镉与豆磺隆复合胁迫下小麦根－土界面镉形态的变化

通过根际箱试验,研究了Cd与豆磺隆复合胁迫下小麦根 土界面Cd形态变化的空间和时间效应.在空间上将根－土界面(0～5 mm)细化到1 mm,在时间上将取样时间分为14、21、28、35和42 d,并将小麦体吸收的Cd与根 土界面各形态Cd作相关分析,从而得出影响小麦体生长的Cd形态.结果表明,在小麦不同的生长时间内,可交换态Cd表现出的空间效应明显不同.在小麦生长的第14天,根－土界面可交换态Cd大体上由根中心区(6.186 mg·kg^-1)向根外区(6.482 mg·kg^-1)逐渐增加;从小麦生长第21天到42天,根-土界面可交换态Cd呈现出由根中心区到某一层升高,之后又由该层到土体下降的趋势.根-土界面各层碳酸盐和铁锰结合态Cd向可交换态Cd转化的趋势由根中心区向根外区逐渐减弱,而向残留态Cd转化的趋势逐渐加强,有机结合态Cd浓度变化在近根区较大.碳酸盐结合态Cd、铁锰结合态Cd、有机结合态Cd浓度随时间而逐渐下降;残留态Cd浓度则表现出明显的上升趋势.相关分析表明,近根层的可交换态Cd和有机结合态Cd是小麦能直接利用的两种Cd形态.豆磺隆对可交换态Cd含量变化以及碳酸盐和铁锰结合态Cd的转化有明显影响.     

农作物对Cd毒害的耐性机理探讨

对生长在Ｃｄ污染条件下的８种作物体内Ｃｄ的存在形态分析表明,作物的耐Ｃｄ性与Ｃｄ的形态分布密切相关,在耐性作物体内,蛋白质（多肽）结合Ｃｄ量的比例低于非耐性作物;有机酸盐和一代磷酸盐态Ｃｄ的比例增加;分离得到Ｃｄ诱导蛋白?其中束缚了一定量的Ｃｄ,限制了Ｃｄ以自由态存在,耐性较低的作物本内,大分子量的蛋白质中富Ｃｄ量较高。     

The influence of zinc on apical uptake of cadmium in the gills and cadmium influx to the circulatory system in zebrafish (Danio rerio)

Zn (0-16 microM) effects on apical Cd uptake from the water into the branchial epithelium and influx of Cd from the water to the circulatory system in zebrafish (Danio rerio) were studied in three experiments. Apical Cd uptake was decreased by Zn in all three experiments. In fish exposed to 1-600 nM Cd (experiment 1), apical Cd uptake showed saturation kinetics at 2 and 4 microM Zn, and a competitive interaction was indicated. At 16 microMZn, Cd uptake increased linearly. Cadmium influx did not show saturation kinetics, but was inhibited by 16 microM Zn at low Cd exposure concentrations. In fish exposed to 0.1-600 nM Cd (experiment 2), Cd uptake was inhibited by 16 microM Zn, whereas at 30 nM Cd uptake was inhibited by 2 microM Zn. Similarly, 2 microM Zn did not influence Cd uptake in fish exposed to 0.1-2 nM Cd (experiment 3), whereas 2 microM Zn inhibited uptake at 8-30 nM Cd. Zinc also inhibited Cd influx at higher Cd concentrations. However, at lower Cd exposures, a Zn-induced increased influx was indicated. Zinc influences the Cd uptake and influx processes at several sites in the branchial epithelial cells, indicating that influx of Zn2+ and Cd2+ occurs through common pathways.     

