Cadmium effects on growth and mineral nutrition of two halophytes: Sesuvium portulacastrum and Mesembryanthemum crystallinum

How does NaCl enhance Cd tolerance in Inula crithmoides L? Insights into Cd uptake, compartmentalization, and chelation

This study investigated the effect of NaCl on the uptake, translocation, compartmentalization, and chelation of cadmium (Cd) in the halophyte Inula crithmoides. Seedlings were subjected hydroponically for 21 days to 25 and 50 μM Cd applied alone or combined with 100 mM NaCl. Findings revealed that, Cd alone induced intense chlorosis and necrosis and altered plant development resulting in diminished biomass production. However, NaCl alleviated Cd-induced toxicity by increasing biomass accumulation, associated with restoration of photosynthesis activity. At the level of whole plant, NaCl reduced Cd concentration in different organs as well as its translocation toward the shoots. At the cellular level, Na Cl changed the cell-compartmentalization of Cd in the shoots and roots by inducing a preferential accumulation into the soluble fraction (vacuole). NaCl increased the chelation of Cd to chloride and nitrate. As compared to Cd alone, salt addition to Cd-treated plants enhanced significantly succinic acid concentration in the leaves suggesting a possible role of this acid in Cd-chelation. Globally, NaCl alleviated Cd-induced phytotoxicity in this halophyte by reducing Cd absorption, translocation and increased Cd fixation to organic acids as well as through the changes in Cd2+ cell compartmentalization. Obtained data suggested that this fast growing halophyte could be used to rehabilitate Cd polluted saline soils.

Exploring the potentials of Sesuvium portulacastrum L. for edibility and bioremediation of saline soils

Sesuvium portulacastrum L. is a flowering succulent halophyte in the ice plant family Aizoaceae. There are various ecotypes distributed in sandy coastlines and salty marshlands in tropical and subtropical regions with the common name of sea purslane. These plants are tolerant to salt, drought, and flooding stresses and have been used for the stabilization of sand dunes and the restoration of coastal areas. With the increased salinization of agricultural soils and the widespread pollution of toxic metals in the environment, as well as excessive nutrients in waterbodies, S. portulacastrum has been explored for the desalination of saline soils and the phytoremediation of metals from contaminated soils and nitrogen and phosphorus from eutrophic water. In addition, sea purslane has nutraceutical and pharmaceutical value. Tissue analysis indicates that many ecotypes are rich in carbohydrates, proteins, vitamins, and mineral nutrients. Native Americans in Florida eat it raw, pickled, or cooked. In the Philippines, it is known as atchara after being pickled. S. portulacastrum contains high levels of ecdysteroids, which possess antidiabetic, anticancer, and anti-inflammatory activities in mammals. In this review article, we present the botanical information, the physiological and molecular mechanisms underlying the tolerance of sea purslane to different stresses, its nutritional and pharmaceutical value, and the methods for its propagation and production in saline soils and waterbodies. Its adaptability to a wide range of stressful environments and its role in the production of valuable bioactive compounds suggest that S. portulacastrum can be produced in saline soils as a leafy vegetable and is a valuable genetic resource that can be used for the bioremediation of soil salinity and eutrophic water.

Physiological response and tolerance of Sesuvium portulacastrum L. to low temperature stress

The plant Sesuvium portulacastrum L., commonly referred to as sea purslane, is a perennial halophytic species with significant potential for development in marine ecological restoration. However, its growth is limited in high-latitude regions with lower temperatures due to its subtropical nature. Furthermore, literature on its cold tolerance is scarce. This study, therefore, focused on sea purslane plants naturally overwintering in Ningbo (29°77'N), investigating their morphological, histological, rooting, and physiological responses to low temperatures (7 °C, 11 °C, 15 °C, and 19 °C). The findings indicated an escalation in cold damage severity with decreasing temperatures. At 7 °C, the plants failed to root and subsequently perished. In contrast, at 11 °C, root systems developed, while at 15 °C and 19 °C, the plants exhibited robust growth, outperforming the 11 °C group in terms of leaf number and root length significantly (P < 0.05). Histological analyses showed a marked reduction in leaf thickness under cold stress (P < 0.05), with disorganized leaf structure observed in the 7 °C group, whereas it remained stable at higher temperatures. No root primordia were evident in the vascular cambium of the 7 and 11 °C groups, in contrast to the 15 and 19 °C groups. Total chlorophyll content decreased with temperature, following the order: 19 °C > 15 °C > 11 °C > 7 °C. Notably, ascorbic acid levels were significantly higher in the 7 and 11 °C groups than in the 15 and 19 °C groups. Additionally, the proline concentration in the 7 °C group was approximately fourfold higher than in the 19 °C group. Activities of antioxidant enzymes-superoxide dismutase, peroxidase, and catalase-were significantly elevated in the 7 and 11 °C groups compared to the 15 and 19 °C groups. Moreover, the malondialdehyde content in the 7 °C group (36.63 ± 1.75 nmol/g) was significantly higher, about 5.5 and 9.6 times, compared to the 15 °C and 19 °C groups, respectively. In summary, 7 °C is a critical threshold for sea purslane stem segments; below this temperature, cellular homeostasis is disrupted, leading to an excessive accumulation of lipid peroxides and subsequent death due to an inability to neutralize excess reactive oxygen species. At 11 °C, although photosynthesis is impaired, self-protective mechanisms such as enhanced antioxidative systems and osmoregulation are activated. However, root development is compromised, resulting in stunted growth. These results contribute to expanding the geographic distribution of sea purslane and provide a theoretical basis for its ecological restoration in high-latitude mariculture.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01429-6.

Effect of Na, K and Ca Salts on Growth, Physiological Performance, Ion Accumulation and Mineral Nutrition of Mesembryanthemum crystallinum

Mesembryanthemum crystallinum L. is an obligatory halophyte species showing optimum growth at elevated soil salinity levels, but the ionic requirements for growth stimulation are not known. The aim of the present study was to compare the effects of sodium, potassium and calcium in the form of chloride and nitrate salts on the growth, physiological performance, ion accumulation and mineral nutrition of M. crystallinum plants in controlled conditions. In a paradoxical way, while sodium and potassium had comparable stimulative effect on plant growth, the effect of calcium was strongly negative even at a relatively low concentration, eventually leading to plant death. Moreover, the effect of Ca nitrate was less negative in comparison to that of Ca chloride, but K in the form of nitrate had some negative effects. There were three components of the stimulation of biomass accumulation by NaCl and KCl salinity in M. crsytallinum: the increase in tissue water content, increase in ion accumulation, and growth activation. As optimum growth was in a salinity range from 20 to 100 mM, the increase in the dry biomass of plants at a moderate (200 mM) and high (400 mM) salinity in comparison to control plants was mostly due to ion accumulation. Among physiological indicators, changes in leaf chlorophyll concentration appeared relatively late, but the chlorophyll a fluorescence parameter, Performance Index Total, was the most sensitive to the effect of salts. In conclusion, both sodium and potassium in the form of chloride salts are efficient in promoting the optimum growth of M. crystallinum plants. However, mechanisms leading to the negative effect of calcium on plants need to be assessed further.

Evaluation of the potential of two halophytes to extract Cd and Zn from contaminated saltwater

The contamination of saltwater by toxic heavy metals has become a worldwide problem. The application of phytoextraction to remove these pollutants seems to be more efficient and cheaper compared to physicochemical methods. In this work, we evaluated the potential of two halophyte species to accumulate cadmium and zinc from contaminated water. Seedlings of Carpobrotus edulis L. and Sesuvium portulacastrum L. were cultivated during 1 month on pots filled with saltwater (200 mM NaCl) containing different concentrations of Cd2+ (0, 50, 100 μM) and of Zn2+ (0, 200, and 400 μM) applied separately. Results showed that both halophytes were more resistant to Zn2+ than Cd2+ and that Sesuvium better tolerates the two metals. Zn2+ and Cd2+ concentrations in the shoot as well as the values of translocation factors suggest that these species are able to absorb and to concentrate Cd2+ and Zn2+ in their roots and shoots. Hence, after 1 month of culture on 50 μM Cd2+, plants were able to extract 31% and 21% of Cd, respectively, in S. portulacastrum and C. edulis. The Zn-extraction efficiency was less important and reached 18 and 19%, respectively, in S. portulacastrum and C. edulis cultivated under 200 μM Zn2+. Given together, data demonstrate the efficiency of the use of halophytes, especially S. portulacastrum, to extract Zn2+ and Cd2+ from salt wastewater.

