Short term toxicity of iron (Fe) and lead (Pb) to the mayfly Leptophlebia marginata (L.) (Insecta) in relation to freshwater acidification

The mayfly Leptophlebia marginata was exposed to different concentrations of Fe²⁺ or Pb²⁺ at pH 4.5 and pH 7.0. The effects of the metals on escape behavior and survival of the mayflies were investigated during an exposure of 120 hours.

1. Whole-body metal loads (Fe; Pb) of the mayflies increased in a dose-dependent way at both pH levels. A significant effect of pH on metal concentration in the mayflies was only found for Pb (p < 0.001).
2. In terms of mortality, both metals were more toxic at pH 4.5 than at pH 7. The 96 h-LC₅₀ values for Fe were 106.3 mg Fe l^-1 at pH 7 and 89.5 mg Fe l^-1 at pH 4.5. Those for Pb were > 5 mg Pb l^-1 at pH 7 and 1.09 mg Pb l^-1 at pH 4.5.
3. The mayflies lost their escape behavior, when exposed to the metals, the effects being more pronounced at low than at circumneutral pH for both metals (p < 0.05). The 96 h-EC₅₀ values for Fe were 70.0 mg Fe l^-1 at pH 7 and 63.9 mg Fe l^-1 at pH 4.5.

     

Kinetic properties of a micronutrient transporter from<Emphasis Type="Italic"> Pisum sativum</Emphasis> indicate a primary function in Fe uptake from the soil

Fe uptake in dicotyledonous plants is mediated by a root plasma membrane-bound ferric reductase that reduces extracellular Fe(III)-chelates, releasing Fe²⁺ ions, which are then absorbed via a metal ion transporter. We previously showed that Fe deficiency induces an increased capacity to absorb Fe and other micronutrient and heavy metals such as Zn²⁺ and Cd²⁺ into pea (Pisum sativum L.) roots [Cohen et al. (1998) Plant Physiol 116:1063–1072). To investigate the molecular basis for this phenomenon, an Fe-regulated transporter that is a homologue of the Arabidopsis IRT1 micronutrient transporter was isolated from pea seedlings. This cDNA clone, designated RIT1 for root iron transporter, encodes a 348 amino acid polypeptide with eight putative membrane-spanning domains that is induced under Fe deficiency and can functionally complement yeast mutants defective in high- and low-affinity Fe transport. Chelate buffer techniques were used to control Fe²⁺ in the uptake solution at nanomolar activities representative of those found in the rhizosphere, and radiotracer methodologies were employed to show that RIT1 is a very high-affinity ⁵⁹Fe²⁺ uptake system (K _m =54–93 nM). Additionally, radiotracer (⁶⁵Zn, ¹⁰⁹Cd) flux techniques were used to show that RIT can also mediate a lower affinity Zn and Cd influx (K _m of 4 and 100 M, for Zn²⁺ and Cd²⁺, respectively). These findings suggest that, in typical agricultural soils, RIT1 functions primarily as a high-affinity Fe²⁺ transporter that mediates root Fe acquisition. This is consistent with recent findings with Arabidopsis IRT1 knockout mutants that strongly suggest that this transporter plays a key role in root Fe uptake and nutrition. However, the ability of RIT1 to facilitate Zn and Cd uptake when these metals are present at elevated concentrations suggests that RIT1 may be one pathway for the entry of toxic metals into the food chain. Furthermore, the finding that plant Fe deficiency status may promote heavy metal uptake via increased expression of this transporter could have implications both for human nutrition and also for phytoremediation, the use of terrestrial plants to sequester toxic metals from contaminated soil.     

Iron plaque formation in the roots of Pistia stratiotes L.: importance in phytoremediation of cadmium

Aquatic macrophytes play an important role in the removal of toxic metals from wastewater. Therefore, the induction of Fe plaque on the roots, and its consequences on Cd tolerance investigated in an aquatic macrophyte Pistia stratiotes L. The presence of Fe²⁺ ion but not Fe³⁺ resulted in Fe plaque formation. Induction of Fe plaque decreased Ca and increased K and Fe accumulations in the root. Plaque formed plants had accumulated less Cd until 50.0?µM CdCl₂ treatments because plaque acted as a barrier to Cd exposure. However, at higher concentrations (500.0?µM CdCl₂), plaque formed plants contained more Cd in the roots. Cadmium inducible ion leakage in the root and lowering of the photosynthetic pigment content were less in plants with a plaque. Stretching of aromatic carbonyl groups and alkyl groups among plaque formed plants upon Cd treatments indicated the putative role of phenolics in Cd detoxification.     

