Use of synchrotron- and plasma-based spectroscopic techniques to determine the uptake and biotransformation of chromium(III) and chromium(VI) by Parkinsonia aculeata

In this study, a combination of inductively coupled plasma optical emission spectroscopy and X-ray absorption spectroscopy (XAS) was used to study the uptake and speciation of chromium in Parkinsonia aculeata, commonly known as Mexican Palo Verde. Plants were treated for 14 days in a modified Hoagland solution containing chromium(III) or chromium(VI) at several concentrations. The results showed that plants treated with 70 mg Cr(III) L(-1) and 30 mg Cr(VI) L(-1) had similar Cr concentrations in leaves (～200 mg kg(-1) dry weight, DW). The results also showed that neither Cr(III) nor Cr(VI) affected the uptake of phosphorus and sulfur. However, the concentration of calcium in the stems of plants treated with Cr(VI) at 40 mg L(-1) (about 6000 mg Ca kg(-1) DW) was significantly higher compared to the Ca concentration (about 3000 mg kg(-1) DW) found in the stems of plants treated with 150 mg Cr(III) L(-1). However, no differences were observed in potassium and magnesium concentrations. The iron concentration (about 1000 mg kg(-1) DW) in roots treated with 40 mg Cr(VI) L(-1) was similar to the iron concentration found in the roots of plants treated with 110 mg Cr(III) L(-1). The XAS data showed that Cr(VI) was reduced to Cr(III) in/on the plant roots and transported as Cr(III) to the stems and leaves. The XAS studies also showed that Cr(III) within plants was present as an octahedral complex.     

Hexavalent chromium uptake and its effects on mineral uptake, antioxidant defence system and photosynthesis in Amaranthus viridis L

The effects of different concentrations (10(-6)M, 10(-5)M and 10(-4)M) of K2Cr2O7Cr(VI) on some minerals (Mn, Fe, Cu and Zn), lipid peroxidation, activities of antioxidant enzymes, photosynthetic function, and chlorophyll fluorescence characteristics were investigated in hydroponically grown Amaranthus viridis L. Results indicated that chromium was accumulated primarily in roots. In the roots and shoots, the Cr content increased with the increasing Cr(VI) concentrations, and induced decrease of Mn, Fe, Cu and Zn. Chromium Cr(VI) induced oxidation stress and lipid peroxidation in A. viridis L. shown by the increased concentration of MDA. The increased activities of POD and SOD indicated that they could serve as important components of antioxidant defense mechanisms to minimize Cr induced oxidative injury. The net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration were reduced only by high Cr(VI) treatments (10(-5)M and 10(-4)M). The chlorophyll fluorescence parameters Fv/Fm, Fv(')/Fm('), Phi PSII and qP, decreased in Cr(VI)-treated, but qN and NPQ showed an increase in Cr(VI) treated plants.     

Accumulation and transport of nickel in relation to organic acids in ryegrass and maize grown with different nickel levels

Difference in Ni tolerance/accumulation in plant genotypes might be used to identify or develop plants for remediation of high Ni soils. Ryegrass was shown to be more sensitive to Ni toxicity and accumulated much more Ni in shoots than maize. The objectives of this study were to examine the relationship of organic acids to Ni accumulation and xylem transport of Ni in ryegrass (Lolium perenne L.) and maize (Zea mays L.). The results showed that accumulation of Ni in shoots was 5 to 7 fold higher in ryegrass than in maize grown at 20 to 80 µM Ni, whereas Ni concentration in ryegrass roots was only 1 to 2 fold higher at 0.1 to 40 µM Ni and 1.5-fold lower at 80 µM Ni than that of maize roots. Xylem transport rates of Ni increased with increasing Ni supply for both species, and were about 2 to 7 times higher in ryegrass than in maize. Shoot concentrations of citric, malic, oxalic and cis-aconitic acids increased at Ni levels above 20 µM, and were about 2 to 6 times higher in ryegrass than in maize. Whereas, maize roots accumulated greater amount of malic, oxalic, and cis-aconitic acids than ryegrass roots, especially at Ni levels of 40-80 µM. The rate of Ni exudation by roots in the two species was significantly correlated with root Ni concentrations. It could be concluded that high Ni accumulation in shoots was closely related to high xylem transport rates of Ni and that the accumulation of organic acids, citric and malic acid in particular. A high root exudate rate of Ni and the enhanced accumulation of organic acids, malic acid in particular, in roots might be among the important factors which are associated with the tolerance of crops to toxic Ni levels.     