Physiological and Cellular Ultrastructural Responses of Sesuvium portulacastrum under Cd Stress Grown Hydroponically

This study aimed to investigate the physiological and cellular mechanisms of Sesuvium portulacastrum under heavy metal stress to evaluate possible tolerance and adaptation mechanisms in a metal-polluted environment. The physiological and cellular ultrastructural responses of S. portulacastrum were studied hydroponically under exposure to a range of cadmium (Cd) concentrations (50 µM to 600 µM) for 28 days. The activity of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), changes in chlorophyll, and cellular ultrastructural content were examined. There was no significant difference in chlorophyll content in the leaf under the stress of 300 μM, but 400 μM and 600 μM Cd stress showed significantly decreased chlorophyll content. The SOD activity indicates an increase under the Cd stress of 100 μM for leaves, 300 μM for stems, and 50 μM for roots; after that, the SOD activity gradually decreased with increasing Cd concentrations. But POD activity was considerably increased with increasing Cd stress. CAT activity showed a gradual increase in concentrations until 300 μM of Cd stress and then decreased sharply in roots, stems, and leaf tissues. Cd stress had a considerable impact on the structure of the roots, stems, and leaves cells, such as distorted and thinner cell walls and the deformation of chloroplasts, mitochondria, and other organelles. Therefore, the increased number of nucleolus in the cell nucleus suggests that cells may be able to maintain their protein synthesis in a stressful environment. This study concludes that SOD is the dominant antioxidant enzyme activity during low Cd toxicity (<100 μM), while POD is the dominant enzyme activity during higher Cd toxicity (>100 μM).

Effects of Salinity Stress on Growth and Physiological Parameters and Related Gene Expression in Different Ecotypes of Sesuvium portulacastrum on Hainan Island

We conducted a study to examine the growth and physiological changes in 12 different ecotypes of Sesuvium portulacastrum collected from Hainan Island in China. These ecotypes were subjected to different concentrations (0, 200, 400, and 600 mmol/L) of sodium chloride (NaCl) salt stress for 14 days. We also analyzed the expression of metabolic genes related to stress response. Under low salt stress, indicators such as plant height in region K (0 mmol/L: 45% and highest at 200 mmol/L: 80%), internode length (0 mmol/L: 0.38, 200 mmol/L: 0.87, 400 mmol/L: 0.25, and 600 mmol/L: 1.35), as well as leaf area, relative water content, fresh weight, and dry weight exhibited an overall increasing trend with the increase in salt concentration. However, as the salt concentration increased, these indicators showed a decreasing trend. Proline and malondialdehyde contents increased with higher salt concentrations. When the NaCl concentration was 400 mmol/L, MDA content in the leaves was highest in the regions E (196.23%), F (94.28%), J (170.10%), and K (136.08%) as compared to the control group, respectively. Most materials demonstrated a significant decrease in chlorophyll a, chlorophyll b, and total chlorophyll content compared to the control group. Furthermore, the ratio of chlorophyll a to chlorophyll b (Rab) varied among different materials. Using principal component analysis, we identified three ecotypes (L from Xinglong Village, Danzhou City; B from Shuigoupo Village, Lingshui County; and J from Haidongfang Park, Dongfang City) that represented high, medium, and low salt tolerance levels, respectively, based on the above growth and physiological indexes. To further investigate the expression changes of related genes at the transcriptional level, we employed qRT-PCR. The results showed that the relative expression of SpP5CS1, SpLOX1, and SpLOX1 genes increased with higher salt concentrations, which corresponded to the accumulation of proline and malondialdehyde content, respectively. However, the relative expression of SpCHL1a and SpCHL1b did not exhibit a consistent pattern. This study contributes to our understanding of the salt tolerance mechanism in the true halophyte S. portulacastrum, providing a solid theoretical foundation for further research in this field.

Overexpression of RtSYP121 confers cadmium colerance by promoting vesicle trafficking, maintaining ion homeostasis, and alleviating photosynthetic inhibition in Arabidopsis

Cadmium (Cd) is a toxic heavy metal in soil that seriously threatens crop production, food security, and human health. Syntaxins, a prototype family of Soluble N-ethyl-maleimide-associated protein receptors (SNAREs) involved in vesicle trafficking, are implicated in resistance to abiotic stresses, including Cd stress, but the molecular mechanisms underlying the involvement of syntaxins in Cd tolerance in plants are unclear. In this study, we isolated and functionally characterized the syntaxin gene RtSYP121 from Reaumuria trigyna to evaluate its potential for phytoremediation. RtSYP121 resides in the plasma membrane. The transcriptional level of RtSYP121 was strongly increased by salt, drought, and Cd stress. Overexpression of RtSYP121 significantly enhanced the Cd tolerance of transgenic Arabidopsis. The Cd tolerance of transgenic plants mainly depended on elevated vesicle trafficking, which increased the content of K⁺ and Ca²⁺ and thus decreased the accumulation of Cd²⁺ by regulating the delivery or activity of ion transporters, channels, and pumps. Moreover, overexpression of RtSYP121 in Arabidopsis ameliorated Cd stress-induced phytotoxic effects, including growth inhibition, ROS burst, photosynthetic impairment, and cell death. Therefore, we suggest that RtSYP121 plays multiple roles in the plant response to Cd stress by promoting vesicle trafficking, maintaining ion homeostasis, and alleviating photosynthetic inhibition.

Halophytes for the sustainable remediation of heavy metal-contaminated sites: Recent developments and future perspectives

Increasing land degradation by high level of metal wastes is of prime concern for the global research communities. In this respect, halophytes having specific features like salt glands, exclusion of excess ions, heavy metals (HMs) compartmentalization, large pool of antioxidants, and associations with metal-tolerant microbes are of great promise in the sustainable clean-up of contaminated sites. However, sustainable clean-up of HMs by a particular halophyte plant species is governed considerably by physico-chemical characteristics of soil and associated microbial communities. The present review has shed light on the superiority of halophytes over non-halophytes, mechanisms of metal-remediation, recent developments and future perspectives pertaining to the utilization of halophytes in management of HM-contaminated sites with the aid of bibliometric analysis. The results revealed that the research field is receiving considerable attention in the last 5-10 years by publishing ∼50-90% documents with an annual growth rate of 15.41% and citations per document of 29.72. Asian (viz., China, India, and Pakistan) and European (viz., Spain, Portugal, Belgium, Argentina) countries have been emerged as the major regions conducting and publishing extensive research on this topic. The investigations conducted both under in vitro and field conditions have reflected the inherent potential of halophyte as sustainable research tool for successfully restoring the HM-contaminated sites. The findings revealed that the microbial association with halophytes under different challenging conditions is a win-win approach for metal remediation. Therefore, exploration of new halophyte species and associated microorganisms (endophytic and rhizospheric) from different geographical locations, and identification of genes conferring tolerance and phytoremediation of metal contaminants would further advance the intervention of halophytes for sustainable ecological restoration.

Mechanisms of Metal Tolerance in Halophytes: A Mini Review

Industrialization and urbanization of coastal wetlands have caused metal pollution worldwide. Phytoremediation has been widely used for treating soil and water pollution, and halophytes are considered a promising remediation method to address metal pollution. However, application of halophytes in phytoremediation is still in its infancy. To increase awareness of halophytes, the metal accumulation, tolerance, and mechanisms of metal detoxification in halophytes are reviewed here. Several halophytes are considered as potential candidates for phytoremediation because they have strong accumulation capacity of metals.