Effects of subacute doses of iron (Fe) on Leptophlebia marginata (Insecta: Ephemeroptera)

1. The objective of this paper was to reveal the toxicity of Fe³⁺ and Fe²⁴ at pH 4.5 and 7 on larvae of the mayfly Leptophlebia marginata, by examining survival, motility, gill ventilation, moulting and feeding in experiments. 2. Fe²⁺ was the dominant metal species at pH 4.5, and Fe³⁺ at pH 7. Precipitation of Fe occurred only at pH 4.5, where Fe-precipitarions were observed on the thorax and the gills of the larvae. 3. Both feeding activity and motility of the animals decreased at pH 4.5 and 10, 20 or 50mg1-^?1 Fe_tot. After a short period of normal feeding, the animals stopped feeding for approximately 2 weeks and did not start to feed again until the end of the experiment. They were constipated. Survival was >95% in all treatments, except at pH 4.5 and 50 mg Fe_tot. In this group, about 20% of the animals died after having been constipated for 2 weeks.     

Siderophores mediate reduced and increased uptake of cadmium by Streptomyces tendae F4 and sunflower (Helianthus annuus), respectively

Aims: As a toxic metal, cadmium (Cd) affects microbial and plant metabolic processes, thereby potentially reducing the efficiency of microbe or plant‐mediated remediation of Cd‐polluted soil. The role of siderophores produced by Streptomyces tendae F4 in the uptake of Cd by bacteria and plant was investigated to gain insight into the influence of siderophores on Cd availability to micro‐organisms and plants. Methods and Results: The bacterium was cultured under siderophore‐inducing conditions in the presence of Cd. The kinetics of siderophore production and identification of the siderophores and their metal‐bound forms were performed using electrospray ionization mass spectrometry. Inductively coupled plasma spectroscopy was used to measure iron (Fe) and Cd contents in the bacterium and in sunflower plant grown in Cd‐amended soil. Siderophores significantly reduced the Cd uptake by the bacterium, while supplying it with iron. Bacterial culture filtrates containing three hydroxamate siderophores secreted by S. tendae F4 significantly promoted plant growth and enhanced uptake of Cd and Fe by the plant, relative to the control. Furthermore, application of siderophores caused slightly more Cd, but similar Fe uptake, compared with EDTA. Bioinoculation with Streptomyces caused a dramatic increase in plant Fe content, but resulted only in slight increase in plant Cd content. Conclusion: It is concluded that siderophores can help reduce toxic metal uptake in bacteria, while simultaneously facilitating the uptake of such metals by plants. Also, EDTA is not superior to hydroxamate siderophores in terms of metal solubilization for plant uptake. Significance and Impact of the Study: The study showed that microbial processes could indirectly influence the availability and amount of toxic metals taken up from the rhizosphere of plants. Furthermore, although EDTA is used for chelator‐enhanced phytoremediation, microbial siderophores would be ideal for this purpose.     

Sulphur deprivation limits Fe-deficiency responses in tomato plants

The aim of this work was to clarify the role of S supply in the development of the response to Fe depletion in Strategy I plants. In S-sufficient plants, Fe-deficiency caused an increase in the Fe(III)-chelate reductase activity, ⁵⁹Fe uptake rate and ethylene production at root level. This response was associated with increased expression of LeFRO1 [Fe(III)-chelate reductase] and LeIRT1 (Fe²⁺ transporter) genes. Instead, when S-deficient plants were transferred to a Fe-free solution, no induction of Fe(III)-chelate reductase activity and ethylene production was observed. The same held true for LeFRO1 gene expression, while the increase in ⁵⁹Fe²⁺ uptake rate and LeIRT1 gene over-expression were limited. Sulphur deficiency caused a decrease in total sulphur and thiol content; a concomitant increase in ³⁵SO₄ ²⁻ uptake rate was observed, this behaviour being particularly evident in Fe-deficient plants. Sulphur deficiency also virtually abolished expression of the nicotianamine synthase gene (LeNAS), independently of the Fe growth conditions. Sulphur deficiency alone also caused a decrease in Fe content in tomato leaves and an increase in root ethylene production; however, these events were not associated with either increased Fe(III)-chelate reductase activity, higher rates of ⁵⁹Fe uptake or over-expression of either LeFRO1 or LeIRT1 genes. Results show that S deficiency could limit the capacity of tomato plants to cope with Fe-shortage by preventing the induction of the Fe(III)-chelate reductase and limiting the activity and expression of the Fe²⁺ transporter. Furthermore, the results support the idea that ethylene alone cannot trigger specific Fe-deficiency physiological responses in a Strategy I plant, such as tomato.     

