Zn Concentration In Solution Research Articles

Separation of Cadmium and Zinc by Supported Liquid Membrane Using TOPS‐99 as Mobile Carrier

The separation of cadmium and zinc from a dilute aqueous sulfate media using supported liquid membrane (SLM) technique has been studied. The microporous polypropylene film Celgard‐2400 was used as the solid support for the liquid membrane and TOPS‐99 was used as the mobile carrier. The effect of different parameters such as flow rate, pH of feed solution, extractant concentration in membrane phase, acid concentration in strip solution, and Cd(II) and Zn(II) concentration in feed solution on metal ion flux was studied. It was observed that a flow rate of 100 mL/min was sufficient to minimize the resistance due to an aqueous boundary layer at the feed solution membrane interface. The zinc flux increased with an increase of pH from 1.5 to 3.5 and then decreased with a further increase of pH. The cadmium permeation started at a higher pH, i.e., 3.0 and increased with increase of pH up to 3.5 followed by a plateau. Also, with an increase of TOPS‐99 concentration in the membrane phase up to 200 mol/m3, J Zn increased and then decreased. The separation factors for cadmium and zinc were calculated at different experimental conditions and are reported.

クエン酸浴から電析されたＺｎＴｅ薄膜のホール効果測定による電気的特性の評価

ZnTe thin films were electrochemically deposited onto Au-coated Cu substrates from an electrolytic solution containing ZnSO4, TeO2, citric acid and sodium citrate. Effects of deposition potentials and solution's Zn concentrations on the structure, chemical composition and electrical property were investigated. The resistivity, conduction type, carrier density, and carrier mobility were evaluated with a Hall effect measurement apparatus. A film with nearly stoichiometric composition was deposited at -0.80 V vs. Ag /AgCl at 353 K from a solution containing 2.0×10-2 kmol m-3-ZnSO4, 1.0×10-4 kmol m-3-TeO2, 9.0×10-2 kmol m-3-citric acid and 2.1×10-1 kmol m-3-sodium citrate and was well crystallized with cubic preferential orientation along the (111) plane. A resistivity, the carrier density and the carrier mobility were 4.1×102 Ωm, 2.9×1019 m-3 and 5.3×10-4 m2 V-1 s-1, respectively.

Soil isotopically exchangeable zinc: A comparison between E and L values

The objectives of the present paper were: (i) to determine isotopically exchangeable zinc using two isotopic exchange methods (E and L values) in a series of polluted and non-polluted Swiss agricultural soils, and (ii) to evaluate the ability of chemical extraction methods to estimate plant-available soil Zn using isotopic techniques. The surface horizon (0–20 cm) of seven polluted and non-polluted soils representing a wide range in physico-chemical properties and Zn contents were sampled. An isotopic exchange kinetics (IEK) approach was used to assess, in a batch experiment, the isotopically exchangeable Zn content (E value). In order to determine the L values, a pot experiment was carried out with Lolium multiflorum (cv. Axis) in a growth chamber using a 65Zn-isotope dilution technique. Total Zn uptake and the isotopic composition (65Zn/stableZn) were determined in Lolium multiflorum for five successive cuts. The amounts of zinc extracted by different chemicals were compared with L values and regression parameters were estimated. The isotopic composition in soil extracted by DTPA and EDTAAc at the end of the pot experiment was also determined. Results showed that the equation describing the increase of isotopically exchangeable Zn with time could be extrapolated to three months for polluted and non-polluted neutral and acidic soils, and that the results were not different from the amount of isotopically exchangeable Zn experimentally determined with Lolium multiflorum (L value). In alkaline soils however, results suggest that either 65Zn sorption occurred in the batch experiment or that the concentration of Zn in the soil solution had been overestimated, leading to an overestimation of the E value compared to the L values. Furthermore, the specific activities measured in DTPA and EDTA extractions at the end of the pot experiment were significantly different compared to the specific activity of the plant, showing that both these chelating agents extract neither all the available soil Zn nor only the available soil Zn for plants.