Magnetic Treatment Improves the Seedling Growth, Nitrogen Metabolism, and Mineral Nutrient Contents in Populus × euramericana ‘Neva’ under Cadmium Stress

This pot experiment was carried out to investigate the mechanism underlying nutrient metabolism and seedling growth responses to magnetic treatment following exposure to cadmium (Cd) stress. A magnetic device of 300 Gs was applied during Cd(NO3)2 solution treatment at 0 and 100 mM·L−1. One-year-old seedlings of Populus × euramericana ‘Neva’ were treated with different Cd(NO3)2 solutions in the presence or absence of magnetic treatment for 30 days. Seedling growth and physiological–biochemical indexes were measured under Cd stress. The contents of ammonium (NH4+–N), nitrate (NO3––N), and total nitrogen (TN) in leaves, as well as NH4+–N and TN in roots, were increased by magnetic treatment combined with Cd stress, although the NO3––N content was decreased. The activities of nitrate reductase (NR), nitrite reductase (NiR), glutathione reductase (GR), and glutamate synthase (GOGAT) in leaves and the activities of NR, glutamine synthetase (GS), and GOGAT in roots were stimulated by magnetic treatment; conversely, the NiR activity in roots was inhibited by magnetic effects. Magnetic treatment improved the synthesis of cysteine (Cys) and glutamine (Gln) in leaves and reduced the contents of glutamic acid (Glu) and glycine (Gly), while the contents of Cys, Glu, Gln, and Gly were increased in roots. The contents of Ca, Mg, Fe, Mn, Zn, and Cu in leaves were increased by magnetic treatment under Cd stress, whereas the content of K was reduced. In roots, the contents of K, Ca, and Fe were increased by magnetic treatment under Cd stress, but the contents of Na, Mg, Mn, Zn, and Cu were decreased. Magnetization could regulate the uptake of mineral nutrients by roots and translocation from the roots to the aboveground parts by affecting root morphology. Magnetic treatment could also improve nitrogen assimilation and the synthesis of free amino acids by stimulating the activities of key enzymes.

Halophytes for phytoremediation of hazardous metal(loid)s: A terse review on metal tolerance, bio-indication and hyperaccumulation

Phytoremediation is a cost-effective and environment friendly method for cleaning metal(loid)s from contaminated soils. Species with exceptionally higher shoot metal concentrations (hyperaccumulators) seem ideal for phytoremediation, though some metal tolerant species with 'above normal' values with higher translocation factor (TF) may also serve the purpose. Halophytes not only remove salts and metalloids from soils but may also be cultivated as non-conventional crops. Nurturing halophytes requires precise understanding of their nature and efficient management for sustainable use. Species with low metal concentrations in their edible parts (especially leaves) may be grown as forage and fodder, but those with metal hyperaccumulation could prove fatal due to their serious health hazards. Like other metallophytes, redundant use of the term 'metal hyperaccumulation' among halophytes needs to be revisited for its ambiguity and potential pitfalls. Similarly, understanding of metal tolerance and shoot accumulation nature of halophytes is needed prior to their use. This review is an attempt to compare halophytes with potential of metal bioindication, phytostabilization and hyperaccumulation (as per definition) as well as their 'obligate' and 'facultative' nature for appropriate uses.

Trace metal accumulation in seagrass and saltmarsh ecosystems of India: comparative assessment and bioindicator potential

Coastal macrophytes serve as bioindicators of coastal trace metal contamination. In this study, trace metal levels in India's seagrass and saltmarsh ecosystems were assessed for their suitability as bioindicators of metal contamination. Trace metal accumulation and bioindicator potential of both seagrasses and saltmarshes were found to be metal and species-specific. Higher concentrations of Cu, Fe, Mg and Mn were found in the tissues of seagrasses, while saltmarshes showed higher accumulation of Cd, Cr, Hg, Ni, Pb and Zn. The leaves of seagrasses are suitable bioindicator of metals in the water column, while the roots and rhizomes of saltmarshes/seagrasses are suitable bioindicators of metals in the sediment. This study proposes the development of a monitoring network using seagrasses and saltmarss as model organisms for short and long-term monitoring of coastal metal contamination. Determining the phytotoxic levels of trace metals in seagrasses and saltmarsh is important for monitoring plant die-offs and loss.

Distribution and dispersion of heavy metals in the rock-soil-moss system of the black shale areas in the southeast of Guizhou Province, China

Black shales are easily exposed due to human activities such as mining, road construction, and shale gas development, which results in several environmental issues including heavy metal (HM) pollution, soil erosion, and the destruction of vegetation. Mosses are widely used to monitor metal pollution in the atmosphere, but few studies on the distribution and dispersion of HMs in the rock-soil-moss system are available. Here, mosses (Pohlia flexuosa Harv. in Hook), growing soils, and corresponding parent rocks were collected from black shale areas. After appropriate pretreatment, samples were analyzed for multiple elemental concentrations by ICP-AES and ICP-MS. The results show that black shale parent rocks have elevated HM concentration and act as a source of multiple metals. The overlying soil significantly inherits and accumulates heavy metals released from black shale. Significant positive correlations between HMs in P. flexuosa and the growing soils indicate that HMs are mainly originating from geological source rather than atmospheric deposition. Differential accumulation of HMs is observed between rhizoids and stems in our study. Moreover, P. flexuosa is able to cope with high concentrations of toxic metals without any visible negative effect on its growth and development. Finally, the bioconcentration factor (BCF) for all the HMs in P. flexuosa is less than 1, indicating that it has a tolerance and exclusion mechanism for these metals, especially for the non-essential elements As and Pb. Therefore, the luxuriant and spontaneous growth of P. flexuosa could be used as a phytostabilization pioneer plant in the black shale outcrop where vascular plants are rare.

Cadmium-Tolerant Rhizospheric Bacteria of the C3/CAM Intermediate Semi-Halophytic Common Ice Plant (Mesembryanthemum crystallinum L.) Grown in Contaminated Soils

The common ice plant, Mesembryanthemum crystallinum L., has recently been found as a good candidate for phytoremediation of heavy-metal polluted soils. This semi-halophyte is a C₃/CAM (Crassulacean acid metabolism) intermediate plant capable of tolerating extreme levels of cadmium in the soil. The aim of the work was to obtain and characterize novel, Cd-tolerant microbial strains that populate the root zone of M. crystallinum performing different types of photosynthetic metabolism and growing in Cd-contaminated substrates. The plants exhibiting either C₃ or CAM photosynthesis were treated for 8 days with different CdCl₂ doses to obtain final Cd concentrations ranging from 0.82 to 818 mg⋅kg^-1 of soil d.w. The CAM phase was induced by highly saline conditions. After treatment, eighteen bacterial and three yeast strains were isolated from the rhizosphere and, after preliminary Cd-resistance in vitro test, five bacterial strains were selected and identified with a molecular proteomics technique. Two strains of the species Providencia rettgeri (W6 and W7) were obtained from the C₃ phase and three (one Paenibacillus glucanolyticus S7 and two Rhodococcus erythropolis strains: S4 and S10) from the CAM performing plants. The isolates were further tested for Cd-resistance (treatment with either 1 mM or 10 mM CdCl₂) and salinity tolerance (0.5 M NaCl) in model liquid cultures (incubation for 14 days). Providencia rettgeri W7 culture remained fully viable at 1 mM Cd, whereas Rh. erythropolis S4 and S10 together with P. glucanolyticus S7 were found to be resistant to 10 mM Cd in the presence of 0.5 M NaCl. It is suggested that the high tolerance of the common ice plant toward cadmium may result from the synergic action of the plant together with the Cd/salt-resistant strains occurring within rhizospheral microbiota. Moreover, the isolated bacteria appear as promising robust microorganisms for biotechnological applications in bio- and phytoremediation projects.

Antioxidant system response, mineral element uptake and safe utilization of Polygonatum sibiricum in cadmium-contaminated soil

Chinese herbal medicine is widely cultivated in Southwest China, where the soil cadmium (Cd) contamination of farmland is more serious than that in China as a whole. In this study, Polygonatum sibiricum was exposed to Cd at concentrations of e⁻¹, e⁰, e², and e⁴ mg/kg for 30, 60, and 90 days, and the physiological stress responses, Cd and mineral element uptake, antioxidant enzyme activities, and content changes of pharmaceutical ingredients (polysaccharides) were analyzed to decipher the feasibility of safe utilization in Cd-contaminated soil. The results show that the activity of antioxidant enzymes (SOD and CAT) in the aboveground part was always higher than that in the underground part. The underground part of Polygonatum sibiricum mobilizes nonenzymatic systems to facilitate the synthesis of polysaccharides (PCP1, PCP2) with antioxidant properties to cope with Cd stress. Mineral elements (P, K, Ca, Mg, Fe, Cu, and Zn) significantly (p < 0.05) changed after 90 d of cultivation. In particular, the changes in the iron and zinc content were significantly correlated (p < 0.05) with the activities of SOD and POD. Soil Cd at e⁰ mg/kg can guarantee the safe production and utilization of Polygonatum sibiricum, and the stimulation of Cd promotes polysaccharide synthesis and biomass growth.