Influence of prior Cd(2+) exposure on the uptake of Cd(2+) and other elements in the phytochelatin-deficient mutant, cad1-3, of Arabidopsis thaliana

In order to test the potential effect of prior exposure to different Cd concentrations on Cd uptake and accumulation, plants of Arabidopsis thaliana, including a phytochelatin-deficient mutant, cad1-3, and the wild type, were compared. For Cd uptake experiments, plants were grown for 1 week in nutrient solution containing different Cd concentrations (0, 0.05, 0.1, 0.25, 0.5, and 1.0 microM Cd(NO(3))(2)). Thereafter they were subjected to 0.5 microM Cd labelled with (109)Cd for 2 h. Uptake experiments with (109)Cd showed that the phytochelatin-deficient mutant cad1-3, accumulated less Cd than the wild type. Both a lower proportion and lower total amount of absorbed Cd were translocated to the shoot in cad1-3 plants compared to wild-type plants. Cadmium exposure also influenced the amounts of nutrients found, whereby after exposure to high Cd concentrations (0.5, 1.0 microM) during growth, cad1-3 roots contained less Fe, K, Mg, P, and S compared to roots of the wild type. In cad1-3 these elements decreased with increasing Cd concentration. The total Cd content in roots and shoots increased significantly with increasing Cd concentration during growth, although the increase was much less in cad1-3 plants. In time-dependent experiments of Cd uptake carried out between 15 and 120 min on plants not previously exposed to Cd, no significant difference in Cd accumulation between the mutant and wild type were found, although a smaller amount of Cd was translocated to the shoot in cad1-3 plants. The possibility that the differences in Cd accumulation in mutant and wild-type lines may be due to the cytosolic Cd regulation, which is inhibited by the complexation of Cd by phytochelatins, is discussed.     

Role of roots in differences in seed cadmium concentration among soybean cultivars—proof by grafting experiment

Soybean cultivars show significant differences in cadmium (Cd) concentrations in seeds, due primarily to genetics, not environmental factors. We previously suggested that low-Cd cultivars accumulate Cd in their roots and thus prevent its translocation to the rest of the plant. Through grafting experiments, we drew the following conclusions about Cd absorption and translocation: (1) The amount of Cd accumulated in shoots is determined by the Cd accumulation capacity of roots: cultivars with a small capacity to accumulate Cd in roots translocate more Cd and accumulate it in shoots; (2) The Cd concentration in shoots is determined by the Cd accumulation capacity of roots and the shoot productive ability of the scion cultivar; (3) The Cd tolerance of shoots differs among cultivars. Enrei, with a high-Cd accumulation capacity of roots, had a low Cd tolerance of shoots compared with Suzuyutaka and Hatayutaka, with a low Cd accumulation capacity of roots; (4) Cultivars differ in their distribution of Cd to seed; (5) These results show that seed Cd concentration is influenced by the differences among cultivars in ease of translocation of Cd to seed and in Cd accumulation capacity of roots.     

Cd accumulation in roots and shoots of durum wheat: the roles of transpiration rate and apoplastic bypass

Cadmium is readily taken up from soils by plants, depending on soil chemistry, and variably among species and cultivars; altered transpiration and xylem transport and/or translocation in the phloem could cause this variation in Cd accumulation, some degree of which is heritable. Using Triticum turgidum var. durum cvs Kyle and Arcola (high and low grain Cd accumulating, respectively), the objectives of this study were to determine if low-concentration Cd exposure alters transpiration, to relate transpiration to accumulation of Cd in roots and shoots at several life stages, and to evaluate the role of apoplastic bypass in the accumulation of Cd in shoots. The low abundance isotope (106)Cd was used to probe Cd translocation in plants which had been exposed to elemental Cd or were Cd-na?ve; apoplastic bypass was monitored using the fluorescent dye PTS (8-hydroxy-1,3,6-pyrenetrisulphonate). Differential accumulation of Cd by 'Kyle' and 'Arcola' could be partially attributed to the effect of Cd on transpiration, as exposure to low concentrations of Cd increased mass flow and concomitant Cd accumulation in 'Kyle'. Distinct from this, exposure of 'Arcola' to low concentrations of Cd reduced translocation of Cd from roots to shoots relative to root accumulation of Cd. It is possible, but not tested here, that sequestration mechanisms (such as phytochelatin production, as demonstrated by others) are the genetically controlled difference between these two cultivars that results in differential Cd accumulation. These results also suggest that apoplastic bypass was not a major pathway of Cd transport from the root to the shoot in these plants, and that most of the shoot Cd resulted from uptake into the stele of the root via the symplastic pathway.     

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.     

