52% of the S present in the flag leaves, older leaves and stems, respectively, at anthesis, was exported during the postanthesis period. These results demonstrate considerable cycling of S within wheat plants, and highlight the importance of S uptake after anthesis to the accumulation of S in grain under the experimental conditions employed.     

Genetic linkage map construction and QTL mapping of cadmium accumulation in radish (Raphanus sativus L.)

Cadmium (Cd) is a widespread soil pollutant and poses a significant threat to human health via the food chain. Large phenotypic variations in Cd concentration of radish roots and shoots have been observed. However, the genetic and molecular mechanisms of Cd accumulation in radish remain to be elucidated. In this study, a genetic linkage map was constructed using an F(2) mapping population derived from a cross between a high Cd-accumulating cultivar NAU-Dysx and a low Cd-accumulating cultivar NAU-Yh. The linkage map consisted of 523 SRAP, RAPD, SSR, ISSR, RAMP, and RGA markers and had a total length of 1,678.2 cM with a mean distance of 3.4 cM between two markers. All mapped markers distributed on nine linkage groups (LGs) having sizes between 134.7 and 236.8 cM. Four quantitative trait loci (QTLs) for root Cd accumulation were mapped on LGs 1, 4, 6, and 9, which accounted for 9.86 to 48.64 % of all phenotypic variance. Two QTLs associated with shoot Cd accumulation were detected on LG1 and 3, which accounted for 17.08 and 29.53 % of phenotypic variance, respectively. A major-effect QTL, qRCd9 (QTL for root Cd accumulation on LG9), was identified on LG 9 flanked by NAUrp011_754 and EM5me6_286 markers with a high LOD value of 23.6, which accounted for 48.64 % of the total phenotypic variance in Cd accumulation of F(2) lines. The results indicated that qRCd9 is a novel QTL responsible for controlling root Cd accumulation in radish, and the identification of specific molecular markers tightly linked to the major QTL could be further applied for marker-assisted selection (MAS) in low-Cd content radish breeding program.     

水稻籽粒发育过程中各器官镉积累量的变化及其与基因型和土壤镉水平的关系

以镉（Cd）积累潜力不同的2个籼稻品种为材料,研究了籽粒发育过程中各器官中Cd含量的动态变化及其与土壤中Cd含量的关系。结果表明,在含Cd的生长环境中,水稻根系中的Cd含量在整个生育期中保持稳定增长的趋势,而茎叶、穗轴和稻壳等器官在营养生长阶段积累了大量的Cd,然后在籽粒充实过程中向外输出,其中旗叶和稻壳中的Cd输出率在50%以上,明显高于其他营养器官。根系和叶片中的Cd含量与土壤中的Cd含量呈高度线性相关,茎秆和籽粒中的Cd含量与土壤中的Cd含量呈抛物线相关,说明根系、茎秆、叶片等营养器官对Cd有储存和“过滤”作用。低积累品种‘X24’穗轴中的Cd含量明显低于高积累品种‘T705’,说明营养器官中的Cd向籽粒中转运的速率是决定籽粒中Cd积累量的关键因素。但是,只有当土壤中的Cd含量为0.3~1.2 mg＆#183;kg-1时,低积累品种精米中的Cd含量才会显著低于高积累品种;当土壤中的Cd含量高于2.4 mg＆#183;kg-1时,2个品种精米中的Cd含量没有显著差异。     

Comparison of heavy metal accumulation and cadmium phytoextraction rates among ten leading tobacco (Nicotiana tabacum L.) cultivars in China

Abstract

Cadmium (Cd) contamination is one of the most serious global environmental problems, and phytoremediation, which uses Cd-accumulator plants, is potentially one of the sustainable solutions. Pot experiments with natural and Cd-amended soils were conducted to investigate the accumulation of heavy metals in 10 leading cultivars of tobacco in China. The extraction ability and profiles of Cd accumulation among plant organs were also analyzed. The tobacco roots accumulated cobalt, nickel, and Cd, while the leaf highly bioaccumulated Cd and lowly accumulated zinc, selenium and mercury. The transport from the tobacco stem to the leaf plays a critical role in the accumulation of these elements. The ratios of Cd concentration in the leaves at lower, middle and upper positions were comparatively stable. The high Cd-extracting cultivars were “Hongda”, “NC89” and “Zhongyan 100” when grown in normal soils, “CuiBi 1” and “Hongda” in moderately contaminated soils, and “YuYan 87”, “LongJiang 851” and “K326” in severely contaminated soils. Tobacco leaves could accumulate about 80% of the total Cd extracted from the soil by the plant. Considering the Cd-extraction limitations exhibited by leading tobacco cultivars, screening of germplasm resources for high or low levels of Cd-accumulation is still an important target for the future.     