Iron assimilation in Chlamydomonas reinhardtii involves ferric reduction and is similar to Strategy I in higher plants

The mechanism of adaptation to Fe-deficiency stress was investigated in the unicellular green alga, Chlamydomonas reinhardtii. Upon removal of nutritional Fe, the activity of a cell surface Fe(III)-chelate reductase was increased by at least 15-fold within 24 h. This increase was negatively corelated with the Fe concentration in the growth media. Incubation of cells in the presence of the Fe²⁺-specific chelator, bathophenanthrolinedisulphonic acid, led to an increased Fe³⁺ reductase activity, even when sufficient Fe was present. Growth of cells in Cu-free media for 48 h led to no statistically significant increase in Fe³⁺ reductase activity. The Fe(III)-chelate reductase activity in Fe-starved cells was saturable with an apparent Km of 31 M and was inhibited by uncouplers of the transmembrane proton gradient but not by SH-specific reagents.Fe uptake was only observed in Fe-deficient cells. Uptake was specific for Fe in that at 100-fold excess of a number of metal ions in the transport assay did not inhibit uptake activity. However, a 100-fold excess of Cu resulted in a 87% inhibition of Fe uptake. The Vmax for Fe³⁺ reduction activity was 250-fold greater than for Fe uptake; although the Km values for both processes differed by only 10-fold. Thus, the rate limiting step in Fe assimilation was transport and not reduction. These results indicate that Fe assimilation in C. reinhardtii involves a reductive step and thus resembles the mechanism of Fe uptake in Strategy I higher plants.Keywords: Ferric chelate reduction, iron assimilation, iron uptake, unicellular green algae, Chlamydomonas.      

Iron-Stress Response in Tomato (Lycopersicon esculentum) 1. Sites of Fe Reduction, Absorption and Transport

T3238fer (Fe-inefficient) and T3238FER (Fe-efficient) tomato plants differ in their ability to utilize Fe and therefore can be used as test genotypes to locate sites of Fe uptake or to characterize changes that occur in roots in response to Fe stress (Fe deficiency). T3238fer does not respond to Fe stress. Release of hydrogen ions and reduction of Fe³⁺ to Fe²⁺ are two primary responses of T3238FER roots to Fe stress. Fe reduction sites were predominately in the young lateral roots, and between the regions of root elongation and maturation of the primary root. The use of BDPS (bathophenanthrolinedisulfonate) to trap Fe²⁺ did not affect the release of H⁺ ions or reduction by T3238FER roots. BPDS did not decrease Fe uptake until it exceeded the Fe concentration in the nutrient solution. A sevenfold increase in BPDS caused a threefold decrease in Fe taken up by the plant. Fe³⁺ is reduced to Fe²⁺ at root sites accessible to BPDS. Adding Zn decreased the response to Fe stress. Iron stress initiates the development of lateral roots, and we propose that most Fe enters the plant through these roots. The iron moves through protoxylem into the metaxylem of the primary root and then to the top of the plant as Fe citrate. Root environmental factors that are competitive or inhibit Fe-stress response, or genotypes that fail to respond to Fe stress, contribute to the development of Fe deficiency in plants.     