Structural, optical and electrical properties of ZnTe thin films electrochemically deposited from a citric acid aqueous solution

ZnTe thin films were electrodeposited onto Au-coated Cu substrates from an electrolytic solution containing ZnSO4, TeO2, citric acid and sodium citrate, and the dependence of structure, composition and surface morphology on the solution's Zn concentration were investigated. Results of examinations for resistivity and optical absorption demonstrated, respectively, that as the Zn concentration in the electrolyte was reduced over the range of 5–50 mmol dm−3, the resistivity of the films obtained continuously decreased and their band gap increased. Quantative analysis of energy dispersive x-ray analysis and inductively coupled plasma results indicated that the composition ratio (Zn : Te) was approximately stoichiometric under all conditions in this study. X-ray diffraction results revealed that all the ZnTe thin films obtained showed a preferred (111) orientation with cubic structure. ZnTe thin films we transferred onto nonconductive epoxy resin also showed the same structure.

Mobilization of metals from riverine suspended matter in seawater

Suspended matter from rivers in the Russian Far East was leached by seawater to assess the extent of Zn, Cd, Pb, and Cu release from the land-derived solids to estuarine and coastal waters. The concentration of solids in the leaching experiments was 0.25 g/l. The suspended matter used varied in metal content from background levels to heavily contaminated. The concentration of Zn, Cd, Pb, and Cu in the leaching solution after 1, 4, 24, 48, and 96 h was determined by anodic stripping voltammetry. Measurable Cd was transferred from river suspended matter to seawater with both background and elevated Cd concentrations, though the amount of Cd released was different. The increase in Zn and Pb in solution was observed only at the enriched concentration of metals in the suspended solids. The Cu transfer into solution was more pronounced from material with elevated Cu concentration, but the amount of degradable organic matter in the solid phase was more important. The losses of metals from the riverine solids varied from 60% to 80% of total concentration for Cd to negligible for Pb, independent of contamination. The Zn loss depended on the initial concentration in the solids and decreased from 11–16% for the heavily contaminated suspended matter to 3–8% for the moderately enriched ones and to negligible for the pristine solids. Cu loss varied from 1% to 30% of total content, with no clear dependence on concentration in the suspended matter. The significance of additional input of dissolved metals to estuarine and coastal waters from remobilization was assessed by comparison with the initial concentration of dissolved metals in river water. Such experiments could be used to assess the water quality impact of atmospheric fallout of contaminated solids and storm drainage from the urbanized areas, in addition to river suspended matter studies.

Zinc Adsorption and Desorption at Low Concentrations by Basaltic Soil on Jos Plateau, Nigeria

Adsorption and desorption studies are useful indices of quantity/intensity (Q/I) relationships for nutrient ions in soil. This study investigated zinc (Zn) adsorption from zinc sulfate (ZnSO4·7H2O) and the desorption of the adsorbed Zn with 0.1 M HCl in the surface (Ap) and subsoil (Bt2) horizons of a Rhodic Kandiustult. Graded concentrations of Zn, from 0 to 154 μM Zn g−1 soil were used for the studies. Zinc adsorption in the soil increased with concentration. The Bt2 horizon adsorbed more Zn than the Ap horizon (P<0.05). At the highest level of added Zn, the concentration of Zn in equilibrium solution could not reach the critical available Zn level of at least 15.4 μM, due to the low concentrations used. Zinc adsorption isotherms were resolved into two parts, part 1 terminating at 25 μM level of added Zn in the Ap horizon and at 62 μM level in the Bt2 horizon. Only part 1 of the isotherm of the Ap horizon adjusted to the Langmuir equation (P<0.01), while both parts fitted the Freundlich isotherm (P<0.05 and P<0.01). Zinc desorption isotherms were also split in two parts with part 1 terminating at 31 μM level of added Zn in both horizons of the soil. The resolution of both adsorption and desorption isotherms into two parts either of which covered similar concentration ranges strongly suggests that more than one mechanism was responsible for Zn adsorption in the soil. The total amount of Zn desorbed was similar between the two horizons of the soil. Only 31% and 29% of total adsorbed Zn (0.1 M HCl extractable Zn+added Zn) was desorbed by three sequential extractions in the Ap and Bt2 horizons respectively, but 151% and 127% of added Zn was desorbed in the two horizons of the soil. A maximum of two extractions, which yielded 122% and 105% of added Zn in the Ap and Bt2 horizons, respectively, are sufficient to remove all the added Zn in the soil.