Tolerance of Ricinus communis L. to Cd and screening of high Cd accumulation varieties for remediation of Cd contaminated soils

Response of castor (Ricinus communis L.) to cadmium (Cd) was assessed by a seed-suspending seedbed approach. Length of total radicle was the most sensitive indicator of Cd tolerance among the tested germination and growth characters. The ED₅₀ value for Cd was 11.87 mg L^?1, indicating high Cd tolerance in castor. A pot experiment was conducted by growing 46 varieties of castor under CK (without Cd) and Cd1 (10 mg kg^?1 of Cd) and Cd2 (50 mg kg^?1 of Cd) treatments to investigate genotype variations in growth response and Cd accumulation of castor under different Cd exposures. Castor possessed high Cd accumulation ability; average shoot and root Cd concentrations of the 46 tested varieties were 21.83 and 185.43 mg kg^?1, and 174.99 and 1181.96 mg kg^?1 under Cd1 and Cd2, respectively. Great variation in Cd accumulation was observed among varieties, and Cd concentration of castor was genotype dependent. The correlation between biomass and Cd accumulation was significantly positive, while no significant correlation was observed between Cd concentration and Cd accumulation, which indicated that biomass performance is the dominant factor in determining Cd accumulation ability.     

蒌蒿对镉的富集特征及亚细胞分布特点

以镉(Cd)富集植物蒌蒿(Artemisia selengensis)为试材, 采用超声波细胞破碎处理和超速离心的方法, 对蒌蒿根和叶中亚细胞水平的Cd分布状况进行研究, 同时测定Cd在蒌蒿不同器官中的富集效果。结果表明, 在30 mg·kg^–1Cd胁迫下, 蒌蒿叶片中Cd的富集浓度是根和茎中的2–3倍, 但因叶片所占植株的生物量比例较小, 其对Cd的积累量远小于茎和根; Cd在蒌蒿叶片细胞壁、胞液和细胞器中含量比为16:5:1。细胞壁固定是叶片对Cd的主要防御机制。随着Cd处理浓度的增加, 细胞壁和胞液中的Cd含量大幅上升, 但细胞器中Cd含量仍维持在较低水平。长时间和高浓度的Cd胁迫可使细胞壁解毒机制失活并诱导细胞器中的Cd含量增加, 导致植株死亡。根中液泡的Cd贮存量较大, 解毒效果显著。     

INCREASED CONCENTRATION OF SOIL CADMIUM AFFECTS ON PLANT GROWTH,DRY MATTER ACCUMULATION,Cd, AND Zn UPTAKE OF DIFFERENT TOBACCO CULTIVARS (NICOTIANA TABACUM L.)

Cadmium (Cd) is a toxic trace metal pollutant for humans, animals, and plants. Tobacco is a wellknown efficient accumulator of Cd and the genotypic differences in Cd uptake and the response to Cd was not determined. The objectives of this study were to investigate: 1) the effects of Cd on the growth and development of different tobacco cultivars; 2) the differences among tobacco cultivars in Cd concentration, uptake, and use for the phytoremediation of polluted soils with Cd; and (3) the interactions between Cd and Zn with respect to concentration and uptake. The Cd level affected the number of leaves and dry matter accumulation, and there were differences among the different cultivars that were used. Furthermore, some cultivars showed a higher reduction in growth than others, indicating that they are more sensitive to Cd level in the soil. Moreover, differences existed among the cultivars for the Cd concentration and uptake. There also were negative correlations between Cd and Zn concentrations; as Cd accumulation increased, Zn accumulation decreased, which showed that the two heavy metals were antagonistic. These results suggest that tobacco cultivars differed greatly in their growth and developmental responses to Cd and in the concentration and uptake of Cd and Zn. In addition, it is possible to use certain tobacco cultivars to lower the Cd concentration in the soil.     

