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.

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.

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.

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.

The present study was planned to explore the bioaccumulation potential of 23 plant species via bioaccumulation factor (BAf), metal accumulation index (MAI), translocation potential (Tf), and comprehensive bioconcentration index (CBCI) for seven heavy metals (cadmium, chromium, cobalt, copper, iron, manganese, and zinc). The studied plants, in the vicinity of ponds at Sahlon: site 1, Chahal Khurd: site 2, and Karnana: site 3 in Shaheed Bhagat Singh Nagar, Punjab (India), were Ageratum conyzoides (L.) L., Amaranthus spinosus L., Amaranthus viridis L., Brassica napus L., Cannabis sativa L., Dalbergia sissoo DC., Duranta repens L., Dysphania ambrosioides (L.) Mosyakin & Clemants, Ficus infectoria Roxb., Ficus palmata Forssk., Ficus religiosa L., Ipomoea carnea Jacq., Medicago polymorpha L., Melia azedarach L., Morus indica L., Malva rotundifolia L., Panicum virgatum L., Parthenium hysterophorus L., Dolichos lablab L., Ricinus communis L., Rumex dentatus L., Senna occidentalis (L.) Link, and Solanum nigrum L. BAf and Tf values showed high inter-site deviations for studied metals. MAI values were found to be more substantial in shoots as compared with that of roots of plants. Maximum CBCI values were observed for M. azedarach (0.626), M. indica (0.572), D. sissoo (0.497), and R. communis (0.474) for site 1; F. infectoria (0.629), R. communis (0.541), D. sissoo (0.483), F. palmata (0.457), and D. repens (0.448) for site 2; D. sissoo (0.681), F. religiosa (0.447), and R. communis (0.429) for site 3. Although, high bioaccumulation of individual metals was observed in herbs like C. sativa, M. polymorpha, and Amaranthus spp., cumulatively, trees were found to be the better bioaccumulators of heavy metals.

Fertilization is an important agricultural strategy for enhancing the efficiency of phytoremediation in copper (Cu)-contaminated soils. In this study, the effects of nitrogen (N) forms, including ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N), on the growth, translocation, and accumulation of Cu in the tissues of Ricinus communis L. were investigated in pot and hydroponic experiments. The results demonstrated that higher biomass and N contents in plants were obtained when N was supplied as NO₃⁻-N rather than NH₄⁺-N. Application of N increased the Cu content in the roots of R. communis, with a higher content after NH₄⁺-N (53.10–64.20 mg kg⁻¹) than NO₃⁻-N (37.62–53.75 mg kg⁻¹) treatment. On the contrary, the levels of Cu translocation factors were much higher in NO₃⁻-fed plants (0.34–0.45) than in NH₄⁺-fed plants (0.28–0.38). The suggested amount of N for fertilizer application is 225 kg hm⁻², which resulted in the highest Cu content in R. communis and optimal plant growth. As the main Cu-binding site, root cell walls accumulated less Cu in plants treated with NH₄⁺-N compared with NO₃⁻-N. Additionally, NH₄⁺-N induced a higher malondialdehyde content and more severe root damage compared with NO₃⁻-N. In the leaf, a larger number of black granules, which could be protein and starch grains involved in the detoxification of Cu in R. communis, were present after NH₄⁺-N than NO₃⁻-N treatment. These results illustrate that N forms are especially important for Cu translocation and accumulation and that immobilization and transformation of Cu in roots were improved more by NH₄⁺-N than NO₃⁻-N. In conclusion, N fertilizers containing the appropriate forms applied at suitable rates may enhance the biomass and Cu accumulation of R. communis and thereby the remediation efficiency of Cu-contaminated soils.

Cadmium (Cd) is considered to be the most phytotoxic heavy metal pollutant. The selection of castor bean cultivars with Cd tolerance and the exploration of the physiological mechanisms involved in Cd tolerance are critical steps for improving phytoremediation performance. In this study, a hydroponic experiment was used to investigate variations in Cd transportation, chelation, and subcellular distribution in two different castor bean cultivars, namely JX-22 and ZB-9. Both cultivars had high tolerance index scores, indicating that both cultivars were tolerant to Cd. The findings of the present study indicate that Cd is significantly more mobile in JX-22 than in ZB-9 during xylem and phloem transportation, resulting in the accumulation of Cd in the shoots of JX-22 was 7.67 times that in ZB-9. Subcellular distribution assessment verified that more Cd was bound to the biologically detoxified metal fractions than the metal sensitive fractions in JX-22. The contents of the non-protein thiol pool and glutathione in the leaves were higher in JX-22 than ZB-9 when exposed to Cd. These results indicate that JX-22 has a greater ability to accumulate Cd, and well-coordinated physiological changes in JX-22 afford greater Cd tolerance in comparison to ZB-9 under Cd exposure, indicating that JX-22 is suitable for use in the remediation of Cd-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%).