Cadmium Uptake and Growth Responses of Seven Urban Flowering Plants: Hyperaccumulator or Bioindicator?

The application of flowering plants is the basis of urban forest construction. A newly-found flowering hyperaccumulator is crucial for remediating urban contaminated soil sustainably by cadmium (Cd). This study evaluated growth responses, Cd uptake and bioaccumulation characteristics of seven urban flowering plants. Based on growth responses of these plants, Calendula officinalis L. showed high tolerance to at least 100 mg kg−1 Cd, in terms of significant increase in biomass and with no obvious changes in height. After 60 d exposure to 100 mg kg−1 Cd, the accumulated Cd in shoots of the plant reached 279.51 ± 13.67 μg g−1 DW, which is above the critical value defined for a hyperaccumulator (100 μg g−1 DW for Cd). Meanwhile, the plant could accumulate Cd to as much as 926.68 ± 29.11 μg g−1 DW in root and 1206.19 ± 23.06 μg g−1 DW in plant, and had higher Cd uptake and bioaccumulation values. According to these traits, it is shown that Calendula officinalis L. can become a potential Cd-hyperaccumulator for phytoremediation. By contrast, Dianthus caryophyllus L. is very sensitive to Cd stress in terms of significantly decreased biomass, height and Cd uptake, indicating the plant is considered as a Cd-bioindicator.

Effect of EDTA chelate, cow manure, elemental sulfur application and Thiobacillus inoculant on Cd, Zn and Fe phytoremediation efficiency in a Cd-polluted soil

Background and Purpose: Phytoremediation efficiency of heavy metals is one of the important points in environmental studies. This research was conducted to investigate the effect of cow manure, elemental sulfur and EDTA on Cd uptake by Indian mustard in a Cd-polluted soil in the presence of Thiobacillus thiooxidans. Materials and Methods: Treatments consisted of applying cow manure (0, 5 and 10 g/kg soil), soil application of elemental sulfur (2 g/kg soil), and Cd-polluted soil (0 and 20 mg Cd/kg soil) with 1.5 mmol EDTA/kg soil in the presence of Thiobacillus spp. After 90 days, Indian mustard plant was harvested and plant Zn, Fe and Cd concentration was measured using atomic absorption spectroscopy. In addition, the soil microbial respiration was measured. Results: The use of 2 g/kg soil of elemental sulfur significantly increased the plant Cd concentration in the presence and absence of Thiobacillus by 14.2 and 11.7%, respectively. Adding cow manure to the soil at the rates of 5 and 10 g/kg soil significantly increased the plant Cd concentration by 15.7 and 18.1%, respectively. Also, the application of EDTA chelate at the rate 0f 1.5 mmol/kg soil significantly increased the Cd concentration of the plants grown in the Cd-polluted soil (20 mg Cd/kg soil) by 13.6%. Conclusion: The results of the present study showed that using elemental sulfur in the Cdpolluted soil can increase the Cd concentration of the plant which was cultivated in the soil amended with cow manure in the presence of Thiobacillus bacteria. However, the role of soil physic-chemical properties on phytoremediation efficiency cannot be ignored.

Risks for Using FeCl3 Under a Submerged Condition, and Different Water Management to Reduce Uptake of Antimony and Cadmium in a Rice Plant

Soil pollution by multiple metal(loid)s is a common problem, and it is not easy to synchronously reduce their uptake in crops. Compounds containing iron (Fe) are often used to efficiently remediate soil metal(loid) pollution; however, its associated risks did not receive much attention especially under unsuitable soil water conditions. Pot experiments were set up using an antimony (Sb) and cadmium (Cd) co-contaminated soil treated with a continued submergence condition plus 5, 10, or 20 mg kg−1 FeCl3 (Experiment I), or treated with different water management including submergence, intermittent irrigation, and dry farming (Experiment II). Our results showed that the continued submergence resulted in excessive accumulation of arsenic (As) in different tissues of rice plants even if the soil As background concentration is low. High soil moisture content increased the available concentrations of Sb and As, but reduced that of Cd in rhizosphere soils, which was in line with their concentrations in different tissues of rice plants (Experiment II). Under a continued submergence condition, FeCl3 significantly stimulated As concentration in the shoots, roots (excluded Fe20 treatment), and husks, but reduced it in the grains. FeCl3 reduced Sb concentration only in the roots and grains, and reduced Cd concentration only in the husks, suggesting a limited efficiency of FeCl3 to reduce Cd uptake under a submergence condition. In this study, the dynamic changes of As, Sb, and Cd concentrations in soil solution, their available concentrations in rhizosphere soils, their accumulation in root iron/manganese plaques, and the relationships among the above parameters were also discussed. We suggested that if FeCl3 would be used to remediate the contaminated soils by Sb and Cd, dry farming for a short time is needed to avoid As accumulation, and intermittent irrigation is a potential choice to avoid the excessive accumulation of As, Sb, and Cd in the edible parts of rice plants.