镉在互花米草中积累、转运及亚细胞的分布

研究了在不同Cd浓度(0、5、100、200μg·g-1)处理下,互花米草花序、叶、茎、根茎、须根中Cd含量、积累量、转运特征,及Cd在互花米草体内的亚细胞分布。结果表明,Cd在互花米草不同器官中的积累能力存在较大差异。茎、根茎、须根中Cd含量及积累量随处理浓度的增加而升高,其中须根中Cd含量及积累量均高于其他器官。Cd处理浓度为100gμ·g-1时,花序和叶中Cd含量达到最大值,分别为8.65和7.82μg·g-1。在Cd处理浓度为200μg·g-1时,须根中Cd含量可高达390.00μg·g-1,积累量达3200μg·株-1。Cd在互花米草体内转运能力极低,绝大部分Cd积累在地下部位。Cd在互花米草亚细胞中的分布规律为细胞壁>胞液>细胞器。随着Cd处理浓度的增加,Cd在细胞壁中的分配比例增大,胞液中Cd分布比例则相应减小,细胞壁和胞液相互协调,增强互花米草对重金属Cd的耐性。     

水稻土施硅对土壤-水稻系统中镉的降低效果

水稻中镉的积累造成人类健康的风险,增加水稻硅素能减轻镉中毒症状,降低稻米镉积累,但是硅对重金属的作用机理尚不清楚。主要研究了在中度和高度镉污染的土壤中,通过施用固态和液态的富硅物质对土壤-水稻系统中镉的吸收和转运的影响,探明决定镉和硅在根与芽的质外体和共质体中的作用机理。试验结果表明:(1)在中度和高度污染的土壤中,镉在土壤-作物系统中的转移和积累情况是不同的,可以通过富硅物质中的单硅酸与镉离子的相互作用,增加镉在硅物质表面的吸附来减少镉在土壤中的流动;(2)富硅物质可以降低水稻根和芽中镉的积累,在高度镉污染的情况下,施用硅可以使镉大量积累在水稻根及其共质体中,并降低根及其共质体中镉的转换和积累;(3)新鲜土壤中水萃取态的单硅酸含量与镉在土壤-作物系统中的流动性、转运以及积累等主要参数密切相关。     

Application of sulfur fertilizer reduces cadmium accumulation and toxicity in tobacco seedlings (<Emphasis Type="Italic">Nicotiana tabacum</Emphasis>)

Greenhouse pot experiments were carried out to illuminate the mitigation effects of sulfur (S) on cadmium (Cd) uptake and toxicity in tobacco using two levels of exogenous sulfur of S1 and S2 containing 47 and 38% total S. Results showed that Cd1 and Cd2 treatments of 1 and 5 mg Cd kg^?1 soil increased leaf Cd concentration and accumulation but reduced plant height, net photosynthetic rate (Pn) and biomass, with a much severe response in Cd2 treatment. Application of S2 fertilizer alleviates Cd toxicity, markedly decreased Cd concentration and improved photosynthesis, compared with Cd1 and Cd2 alone treatment; but S1 only alleviated Cd toxicity when tobacco was subjected to Cd1 stress. Our results suggest that S2 fertilizer was more effective in reducing Cd accumulation and toxicity in Tobacco than S1, and highlights a promising approach of S fertilizer application to lower leaf Cd accumulation in order to ensure product safety of tobacco grown in Cd polluted soils.     

Glucose alleviates cadmium toxicity by increasing cadmium fixation in root cell wall and sequestration into vacuole in Arabidopsis

Glucose(Glu) is involved in not only plant physiological and developmental events but also plant responses to abiotic stresses. Here, we found that the exogenous Glu improved root and shoot growth, reduced shoot cadmium(Cd) concentration, and rescued Cdinduced chlorosis in Arabidopsis thaliana(Columbia ecotype,Col-0) under Cd stressed conditions. Glucose increased Cd retained in the roots, thus reducing its translocation from root to shoot signi fi cantly. The most Cd retained in the roots was found in the hemicellulose 1. Glucose combined with Cd(Glu t Cd) treatment did not affect the content of pectin and its binding capacity of Cd while it increased the content of hemicelluloses 1 and the amount of Cd retained in it signi fi cantly. Furthermore, Leadmium Green staining indicated that more Cd was compartmented into vacuoles in Glu t Cd treatment compared with Cd treatment alone, which was in accordance with the R e ssigni fi cant upregulation of the expression of tonoplastlocalized metal transporter genes, suggesting that compartmentation of Cd into vacuoles also contributes to the Glu-alleviated Cd toxicity. Taken together, we demonstrated that Glu-alleviated Cd toxicity is mediated through increasing Cd fi xation in the root cell wall and sequestration into the vacuoles.     

