Due to the potential toxicity of polycyclic aromatic hydrocarbons (PAHs) to humans, the uptake and translocation of PAHs in food crops have gained much attention. However, it is still unclear whether phloem participates in the acropetal translocation of PAHs in plants. Herein, the evidence for acropetal translocation of phenanthrene (a model PAH) via phloem is firstly tested. Wheat (Triticum aestivum L.) new leaves contain significantly higher phenanthrene concentration than old leaves (P < 0.05), and the inhibitory effect on phenanthrene translocation is stronger in old leaves after abscisic acid and polyvinyl alcohol (two common transpiration inhibitors) application. Phenanthrene concentration in xylem sap is slightly higher than in phloem sap. Ring-girdling treatment can significantly reduce phenanthrene concentration in castor bean (Ricinus communis L.) leaves. Two-photon fluorescence microscope images indicate a xylem-to-phloem and acropetal phloem translocation of phenanthrene in castor bean stem. Therefore, phloem is involved in the acropetal translocation of phenanthrene in wheat seedlings, especially when the xylem is not mature enough in scattered vascular bundle plants. Our results provide a deeper understanding of PAH translocation in plants, which have significant implications for food safety and phytoremediation enhancement of PAH-contaminated soil and water.

The isotopic fractionation could contribute to understanding the Cd accumulation mechanisms in plant species. However, there are few of systematical investigations with regards to the Cd isotope fractionation in hyperaccumulator plants. The Cd tolerant Ricinus communis and hyperaccumulator Solanum nigrum were cultivated in nutrient solutions with varying Cd and EDTA concentrations. Cd isotope ratios were determined in the solution, root, stem and leaf. The two investigated plants were systematically enriched in light isotopes relative to their solutions (Δ114/110Cdplant-solution = −0.64‰ to −0.29‰ for R. communis and −0.84‰ to −0.31‰ for S. nigrum). Cd isotopes were markedly fractionated among the plant tissues. For both plant species, an enrichment in light Cd isotopes from solution to root was noted, followed by a slight depletion in light Cd isotopes from root to shoot. Noticeably, the chelation process has caused lighter Cd isotope enrichment in the root of R. communis and S. nigrum. Further, the good fits between △114/110Cdroot-plant and ln Froot (or between △114/110Cdshoot-plant and ln Fshoot) indicate that Cd isotopic signatures can be used to study Cd transportation during the metabolic process of plants. This study suggests that knowledge of the Cd isotope ratios could also provide new tool for identifying the Cd-avoiding crop cultivars.

Arsenic (As) is a toxic environmental pollutant. Growing Ricinus communis (castor) on As-contaminated land has the advantage that in addition to revegetation of contaminated land, it can produce bioenergy. To date, As tolerance mechanisms of this plant are not fully understood. In our previous study, we screened tolerant and sensitive genotypes of castor and reported higher total As concentration, enhanced reactive oxygen species (ROS) generation, and oxidative stress in sensitive genotypes of castor GCH 2 and GCH 4 in comparison to tolerant genotypes WM and DCH 177. In the present study, we compared the activity, isoenzyme profile, and gene expression of ROS-scavenging enzymes, proline content, and expression of nicotianamine synthase genes (RcNAS1, RcNAS2, and RcNAS3) in As-tolerant and As-sensitive genotypes of castor. SOD and GPX activity increased significantly in roots of tolerant genotype WM but remained the same or decreased in sensitive genotype GCH 2 and GCH 4 at 200 μM arsenate [As(V)] treatment indicating their important role in As tolerance in castor. CAT activity and proline content increased in sensitive genotypes but remained the same in tolerant genotypes due to As(V) treatment. APX activity showed no significant change in roots and leaves of both tolerant and sensitive genotypes. NAS genes (RcNAS1, RcNAS2, and RcNAS3) encode enzymes that catalyze trimerization of S-adenosylmethionine to form nicotianamine and are critical for metal chelation and heavy metal tolerance. Differential responses of RcNAS1, RcNAS2, and RcNAS3 genes in WM and GCH 2 due to As(V) treatment suggest their role in As(V) tolerance.