Effect of cadmium on iron uptake in cucumber roots: A Mössbauer-spectroscopic study

The uptake and accumulation of iron in cucumber roots exposed to cadmium were investigated with Fe sufficient and deficient cucumber plants using Mössbauer spectroscopy, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and ferric chelate reductase activity measurements. Both Fe sufficient and Fe deficient plants were applied. In the case of Fe sufficient cucumber roots grown in nutrient solution with 10 μM Cd no changes were found in the occurrence of Fe species (mostly hydrous ferric oxides and ferric-carboxylate complexes) compared to the control where no Cd was added. In the Fe deficient roots pretreated with 0, 0.1, 1, 10 and 100 μM Cd for 3 h then supplied also with 0.5 mM ⁵⁷Fe-citrate for 30 min, Fe^II was identified in a hexaaqua complex form. The relative amount of Fe^II was decreasing simultaneously with increasing Cd concentration, while the relative occurrence of Fe^III species and total Fe concentration were increasing. The results support the inhibitory effect of Cd on Fe-chelate reduction. Although the reductase activity at 10 and 100 μM Cd treatment was lower than in the iron sufficient control plants, Fe^II could be identified by Mössbauer spectroscopy whereas in the Fe sufficient control, this form was below detection limit. These data demonstrate that the influx and the reoxidation of Fe^II was decreased by Cd, consequently, they refer to the competition of Cd²⁺ and Fe²⁺ during the membrane transport and the inhibition of the reoxidation process.     

Uptake and degradation of EDTA by Escherichia coli

It was found that Escherichia coli exhibited a growth by utilization of Fe(III)EDTA as a sole nitrogen source. No significant growth was detected when Fe(III)EDTA was replaced by EDTA complexes with other metal ions such as Ca²⁺, Co²⁺, Cu²⁺, Mg²⁺, Mn²⁺, and Zn²⁺. When EDTA uptake was measured in the presence of various ions, it was remarkable only when Fe³⁺ was present. The cell extract of E. coli exhibited a significant degradation of EDTA only in the presence of Fe³⁺. It is likely that the capability of E. coli for the growth by utilization of Fe(III)EDTA results from the Fe³⁺-dependent uptake and degradation of EDTA.     

Effects of acidity on the growth of two Euglena species

Our study separates the effects of elevated protons (at pH <3) and elevated metals (Al, Cd, Cu, Fe, Ni, Zn) on the growth of E. mutabilis Schmitz, a pioneering phototroph in acid mine drainage (AMD) and E. gracilis Klebs, a closely-related species rarely found in severely AMD-impacted sites. Both species were acid tolerant, growing optimally at pH 2.5–7. At pH values typical of AMD (pH 2.5–4) in the absence of elevated metals, E. gracilis outcompeted E. mutabilis (growth rates of 1.0 and 0.8 div d^–1, respectively). Relative metal toxicities were evaluated based on the Effective Exposure causing 50% growth reduction (= EE50). With total metal additions similar to AMD levels, E. mutabilis demonstrated significantly greater tolerance to all metals, except Cu. E. gracilis showed two-fold higher tolerance to Cu²⁺ than E. mutabilis (EE50 of 91.6 vs. 45.7 pmol cell^–1). The EE50 for Zn²⁺ was similar for both species (368 pmol cell^–1 for E. gracilis and 423 pmol cell^–1 for E. mutabilis). With Cd and Ni, E. mutabilis tolerated an order of magnitude higher exposure than E. gracilis(EE50 of 1.6 vs. 0.2 pmol Cd²⁺ cell^–1; EE50 of 942 vs. 87 pmol Ni²⁺ cell^–1). Al and Fe were tolerated at high total metal concentrations (up to 100 mM) by E. mutabilis, but toxicity was evident with E. gracilisat much lower levels. E. mutabilis grew at double the Al³⁺ exposure tolerated by E. gracilis (EE50 of 398 vs. 188 pmol Al³⁺ cell^–1). There was an 18-fold difference in Fe tolerance levels between E. mutabilis and E. gracilis with EE50s of 8773 and 502 pmol Fe²⁺ cell^–1, respectively. We conclude that differential metal tolerance, particularly to Fe²⁺, accounts for the mutually exclusive distribution of E. gracilis and E. mutabilis in AMD-impacted habitats.     