Utilisation of Zinc from Zinc Fertiliser Bands: Effect of Concentrations of Zinc in Zinc Chloride and Zinc-Edta Fertiliser Bands on Root Growth and Zinc Uptake by Wheat

Factors affecting the extent of utilisation of zinc from zinc fertiliser bands include the extent of solubility and movement of zinc from the fertiliser band, the extent of root to zinc fertiliser contact as well as the concentration and extent root growth in the fertiliser band. Growth chamber and incubation studies were conducted to compare the extent of root growth and zinc uptake by wheat (Triticum aestivum var Columbus) from Zinc chloride (ZnCl2) and Zn-EDTA fertiliser bands when applied to a non-calcareous soil of pH 7.0 and low in DTPA-extractable Zn. The Zn fertilisers were uniformly mixed with 200 g of soil contained in acrylic cylinders. Concentrations and forms of water-soluble Zn in the fertiliser bands were determined from the incubation studies. Concentrations of Zn in the Zn-EDTA fertiliser bands were much greater than concentrations of Zn in the ZnCl2 bands. Whereas root growth was stimulated by low rates of ZnCl2, high rates of Zn (>0.8 mg Zn/cm3) resulted in a decrease in the amount as well as quality of roots recovered from the fertiliser band. Zn-EDTA reduced root growth into the fertiliser band for most of the rates of Zn applied. The results obtained showed that root growth into the fertiliser bands was closely related to the concentration of Zn in soil solution. Concentrations of Zn in the plant shots increased with increase in the amount of roots in the fertiliser bands as well as the concentration of Zn in soil solution.

Metal accumulation and competitive ability in metallicolous and non-metallicolous Thlaspi caerulescens fed with different Zn salts

A non-metallicolous (NM) ecotype of Thlaspi caerulescens from Luxembourg and a metallicolous (M) ecotype from Prayon (E Belgium) are compared for growth and Zn, Mg and Ca accumulation in shoot in a pot experiment in six soil conditions with contrasting Zn availability. The soils were spiked with 2000 mg kg−1 Zn as monometallic salts of contrasting solubility. Both ecotypes were grown in pure and mixed culture in order to assess competitive ability. Both ecotypes had similar growth on all substrates except the one spiked with Zn-sulphate, where ecotype M grew better and had higher competitive ability than ecotype NM. Ecotype NM had higher Zn concentrations than M in all treatments and the difference varied with Zn availability, being largest with Zn-oxide (NM: 31 300 mg kg−1 Zn; M: 5900 mg kg−1 Zn). The results thus indicate that ecotype NM has constitutively higher Zn uptake capacity and may have a higher ability to obtain Zn from specific Zn salts. However, ecotype NM does not appear to be more efficient in obtaining Zn from little available forms. Mg concentration was also higher in ecotype NM. Zn mass per plant was higher in ecotype NM compared to ecotype M in all substrates except Zn-sulphate where the reverse was true. Accordingly, ecotype NM could prove to be a better phytoextractor of Zn for phytoremediation, except in substrates with low pH and high concentration of free Zn in the soil solution.

Applying a solute transfer model to phytoextraction: Zinc acquisition by Thlaspi caerulescens

Phytoextraction is the removal of metals from contaminated soils into harvested plant tissues. The rate of phytoextraction is governed by both soil and plant characteristics. Most effort has focused on identifying appropriate plants for phytoextraction, but the benefits from this effort will be marginal unless the metals are in phytoavailable forms in the rhizosphere. The concentration of a metal in the rhizosphere can be estimated using solute transfer models that incorporate: the metal concentration in the bulk soil solution, the buffer power of the soil, diffusion coefficient for the metal, water movement, root size and morphology, and the rate of entry of metal into the roots. Here a solute transfer model is developed to predict the concentration of Zn in the rhizosphere solution ([Zn]ext) of Thlaspi caerulescens, a hyperaccumulator species that could be exploited for Zn phytoextraction. The model predicts that Zn accumulation by T. caerulescens is sub-optimal when the Zn concentration in the bulk soil solution is <27 μM. Such a high [Zn]ext is rare in contaminated agricultural soils, but is possible in the metalliferous substrates where T. caerulescens is endemic. Sensitivity analyses indicate that Zn diffusion is more important than transpiration-driven mass flow for Zn delivery to the root, implying that management of soil physical and hydrological properties will improve phytoextraction. Sensitivity analyses also imply that strategies to enhance the Zn absorption power of the root will not necessarily be successful for enhancing phytoextraction per se. Thus, research into enhancing Zn availability and mobility in soil will be as important as understanding and manipulating Zn uptake by plants. In general, such models can be used to identify constraints to efficient phytoextraction (whether plant or soil) and to determine whether commercial phytoextraction is feasible.