The present study was conducted to assess the effect of toxicity of cadmium (Cd) on growth, tolerance index (TI), antioxidant activities, and malondialdehyde (MDA) content in two contrasting wild castor accessions (16-024 and S2-4) via hydroponic experiment (0 and 100 mg/L Cd). The results showed that Cd significantly reduced the growth rate, seedling height, root length, and shoot length of the castor accessions compared to the control, with the Cd effect being more severe in S2-4 than in 16-024. In addition, biomass response including the root and shoot fresh weight and root dry weight decreased in both accessions compared to the control. Compared to the control group, the shoot dry weight of accession S2-4 declined by 21.7%, whereas there was no change in 16-024, suggesting a level of tolerance in 16-024. Analysis of TI on all the growth parameters and biomass content revealed that accession 16-024 was highly tolerant to Cd stress than S2-4. The results further revealed that the expression of the antioxidant enzymes, viz., superoxide dismutase (SOD), catalase (CAT), non-enzymatic antioxidant, glutathione, and MDA content, was influenced by genotype. S2-4 exhibited a higher antioxidant activity (SOD, CAT) and lipid peroxidation activity than 16-024, indicative of oxidative damage from Cd stress.

Heavy metals are well known for their toxicity and become significant environmental pollution with a continually rising technology and public outcry to ensure the safest and healthiest environment. The present study aims to investigate the uptake capability of heavy metals and its impact on the growth dynamics of Ricinus communis L. (castor bean), along various habitats in Qalyubia Province, Egypt. Three composite plants and soil samples were collected from four different habitats: urban (residential area), canal banks, field edges, and drain banks. The samples were analyzed for nutrients and heavy metals. At the same time, forty quadrats (5 × 5 m) were selected to represent the micro-variations of castor bean in the selected habitats to determine its growth criteria and normalized vegetation index (NDVI). The lowest size index, volume, and number of leaves of castor bean were recorded along canal banks and they were characterized by high soil heavy metal concentration, especially Zn, Cu, and Ni, while the highest values were recorded along field edges with lower heavy metal concentration. Moreover, the NDVI indicated that castor bean from most studied habitats, except field edges, was healthy population. This study revealed that the leaves collected from all habitats were considered to be toxic with Cu. The bioconcentration factor (BF) of the investigated heavy metals was greater than 1. The BF order for heavy metals uptake by castor bean leaves was Fe > Ni > Mn > Cu > Zn. Consequently, the species selected in the present study can be used as a biomonitor of these heavy metal polluted soils. Moreover, it could be used as a phytoremediator, taken into consideration its use in all medicinal purposes.

Heavy metal-polluted water has become a problem for sustainable environment, agriculture, and human health. Phyto-accumulation is an eco-friendly technique for decontamination of metal-polluted water and soil. The efficiency of phyto-accumulation and rhizo-filtration can be enhanced by the application of certain nutrients to accumulator plants. In this study, we focused on the role of iron (Fe) in rhizo-filtration and phyto-accumulation of cadmium (Cd) from polluted water/media, using Ricinus communis plant. Medium was contaminated with 10 ppm Cd while Fe (2.50, 5.00, and 7.50 ppm) was applied both as foliar spray and medium addition separately. Accumulation of Cd and concentrations of soluble proline, phenolic compounds, and chlorophylls were measured in plant tissues. Addition of Fe into media significantly increased biomass in the plants but decreased Cd absorption by roots and its accumulation in other tissues of the plants. Foliar application of Fe, especially 7.5 ppm, significantly increased biomass as well as accumulation of Cd in tissues of the plants. Contents of soluble proline (41.88 ± 3.56 ppm) and phenolics (171.00 ± 4.98 ppm) in leaves were highly increased by foliar spray of 7.5 ppm Fe on the plants. On the other hand, highest concentrations of free proline (67.00 ± 2.00 ppm) and total phenolics (82.67 ± 2.52 ppm) in plant roots were observed in 7.5 ppm Fe added to media and as foliar spray, respectively. Strong correlations were observed between phenolics content in roots and leaves with Cd accumulation after foliar application of 7.5 ppm Fe.

Contamination levels of copper (Cu) and other heavy metals are very high in the soils of the abandoned copper mine of Lasail in the north western Hajar Mountains of Oman. Environment-friendly approaches such as phytoremediation are needed to clean and rehabilitate these areas to their natural status. In the present study, the phytoremediation potential of castor, Ricinus communis L., was evaluated for copper and other heavy metals by growing it in different types of Cu-mine soils and slags. Growth parameters such as shoot height and biomass weight (fresh and dry) were evaluated. Castor showed a high tolerance index (TI) in Cu-mine soils. The highest TI was calculated for the fresh mass of castor shoots in E soil with a percentage of 405.99. The translocation factor (TF) of all the metals except boron (B) and manganese (Mn) was < 1, which reveals that these metals are stabilised in the root portion of the castor. Bioconcentration factor (BCF) value < 1 for Cu indicates that castor is not a hyperaccumulator plant for copper. In addition to high concentrations of copper, other heavy metals such as arsenic (As), iron (Fe), and zinc (Zn) were observed in the roots than in shoots. Castor grown in slag accumulated Cu in the shoots, roots, and entire plant with the values of 25, 1184, and 1209 mg kg⁻¹, respectively. Similarly, castor cultivated in A soil accumulated 18, 901, and 919 mg kg⁻¹ of copper in shoots, roots, and entire plant, respectively. The calculated plant effective number (PENt) indicated the need for 253 castor plants to remove 1 g of Cu from E soil. The ability of castor to grow well in Cu-mine soils suggests that it can be used for the removal of Cu and other heavy metals. Additionally, the shoot portion could potentially be used for oil production since the phytoaccumulation levels of heavy metal concentration in the shoots were below the standard toxicity limits.