Zn,Cd, S and trace metal bioaccumulation in willow (Salix spp.) cultivars grown hydroponically

Willows (Salix spp.) can be used to phytoremediate soils contaminated by Zn and Cd under certain conditions. In this study, the ability of 14 Salix cultivars to concentrate Cd, Zn and S in leaves was measured in hydroponic culture with 10 and 200 µM Cd and Zn, respectively, in the nutrient medium. The cultivars showed a wide range of biomass yields, tolerance to metals, and foliar concentrations of Zn and Cd, with some cultivars accumulating up to 1000 mg kg^?1 Zn, 70 mg kg^?1 Cd and 10,000 mg kg^?1 S with only mild phytotoxicity symptoms attributable to excess Zn. Cultivars with higher foliar Zn concentrations tended to have higher foliar Cd concentrations as well, and competition between Zn and Cd for uptake was observed. Exposure of Salix cultivars to Cd and Zn did not affect foliar concentrations of secondary metabolites such as polyphenols, but trace metal concentrations in leaves were significantly reduced (Fe and Cu) or increased (Mn) by exposure to excess Zn and Cd. Sulfur-XANES spectroscopy showed foliar S to be predominantly in highly oxidized (sulfate plus sulfonate) and reduced (thiol) forms, with oxidized S more prevalent in willows with the highest total S content.     

Bioaccumulation of heavy metals in Channa punctatus (Bloch) in river Ramganga (U.P.), India

Ganga is the largest riverine system of India with a fragile ecosystem. Its prone to anthropogenic disturbances because of its cultural, economic and environmental values. The contamination of river Ganga by heavy metals (HM) is due to biotic (anthropogenic sources) and abiotic (pesticides, fertilizers) sources that poses a devastating health hazard to human, plant and edible fish life. The chemical analysis with the help of atomic absorption spectrometer performed on its water samples demonstrated the accumulation of heavy metals such as Arsenic (As), Lead (Pb), Cadmium (Cd), Iron (Fe), Zinc (Zn). Moreover, the spectrophotometric analysis indicated clearly the accumulation of heavy metals in order of occurrence (Fe > As > Cd > Zn > Pb) in liver and (Zn > Fe > As > Cd > Pb) in kidney of edible fish Channa punctatus. The present study has be used to sensitively monitor the extent of heavy metals pollution in the biotic aqua life of river Ramganga system and its suggested that the bioaccumulation of heavy metal in Channa punctatus has reached above permissible limits for human consumption, indicating potential health risks. Necessary biological steps should be taken to handle such food pollution and prevent the environmental risk and food chain disruption.     

Free metal activity and total metal concentrations as indices of micronutrient availability to barley [Hordeum vulgare (L.) ‘Klages’]

The form in which a micronutrient is found in the rhizosphere affects its availability to plants. We compared the availability to barley of the free hydrated cation form of Fe³⁺, Cu²⁺, Zn²⁺, and Mn²⁺ versus their total metal concentrations (free ion plus complexes) in chelator-buffered solutions. Free metal ion activities were estimated using the chemical equilibrium program GEOCHEM-PC with the corrected database. In experiment 1, barley was grown in nutrient solutions with different Fe³⁺ activities using chelators to control Fe levels. Chlorosis occurred at Fe³⁺ activities of 10^–18 and 10^–19 M for barley grown in HEDTA and EDTA solutions, respectively. In experiment 2, barley was grown in nutrient solutions with the same calculated Fe³⁺ activity and the same chelator, but different total Fe concentrations. Leaf, root and shoot Fe concentrations were higher from CDTA buffered solutions which had the higher total Fe concentration indicating the importance of the total Fe concentration on Fe uptake. Results from treatments using EDTA or HEDTA, with one exception, were similar to the results from the CDTA treatment. This suggests differences in critical Fe³⁺ activities found in experiment 1 were due to differences in the total Fe concentration and not errors in chelate formation constants used to estimate the critical activities. Results for Cu, Zn, and Mn were similar to Fe; despite solutions with equal free Cu²⁺, Zn²⁺ and Mn²⁺ activities, plant concentrations of these metals were generally greater when grown in the solutions with the greater total amount of Cu, Zn, or Mn. When the free Zn²⁺ activity was kept constant while the total amount of Zn was increased from 4.4 to 49 M, leaf Zn concentration increased from 77 to 146 g g^-1. In order to predict metal availability to barley and other species in chelator-buffered nutrient solutions, both free and total metal concentrations in solution must be considered. The critical Fe³⁺ activities required by barley in this study are much higher than those from tomato and soybean, 10^-28 M, which strongly supports the Strategy 2 model of Fe uptake for Poaceae. This is related to the importance of the Fe³⁺ (barley) and the Fe²⁺ (tomato and soybean) ions in Fe uptake. Fe-stressed barley is known to release phytosiderophores which compete for Fe³⁺ in the nutrient solution, while tomato and soybean reduce Fe³⁺ to Fe²⁺ at the epidermal cell membranes to allow uptake of Fe²⁺ from Fe³⁺ chelates in solution.Abbreviations CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetracetic acid - DTPA diethylenetriaminepentacetic acid - EDTA ethylenediaminetetracetic acid - EDDHA ethylenediamine-di(o-hydroxyphenylacetic acid) - HBED-N,N di(2-hydroxybenzoyl)-ethylenediamine-N,N-diacetic acid - HEDTA-N hydroxyethylenediaminetriacetic acid - MES-2 (N-morpholino)ethanesulfonic acid - NTA nitrilotriacetic acid     