GROWING RICE GRAIN WITH CONTROLLED CADMIUM CONCENTRATIONS

ABSTRACT Two solution studies were conducted a) to investigate the uptake of zinc (Zn) and cadmium (Cd) by rice plants (Oryza sativa L.) and interaction between these elements, and b) to determine experimental conditions for growing rice grain with desired Cd concentration for an animal feeding study. In both studies, free metal activities of cadmium and cationic microelements were buffered by an excess of chelating agents. The first study was a factorial design with two Zn levels (1.0 and 3.89 ÂμM) and four Cd levels (0.81, 1.44, 2.56 and 4.55 ÂμM) in the solution. In the second study, rice was grown in two solutions of different micro- and macro-element compositions and three Cd levels (0.0, 0.5, and 2.0 ÂμM). In the first study, solution Zn concentration of 3.89 ÂμM and corresponding free metal activity (pZn2+) of 6.00 was toxic to young rice plants. With time, Zn concentrations in rice plants decreased while Cd concentrations increased. Toxic concentration of Cd in roots (about 100 mg kg−1) associated with a 20% reduction in the root dry matter occurred at the free Cd2+ activities in the solution (pCd2+) in the range of 10.25–9.75. Sufficient Zn level in plants slightly stimulated Cd transfer from roots to shoots as opposed to barely sufficient or slightly deficient Zn concentration in shoots. However, the better Zn status in plants clearly diminished severity of Cd toxicity symptoms in shoots. The use of nutrient solutions adapted for rice growth allowed the rice grown in the second experiment to produce grain under controlled conditions. Cadmium in the brown rice grain was 0.1 to 0.8 mg kg−1, covering the range needed for feeding experiments relevant to rice Cd risk to humans. Composition of the nutrient solutions, in addition to solution Cd level, had a significant effect on Cd concentration in grain. Correlation of grain Cd concentration with solution Cd2+ activity was much stronger than with total solution Cd. Results of both experiments supported hypothesis that Cd uptake and transport within rice plants is an active process.

INTER-RELATIONSHIPS BETWEEN ZINC NUTRITION, GROWTH PARAMETERS, AND NUTRIENT PHYSIOLOGY IN A HYDROPONICALLY GROWN TOMATO CULTIVAR

An experiment was carried out to investigate the role of zinc (Zn) in physiological and nutritional development of the tomato (Lycopersicon esculentum Mill.) cultivar Moneymaker grown hydroponically in a controlled temperature room. The experiment was a 2 × 3 factorial arrangement with two levels of nutrient solution Zn concentrations, 0.01 or 0.10 mg L−1, and three levels of foliar applied Zn, 0, 23, or 230 mg L−1. Low (0.01 mg L−1) nutrient solution Zn seems to be inadequate for optimum plant growth; in this treatment plants showed Zn deficiency symptoms. Foliar application of Zn at 23 mg L−1 alleviated Zn deficiency by enhancing Zn concentration in the leaves, dry weight, and chlorophyll content. However, foliar application of Zn at 230 mg L−1 Zn caused adverse effects on both dry weight and chlorophyll content. Phosphorus (P) reached a detrimental level in the leaves of plants grown at 0.01 mg L−1 Zn in nutrient solution, but 23 mg L−1 Zn applied via the leaves reduced P concentrations from toxic to adequate levels. Concentration of iron (Fe) was also higher but not detrimental in the leaves of plants grown at 0.01 mg L−1 Zn. Foliar application of both concentrations of Zn slightly reduced Fe concentration in the leaves. Membrane permeability was impaired both in the plants grown at low Zn and in those receiving the high (230 mg L−1) foliar application. Foliar Zn at 23 mg L−1 maintained membrane permeability by decreasing electrolyte leakage from leaves of Zn-deficient plants. Low (0.01 mg L−1) Zn in the nutrient solution resulted in plants with very leaky root systems as measured by high potassium (K) efflux; this leakiness was ameliorated by foliar application of Zn at 23 mg L−1. Cumulative potassium release from intact roots was higher in both the Zn-deficient plants (0.01 mg L−1) and those receiving high (230 mg L−1) foliar Zn. These data show that Zn nutritional status affects root membrane integrity.