Chromium-reducing and plant growth-promoting Mesorhizobium improves chickpea growth in chromium-amended soil

Mesorhizobium strain RC3, isolated from chickpea nodules, tolerated chromium up to 500 μg/ml and reduced it by 90% at pH 7 after 120 h. It produced plant growth-promoting substances, both in the presence and absence of chromium. Strain RC3 produced 35 μg indole acetic acid/ml in Luria Bertani broth with 100 mg tryptophan/ml, which decreased with an increase in chromium concentration. Chromium application to soil at 136 mg/kg was toxic to chickpea plants but when RC3 at 136 mg/kg was also added, it increased the dry matter accumulation, number of nodules, seed yield and grain protein by 71, 86, 36 and 16%, respectively, compared to non-inoculated plants. Nitrogen in roots and shoots were increased by 46 and 40%, respectively, at 136 mg Cr/kg. The bio-inoculant decreased the uptake of chromium by 14, 34 and 29% in roots, shoots and grains, respectively.     

不同浓度硝态氮供应下小麦生长、硝态氮累积及根系钙信号特征

以小麦品种‘石麦15’和‘衡观35’为材料进行营养液水培试验,研究不同浓度硝态氮供应对小麦苗期根系形态、钙离子流特征及钙调蛋白(CaM)含量的影响。结果表明,与适宜浓度硝态氮处理(2.5mmol/L)相比,无外源硝态氮供应时小麦地上部鲜重、硝态氮含量均降低,侧根数量显著减少;高浓度硝态氮处理(50mmol/L)下两个小麦品种地上部硝态氮含量升高,根系总长度降低,‘石麦15’侧根数量减少。无硝态氮和高浓度硝态氮处理下,根系中钙调蛋白含量降低,且‘衡观35’的降低幅度大于‘石麦15’。无外源硝态氮供应时小麦根尖表现出较为明显的钙离子外流特征;与适宜浓度硝态氮处理相比,高硝态氮处理下小麦根尖Ca2+的内流速度显著下降。说明硝态氮供应不足和高浓度硝态氮供应会影响小麦根系生长,根系Ca2+外流或Ca2+内流速度下降,CaM含量减少,Ca2+/CaM可能介导硝态氮调控小麦根系生长发育。     

The Ameliorative Role of 5-Aminolevulinic Acid (ALA) Under Cr Stress in Two Maize Cultivars Showing Differential Sensitivity to Cr Stress Tolerance

Heavy metal (HM) contamination of the environment is a serious threat to sustainable crop production. Among the HMs, chromium (Cr) is one of the most toxic HMs that is known to negatively affect growth and metabolic activities of diverse crop plants. The present study was designed to investigate the ameliorative role of 5-aminolevulinic acid (ALA) under Cr stress in two maize (Zea mays L.) cultivars showing differential sensitivity to Cr tolerance. ALA is a biosynthesis precursor and it has a dominant regulatory effect related to physiological, respiratory, and photosynthesis processes in various plant species. Three concentrations of Cr (0, 5, and 10 mg kg⁻¹) were tested under the graded levels of ALA application (0, 12.5, and 25 mg L⁻¹). The results indicated that Cr stress differentially reduced plant growth attributes, gas exchange characteristics, photosynthetic pigments, and biomass in both the cultivars. Oxidative stress increased as evidenced in the form of electrolyte leakage, malondialdehyde, and hydrogen peroxide (H₂O₂) accumulation in plants. The anti-oxidative enzyme activities, that is, catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) both in the leaves and roots of maize cultivars decreased due to Cr stress. The concentration of Cr increased in roots and shoots of maize under Cr levels without ALA. Under Cr stress, ALA exogenous application markedly enhanced plant growth, photosynthetic pigments, gas exchange capacity, and biomass. Furthermore, ALA application decreased the Cr-induced oxidative stress in maize cultivars by improving the activities of CAT, POD, and SOD in plants. After ALA application, the Cr concentrations and total Cr uptake by plants differently decreased in both cultivars. The 6103 cultivar of maize was found to be a tolerant cultivar against Cr stress due to its strong defensive system with a higher rate of antioxidant enzyme activities. On the other hand, the other maize cultivar (9108) was found to be a sensitive cultivar against Cr stress due to its weak defense system with higher contents of reactive oxygen species. These findings suggest that ALA can play a regulatory role in maintaining optimum plant growth and efficient photosynthetic processes under Cr-challenged habitats in maize. Thus, ALA application may be used as a sustainable remedial strategy to alleviate Cr-induced stress in maize cultivars.