Effect of arsenate [As(V)] on biomass, photosynthetic rate, stomatal conductance, transpiration, oxidative stress, accumulation of As, Fe, Zn, Cu and Mn and expression of NRAMP genes was investigated in As(V) tolerant and sensitive genotypes of bioenergy crop Ricinus communis. As(V) treatments (100 and 200 μM) led to significant reduction in root and leaf biomass, photosynthetic rate, stomatal conductance and transpiration in GCH 2 and GCH 4 genotypes but no significant change or increase was observed in WM and DCH 177 genotypes. No significant difference was observed in hydrogen peroxide content and lipid peroxidation in As(V)-treated tolerant genotypes compared to control, whereas these parameters enhanced significantly in As(V)-treated sensitive genotypes. GCH 2 accumulated around two times As in leaves and showed significant reduction in concentration of Zn and Mn in the leaves and roots due to 200 μM As(V) treatment compared to WM. NRAMP genes are critical for uptake and distribution of essential divalent metal cations, photosynthesis and controlled production of reactive oxygen species in plants. RcNRAMP2, RcNRAMP3 and RcNRAMP5 genes showed differential expression in response to 200 μM As(V) in GCH 2 and WM suggesting that NRAMP genes are associated with differential responses of WM and GCH 2 genotypes to As(V) stress.

Application of fertilizers to supply appropriate nutrients has become an essential agricultural strategy for enhancing the efficiency of phytoremediation in heavy metal contaminated soils. The present study was conducted to investigate the beneficial effects of three types of phosphate fertilizers (i.e., oxalic acid-activated phosphate rock (APR), Ca(H₂PO₄)₂, and NaH₂PO₄) in the range of 0–600 mg P kg⁻¹ soil, on castor bean growth, antioxidants [antioxidative enzymes and glutathione (GSH)], and Cu uptake. Results showed that with the addition of phosphorus fertilizers, the dry weight of castor bean and the Cu concentration in roots increased significantly, resulting in increased Cu extraction. The phosphorus concentration in both shoots and roots was increased as compared with the control, and the Ca(H₂PO₄)₂ treatment had the greatest effect. Application of APR, NaH₂PO₄, and Ca(H₂PO₄)₂ reduced the malondialdehyde (MDA) content, and the activity of the two antioxidant enzymes superoxide dismustase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6) in the leaves of castor bean. GSH concentration in leaves increased with the increasing levels of phosphorus applied to soil as well as the accumulation of phosphorus in shoots, compared to the control. These results demonstrated that the addition of phosphorus fertilizers can enhance the resistance of castor bean to Cu and increase the Cu extraction efficiency of the plant from contaminated soils.

We have previously reported that Ricinus communis is a good candidate for the phytoremediation of Cd- and Zn-contaminated soil and for fuel production. In this study, changes in the activity of antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; and guaiacol peroxidase, POD) and the contents of chlorophyll and malondialdehyde (MDA) in R. communis leaves under Cu, Zn, and Cd stress were examined. Compounds from the exudate of R. communis roots were collected and analyzed using GC-MS chromatograms. The results of enzyme activity showed that Cd treatment significantly increased the SOD content of R. communis leaves and slightly elevated the CAT content, whereas the POD content increased markedly at low Cd treatment concentrations and decreased as Cd concentrations increased. Zn treatment distinctly elevated SOD and POD content in R. communis leaves but had no great influence on CAT content. Cu treatment slightly increased CAT activity, while Cu did not evidently change SOD and POD activity. We found 17, 29, 18, 18, and 33 different compounds in the R. communis root exudates from the control group and Cd, Cu, Zn, and Cd+Cu+Zn treatment groups, respectively. The root exudates mainly included ester, alcohol, ether, amide, acid, phenol, alkanes, ketone, aromatic hydrocarbon, and nitrile compounds. However, the root exudates of R. communis grown in uncontaminated soils were dominated by esters, alcohols, and ethers. Single Cu or Zn treatment slightly changed the root exudates, which were dominated by esters, alcohols, and amides. In the Cd and Cd+Cu+Zn treatment groups, the compositions of root exudates apparently increased, with alkanes as the major species (> 88%).