RESPONSES OF TOMATO CVs GROWN TO FRUIT–HARVEST STAGE UNDER ZINC STRESS IN GLASSHOUSE CONDITIONS

A glasshouse pot experiment, with three tomato cultivars (CVs), Blizzard, Calypso, and Liberto, was carried out to investigate the effects of three levels of Zn (0.01, 0.5, and 30 mg L−1), supplied in nutrient solution, on plant growth, fruit yield, and nutrient composition. Maximum plant growth and fruit yield in all three cultivars were in the 0.5 mg L−1 treatments. Plants treated with Zn at 30 mg L−1 had lower total dry matter content at fruit set as compared with the 0.5 mg L−1 treatment (reduced by 34% in Calypso, 30% in Blizzard, and 21% in Liberto). The percent reduction was similar at the low-Zn treatment (27, 27, and 29%, respectively), and plants exhibited Zn deficiency symptoms. Fruit yield was significantly reduced under both conditions of Zn stress, but only in Calypso was the yield significantly lower at the high rather than at the low-Zn treatment. Zinc concentrations in all plant parts (in particular in roots) increased with increasing Zn concentration in nutrient solution. Concentrations of phosphorus (P), magnesium (Mg), potassium (K), and iron (Fe) in the leaves and fruit of plants grown at 30 mg L−1 Zn were significantly lower than the 0.5 mg L−1 treatment and, with the exception of P in the leaves, lower than in the 0.01 mg L−1 treatment. Zinc stress appears to affect the distribution of macro- and micronutrients in tomato plants with corresponding adverse effects on growth and yield. In this experiment there were no consistent cultivar differences at these zinc concentrations.

Seedling responses of three Australian tree species to toxic concentrations of zinc in solution culture

A frequently desired outcome when rehabilitating Zn toxic sites in Australia is to establish a self-sustaining native ecosystem. Hence, it is important to understand the tolerance of Australian native plants to high concentrations of Zn. Very little is known about the responses of Australian native plants, and trees in particular, to toxic concentrations of Zn. Acacia holosericea, Eucalyptus camaldulensis and Melaleuca leucadendra plants were grown in dilute solution culture for 10 weeks. The seedlings (42 days old) were exposed to six Zn treatments viz., 0.5, 5, 10, 25, 50 and 100 μM. The order of tolerance to toxic concentrations of Zn was E. camaldulensis> A. holosericea> M. leucadendra, the critical external concentrations being approximately 20, 12 and 1.5 μM, respectively. Tissue Zn concentrations increased as solution Zn increased for all species. Root tissue concentrations were higher than shoot tissue concentrations at all solution Zn concentrations. The critical tissue Zn concentrations were approximately 85 and 110 μg g−1 DM for M. leucadendra, 115 and 155 μg g−1 DM for A. holosericea and 415 and 370 μg g−1 DM for E. camaldulensis for the youngest fully expanded leaf and total shoots, respectively. The results from this paper provide the first comprehensive combination of growth responses, critical external concentrations, critical tissue concentrations and plant toxicity symptoms for three important Australian genera, viz., Eucalyptus, Acacia and Melaleuca, for use in the rehabilitation of potentially Zn toxic sites.

Metal availability in contaminated soils: I. Effects of floodingand organic matter on changes in Eh, pH and solubility of Cd, Ni andZn

Soil pH and Eh play an important role in reducing heavy metal solubility in paddy soils. To assess the effects of flooding and organic matter application on changes in Eh, pH and solubility of Cd, Ni and Zn in contaminated soils, a growth chamber experiment with rice plants(Oryza sativa L.) was conducted. Eh values decreased with flooding time in all three soils. The changes of Eh values were more negative in the tannery and alum shale contaminated soils and stabilized after 30 days of submergence. The Eh changes were not so large in the city sewage contaminated soil as in the other two soils. Soil pH increased with flooding time. During the 65 days of submergence, pH increase was about 2, 1 and 0.6units in the tannery, city sewage and alum shale soils, respectively. In all three soils, organic matter treated soil showed lower Eh and higher pH values compared to untreated soil. Concentration of Cd, Ni and Zn in soil solution decreased with flooding time. The solution concentration of Cd and Zn in the city sewage soil and of Ni in the tannery soil was higher than in the alum shale soil. The soluble metal concentration in all three soils was lower inorganic matter treated soils. Reduced solubility of metals in the organic matter treated soils was related to larger changes of Eh and pH values in these soils. Correlation coefficient calculations also showed that metal solubility decreased with decreased Eh and increased pH in the soil solution.