     

Evaluation of Lupinus Species to Accumulate Heavy Metals From W aste Waters

Several Lupinus species, for example, Lupinus albus, Lupinus luteus, Lupinus angustifolius, and Lupinus hispanicus were used to accumulate Mn(II), Cd(II), Pb(II), Cr(III), Cr(VI), Hg²⁺, and CH₃Hg⁺ from waste waters. The influence of different species concentrations (50 and 100 mg L^-1) and pH on growing behavior as well as the resulting distribution of metals in the plants were investigated. The results obtained showed that lupins were able to germinate and to grow in the presence of the metals mentioned above, even when they were present at levels as high as 50 mg L^-1. Accumulation of Pb(II), Cr(III), and Cd(II) was higher in roots than in shoots. As far as mercury is concerned, the highest CH₃Hg and Hg²⁺ accumulation was detected in roots, but fast transport toward the leaves was noticed. In contrast to mercury, the uptake of chromium seems to be influenced by the chemical form of the analyte, remaining Cr(VI) in solution. No differences in growing behavior and accumulation were observed for the four Lupinus species studied. Even though plants were exposed only a relatively short time to the metal solutions, metal concentrations of approximately 2 g/kg of dry matter were detected in the young lupins plants. The feasibility of utilizing Lupinus plants for the removal of heavy metals from wastewater was also investigated. Lupins were able to grow under extreme conditions (wastewater, pH lower than 2) and to remove 98% of the initial amount of toxic metals present in the sample.     

Cr Localization and Speciation in Roots of Chromate Fed Helianthus annuus L. Seedlings Using Synchrotron Techniques

In order to gain knowledge on the potential use of Helianthus annuus L. for the remediation of Cr(VI) polluted waters, hydroponics experiments were set up to determine Cr uptake and tolerance in different Cr(VI)-sulfate conditions, and Cr biotransformations. Results indicated that Cr(VI) promoted seed germination, and plant tolerance was higher at younger plant stages. Cr uptake was dependent on sulfate concentrations. The highest Cr levels in roots and shoots (13,700 and 2,500 mg kg^–1dry weight (DW), respectively) were obtained in 1 mM sulfate. The lowest Cr uptake in roots (10,600 mg kg^–1DW) was observed in seedlings treated with no sulfate. In shoots, Cr concentration was of 1,500 mg kg^–1DW for the 1 mM sulfate treatment, indicating a different level of interaction between chromate and sulfate in both tissues. For the first time, using micro X-ray florescence (μXRF), we demonstrated Cr reaches the root stele and is located in the walls of xylem vessels. Bulk and micro X-ray Absorption Near-Edge Structure (μXANES) results showed that Cr in the roots is mostly in the form of Cr(III) phosphate (80%), with the remainder complexed to organic acids. Our results suggest this plant species may serve for Cr(VI) rhizofiltration purposes.     

Impact of picolinic acid on the chromium accumulation in fodder radish and komatsuna