Rootstock-Mediated Genetic Variance in Cadmium Uptake by Juvenile Cacao (Theobroma cacao L.) Genotypes, and Its Effect on Growth and Physiology

Grafting typically offers a shortcut to breed tree orchards throughout a multidimensional space of traits. Despite an overwhelming spectrum of rootstock-mediated effects on scion traits observed across several species, the exact nature and mechanisms underlying the rootstock-mediated effects on scion traits in cacao (Theobroma cacao L.) plants often remain overlooked. Therefore, we aimed to explicitly quantify rootstock-mediated genetic contributions in recombinant juvenile cacao plants across target traits, specifically cadmium (Cd) uptake, and its correlation with growth and physiological traits. Content of chloroplast pigments, fluorescence of chlorophyll a, leaf gas exchange, nutrient uptake, and plant biomass were examined across ungrafted saplings and target rootstock × scion combinations in soils with contrasting levels of Cd. This panel considered a total of 320 progenies from open-pollinated half-sib families and reciprocal full-sib progenies (derived from controlled crosses between the reference genotypes IMC67 and PA121). Both family types were used as rootstocks in grafts with two commercial clones (ICS95 and CCN51) commonly grown in Colombia. A pedigree-based best linear unbiased prediction (A-BLUP) mixed model was implemented to quantify rootstock-mediated narrow-sense heritability (h2) for target traits. A Cd effect measured on rootstocks before grafting was observed in plant biomass, nutrient uptake, and content of chloroplast pigments. After grafting, damage to the Photosystem II (PSII) was also evident in some rootstock × scion combinations. Differences in the specific combining ability for Cd uptake were mostly detected in ungrafted rootstocks, or 2 months after grafting with the clonal CCN51 scion. Moderate rootstock effects (h2> 0.1) were detected before grafting for five growth traits, four nutrient uptake properties, and chlorophylls and carotenoids content (h2 = 0.19, 95% CI 0.05–0.61, r = 0.7). Such rootstock effects faded (h2< 0.1) when rootstock genotypes were examined in soils without Cd, or 4 months after grafting. These results suggest a pervasive genetic conflict between the rootstock and the scion genotypes, involving the triple rootstock × scion × soil interaction when it refers to Cd and nutrient uptake, early growth, and photosynthetic process in juvenile cacao plants. Overall, deepening on these findings will harness early breeding schemes of cacao rootstock genotypes compatible with commercial clonal scions and adapted to soils enriched with toxic levels of Cd.

Application of Organic Amendments to Cadmium-contaminated Soils for Phytostabilization Potential by Dicotyledonous Bioenergy Crops

Acacia (Acacia mangium), jatropha (Jatropha curcas), and cassava (Manihot esculenta) were cultivated in a greenhouse to see how organic amendments affected plant survival and accumulation of cadmium (Cd) in plant tissues. The study plants are bioenergy crops, which are advantageous to phytomanagement because they provide a significant economic benefit to local residents and farmers in the agricultural sector while also simultaneously reducing Cd entry into food webs through consumption. In this study, bone meal/bat manure and leonardite/bat manure were the key organic amendments that promoted the best growth performances in acacia (growth rate in dry biomass; GRDB 24.2) and cassava (GRDB 22.2), respectively, while jatropha preferred bone meal (GRDB 17.2). However, leonardite/bone meal considerably reduced Cd uptake values in whole plant tissues of acacia and cassava (35 mg plant-1 and 119.1 mg plant-1, respectively), while bone meal/chicken manure greatly reduced Cd uptake values in jatropha (127.8 mg plant-1). Cassava is a Cd excluder; however, it may not be a useful bioenergy crop to cultivate in Cd soil because it displayed toxicity symptoms after harvesting. The best plant for phytomanagement in this study was jatropha, which demonstrated substantial growth biomass and Cd accumulation, followed by acacia.

Study of the Physiological Dynamics of Cadmium Accumulation in Two Varieties of Rice with Different Cadmium-Accumulating Properties

This study focused on cadmium (Cd) uptake by two rice varieties, Yuzhenxiang (YZX) and Xiangwanxian 12 (XWX), which differ in their capacity to accumulate Cd, i.e., XWX > YZX. Treatments with three different gradients of soil Cd concentrations showed that with the increase in soil Cd concentration gradient, the Cd content in each rice plant organ also increased, i.e., Cd-3 > Cd-2 > Cd-1. The trend in the Cd content of each organ was such that the farther the organ from the root, the lower its Cd content, i.e., root > stem and sheath > leaf > grain. We observed that for all four growth stages, the booting stage is the key stage in terms of Cd absorption, where the highest levels of accumulation are observed, that is, booting stage > full heading stage > tillering stage > maturity stage. Of the two cultivars, XWX had higher SOD, POD, and CAT activities but lower MDA content. In contaminated soils, SOD, POD, and CAT activities increased gradually with the increase in Cd concentration, while MDA content decreased, which indicated that the low Cd variety XWX had an advantage over the high Cd variety YZX. Through the comparative analysis of photosynthetic physiology, it was found that the low-Cd-accumulating rice variety XWX appeared more tolerant to Cd, while the high-Cd-accumulating rice YZX was more sensitive. Therefore, the low Cd rice variety XWX was more suitable for planting safe rice in Cd-polluted paddy fields.