Characterization of cadmium accumulation and phytoextraction in three species of the genus Atriplex (canescens, halimus and nummularia) in the presence or absence of salt

This study aims to establish for the first time a comparison between the resistance to cadmium (Cd) stress of three halophyte species, Atriplex canescens, Atriplex halimus and Atriplex nummularia in addition to their already known tolerance for salt and drought. Plants were exposed to CdCl₂ (20 and 50 μM) in the presence or in the absence of salt (50 mM NaCl) for one and two months. The amount of accumulated Cd was determined in the roots and leaves as well as the amount excreted on the surface of the leaves. Physiological parameters such as chlorophyll content and stress biomarkers, including malondialdehyde and enzymatic activities, were then analyzed. The results show that these plants are able to neutralize the excess of reactive oxygen species resulting from treatments by activating the antioxidant defense mechanisms in order to restore the homeostasis of cells. All three species are also able to accumulate high amounts of Cd in the leaves (several hundred mg of Cd/kg of dry leaves) and this phenomenon is amplified in the presence of salt. All together our results allow to consider the three Atriplex species as hyperaccumulators in the presence/absence of salt and as good candidates in a strategy of Cd phytoextraction in the presence of low concentrations of the pollutant. Nevertheless, both A. canescens and A. nummularia species seem to have a higher capacity to hyper-accumulate Cd when the concentration of Cd reaches higher level of contamination.

Salinity alleviates the toxicity level of ozone in a halophyte Mesembryanthemum crystallinum L

Mesembryanthemum crystallinum (Ice plant) is an annual halophytic plant species spread in the coastal areas of the Mediterranean Sea, Egypt. Information about the behaviour of halophytes under the future concentration of ozone (O₃) is scanty. Therefore, we have assessed the effects of elevated O₃ (ambient + 20 ppb), moderate salinity (200 mM NaCl), and their combined treatment (salinity + elevated O₃) on various morphological, growth, physiological, biochemical and anatomical parameters of Egyptian ice plant. Under salinity stress, plant growth, percentage of pigmented leaf and its thickness, ROS levels, antioxidative enzymes, and ROS scavenging activities were increased, while photosynthetic pigments and efficiency were decreased compared to the control. Elevated O₃ exposure led to reductions in most of the growth parameters and pigments, while ROS levels, histochemical localization of H₂O₂ and ·O₂^-, antioxidative enzymes and non-enzymatic antioxidants (betacyanin, phenolics, thiols and ascorbic acid) showed increases. Surprisingly, salinity alleviated the oxidative stress of elevated O₃ due to the rise of SOD activity, antioxidant compounds, and a decrease of ·O₂^- production rate with concomitant increases of most of the growth parameters. Thick lower collenchyma and enhancement of xylem parenchyma under O₃ and combined treatment suggested that anatomical acclimation also operated under O₃ stress and salinity played a vital role in the growth of this plant under combined stress. Results showed that salt is essential for the optimum development of this species and its role is extended to alleviate the oxidative damage caused by elevated O₃. The results further recommend the use of Egyptian M. crystallinum as a O₃ tolerant crop for saline areas along the Mediterranean Sea coast.

Phytoremediation: a sustainable environmental technology for heavy metals decontamination

Toxic metal contamination of soil is a major environmental hazard. Chemical methods for heavy metal's (HMs) decontamination such as heat treatment, electroremediation, soil replacement, precipitation and chemical leaching are generally very costly and not be applicable to agricultural lands. However, many strategies are being used to restore polluted environments. Among these, phytoremediation is a promising method based on the use of hyper-accumulator plant species that can tolerate high amounts of toxic HMs present in the environment/soil. Such a strategy uses green plants to remove, degrade, or detoxify toxic metals. Five types of phytoremediation technologies have often been employed for soil decontamination: phytostabilization, phytodegradation, rhizofiltration, phytoextraction and phytovolatilization. Traditional phytoremediation method presents some limitations regarding their applications at large scale, so the application of genetic engineering approaches such as transgenic transformation, nanoparticles addition and phytoremediation assisted with phytohormones, plant growth-promoting bacteria and AMF inoculation has been applied to ameliorate the efficacy of plants as candidates for HMs decontamination. In this review, aspects of HMs toxicity and their depollution procedures with focus on phytoremediation are discussed. Last, some recent innovative technologies for improving phytoremediation are highlighted.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae]

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.

Effect of foliar application of selenium on morphological and physiological indices of savory (Satureja hortensis) under cadmium stress

Cadmium is a heavy metal that pollutes the environment and affects plants physiologically and morphologically. Selenium is considered as a beneficial element, with effective roles in increasing plant tolerance to environmental stresses. A greenhouse factorial pot experiment was conducted to study the impact of selenium on traits of Savory plants under Cd stress. Experimental factors included soil contamination with cadmium (0, 75, 100, and 150 μM) and foliar spraying of selenium (0, 10, 20, and 40 μM of Sodium Selenate). Biomass, photosynthetic pigments including chlorophyll a, chlorophyll b, total chlorophyll, proline, total soluble solids, cell membrane leakage, relative water content of leaves antioxidant enzymes, and Cd and Zn concentration in shoot and root were recorded. Results revealed that Cd stress decreased vegetative growth criteria, photosynthetic pigments include chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid almost, 55%, 57%, 57%, and 68%, respectively, while poline, cell membrane leakage, peroxidase (POD), and catalase (CAT) antioxidant enzymes were increased with increasing Cd concentrations. Foliar spray of selenium reduced the toxic effects of Cd stress on savory plants via enhancing of proline content and stimulation of CAT and POD enzymes and limitation of cell membrane leakage. Also, selenium foliar spray improved chlorophyll content under Cd stress condition and decreased cadmium accumulation 29% in root, respectively. In general, these results suggest that foliar application of selenium could mitigate Cd toxicity and improve growth and antioxidant capacity of savory under different level of cadmium heavy metal stress.

The Common Ice Plant (Mesembryanthemum crystallinum L.)-Phytoremediation Potential for Cadmium and Chromate-Contaminated Soils

The common ice plant (Mesembryanthemum crystallinum L.) is a widely studied model due to its tolerance to numerous biotic and abiotic stresses. In this study, carried out in model pots, the plants were treated with variant doses of Cd(II) and Cr(VI) and proved resistant to extreme levels of these heavy metals. Initial toxicity symptoms were observed upon final concentrations of 818 mg Cd kg-1 soil d.w., and 1699 mg Cr kg-1 applied as potassium chromate. Biometric analyses revealed that none of the Cr(VI) doses affected dry weight of the plant organs thus maintaining the shoot-to-root ratio. The Cd and Cr hypertolerance strategies were divergent and resulted in different accumulation patterns. For the case of Cd(II), an excluder-like mechanism was developed to prevent the plant from toxicity. For chromate, high accumulation potential together with Cr(VI) root-to-shoot translocation at sublethal concentrations was revealed (up to 6152 mg Cr kg-1 shoot at 4248 mg Cr kg-1 soil). It is concluded that M. crystallinum reveals considerable phytoremediation capabilities due to unique growth potential in contaminated substrates and is suitable for bioreclamation of degraded soils. The plant is especially applicable for efficient phytoextraction of chromate-contamination, whereas for Cd-affected areas it may have a phytostabilizing effect.

High NaCl tolerance potential of Bruguiera cylindrica (L.) Blume compromised by mild CuSO4 concentration as evidenced by unique physiochemical features

Differential response of Bruguiera cylindrica to individual (CuSO₄) and combined (CuSO₄ NaCl) effect was evaluated. The plantlets were treated with control, 0.15 mM CuSO₄, 400 mM NaCl and 0.15 mM CuSO₄ + 400 mM NaCl. Under combined stress, higher accumulation of Cu in the roots indicate that the roots are the primary site of Cu accumulation and thus the plant perform as an excluder and photosynthetic efficiency reduced drastically and significant enhancement in the superoxide and hydroxyl free radicals which increase membrane lipid peroxidation, leading to cellular damage and destruction. As evidenced from SEM-EDXMA, increase in Cu and Na⁺ levels in xylem and pith regions of leaf and stem and the presence of deeply stained structures, denoting the probable formation of complex containing the metal. Increased CaOx crystal forming cells (crystal idioblasts) reveals the regulation of bioaccumulated Cu and Na⁺ by complexing with CaOx. Thus the study suggested that, 400 mM NaCl and 0.15 mM CuSO₄ treatments does not have negative impact on plant growth, the NaCl tolerance potential compromised in the presence of mild CuSO₄ concentration during combined stress.