Effects of picolinic acid (2-pyridinecarboxylic acid) and chromium(III) picolinate was studied on the chromium (Cr) accumulation of fodder radish (Raphanus sativus L. convar. oleiformis Pers., cv. Leveles olajretek) and komatsuna (Brassica campestris L. subsp. napus f. et Thoms. var. komatsuna Makino, cv. Kuromaru ) grown in a pot experiment. Control cultures, grown in an uncontaminated soil (UCS; humous sand with pH_KCl 7.48, sand texture with 12.4% clay+silt content, organic carbon 0.56%, CaCO₃ 2.2%, CEC 6.2 cmol_c kg^–1, Cr 10.6 mg kg^–1), accumulated low amounts of chromium (less than 5.4 g g^–1) in their roots or shoots. When this UCS was artificially contaminated with 100 mg kg^–1 Cr (CrCl₃) later picolinic acid treatment promoted the translocation of chromium into the shoots of both species. In fodder radish shoots Cr concentration reached 30.4 g g^–1 and in komatsuna shoots 44.5 g g^–1. Application of ethylene diamine tetra-acetic acid (EDTA) to this Cr contaminated soil had similar effect to picolinic acid. When the UCS was amended with leather factory sewage sediment (which resulted in 853 mg kg^–1 Cr in soil), Cr mobilization was observed only after repeated soil picolinic acid applications. From a galvanic mud contaminated soil (brown forest soil with pH_KCl 6.77, loamy sand texture with 26.6% clay+silt content, organic carbon 1.23%, CaCO₃ 0.7%, CEC 24.5 cmol_c kg^–1, Cd 5.0 mg kg^–1, Cr 135 mg kg^–1, and Zn 360 mg kg^–1) the rate of Cr mobilization was negligible, only a slight increase was observed in Cr concentration of fodder radish shoots after repeated picolinic acid treatments of soil. Presumably picolinic acid forms a water soluble complex (chromium(III) picolinate) with Cr in the soil, which promotes translocation of this element (and also Cu) into the shoots of plants. The rate of complex formation may be related to the binding forms and/or concentration of Cr in soil and also to soil characteristics (i.e. pH, CEC), since the rate of Cr translocation was the following: artificially contaminated soil > leather factory sewage sediment amended soil > galvanic mud contaminated soil. Four times repeated 10 mg kg^–1 chromium(III) picolinate application to UCS multiplied the transport of chromium to shoots, as compared to single 10 mg kg^–1 CrCl₃ treatment. This also suggests that chromium(III) picolinate is forming in the picolinic acid treated Cr-contaminated soils, and plants more readily accumulates and translocates organically bound Cr than ionic Cr. Picolinic acid promotes Cr translocation in soil-plant system. This could be useful in phytoextraction (phytoremediation) of Cr contaminated soils or in the production of Cr enriched foodstuffs.     

Effect of intermediates on ascorbic acid and oxalate biosynthesis of rice and in relation to its stress resistance.

Rice (Oryza sativa L.) roots were fed with L-ascorbic acid (AsA) and its putative precursors to observe AsA and oxalate concentrations and the resistance of rice to chilling, water stress, and Al toxicity. AsA concentration was significantly enhanced in both shoots and roots of rice seedlings by feeding with D-glucose or L-galactono-gamma-lactone. AsA or L-galactono-gamma-lactone treatment increased accumulation of oxalate mainly in soluble form, while these treatments decreased electrolyte leakage from root cells, H2O2 and lipid peroxidation level in rice seedlings subjected to chilling, water stress, and Al toxicity. They also alleviated the inhibition on root growth by Al. These results indicated that AsA and its immediate precursor protected plants against the oxidative damages induced by various stresses. However, 0.5 mM AsA and 10 mM L-galactono-gamma-lactone treatment had no significant effect on superoxide dismutase and catalase activity and ascorbate-peroxidase activities. Enhanced Al resistance caused by AsA and L-galactono-gamma-lactone may possibly be resulted from increased level of oxalate, which acts as metal chelator. Thus it is proposed that manipulation of AsA and oxalate biosynthesis through enhancement of L-galactono-gamma-lactone level in plants could be a strategy for improving abiotic stress tolerance.     

Inhibition of iron deficiency stress response in cucumber by rare earth elements.

We investigated the influence of the trivalent scandium (Sc), chromium (Cr), gallium (Ga), yttrium (Y) and lanthanum (La) on both the function and activity of ferric chelate reductase (FCR) in cucumber (Cucumis sativus L.) roots. Cucumber seedlings were grown for 1week in a nutrient solution without Fe or in some experiments with 10microM FeEDTA. Intact root systems were assayed for FCR activity in a medium at pH 5.0 containing 100microM FeEDTA with the ferrous chelating agent Ferrozine. Addition of 100microM concentrations of the EDTA complexes of Sc, Cr, Ga, Y and La did not inhibit FCR in Fe-deficient roots. When Fe-deficient roots were grown with 10microM LaCl(3), ScCl(3), or YCl(3) for 3days, FCR activity decreased to 23%, 15% and 1%, respectively, of the activity of Fe-deficient plants grown without trivalent metal addition. Additionally, these trivalent metals suppressed proton secretion. Growth of Fe-deficient plants with 80microM Ga(2)(SO(4))(3) decreased FCR activity to 35% of the control activity while 80microM CrEDTA did not affect FCR activity. With the addition of either FeEDTA or YCl(3), FCR activity decreased to less than 5% of the activity of the Fe-deficient control roots in 3days. Addition of FeEDTA, but not Y, resulted in recovery from Fe deficiency as indicated by increasing chlorophyll content of leaves.     