The timing of grain Cd accumulation in rice plants: the relative importance of remobilisation within the plant and root Cd uptake post-flowering

The pathways by which Cd is accumulated in rice grain are not well understood, in particular the components attributable to direct transfer from the root, and to remobilisation of Cd previously accumulated in other plant parts. In order to observe the timing of Cd accumulation in rice plants and determine the major period for accumulation of Cd which can be translocated to the grain, Cd was supplied to the roots of rice plants grown under static hydroponic conditions at a non-toxic, environmentally relevant concentration (50 nM), according to three different timing regimes: (1) Pre-flowering Cd, (2) Post-flowering Cd, or (3) Continuous Cd. The rate of accumulation of Cd in the developing grain was monitored by harvesting immature rice panicles at four time points prior to a final harvest. Nearly all grain Cd was accumulated within 16 days of anthesis and the contribution of post-flowering Cd uptake was evident from 7 days after flowering. It was estimated that 60% of the final grain Cd content was remobilised from that accumulated by the plant prior to flowering and the other 40% came from uptake during grain maturation. This study shows that Cd uptake from the root to the grain in rice is indeed possible post-flowering and it is an important source of grain Cd.     

Zinc or Magnesium Supplementation Modulates Cd Intoxication in Blood,Kidney, Spleen,and Bone of Rabbits

The objective of this study was to examine the influence of oral supplementation with Zn or Mg on Cd content in the blood and organs of rabbits exposed to prolonged Cd intoxication. Rabbits were divided into the following groups: Cd group-received orally every day for 4 weeks 10 mg Cd/kg body weight (b.w.), Cd+Zn group and Cd+Mg group-exposed to Cd and supplemented with 20 mg Zn/kg b.w. or 40 mg Mg/kg b.w. 1 h after Cd treatment. Cd content in biological material was determined by atomic absorption spectrophotometry. Blood Cd concentration was determined in all investigated groups at time 0 and after 10, 14, 18, 22, 25, and 28 days, whereas Cd content in the brain, heart, lungs, liver, kidney, spleen, pancreas, skeletal muscle, and bone was determined after 28 days. Blood Cd concentration was significantly increased in all groups from the 14th day of Cd intoxication and lasted till the end of the experiment. Zn or Mg supplementation significantly reduced blood Cd content on the 18th and 25th days. Supplementation with Zn or Mg significantly decreased Cd concentration in the kidney, spleen, and bone and, in addition, Zn reduced Cd content in the brain. Supplementation with Zn or Mg in Cd-intoxicated rabbits caused similar reduction of blood Cd concentration; however, reduction of tissue Cd content was more pronounced in Zn- than in Mg-supplemented group.     

Cadmium accumulation in sunflower plants influenced by arbuscular mycorrhiza

In order to investigate the cadmium (Cd) accumulation patterns and possible alleviation of Cd stress by mycorrhization, sunflower plants (Helianthus annuus L.) were grown in the presence or absence of Cd (20 micromol L(-1)) and inoculated or not inoculated with the arbuscular mycorrhizal fungus (AMF) Glomus intraradices. No visual symptoms of Cd phytotoxicity were observed; nevertheless, in non-mycorrhizal plants the presence of Cd decreased plant growth. The addition of Cd had no significant effect on either mycorrhizal colonization or the amount of extra-radical mycelia that was produced by the AMF. Cd accumulated mainly in roots; only 22% of the total Cd absorbed was translocated to the shoots, where it accumulated to an average of 228 mg Cd kg(-1). Although the shoot-to-root ratio of Cd was similar in both the AMF inoculated and non-inoculated plants, the total absorbed Cd was 23% higher in mycorrhizal plants. Cd concentration in AMF extra-radical mycelium was 728 microg g(-1) dry weight. Despite the greater absorption of Cd, mycorrhizal plants showed higher photosynthetic pigment concentrations and shoot P contents. Cd also influenced mineral nutrition, leading to decreased Ca and Cu shoot concentrations; N, Fe and Cu shoot contents; and increased S and K shoot concentrations. Cd induced guaiacol peroxidase activity in roots in both mycorrhizal and non-mycorrhizal plants, but this increase was much more accentuated in non-mycorrhizal roots. In conclusion, sunflower plants associated with G. intraradices were less sensitive to Cd stress than non-mycorrhizal plants. Mycorrhizal sunflowers showed enhanced Cd accumulation and some tolerance to excessive Cd concentrations in plant tissues.     