Phytoremediation capacity, growth and physiological responses of Crambe abyssinica Hochst on soil contaminated with Cd and Pb

The search for vegetal species regarding effectiveness in the phytoremediation of soils is of great importance, mainly in function of the great environmental problems, such as soil contamination with heavy metals, the necessity of producing more food, among others that mankind face today. This work aimed (i) to evaluate phytoremediation capacity of Crambe abyssinica Hochst and its growth in soil artificially contaminated with Cd and Pb, and (ii) to evaluate the possible impacts of crambe cultivation in contaminated soil conditions, in order to evaluate, to test, and to question the Brazilian CONAMA 420, providing important information that can be useful for governmental and environmental purposes. Two simultaneous experiments were developed, one for each metal. The soils were contaminated with salts of CdCl₂ and PbCl₂H₂O in five doses based on the investigation values (IV) of CONAMA Resolution 420, resulting in 0; 1.5; 3; 9 and 30 mg kg^-1 for Cd and 33; 90; 180; 540 and 1800 mg kg^-1 for Pb. Gaseous exchange, development, nutritional composition and production of plant components, as well as phytoavailability of metals, were evaluated. The contamination with metals reduced photosynthesis, increased breathing as well as leading to a negative effect on the mineral nutrition and productivity in general; Plants cultivated in soil with Cd presented higher phytoavailability when compared to those cultivated in the Pb conditions, being found metals in all parts of the crambe plants from 1.5 mg kg^{- 1} of Cd in the soil; and Pb was retained only in roots, not being translocated in the plant. Cd showed higher phytoavailability, being found in all parts of the plant and Pb was retained only in the roots. Cd showed a higher phytoavailability when compared to Pb, also being found in all parts of crambe plants from dose 1.5 mg kg-1 of Cd in soil, which is an environmental problem, since in these concentrations the cultivation of crops is allowed by Brazilian legislation CONAMA 420.

Expression of Genes Involved in Heavy Metal Trafficking in Plants Exposed to Salinity Stress and Elevated Cd Concentrations

Many areas intended for crop production suffer from the concomitant occurrence of heavy metal pollution and elevated salinity; therefore, halophytes seem to represent a promising perspective for the bioremediation of contaminated soils. In this study, the influence of Cd treatment (0.01-10.0 mM) and salinity stress (0.4 M NaCl) on the expression of genes involved in heavy metal uptake (irt2-iron-regulated protein 2, zip4-zinc-induced protein 4), vacuolar sequestration (abcc2-ATP-binding cassette 2, cax4-cation exchanger 2 pcs1-phytochelatin synthase 1) and translocation into aerial organs (hma4-heavy metal ATPase 4) were analyzed in a soil-grown semi-halophyte Mesembryanthemum crystallinum. The upregulation of irt2 expression induced by salinity was additionally enhanced by Cd treatment. Such changes were not observed for zip4. Stressor-induced alterations in abcc2, cax4, hma4 and pcs1 expression were most pronounced in the root tissue, and the expression of cax4, hma4 and pcs1 was upregulated in response to salinity and Cd. However, the cumulative effect of both stressors, similar to the one described for irt2, was observed only in the case of pcs1. The importance of salt stress in the irt2 expression regulation mechanism is proposed. To the best of our knowledge, this study is the first to report the combined effect of salinity and heavy metal stress on genes involved in heavy metal trafficking.

Why Does the Halophyte Mesembryanthemum crystallinum Better Tolerate Ni Toxicity than Brassica juncea: Implication of Antioxidant Defense Systems

The implication of enzymatic and non-enzymatic antioxidative systems in response to Ni was evaluated in the halophyte Mesembryanthemum crystallinum in comparison with the metal tolerant glycophyte species Brassica juncea. Seedlings of both species were hydroponically subjected during 21 days to 0, 25, 50, and 100 µM NiCl2. Growth parameters showed that the halophyte M. crystallinum was more tolerant to Ni than B. juncea. Malondialdehyde (MDA) content increased to a higher extent in B. juncea than in M. crystallinum. Antioxidant enzymesactivities were differently affected by Ni in both species. Nickel increased shoot superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in B. juncea, whereas these activities were reduced in M. crystallinum when exposed to metal stress. The root SOD, APX and guaiacol peroxidase (GPX) activities increased upon Ni treatments for both species. The content of non-enzymatic antioxidative molecules such as glutathione, non-protein thiols and proline increased in Ni-treated plants, except for GSH content in the shoot of B. juncea. Based on the oxidative balance, our findings confirm the higher tolerance of the halophyte M. crystallinum to Ni-induced oxidative stress comparatively to B. juncea. We suggest that M. crystallinum is able to overcome the produced ROS using the non-enzymatic system, while Ni-induced oxidative stress was more acute in B. juncea, leading this species to mainly use the enzymatic system to protect against reactive oxygen species.

Assessment of phytoremediation potential of native plant species naturally growing in a heavy metal-polluted saline-sodic soil

Many areas throughout the world, mainly arid and semi-arid regions, are simultaneously affected by salinity stress and heavy metal (HM) pollution. Phytoremediation of such environments needs suitable plants surviving under those combined stresses. In the present study, native species naturally growing under an extreme condition, around Qaleh-Zari copper mine located in the eastern part of Iran, with HM-contaminated saline-sodic soil, were identified to find suitable plant species for phytoremediation. For this purpose, the accumulation of HMs (Cu, Zn, Cd, and Pb) in the root and shoot (stem and leaf) of the plants and their surrounding soils was determined to find their main phytoremediation strategies: phytoextraction or phytostabilization. Seven native species surviving in such extreme condition were found, including Launaea arborescens (Batt.) Murb, Artemisia santolina Schrenk, Pulicaria gnaphalodes (Vent.) Boiss, Zygophyllum eurypterum Boiss. & Buhse, Peganum harmala L., Pteropyrum olivieri Jaub. & Spach, and Aerva javanica (Burm. f.) Juss. ex Schult. Evaluation of phytoremediation potential of the identified species based on the calculated HM bioconcentration in roots, HM translocation from roots to shoots, and HM accumulation in the shoots revealed that all of the species were metal phytostabilizers rather than hyperaccumulators. Therefore, these native species can be used for phytostabilization in the HM-contaminated saline soils to prevent HMs entering the uncontaminated areas and groundwater. Compared with the biennial low-biomass hyperaccumulators, some native species such as Z. eurypterum and A. javanica may have more economic value for phytoremediation because of a significant accumulation of HMs in their relatively higher biomass.

Halophyte Common Ice Plants: A Future Solution to Arable Land Salinization

The problems associated with the salinization of soils and water bodies and the increasing competition for scarce freshwater resources are increasing. Current attempts to adapt to these conditions through sustainable agriculture involves searching for new highly salt-tolerant crops, and wild species that have potential as saline crops are particularly suitable. The common ice plant (Mesembryanthemum crystallinum L.) is an edible halophyte member of the Aizoaceae family, which switches from C3 photosynthesis to crassulacean acid metabolism (CAM) when exposed to salinity or water stress. The aim of this review was to examine the potential of using the ice plant in both the wild and as a crop, and to describe its ecology and morphology, environmental and agronomic requirements, and physiology. The antioxidant properties and mineral composition of the ice plant are also beneficial to human health and have been extensively examined.

The response of a model C3/CAM intermediate semi-halophyte Mesembryanthemum crystallinum L. to elevated cadmium concentrations

Many areas exhibiting increased concentrations of soluble salts are simultaneously polluted with heavy metals (HM), and halophytes with extended tolerance to heavy metal toxicity seem to represent a promising tool for their phytoremediation. In this study, the response of the soil-grown C₃-CAM (Crassulacean acid metabolism) intermediate halophyte Mesembryanthemum crystallinum (common ice plant) to increased concentrations of Cd (0.01-1 mM) was investigated. None of the tested Cd treatments affected growth parameters or tissue water content of either C₃ or CAM-performing plants. Chlorophyll a fluorescence confirmed high tolerance of the photosynthetic apparatus of both metabolic states towards Cd. Plants performing both photosynthesis types accumulated significant Cd amounts only under the highest (1 mM) treatment, and the metal was primarily deposited in the roots, which are features typical of an excluding strategy. Upon the application of 1 mM Cd solution CAM-performing plants, due to the NaCl pre-treatment applied for CAM induction, were exposed to significantly higher amounts of bioavailable Cd in comparison with those of C₃-performing plants. As a result, roots of CAM plants accumulated over 4-fold higher Cd amounts when compared with C₃ plants. In our opinion, enhanced Cd-accumulating potential observed in CAM-performing plants was the effect of osmotic stress episode and resulting modifications e.g. in the detoxifying capacity of the antioxidative system. Increased antioxidative potential of NaCl pre-treated plants was pronounced with significantly higher activity of CuZnSOD (copper-zinc superoxide dismutase), not achievable in C₃ plants subjected to high Cd concentrations. Moreover, the applied Cd doses induced SOD activity in a compartment-dependent manner only in C₃ plants. We confirmed that none of the applied Cd concentrations initiated the metabolic shift from C₃ to CAM.