六价铬对薏米人工湿地微生物群落数量的影响

通过桶栽构筑微型模拟垂直流薏米人工湿地(CAW),以1/2 Hoaglands营养液为营养源,在营养液中添加不同浓度的Cr6+(0,5,20,40,60mg/L,以K2Cr2O7配置),各浓度处理均以不种植薏米湿地(NPW)为对照,以研究铬(Cr6+)对薏米人工湿地基质真菌、细菌及放线菌群落结构数量的影响。结果表明:(1)真菌、细菌、放线菌的数量在薏米人工湿地(CAW)中明显多于无植物对照处理(NPW);(2)中低浓度(5、20mg/L)Cr6+对CAW真菌,对NPW细菌、放线菌数量有促进作用,薏米人工湿地微生物对低中浓度的Cr6+胁迫有一定的耐受能力。     

Role of iron plaque in Cd uptake by and translocation within rice (Oryza sativa L.) seedlings grown in solution culture

A hydroponics culture experiment was conducted to investigate the effect of iron plaque on Cd uptake by and translocation within rice seedlings grown under controlled growth chamber conditions. Rice seedlings were pre-cultivated for 43 days and then transferred to nutrient solution containing six levels of Fe (0, 10, 30, 50, 80 and 100 mg L⁻¹) for 6 days to induce different amounts of iron plaque on the root surfaces. Seedlings were then exposed to solution containing three levels of Cd (0, 0.1 and 1.0 mg L⁻¹) for 4 days. In order to differentiate the uptake capability of Cd by roots with or without iron plaque, root tips (white root part without iron plaque) and middle root parts (with iron plaque) of pre-cultivated seedlings treated with 0, 30 and 50 mg L⁻¹ Fe were exposed to ¹⁰⁹Cd for 24 h. Reddish iron plaque gradually became visible on the surface of rice roots but the visual symptoms of the iron plaque on the roots differed among treatments. In general, the reddish color of the iron plaque became darker with increasing Fe supply, and the iron plaque was more homogeneously distributed all along the roots. The Fe concentrations increased significantly with increasing Fe supply regardless of Cd additions. The Cd concentrations in dithionite–citrate–bicarbonate (DCB)-extracts and in shoots and roots were significantly affected by Cd and Fe supply in the nutrient solution. The Cd concentrations increased significantly with increasing Cd supply in the solution and were undetectable when no Cd was added. The Cd concentrations in DCB-extracts with Fe supplied tended to be higher than that at Fe₀ at Cd_0.1, and at Cd_1.0, DCB-Cd with Fe supplied was significantly lower. Cd concentrations in roots and shoots decreased with increasing Fe supply at both Cd additions. The proportion of Cd in DCB-extracts was significantly lower than in roots or shoots. Compared to the control seedlings without Fe supply, the radioactivity of ¹⁰⁹Cd in shoots of seedlings treated with Fe decreased when root tips were exposed to ¹⁰⁹Cd and did not change significantly when middle parts of roots were exposed. Our results suggest that root tissue rather than iron plaque on the root surface is a barrier to Cd uptake and translocation within rice plants, and the uptake and translocation of Cd appear to be related to Fe nutritional levels in the plants.     