Influence of external zinc and phosphorus supply on Cd uptake by rice (Oryza sativa L.) seedlings with root surface iron plaque

Rice seedlings were grown in hydroponic culture to determine the effects of external Zn and P supply on plant uptake of Cd in the presence or absence of iron plaque on the root surfaces. Iron plaque was induced by supplying 50 mg l⁻¹ Fe²⁺ in the nutrient solution for 2 day. Then 43-day-old seedlings were exposed to 10 μmol l⁻¹ Cd together with 10 μmol l⁻¹ Zn or without Zn (Zn–Cd experiment), or to 10 μmol l⁻¹ Cd with 1.0 mmol l⁻¹ P or without P (P–Cd experiment) for another 2 day. The seedlings were then harvested and the concentrations of Fe, Zn, P and Cd in dithionite–citrate–bicarbonate (DCB) extracts and in roots and shoots were determined. The dry weights of roots and shoots of seedlings treated with 50 mg l⁻¹ Fe were significantly lower than when no Fe was supplied. Adsorption of Cd, Zn and P on the iron plaque increased when Fe was supplied but Cd concentrations in DCB extracts were unaffected by external Zn or P supply levels. Cd concentrations in shoots and roots were lower when Fe was supplied. Zn additions decreased Cd concentrations in roots but increased Cd concentrations in shoots, whereas P additions significantly increased shoot and root Cd concentrations and this effect diminished when Fe was supplied. The percentage of Cd in DCB extracts was significantly lower than in roots or shoots, accounting for up to 1.8–3.8% of the plant total Cd, while root and shoot Cd were within the ranges 57–76% and 21–40% respectively in the two experiments. Thus, the main barrier to Cd uptake seemed to be the root tissue and the contribution of iron plaque on root surfaces to plant Cd uptake was minor. The changes in plant Cd uptake were not due to Zn or P additions altering Cd adsorption on iron plaque, but more likely because Zn or P interfered with Cd uptake by the roots and translocation to the shoots.     

Mycorrhizal symbiosis and phosphorus fertilization effects on Zea mays growth and heavy metals uptake

Arbuscular mycorrhizal fungi (AMF) can promote plant growth and reduce plant uptake of heavy metals. Phosphorus (P) fertilization can affect this relationship. We investigated maize (Zea mays L.) uptake of heavy metals after soil AMF inoculation and P fertilization. Maize biomass, glomaline and chlorophyll contents and uptake of Fe, Mn, Zn, Cu, Cd and Pb have been determined in a soil inoculated with AMF (Glomus aggregatum, or Glomus intraradices) and treated with 30 or 60 µg P-K₂HPO₄ g^?1 soil. Consistent variations were found between the two mycorrhizal species with respect to the colonization and glomalin content. Shoot dry weight and chlorophyll content were higher with G. intraradices than with G. aggregatum inoculation. The biomass was highest with 30 µg P g^?1 soil. Shoot concentrations of Cd, Pb and Zn decreased with G. aggregatum inoculation, but that of Cd and Pb increased with G. intraradices inoculation. Addition of P fertilizers decreased Cd and Zn concentrations in the shoot. AMF with P fertilization greatly reduced maize content of heavy metals. The results provide that native AMF with a moderate application rate of P fertilizers can be exploited in polluted soils to minimize the heavy metals uptake and to increase maize growth.     