Characterization of physiochemical and anatomical features associated with enhanced phytostabilization of copper in Bruguiera cylindrica (L.) Blume

Copper is an essential micronutrient for normal plant metabolism and it is involved in number of physiological processes in plants but at the same time, at concentrations above threshold level, it acts as a potential stress factor. In this study, the phytoremediation potential of Bruguiera cylindrica (L.) Blume with respect to Cu was evaluated for the first time. Various physiochemical and anatomical parameters were analyzed in three-month-old healthy plantlets of B. cylindrica on exposure to different concentrations of CuSO₄ (0, 0.05, 0.15, and 0.25 mM)for 20 d. Higher uptake and accumulation of Cu in the roots indicates that the roots are the primary site of Cu accumulation and thus the plant perform as an excluder. Tolerance index values (TI > 60) reveals the phytoremediation potential of this plant. Metabolites are accumulated in plants to cope up with the oxidative damage due to Cu stress. Increased rate of proline and free amino acids content and soluble sugar content especially in leaves of B. cylindrica subjected to CuSO₄ contributes toward higher osmolality so as to counter the reduced water transport from roots. Nonenzymatic antioxidants like ascorbic acid, glutathione, and phenolics are the ROS scavenging compounds in the Defense system of B. cylindrica toward higher concentrations of CuSO₄, and of these, phenolics accumulation plays greater role in the antioxidative function in B. cylindrica in response to Cu stress. The histochemistry of B. cylindrica revealed the prominent occurrence of star-shaped calcium oxalate crystals when exposed to 0.25 mM CuSO₄, and it seems to be a prominent defense mechanism under Cu stress. Also a remarkable finding was the accumulation of Cu in the xylem vessels of plants on exposure of 0.25 mM CuSO₄ as compared to control. The infrared spectra were analyzed to compare the functional groups in the phenolics and carbohydrate constituents of control and CuSO₄-treated B. cylindrica plantlets and it indicated that carboxyl and hydroxyl groups are involved in the Cu binding so as to achieve tolerance to Cu. Thus this study revealed the potential role of B. cylindrica as a promising candidate for phytostabilization of copper.

Detection of Cadmium Risk to the Photosynthetic Performance of Hybrid Pennisetum

Photosynthesis plays an essential role in plant growth and crop yield, and the mechanisms of the effects of cadmium (Cd) on photosynthetic performance require more attention. The acute toxicity of Cd in soil to the photosynthetic capacity of Hybrid Pennisetum was evaluated using gas exchange parameters, A/Ci curves, light response curves, and chlorophyll a fluorescence transients after exposure to elevated Cd concentrations (0, 10, 20, 50, 70, and 100 mg kg-1) for a 3-month period. The results indicated that leaf Cd concentration in Hybrid Pennisetum increased with the strength of soil Cd stress and ranged from 4.9 to 15.8 μg g-1 DW. The accumulation of leaf Cd severely restricted photosynthesis and its non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum. The leaf chloroplasts at 10 and 20 mg kg-1 Cd concentrations showed no noticeable change, but the chlorophyll content significantly decreased by 9.0-20.4% at 50-100 mg kg-1 Cd concentrations. The Cd treatments also decreased plant ribulose-1,5-bisphosphate (RuBP) activity (Vcmax ) and regeneration capacity (Jmax ), triose phosphate utilization (TPU), light-saturated photosynthesis (Amax ), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (Rday ), but Cd treatment increased the light compensation point (LCP). The shape of chlorophyll a fluorescence transients in leaves was altered under different Cd treatments. The increased OJ phase and the decreased IP phase in fluorescence induction curves suggested that Cd toxicity inhibited both light use efficiency and photodamage avoidance ability. These results suggested that the decrease in photosynthesis through exposure to Cd may be a result of the decrease in leaf chlorophyll content, Rubisco activity, and RuBP regeneration, inhibition of triose phosphate utilization, reduction of the ability to use light and provide energy, and restrictions on electron transport in PSII.

Characteristics of cadmium accumulation and isotope fractionation in higher plants

Cadmium (Cd) pollution of the soil is an important global environmental issue owing to its great toxicity. The study of metal isotope fractionation is a novel technique that could be used to identify and quantify metal uptake and transport mechanisms in plant. In this study, cadmium tolerant Ricinus communis and hyperaccumulator Solanum nigrum have been cultured in different Cd concentration nutrient solutions. The Cd isotope values, metal elements concentrations in the organs (root, stem and leaf) in the two plant species have been measured during the growth periods (10d, 15d, 20d, 25d, and 30d). The results indicate that the organs of S. nigrum could be enriched with lighter Cd isotopes compared with R. communis. In addition, the Cd isotope fractionation become smaller when the plants were subjected to high Cd toxicity, which indicates that Cd isotope fractionation reflected the extent of Cd toxicity to plants. This study advances our current view of Cd translocation machination in plants.

Physiological response and mineral elements accumulation pattern in Sesuvium portulacastrum L. subjected in vitro to nickel

Sesuvium portulacastrum, a halophyte with high tolerance to heavy metals like Cd, Pb and Ni is considered for phytoremediation of metal contaminated saline soils. The tolerance to a selected metal ion could, by hypothesis, be stimulated through in vitro adaptation and regeneration of the plant. Seedlings obtained by in vitro micro-propagation, were exposed to 0, 25 and 50 μM Ni, as NiCl₂, in agar-based medium for 30 days. Growth parameters, plant water content, the concentration of photosynthetic pigments, proline and malondialdehyde (MDA) concentrations were determined. Nickel and nutrients distribution in leaves was studied by micro-Proton-Induced-X-ray-Emission (μ-PIXE). The results showed that Ni was mainly accumulated in vascular bundles, next in water storage tissues and chlorenchyma. Ni concentrations in chlorenchyma increased with increasing Ni in culturing medium, in direct relation to decrease of photosynthetic pigments and increase of oxidative stress. As compared to control plants, Ni induced substantial increase in MDA and proline accumulation. Plants exposed to 50 μM Ni accumulated up to 650 μg g^-1 of Ni in the shoots, exhibiting chlorosis and necrosis and a drastically reduced plant growth. Perturbations in uptake and distribution of nutrients were observed, inducing mineral deficiency, probably through membrane leakage. The mineral nutrient disturbances induced by Ni could be highly implicated in the restriction of S. portulacastrum development under the acute 50 μM Ni level.

Cadmium Accumulation and Tolerance in Seven Ornamental Willow Genotypes

Cadmium (Cd) accumulation and tolerance in seven ornamental willow genotypes were investigated at different Cd levels (0, 5, 25, 50, and 100 µM) through hydroponic culture in a greenhouse. Severe phytotoxic symptoms in leaves and roots including leaf chlorosis and root browning were noticed when grown at ≥ 50 µM Cd. All genotypes showed high tolerance to low Cd levels (5 µM and 25 µM Cd), in contrast to high Cd levels (i.e. 50 µM and 100 µM) that severely disturbed plant growth of all genotypes. Cadmium concentrations reached 165.0-1251.0 µg g^-1 DW in leaves, 22.9-331.2 µg g^-1 in stems among the genotypes at 5-25 µM Cd, and Cd mainly accumulated in the roots. Although willow tissues can accumulate relatively high Cd concentrations under high Cd levels (≥ 50 µM), they are not suitable for phytoextraction of Cd due to poor growth.

Tolerance and bioaccumulation of combined copper, zinc, and cadmium in Sesuvium portulacastrum

Sesuvium portulacastrum was treated with mixture of copper, zinc, and cadmium for 60 days, with the concentration of each metal ranging from 0 to 20 mg/L. The tolerance of plants and bioaccumulation of heavy metals were then investigated. The height of S. portulacastrum decreased significantly with increasing heavy metal concentrations from 1 to 20 mg/L. The biomass was adversely impacted when the concentration exceeded 5 mg/L. There were no significant differences in malondialdehyde (MDA) concentration among different treatment groups, while the soluble protein content and superoxide dismutase (SOD) activity decreased with increasing heavy metal concentration. However, the BCF values of the three metals were all higher than 10 and the tolerance in root was up to 1000 mg/kg without causing significant growth inhibition, suggesting that S. portulacastrum should be a potential candidate for phytostabilization for the phytoremediation of polymetallic contaminations in coastal environments.