Chromium accumulation, translocation and chemical speciation in vegetable crops

Trivalent chromium (Cr³⁺) is essential for animal and human health, whereas hexavalent Cr (CrO₄ ²⁻) is a potent carcinogen and extremely toxic to animals and humans. Thus, the accumulated Cr in food plants may represent potential health hazards to animals and humans if the element is accumulated in the hexavalent form or in high concentrations. This study was conducted to determine the extent to which various vegetable crops absorb and accumulate Cr³⁺ and CrO₄ ²⁻ into roots and shoots and to ascertain the different chemical forms of Cr in these tissues. Two greenhouse hydroponic experiments were performed using a recirculating-nutrient culture technique that allowed all plants to be equally supplied with Cr at all times. In the first experiment, 1 mg L⁻¹ Cr was supplied to 11 vegetable plant species as Cr³⁺ or CrO₄ ²⁻, and the accumulation of Cr in roots and shoots was compared. The crops tested included cabbage (Brassica oleracea L. var. capitata L.), cauliflower (Brassica oleracea L. var. botrytis L.), celery (Apium graveolens L. var. dulce (Mill.) Pers.), chive (Allium schoenoprasum L.), collard (Brassica oleracea L. var. acephala DC.), garden pea (Pisum sativum L.), kale (Brassica oleracea L. var. acephala DC.), lettuce (Lactuca sativa L.), onion (Allium cepa L.), spinach (Spinacia oleracea L.), and strawberry (Fragaria ×  ananassaDuch.). In the second experiment, X-ray absorption spectroscopy (XAS) analysis on Cr in plant tissues was performed in roots and shoots of various vegetable plants treated with CrO₄ ²⁻ at either 2 mg Cr L⁻¹ for 7 d or 10 mg Cr L⁻¹ for 2, 4 or 7 d. The crops used in this experiment included beet (Beta vulgaris L. var. crassa (Alef.) J. Helm), broccoli (Brassica oleracea L. var. Italica Plenck), cantaloupe (Cucumis melo L. gp. Cantalupensis), cucumber (Cucumis sativus L.), lettuce, radish (Raphanus sativus L.), spinach, tomato (Lycopersicon lycopersicum (L.) Karsten), and turnip (Brassica rapa L. var. rapifera Bailey). The XAS speciation analysis indicates that CrO₄ ²⁻ is converted in the root to Cr³⁺ by all plants tested. Translocation of both Cr forms from roots to shoots was extremely limited and accumulation of Cr by roots was 100-fold higher than that by shoots, regardless of the Cr species supplied. Highest Cr concentrations were detected in members of the Brassicaceae family such as cauliflower, kale, and cabbage. Based on our observations and previous findings by other researchers, a hypothesis for the differential accumulation and identical translocation patterns of the two Cr ions is proposed. Received: 27 February 1998 / Accepted: 2 April 1998     

Arsenic tolerance in mesquite (Prosopis sp.): Low molecular weight thiols synthesis and glutathione activity in response to arsenic

The effects of arsenic stress on the production of low molecular weight thiols (LMWT), glutathione S-transferase activity (GST) and sulfur metabolism of mesquite plant (Prosopis sp.) were examined in hydroponic culture at different arsenic [As(III) and (V)] concentrations. The production of LMWT was dependent on As speciation and concentration in the growth medium. The roots of As(III) treated plants produced significantly higher LMWT levels than As(V) treated roots at the same concentration of As applied. In leaves, the thiols content increased with increasing As(III) and (V) concentrations in the medium. Hypersensitivity of the plant to high As concentrations was observed by a significant decrease of LMWT produced in the roots at 50 mg/L treatment in both As(III) and (V) treatments. Sulfur was translocated from roots and accumulated mainly in the shoots. In response to As-induced phytotoxicity, the plants slightly increased the sulfur content in the roots at the highest As treatment. Compared with As(V)-treated plants, As(III)-treated roots and leaves showed significantly higher GST activity. The roots of both As(III) and (V) treated plants showed an initial increase in GST at low As concentration (5 mg/L), followed by significant inhibition up to 50 mg/L. The leaves had the highest GST activity, an indication of the ability of the plant to detoxify As in the leaves than in the roots. The correlation between LMWT content, S content and GST activity may be an indication these parameters may be used as biomarkers of As stress in mesquite.     

Effects of Reductants on Phytoextraction of Chromium (VI) by Ipomoea aquatica

Reductants are often used to reduce Cr(VI) in chemical treatments, yet the effects of the reductants on Cr(VI) phytoremediation are not fully understood. This study investigates the effects of different reductants on Cr(VI) phytoremediation by Ipomoea aquatica in simulated solution with 3 mg L^?1 of Cr(VI), pH₀ of 6, and an incubation time of 5 days. Results indicate that the applications of S₂O₃^2?, Fe⁰, and Fe²⁺ at low doses notably increased root Cr concentrations, which were obviously higher than that those in the control (Cr⁶⁺ alone). However, high reductant concentrations decreased bioaccumulation of Cr in the roots and shoots of the plant.

Statistical results indicate that Cr concentrations were significantly and negatively correlated with Fe concentrations in the roots and shoots of the plant (p < 0.05). This suggest that Fe accumulation inhibited Cr accumulation in the plant. A Cr(VI) concentration of 3 mg L^?1 caused short, brown lateral roots with tip necrosis, leaf chlorosis, and noticeable shoot wilting. The leaf necrosis and shoot wilting is caused by oxidative damage of lateral roots by Cr(VI) rather than by the reactive oxygen species generated by the oxidative stress. Addition of the reductants effectively reduced these plant injuries.     