Cadmium Speciation Distribution Responses to Soil Properties and Soil Microbes of Plow Layer and Plow Pan Soils in Cadmium-Contaminated Paddy Fields

Cadmium (Cd) speciation ratio in arable land determines the Cd exposure risk and Cd uptake in crops. However, the driving mechanisms of Cd speciation change on the vertical scale of paddy fields remain poorly understood. In this study, the effects of plow layer and plow pan on Cd speciation distribution were investigated in a long-term Cd-contaminated rice ecosystem. The Cd accumulative effect within rice grain was enhanced with high levels of activated Cd speciation ratios in soils. Activated Cd speciation ratios were higher in plow layer soils, while stabilized Cd speciation ratios were elevated in plow pan soils. Soil physicochemical properties and soil microbes synergistically affected the Cd speciation changes in different ways between the two soil layers. Soil pH and organic elements in plow layer environment directly hindered the transformation of stabilized Cd speciation, while in plow pan environment, soil pH and organic elements indirectly decreased activated Cd speciation ratios and resulted in the accumulation of stabilized Cd speciation via regulating the predominant bacterial taxa. This study will improve our understanding of how soil environments regulate Cd speciation distributions in rice ecosystems and help to seek effective remediation methods of Cd-contaminated paddy fields to reduce the Cd accumulation in rice.

Cd and Zn Concentrations in Soil and Silage Maize following the Addition of P Fertilizer

Studies of soil Cd and Zn are often performed on sites that are contaminated or have deficient Zn conditions. Soil characteristics and crop management could impact the soil mobility and uptake of Cd and Zn, even when considering unpolluted Cd soils and adequate soil Zn levels. The concentrations of these two metals were assessed in soil and silage maize under five P fertilization treatments at two growth stages under low Cd and sufficient Zn conditions. Pearson correlation coefficients and stepwise linear regressions were calculated to investigate the soil characteristics influencing the bioavailable metal fraction in soil and the metal concentration in silage maize. P treatments did not impact Cd accumulation in maize; however, the Zn uptake was affected by P placement at the leaf development stage. From early development to maturity, the Cd level in maize decreased to 10% of the initial uptake, while the Zn level decreased to 50% of the initial uptake. This reduction in both metals may be attributed to a dilution effect derived from high biomass production. Silage maize could alleviate the initial Cd uptake while diminishing the depressant effect of P fertilizer on Zn concentration. Further research is required to understand the effect of P fertilizer on Cd uptake and its relation to Zn under field conditions at early and mature stages.

Regulating role of abscisic acid on cadmium enrichment in ramie (Boehmeria nivea L.)

Abscisic acid (ABA) is known as an important hormone regulating plant stress resistance, such as salt, drought and heavy metal resistance. However, the relationship between ABA and cadmium (Cd) enrichment in ramie (Boehmeria nivea L.) is still unclear to date. This study aimed to reveal the effect of ABA on Cd enrichment in ramie, and we received the following results: (1) Under Cd treatment, the Cd uptake of ramie increased with the increase of Cd concentration, but the chlorophyll content decreased. Under Cd treatment, the ABA content was highest in roots of ramie, followed by that in old leaves, and lowest in new leaves. Long-time treatment of high Cd concentration reduced the ability of endogenous ABA biosynthesis. (2) Spraying ABA on ramie plants (SORP) and adding ABA directly to the culture solution (ADCS) with low concentration can promote the growth of ramie and increase the amount of Cd uptake, and the effect of SORP is better. (3) The molecular reason for the decrease of chlorophyll content due to Cd stress, may be resulted from the down-regulated expression of the chlorophyll synthesis genes (BnPAO and BnNYC1) and the up-regulated expression of the chlorophyll degradation genes (BnCHLH, BnCHLG, BnHAP3A and BnPPR1). The elevated ABA content in ramie plants may due to the up-regulated expression of the ABA synthesis related genes (BnABA1, BnNCED3, and BnNCED5) and the genes (BnABCG40, BnNFXL2, BnPYL9, BnGCR2, BnGTG1, BnBGLU1, BnUTG1, BnVHAG1 and BnABI5) that encoding ABA transport and response proteins, which was consistent with the enhance the Cd uptake in ramie. Our study revealed the relationship between ABA and Cd uptake in ramie, which provided a reference for improving the enrichment of Cd in ramie.

Challenges and opportunities to regulate mineral transport in rice

Iron (Fe) is an essential mineral for plants and its deficiency as well as toxicity severely affects plant growth and development. Although Fe is ubiquitous in mineral soils, its acquisition by plants is difficult to regulate particularly in acidic and alkaline soils. Under alkaline conditions, where lime is abundant, Fe and other mineral elements are sparingly soluble. In contrast, under low pH conditions, especially in paddy fields, Fe toxicity could occur. Fe uptake is complicated and could be integrated with copper (Cu), manganese (Mn), zinc (Zn), and cadmium (Cd) uptake. Plants have developed sophisticated mechanisms to regulate the Fe uptake from soil and its transport to root and above-ground parts. Here, we review recent developments in understanding metal transport and discuss strategies to effectively regulate metal transport in plants with a particular focus on rice.