Towards saving freshwater: halophytes as unconventional feedstuffs in livestock feed: a review

Water represents 71% of all earth area and about 97% of this water is salty water. So, only 3% of the overall world water quantity is freshwater. Human can benefit only from 1% of this water and the remaining 2% freeze at both poles of earth. Therefore, it is important to preserve the freshwater through increasing the plants consuming salty water. The future prosperity of feed resources in arid and semi-arid countries depends on economic use of alternative resources that have been marginalized for long periods of time, such as halophytic plants, which are one such potential future resource. Halophyte plants can grow in high salinity water and soil and to some extent during drought. The growth of these plants depends on the contact of the salted water with plant roots as in semi-desert saline water, mangrove swamps, marshes, and seashores. Halophyte plants need high levels of sodium chloride in the soil water for growth, and the soil water must also contain high levels of salts, as sodium hydroxide or magnesium sulfate. There are many uses for halophyte plants, including feed for animals, vegetables, drugs, sand dune stabilizers, wind shelter, soil cover, wetland cultivation, laundry detergents, and paper production. This paper will focus on the use of halophytes as a feed additive for animals. In spite of the good nutritional value of halophytes, some anti-nutritional factors as nitrates, nitrite complexes, tannins, glycosides, phenolic compounds, saponins, oxalates, and alkaloids may be present in some of them. The presence of such anti-nutritional agents makes halophytes unpalatable to animals, which tends to reduce feed intake and nutrient use. Therefore, the negative effects of these plants on animal performance are the only objection against using halophytes in animal feed diets. This review article highlights the beneficial impact of considering halophytes in animal feeding on saving freshwater and illustrates its nutritive value for livestock from different aspects.

Tolerance and bioaccumulation of Cd and Cu in Sesuvium portulacastrum

In order to investigate the tolerance and bioaccumulation of Cd and Cu in the halophyte Sesuvium portulacastrum, seedlings were hydroponically cultured for 30 days using the modified 1/2 Hoagland nutrient solution with different concentrations of Cd (0, 5, 10, 15, and 20mgL^-1) and Cu (0, 2.5, 5, 7.5, and 10mgL^-1). Afterwards, the seedling height, leaf area, biomass, and mineral element contents (Fe, Mg, Cu, and Zn) in the roots, stems and leaves were measured, and the tolerance index, bioconcentration factor (BCF), transportation index, and removal rate were calculated. The effects of salinity (0‰-30‰) on the growth and bioaccumulation ability of S. portulacastrum under combined Cu/Cd (5mgL^-1) exposure were also determined. The results showed that, with an increasing Cd concentration, the biomass and seedling height of S. portulacastrum initially increased and then decreased. The highest leaf biomass and seedlings height was observed in the 10mgL^-1 and 5mgL^-1 Cd treatment group, respectively. Salinity did not affect the biomass of S. portulacastrum but decreased Cd concentration in roots and aboveground tissues and Cu concentration in roots of S. portulacastrum. Cu treatment significantly facilitated the absorption of Mg, Cu, and Zn in roots. With an increasing Cu concentration, the Mg and Fe contents increased in the leaves of S. portulacastrum. In comparison to the above-ground portions, the root showed a higher bioaccumulation ability of Cd and Cu, with the BCF of 341.5 and 211.9, respectively. The BCF and translocation factor (TF) values indicated that S. portulacastrum was not a hyperaccumulator for Cd and Cu, but could be used as a phytostablization plant in heavy metal contaminated coastal environments.

Nickel tolerance, accumulation and subcellular distribution in the halophytes Sesuvium portulacastrum and Cakile maritima

It has been shown that halophytes are able to successfully cope with heavy metal toxicity, suggesting their possible use for remediation of metal contaminated soils. In this work, Ni tolerance and accumulation in two halophytes, Sesuvium portulacastrum (L.) L. and Cakile maritima Scop. was investigated. Seedlings of both species were subjected hydroponically during 21 days to 0, 25, 50, and 100 μM of NiCl₂. The growth and photosynthesis parameters revealed that S. portulacastrum tolerates Ni better than C. maritima. The photosynthesis activity, chlorophyll content and photosystem II integrity were less impacted in Ni-treated S. portulacastrum as compared to C. maritima, although, Ni accumulated in higher concentrations in the shoots of S. portulacastrum (1050 μg g^-1 DW) than in those of C. maritima (550 μg g^-1 DW). The subcellular fractionation of Ni in the shoots of both species showed that C. maritima accumulated about 65% of Ni in the soluble fraction, while 28% was associated with the cell walls. In S. portulacastrum 44% of the total cellular Ni was seen in the soluble fraction and 43% was bound to the cell walls. It can be concluded that S. portulacastrum tolerates Ni better than C. maritima, most probably due to a better ability to sequester Ni in the cell walls, restricting its accumulation in the soluble fraction.

High salinity helps the halophyte Sesuvium portulacastrum in defense against Cd toxicity by maintaining redox balance and photosynthesis

NaCl alleviates Cd toxicity in Sesvium portulacastrum by maintaining plant water status and redox balance, protecting chloroplasts structure and inducing some potential Cd (2+) chelators as GSH and proline. It has been demonstrated that NaCl alleviates Cd-induced growth inhibition in the halophyte Sesuvium portulacastrum. However, the processes that mediate this effect are still unclear. In this work we combined physiological, biochemical and ultrastructural studies to highlight the effects of salt on the redox balance and photosynthesis in Cd-stressed plants. Seedlings were exposed to different Cd concentrations (0, 25 and 50 µM Cd) combined with low (0.09 mM) (LS), or high (200 mM) NaCl (HS) in hydroponic culture. Plant-water relations, photosynthesis rate, leaf gas exchange, chlorophyll fluorescence, chloroplast ultrastructure, and proline and glutathione concentrations were analyzed after 1 month of treatment. In addition, the endogenous levels of stress-related hormones were determined in plants subjected to 25 µM Cd combined with both NaCl concentrations. In plants with low salt supply (LS), Cd reduced growth, induced plant dehydration, disrupted chloroplast structure and functioning, decreased net CO2 assimilation rate (A) and transpiration rate (E), inhibited the maximum potential quantum efficiency (Fv/Fm) and the quantum yield efficiency (Φ PSII) of PSII, and enhanced the non-photochemical quenching (NPQ). The addition of 200 mM NaCl (HS) to the Cd-containing medium culture significantly mitigated Cd phytotoxicity. Hence, even at similar internal Cd concentrations, HS-Cd plants were less affected by Cd than LS-Cd ones. Hence, 200 mM NaCl significantly alleviates Cd-induced toxicity symptoms, growth inhibition, and photosynthesis disturbances. The cell ultrastructure was better preserved in HS-Cd plants but affected in LS-Cd plants. The HS-Cd plants showed also higher concentrations of reduced glutathione (GSH), proline and jasmonic acid (JA) than the LS-Cd plants. However, under LS-Cd conditions, plants maintained higher concentration of salicylic acid (SA) and abscisic acid (ABA) than the HS-Cd ones. We conclude that in S. portulacastrum alleviation of Cd toxicity by NaCl is related to the modification of GSH and proline contents as well as stress hormone levels thus protecting redox balance and photosynthesis.

Moderate salinity reduced phenanthrene-induced stress in the halophyte plant model Thellungiella salsuginea compared to its glycophyte relative Arabidopsis thaliana: Cross talk and metabolite profiling

It was shown that halophytes experience higher cross-tolerance to stresses than glycophytes, which was often associated with their more powerful antioxidant systems. Moreover, salinity was reported to enhance halophyte tolerance to several stresses. The aim of the present work was to investigate whether a moderate salinity enhances phenanthrene stress tolerance in the halophyte Thellungiella salsuginea. The model plant Arabidopsis thaliana, considered as its glycophyte relative, was used as reference. Our study was based on morpho-physiological, antioxidant, and metabolomic parameters. Results showed that T. salsuginea was more tolerant to phenanthrene stress as compared to A. thaliana. An improvement of phenanthrene-induced responses was recorded in the two plants in the presence of 25 mM NaCl, but the effect was significantly more obvious in the halophyte. This observation was particularly related to the higher antioxidant activities and the induction of more adapted metabolism in the halophyte. Gas Chromatography coupled with Mass Spectrometry (GC-MS) was used to quantify alcohols, ammonium, sugars, and organic acids. It showed the accumulation of several metabolites, many of them are known to be involved in signaling and abiotic stress tolerance. Moderate salinity and phenanthrene cross-tolerance involved in these two stresses was discussed.