Cadmium translocation and accumulation in developing barley grains

Soil cadmium (Cd) contamination has posed a serious problem for safe food production and become a potential agricultural and environmental hazard worldwide. In order to study the transport of Cd into the developing grains, detached ears of two-rowed barley cv. ZAU 3 were cultured in Cd stressed nutrient solution containing the markers for phloem (rubidium) and xylem (strontium) transport. Cd concentration in each part of detached spikes increased with external Cd levels, and Cd concentration in various organs over the three Cd levels of 0.5, 2, 8 μM Cd on 15-day Cd exposure was in the order: awn > stem > grain > rachis > glume, while the majority of Cd was accumulated in grains with the proportion of 51.0% relative to the total Cd amount in the five parts of detached spikes. Cd accumulation in grains increased not only with external Cd levels but the time of exposure contrast to stem, awn, rachis and glume. Those four parts of detached spike showed increase Cd accumulation for 5 days, followed by sharp decrease till day 10 and increase again after 12.5 days. Awn-removal and stem-girdling markedly decreased Cd concentration in grains, and sucrose or zinc (Zn) addition to the medium and higher relative humidity (RH) also induced dramatic reduction in Cd transport to developing grains. The results indicated that awn, rachis and glume may involve in Cd transport into developing grains, and suggested that Cd redistribution in maturing cereals be considered as an important physiological process influencing the quality of harvested grains. Our results suggested that increasing RH to 90% and Zn addition in the medium at grain filling stage would be beneficial to decrease Cd accumulation in grains.     

矿区分离丛枝菌根真菌对万寿菊吸Cd潜力影响

盆栽试验研究土壤不同施Cd水平(0、5、20、50μg/g)下,接种矿区污染土壤中丛枝菌根真菌对万寿菊根系侵染率、植株生物量及Cd吸收与分配的影响。结果表明:接种丛枝菌根真菌显著提高Cd胁迫下万寿菊的根系侵染率和植株生物量;随着施Cd水平提高,各处理植株Cd浓度显著增加。各施Cd水平下万寿菊地上部Cd吸收量远远高于根系Cd吸收量,在土壤施Cd量达到50μg/g时,接种处理地上部Cd吸收量是根系的3.48倍,对照处理地上部Cd吸收量是根系的1.67倍;同一施Cd水平下接种处理植株Cd吸收量要显著高于对照。总体上,试验条件下污染土壤中分离的丛枝菌根真菌促进了万寿菊对土壤中Cd的吸收,并在一定程度上增加Cd向地上部分的运转,表现出植物提取的应用潜力。     

Cadmium distribution in maize inbred lines: Effects of pH and level of Cd supply

In order to investigate the physiological basis of the differential Cd distribution and the degree of variation of this Cd distribution among maize inbred lines, six inbreds designated earlier as ‘shoot Cd excluders’ (B73, H99, and H96) and ‘non-shoot Cd excluders’ (B37, H98, and N28) were grown in nutrient solution culture at different external Cd levels or at different pH. The characterization of the inbreds according to their shoot/root partitioning of Cd was consistent, independent of pH or level of Cd supply. The Cd concentrations in the plants were highest at the highest pH of the solution cultures. Generally, there was a positive correlation between the Cd concentrations in shoots and xylem exudates. It was shown that the Cd concentration in the roots is particularly important in the Cd distribution process. Above a ‘critical’ internal Cd concentration in the roots, specific for each inbred, the ability to retain Cd is strongly diminished. It is concluded that structural and/or physiological characteristics of the roots are involved in Cd partitioning.