Phosphorus and acid phosphatase enzyme activity in leaves of tomato cultivars in relation to zinc supply

Tomato cultivars Blizzard and Liberto were grown hydroponically in a controlled temperature (C.T.) room for 35 days. The objective was to investigate the relationship between phosphorus (P) concentration and acid phosphatase enzyme [EC.3.1.3.2.] (APE) activity in leaves in relation to zinc (Zn) concentration in nutrient solution. Zinc was added at concentrations of 0.01,0.5, and 5 mg L‐1. The 0.01 and 5 mg L‐1 Zn treatments led to a significant reduction in dry matter and total chlorophyll content compared with 0.5 mg L‐1 for both cultivars. Zinc concentration was considered inadequate in the leaves of plants subjected to 0.01 mg L‐1 Zn, while it was at toxic level in those in the 5 mg L‐1 Zn treatment according to values stated for tomato plants. Optimal results for all criteria tested in this experiment were for plants grown in 0.5 mg L‐1 Zn treatment. In the leaves of plants grown at 0.01 mg L‐1 Zn, APE concentrations were significantly the lowest and concentrations of P were at a toxic level. The APE activity was noticeably higher in the P‐deficient plants of both cultivars grown in the solutions with high Zn (5 mg L‐1).

Changes in soil solution Zn and pH and uptake of Zn by arbuscular mycorrhizal red clover in Zn-contaminated soil

Red clover plants inoculated with Glomus mosseae were grown in a sterile pasture soil containing 50 mg Zn kg −1 in ‘Plexiglas’ (acrylic) containers with nylon net partitions (30 μm mesh) designed to separate the soil into a central root zone and two outer zones for hyphal growth with no root penetration. Two porous plastic soil moisture samplers were installed in each pot, one in the root compartment and the other in one of the hyphal compartments. The soil in the outer compartments was amended with one of the four application rates of Zn (as ZnSO 4) ranging from 0 to 1000 mg kg −1. Non-mycorrhizal controls were included, and there were five replicates of each treatment in a randomised block in a glasshouse. Uninoculated plants received supplementary P to avoid yield limitation due to low soil P status. Plants grew in the central compartment for nine weeks. Soil moisture samples were collected 4, 24 and 62 days after sowing to monitor changes in the Zn concentration and pH of the soil solution. At harvest, the mean mycorrhizal infection rate of inoculated plants ranged from 29% to 34% of total root length and was little affected by Zn application. Root and shoot yields were not affected by mycorrhizal infection. Plant Zn concentration and uptake were lower in mycorrhizal plants than non-mycorrhizal controls, and this effect was more pronounced with increasing Zn application rate to the soil. Soil solution Zn concentrations were lower and pH values were higher in mycorrhizal treatments than non-mycorrhizal controls and the mycorrhiza effect was more pronounced at higher Zn application rates. The protective effect of mycorrhiza against plant Zn uptake may have been associated with changes in Zn solubility mediated by changes in the soil solution pH, or by immobilisation of Zn in the extraradical mycelium.

Short‐term relationships between membrane permeability and growth parameters in three tomato cultivars grown at low and high zinc

An experiment was carried out in a controlled temperature (CT) room for five weeks with tomato cvs., Moneymaker, Liberto, and Calypso, to investigate possible relationships between zinc (Zn) deficiency or toxicity and electrolyte leakage in plant leaves. The concentrations of Zn in nutrient solution were 0.01, 0.5, and 5.0 mg L−1, respectively. There were significant reductions in the dry matter and chlorophyll content of all three cultivars grown both at 0.01 (low) and 5 mg L−1 (high) Zn compared to 0.5 mg L−1. The concentration of Zn at 0.01 mg L−1 was not sufficient to provide for optimal plant growth, while 5 mg L−1 in nutrient solution was detrimental to plant growth for all three cultivars. Dry matter production was generally lowest in the plants grown at low (0.01 mg L−1) Zn except for Moneymaker where the lowest biomass was in the high Zn treatment. Zinc concentration was increased in the leaves and roots with increasing Zn concentration in nutrient solution. Phosphorus concentration was toxic in the leaves of the plants grown at low (0.01 mg L−1) and was deficienct at high Zn (5 mg L−1). The electrolyte leakage (%) gradually increased in the plants grown at low and high Zn concentrations and these increases were greatest in the leaves of plants grown at low Zn (except for Moneymaker grown at high Zn where reduction in dry matter was less). The best results for all growth parameters tested were for the plants grown at 0.5 mg L−1 Zn. The results of this short‐term experiment show that electrolyte leakage which is relatively simple and easy to measure may be a good indicator of cultivar tolerance to Zn deficiency and toxicity.