Carbon isotope discrimination and mineral composition of three organs in durum wheat genotypes grown under Mediterranean conditions

Carbon isotope discrimination (delta) has been proposed as a good criterion for transpiration efficiency and grain yield improvement. Its measurement, however, remains very expensive. Ash content (ma) has been proposed as an alternative criterion for delta in bread wheat and barley. The aims of this study were (i) to analyse the relationships between delta and mineral composition in different durum wheat plant parts and (ii) to compare the variation of these traits between landraces and improved varieties from different geographic origins. For this purpose, delta, ma, and composition in four minerals (K, Mg, P and Si) were assessed in flag leaves and awns at anthesis, and in mature grains of ten durum wheat genotypes grown under rainfed Mediterranean conditions. The three plant parts differed significantly for the measured traits. Significant correlations were noted between delta and ma in the flag leaf and in the grain. Silicon content in flag leaves and potassium content in awns were also positively related to delta of the considered plant part. The coefficient of correlation between delta and ma was generally higher than that observed between individual mineral content and delta, suggesting that ma is the better alternative criterion for delta. In addition, grain yield was related to grain delta and both ma and potassium content in awns. Harvest index was correlated with delta and ma of grain and flag leaf. These results emphasised that ma values in flag leaf and grain represent the efficiency of carbon partitioning to the grain. Improved varieties showed higher delta and ma values than landraces. Differences between Middle-East and West Mediterranean genotypes for the measured traits were also presented and discussed.     

Quantitative Trait Loci and Inter-Organ Partitioning for Essential Metal and Toxic Analogue Accumulation in Barley

The concentrations of both essential nutrients and chemically similar toxic analogues accumulated in cereal grains have a major impact on the nutritional quality and safety of crops. Naturally occurring genetic diversity can be exploited for the breeding of improved varieties through introgression lines (ILs). In this study, multi-element analysis was conducted on vegetative leaves, senesced flag leaves and mature grains of a set of 54 ILs of the wild ancestral Hordeum vulgare ssp. spontaneum in the cultivated variety Hordeum vulgare ssp. vulgare cv. Scarlett. Plants were cultivated on an anthropogenically heavy metal-contaminated soil collected in an agricultural field, thus allowing simultaneous localization of quantitative trait loci (QTL) for the accumulation of both essential nutrients and toxic trace elements in barley as a model cereal crop. For accumulation of the micronutrients Fe and Zn and the interfering toxin Cd, we identified 25, 16 and 5 QTL, respectively. By examining the gene content of the introgressions, we associated QTL with candidate genes based on homology to known metal homeostasis genes of Arabidopsis and rice. Global comparative analyses suggested the preferential remobilization of Cu and Fe, over Cd, from the flag leaf to developing grains. Our data identifies grain micronutrient filling as a regulated and nutrient-specific process, which operates differently from vegetative micronutrient homoeostasis. In summary, this study provides novel QTL for micronutrient accumulation in the presence of toxic analogues and supports a higher degree of metal specificity of trace element partitioning during grain filling in barley than previously reported for other cereals.     

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.     

Characterization of Cadmium Binding, Uptake, and Translocation in Intact Seedlings of Bread and Durum Wheat Cultivars

High Cd content in durum wheat (Triticum turgidum L. var durum) grain grown in the United States and Canada presents potential health and economic problems for consumers and growers. In an effort to understand the biological processes that result in excess Cd accumulation, root Cd uptake and xylem translocation to shoots in seedlings of bread wheat (Triticum aestivum L.) and durum wheat cultivars were studied. Whole-plant Cd accumulation was somewhat greater in the bread wheat cultivar, but this was probably because of increased apoplastic Cd binding. Concentration-dependent ¹⁰⁹Cd²⁺-influx kinetics in both cultivars were characterized by smooth, nonsaturating curves that could be dissected into linear and saturable components. The saturable component likely represented carrier-mediated Cd influx across root-cell plasma membranes (Michaelis constant, 20–40 nm; maximum initial velocity, 26–29 nmol g⁻¹ fresh weight h⁻¹), whereas linear Cd uptake represented cell wall binding of ¹⁰⁹Cd. Cd translocation to shoots was greater in the bread wheat cultivar than in the durum cultivar because a larger proportion of root-absorbed Cd moved to shoots. Our results indicate that excess Cd accumulation in durum wheat grain is not correlated with seedling-root influx rates or root-to-shoot translocation, but may be related to phloem-mediated Cd transport to the grain.     