Implication of citrate, malate and histidine in the accumulation and transport of nickel in Mesembryanthemum crystallinum and Brassica juncea

Citrate, malate and histidine have been involved in many processes including metal tolerance and accumulation in plants. These molecules have been frequently reported to be the potential nickel chelators, which most likely facilitate metal transport through xylem. In this context, we assess here, the relationship between organics acids and histidine content and nickel accumulation in Mesembryanthemum crystallinum and Brassica juncea grown in hydroponic media added with 25, 50 and 100 µM NiCl2. Results showed that M. crystallinum is relatively more tolerant to Ni toxicity than B. juncea. For both species, xylem transport rate of Ni increased with increasing Ni supply. A positive correlation was established between nickel and citrate concentrations in the xylem sap. In the shoot of B. juncea, citric and malic acids concentrations were significantly higher than in the shoot of M. crystallinum. Also, the shoots and roots of B. juncea accumulated much more histidine. In contrast, a higher root citrate concentration was observed in M. crystallinum. These findings suggest a specific involvement of malic and citric acid in Ni translocation and accumulation in M. crystallinum and B. juncea. The high citrate and histidine accumulation especially at 100µM NiCl2, in the roots of M. crystallinum might be among the important factors associated with the tolerance of this halophyte to toxic Ni levels.

SpBADH of the halophyte Sesuvium portulacastrum strongly confers drought tolerance through ROS scavenging in transgenic Arabidopsis

Glycine betaine (GB) accumulation is involved in abiotic stress. However, it is not known whether BADH, the key enzyme of GB synthesis, utilizes the antioxidant system to confer drought stress tolerance. In this study, a novel member of the ALDH10 gene family, SpBADH, was isolated from Sesuvium portulacastrum. The expression of this gene was up-regulated by NaCl, PEG6000, H2O2, ABA and high temperature in S. portulacastrum. SpBADH overexpression in Arabidopsis resulted in higher BADH activity and GB content and might increase tolerance to drought/osmotic stresses, specifically strong tolerance to drought stress. Transgenic lines exhibited lower MDA and H2O2 contents but higher proline, POD, SOD and CAT contents than the wild type under drought and osmotic stresses. SpBADH overexpression in Arabidopsis also enhanced the expression of ROS-related genes including AtSOD, AtPOD, AtCAT, AtAPX and Atpsb under drought and osmotic stresses. Thus, SpBADH increases plant tolerance to drought or osmotic stresses by reducing H2O2, increasing proline, and activating antioxidative enzymes to improve ROS scavenging.

Architectural and biochemical changes in embryonic tissues of maize under cadmium toxicity

Heavy metals greatly alter plant morphology and architecture, however detailed mechanisms of such changes are not fully explored. Two experiments were conducted to investigate the influence of cadmium (CdCl2 · 2.5H2 O) on some germination, morphological, biochemical and histological characteristics of developing embryonic tissue of maize. In the first experiment, maize seeds were germinated in increasing levels of CdCl2 (200-2000 μm) in sand and measurements were taken of changes in germination and seedling development attributes. Based on these parameters, 1000 μM CdCl2 was chosen for detailed biochemical and histological measurements. In the second experiment, seeds were germinated in Petri dishes and supplied with 0 (control) or 1000 μM CdCl2 (Cd-treated). Radicle, plumule, coleoptile and coleorhiza were measured for biochemical and histological changes. The highest amount of Cd was in the coleorhiza and radicle. Free proline, soluble sugars, anthocyanin, soluble phenolics, ascorbic acid, H2 O2 and MDA were significantly higher in coleorhizae, followed by the coleoptile, radicle and plumule. Although the radicle and coleorhiza were relatively poor targets of Cd than the other tissues, Cd stress reduced cortical cell size and vascular tissues, and deformed xylem and phloem parenchyma in all plant parts. In conclusion, the main reason for reduced germination was the influence of Cd on architecture of the coleorhiza and coleoptile, which was the result of oxidative stress and other physiological changes taking place in these tissues.

Hairy root induction and phytoremediation of textile dye, Reactive green 19A-HE4BD, in a halophyte, Sesuvium portulacastrum (L.) L

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Hairy roots induction from A. rhizogenes NCIM 5140 strain in Sesuvium.

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Textile dye degradation and color removal using hairy roots.

In this study, we report phytoremediation of textile dyes using hairy roots derived through Agrobacterium rhizogenes (NCIM 5140) infection of in vitro leaf and stem explants of a halophyte Sesuvium portulacastrum (L.) L. Leaf explants showed higher frequency of hairy root induction (70%) than stem explants (30%), and maximum number of roots (leaf 42.3 ± 2.4 and stem 50.3 ± 1.7). Transformed nature of hairy roots was ascertained by amplifying 970 bp region of T-DNA of Ri plasmid. Hairy roots were screened for phytoremediation of various textile dyes and results showed that HRs were able to degrade Reactive green 19A HE4BD upto 98% within 5 days of incubation. Spectrophotometric analysis showed decrease in dye concentration while HPLC and FTIR analysis confirmed its degradation. Seed germination assay demonstrated non-toxic nature of the extracted metabolites. This is the first report on induction of hairy root culture in Sesuvium portulacastrum and phytoremediation of textile dyes.

NaCl alleviates Cd toxicity by changing its chemical forms of accumulation in the halophyte Sesuvium portulacastrum

It has previously been shown that certain halophytes can grow and produce biomass despite of the contamination of their saline biotopes with toxic metals. This suggests that these plants are able to cope with both salinity and heavy metal constraints. NaCl is well tolerated by halophytes and apparently can modulate their responses to Cd. However, the underlying mechanisms remain unclear. This study explores the impact of NaCl on growth, Cd accumulation, and Cd speciation in tissues of the halophyte Sesuvium portulacastrum. Seedlings of S. portulacastrum were exposed during 1 month to 0, 25, and 50 μM Cd combined with low salinity (LS, 0.09 mM NaCl) or high salinity (HS, 200 mM NaCl) levels. Growth parameters and total tissue Cd concentrations were determined, in leaves, stems, and root. Moreover, Cd speciation in these organs was assessed by specific extraction procedures. Results showed that, at LS, Cd induced chlorosis and necrosis and drastically reduced plant growth. However, addition of 200 mM NaCl to Cd containing medium alleviated significantly Cd toxicity symptoms and restored plant growth. NaCl reduced the concentration of Cd in the shoots; nevertheless, due to maintenance of higher biomass under HS, the quantity of accumulated Cd was not modified. NaCl modified the chemical form of Cd in the tissues by increasing the proportion of Cd bound to pectates, proteins, and chloride suggesting that this change in speciation is involved in the positive impact of NaCl on Cd tolerance. We concluded that the tolerance of S. portulacastrum to Cd was enhanced by NaCl. This effect is rather governed by the modification of the speciation of the accumulated Cd than by the reduction of Cd absorption and translocation.

Cd and Ni transport and accumulation in the halophyte Sesuvium portulacastrum: implication of organic acids in these processes

The implication of organic acids in Cd and Ni translocation was studied in the halophyte species Sesuvium portulacastrum. Citric, fumaric, malic, and ascorbic acids were separated and quantified by HPLC technique in shoots, roots and xylem saps of plants grown on nutrient solutions added with 50 μM Cd, 100 μM Ni and the combination of 50 μM Cd + 100 μM Ni. Results showed that Cd had no significant impact on biomass production while Ni and the combination of both metals drastically affected plant development. Cadmium and Ni concentrations in tissues and xylem sap were higher in plants subjected to individual metal application than those subjected to the combined effect of Cd and Ni suggesting a possible competition between these metals for absorption. Both metals applied separately or in combination induced an increase in citrate concentration in shoots and xylem sap but a decrease of this concentration in the roots. However, a minor relationship was observed between metal application and fumaric, malic, and ascorbic acids. Both observations suggest the implication of citric acid in Cd, Ni translocation and shoot accumulation in S. portulacastrum. The relatively high accumulation of citric acid in xylem sap and shoot of S. portulacastrum could be involved in metal chelation and thus contributes to heavy metal tolerance in this species.