Response ofAnabaena doliolum to bimetallic combinations of Cu,Ni and Fe with special reference to sequential addition

This study reports physiological features of a N₂-fixing cyanobacteriumAnabaena doliolum in response to metal mixtures. Exposure of the cyanobacterium to Cu, Ni and Fe individually, as well as in combinations (Cu + Ni, Cu + Fe, Ni + Fe), showed marked differences in growth inhibition, nutrient uptake (NH₄ ⁺ and NO₃ ⁻), photosynthesis, ATP content, nitrate reductase, glutamine synthetase and urease activities. The response to metal combinations was also dependent upon the order in which the metals were added. The Cu-Ni combination resulted in synergistic interaction, in contrast to the antagonism of Cu-Fe and Ni-Fe. Pre-addition of Fe protected the cyanobacterium against Cu and Ni toxicity. Statistically significant (P < 0.005) inhibition of all the processes following metal supplementation was observed. This study suggests that carbon fixation is the most suitable variable for assessing heavy metal toxicity.     

Magnesium and iron deficiencies alter Cd accumulation in Salix viminalis L.

Evidence exists that Cd and certain nutrient elements, such as Fe and Mg, could share similar mechanisms of plant uptake and accumulation. Here we report that Mg and Fe deficiency in mature plants of Salix viminalis, grown in hydroponic solutions containing 5 µg ml^?1 of Cd, caused a significant increase in Cd accumulation in roots, stems and leaves. Cd (µg g^?1 dry weight) was determined following three treatments: 1) Cd treatment in complete nutrient solution; 2) Cd treatment with Fe deficiency; and 3) Cd treatment with Mg deficiency, yielding, respectively: in young leaves (65.3, 76.1, and 92.2), mature leaves (51.5 to 76.3 and 87.1), upper stems (80.6, 116.8, and 130.6) lower stems (67.2, 119, and 102.3), roots (377.1, 744.8, and 442,5). Our results suggest that Cd utilizes the same uptake and transport pathways as Mg and Fe. Evidence exists that Mg and Fe uptake and translocation could be further facilitated by plants as an adaptive response to deficiency of these elements. Such physiological reaction could additionally stimulate Cd accumulation. Although Cd uptake was mostly confined in roots, high Cd content in aerial plant parts (51.5–130.6 µg g^?1) indicates that the analysed Salix viminalis genotype is suitable for phytoextraction.     

Growth responses of <Emphasis Type="Italic">Sagittaria sagittifolia</Emphasis> L. plants to water contamination with cadmium

Cadmium accumulation, the relative content of different chemical forms of Cd, as well as the toxic effect of Cd on nutrient element uptake, physiological parameters, and ultrastructure of Sagittaria sagittifolia L. seedlings were determined after the seedlings were exposed to different Cd concentrations for 4 days. The results showed that S. sagittifolia had the ability to accumulate large amounts of Cd. In the root, stem, and bulb, the predominant chemical Cd forms were NaCl extractable. With an increase in the Cd²⁺ concentration, the chlorophyll content, the relative membrane penetrability (RMP) of root cells, peroxidase (POD) activity, superoxide dismutase (SOD) activity in leaves, malondiadehyde (MDA) content and the superoxide anion (O₂⁻) generation rate in roots all decreased following an initial increase. On the other hand, catalase (CAT) activity, SOD activity in roots, MDA content, and the generation rate of O₂⁻ in leaves all increased gradually. The toxic effect of Cd²⁺ was more severe on roots than on leaves at the same concentration. Cadmium affected the mineral nutrition balance; mainly, it promoted the uptake of Ca, Cu, Mn, and Fe, while inhibited Mg, Na, and K uptake. The physiological toxic effect of Cd²⁺ was close to the ultrastructural damage induced by Cd contamination. A significant correspondence was observed between the Cd dose and its toxic effect. Cadmium could destroy the normal ultrastructure, disturb the ion balance, and interfere with cell metabolism.