Contributions of apoplasmic cadmium accumulation,antioxidative enzymes and induction of phytochelatins in cadmium tolerance of the cadmium-accumulating cultivar of black oat (<Emphasis Type="Italic">Avena strigosa</Emphasis> Schreb.)

The contributions of cadmium (Cd) accumulation in cell walls, antioxidative enzymes and induction of phytochelatins (PCs) to Cd tolerance were investigated in two distinctive genotypes of black oat (Avena strigosa Schreb.). One cultivar of black oat ‘New oat’ accumulated Cd in the leaves at the highest concentration compared to another black oat cultivar ‘Soil saver’ and other major graminaceous crops. The shoot:root Cd ratio also demonstrated that ‘New oat’ was the high Cd-accumulating cultivar, whereas ‘Soil saver’ was the low Cd-accumulating cultivar. Varied levels of Cd exposure demonstrated the strong Cd tolerance of ‘New oat’. By contrast, low Cd-accumulating cultivar ‘Soil saver’ suffered Cd toxicity such as growth defects and increased lipid peroxidation, even though it accumulated less Cd in shoots than ‘New oat’. Higher activities of ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase (EC 1. 15. 1. 1) were observed in the leaves of ‘New oat’ than in ‘Soil saver’. No advantage of ‘New oat’ in PCs induction was observed in comparison to Cd-sensitive cultivar ‘Soil saver’, although Cd exposure increased the concentration of total PCs in both cultivars. Higher and increased Cd accumulation in cell wall fraction was observed in shoots of ‘New oat’. On the other hand, in ‘Soil saver’, apoplasmic Cd accumulation showed saturation under higher Cd exposure. Overall, the present results suggest that cell wall Cd accumulation and antioxidative activities function in the tolerance against Cd stress possibly in combination with vacuolar Cd compartmentation.     

Comparative physiological responses of Solanum nigrum and Solanum torvum to cadmium stress

? Under cadmium (Cd) stress, Solanum nigrum accumulated threefold more Cd in its leaves and was tolerant to Cd, whereas its low Cd-accumulating relative, Solanum torvum, suffered reduced growth and marked oxidative damage. However, the physiological mechanisms that are responsible for differential Cd accumulation and tolerance between the two Solanum species are largely unknown. ? Here, the involvement of antioxidative capacity and the accumulation of organic and amino acids in response to Cd stress in the two Solanum species were assessed. ? Solanum nigrum contains higher antioxidative capacity than does S.?torvum under Cd toxicity. Metabolomics analysis indicated that Cd treatment also markedly increased the production of several organic and amino acids in S.?nigrum. Pretreatment with proline and histidine increased Cd accumulation; moreover, pretreatment with citric acid increased Cd accumulation in leaves but decreased Cd accumulation in roots, which indicates that its biosynthesis could be linked to Cd long-distance transport and accumulation in leaves. ? Our data provide novel metabolite evidence regarding the enhancement of citric acid and amino acid biosynthesis in Cd-treated S.?nigrum, support the role of these metabolites in improving Cd tolerance and accumulation, and may help to provide a better understanding of stress adaptation in other Solanum species.     

Variation in cadmium accumulation potential and tissue distribution of cadmium in tobacco

Variation in Cd accumulation between Nicotiana species but not varieties has been observed in seedlings grown in solution culture with moderate-to-low levels of Cd. Nicotiana tabacum has been characterized as a leaf and root accumulator while Nicotiana rustica is shown to be primarily a root accumulator, having about half the leaf Cd per gram dry weight of N. tabacum. This phenotype is retained in the mature N. rustica plant. To characterize these two species which differ in their modes of Cd accumulation, tissue Cd distribution, partitioning of metal in soluble and insoluble fractions and the contribution of soluble Cd-binding proteins (peptides) to total plant Cd was assessed using mature solution cultured plants. Metal accumulation was highest in the most mature leaves and in young roots. The preponderance of young roots in N. rustica may, in part, account for low leaf/high root Cd accumulation in this species. While Cd-binding peptides appear to be a principal form of Cd in leaves and roots of seedlings and these also occur in mature leaves, Cd is equally distributed between soluble (about 80% as Cd-binding peptide) and uncharacterized insoluble forms in mature plant roots.