Toxicity of arsenic (III) and (V) on plant growth, element uptake, and total amylolytic activity of mesquite (Prosopis juliflora x P. velutina)

The effects of arsenite [As(III)] and arsenate [As(V)] on the growth of roots, stems, and leaves and the uptake of arsenic (As), micro- and macronutrients, and total amylolytic activity were investigated to elucidate the phytotoxicity of As to the mesquite plant (Prosopis juliflora x P. velutina). The plant growth was evaluated by measuring the root and shoot length, and the element uptake was determined using inductively coupled plasma optical emission spectroscopy. The root and leaf elongation decreased significantly with increasing As(III) and As(V) concentrations; whereas, stem elongation remained unchanged. The As uptake increased with increasing As(III) or As(V) concentrations in the medium. Plants treated with 50 mg/L As(III) accumulated up to 920 mg/kg dry weight (d wt) in roots and 522 mg/kg d wt in leaves, while plants exposed to 50 mg/L As(V) accumulated 1980 and 210 mg/kg d wt in roots and leaves, respectively. Increasing the As(V) concentration up to 20 mg/L resulted in a decrease in the total amylolytic activity. On the contrary, total amylolytic activity in As(III)-treated plants increased with increasing As concentration up to 20 mg/L. The macro- and micronutrient concentrations changed in As-treated plants. In shoots, Mo and K were reduced but Ca was increased, while in roots Fe and Ca were increased but K was reduced. These changes reduced the size of the plants, mainly in the As(III)-treated plants; however, there were no visible sign of As toxicity.     

Aluminium-induced changes in the profiles of both organic acids and phenolic substances underlie Al tolerance in Rumex acetosa L.

The common sorrel, Rumex acetosa L. is well adapted to acid mineral soils with high availability of phytotoxic Al species. The mechanisms of Al resistance in this species are not established. Our goal was to assess the possible implications of organic acids and phenolic substances in Al detoxification in roots and shoots of this plant. R. acetosa plants were exposed in nutrient solution (pH 4.3) to a non-growth reducing Al concentration of 50 μM Al for 5 days. Exclusion of Al from root tips was visualized by haematoxylin staining. Tissue Al and Ca concentrations were analysed by ICP ES. Root and shoot concentrations of organic acids and phenolic substances were analysed by HPLC. A time-dependent (model II type) Al exclusion pattern in root tips was observed. Nonetheless, high Al concentrations accumulated in roots (1170 μg/g) and shoots (275 μg/g). Aluminium supply enhanced root citrate concentrations but decreased shoot organic acid levels. Aluminium induced high levels of anthraquinone in roots and of catechol, catechin and rutin in shoots. Aluminium resistance in R. acetosa implies both exclusion of Al from root tips and tolerance to high Al tissue concentrations. Citrate in roots and phenolics in shoots may bind Al in non-toxic form. Anthraquinones, as strong antioxidants, may play a role in a general defence response to the root stress.     

Growth and translocation of C and N in wheat (Triticum aestivum) grown with a split root system

The root system of wheat seedlings ( Triticum aestivum L. SUN 9E) was pruned to two seminal roots. One of the roots was supplied with different levels of NO₃, the other was deprived of N. Root respiration and the increment of C and N in roots and shoots were measured to determine the C/N ratio of the phloem sap feeding the N-deprived roots. Thus it was possible to determine translocation of N from the shoots to the roots. It was calculated that the C/N ratio of phloem sap feeding roots of plants growing at optimal and suboptimal N supply was ca 54. A supra-optimal N supply reduced, whilst shading increased, the C/N ratio of phloem sap. At optimal N supply 11% of all N transported to the shoots was retranslocated to the roots. Both a supra-optimal and a limiting N supply increased translocation of N back to the roots to 18% of the N translocated to the shoot, whilst shading of the plants decreased the proportion cycled to 7%. At the optimal N supply, 40% more N was translocated to the roots from the shoot than was incorporated by them. At a lower supply of N, 80% more N was imported from the shoots than was incorporated by these roots. It is suggested that the distribution of N between roots and shoots predominantly occurs in the shoots. The specific mass transfer rate in seminal roots was determined. The highest value was found for roots grown with an optimal N supply: 1.1 mg carbohydrate s⁻¹ cm⁻² (sieve tube) which is well within the range observed for other plant organs. Roots supplied with NO₃ produced more and longer laterals than N-deprived roots. It is suggested that this is due to the effect of NO₃ on import of carbon and other components transported in the mass flow with carbon.