539 Assessing Soil and Water Thresholds for Potential Dietary Zinc Toxicity in Selected Vegetable Crops

Vegetables play an important role in the human diet, and production in suburban areas has increased as populations have become more urbanized. However, heavy metal pollution of soils has enhanced in such areas, and metal accumulation in vegetables may pose a human health risk when consumed. Zinc is an essential micronutrient for plants and humans, but it is toxic to plants and humans at high levels. Although a maximum Zn tolerance for human health has been established for edible parts of vegetables (20 mg/kg DW), little information is available for predicting vegetable Zn concentration based on soil and water Zn levels. The objectives of this study were to determine the critical Zn concentrations in nutrition solution and soil to reach maximum Zn tolerance concentrations in Chinese cabbage, bok choy, and celery. Five Zn levels were used for both solution and soil culture experiments, with three replicates of each. Shoot growth was significantly inhibited at Zn concentrations above 50 mg/L in nutrition solution and above 180 mg/kg in soil. The sensitivity of crops to zinc toxicity, in term of shoot and root growth, decreased in the order: celery &gt; Chinese cabbage &gt; bok choy. Zinc accumulation in shoots and edible parts varied with Zn supply levels and type of vegetables. A negative correlation was noted between Zn accumulation and dry matter yields, with r-squared values of 0.980** for nutrient solution and 0.960* for soil culture. Zinc concentrations in shoots or edible vegetable parts were below 20 mg/kg (human health threshold) when they were grown at DTPA extractable Zn in the soil less than 75, 100, and 175 mg/kg for bok choy, celery, and Chinese cabbage, respectively.

Copper and Zinc Competitive Adsorption: Desorption in Calcareous Soils

The behavior in competitive adsorption-desorption reactions of Cu and Zn was studied in four calcareous soils. Cu and Zn were added to the soil by Cu, Zn, and Cu+Zn sulfate solutions in a CaSO4 background. Soil sorption of these cations was described by equilibrium isotherms that fitted either Freundlich- or Langmuir type equations, although Cu desorption data fitted only Freundlich isotherms. Cu and Zn competition was quantified by distribution coefficients, Kd, relating cation distribution between soil and solute and by the competitive Langmuir equation. The competitive Langmuir equation was the better suited to describe the Cu-Zn competitive adsorption in these soils. Distribution coefficients presented lower values when both cations were present, decreasing when the Cu and Zn concentration in solution increased (decreasing soil affinity for these cations), thereby increasing their mobility through the soil. However, the distribution coefficient of specifically adsorbed Cu in equilibrium with cations extracted by a Mg (NO) solution increased with Cu concentration. Cu adsorption was more depres 3 se 2 d by Zn than Zn adsorption by Cu. The different behavior of Cu and Zn seems dependent on the percentage Ca (CO) and, to a lesser degree, on Cu and Zn organic matter complexes, free iro 3 n 2 content, and surface precipitation on oxides and carbonates.

Effects of nutrient solution zinc activity on net uptake, translocation, and root export of cadmium and zinc by separated sections of intact durum wheat (Triticum turgidum L. var durum) seedling roots

Cd accumulation in durum wheat presents a potential health risk to consumers. In an effort to understand the physiological mechanisms involved with Cd accumulation, this study examined the effects of Zn on Cd root uptake and phloem translocation in a split– root system. Durum wheat seedlings were grown in chelate-buffered nutrient solution with intact root systems divided into two sections. Each root section grew in a separate 1 l pot, one of which contained 0.2 μM CdSO4. In addition, each two-pot system contained ZnSO4 in the following combinations (in μm) (for -cd root system: +cd root system): 1:1, 1:10, 10:1,10:10, 1:19, and 19:1. Harvested plant material was analyzed for Cd and Zn. In addition, rates of Cd and Zn net uptake, translocation to the shoot, and root export (translocation from one root segment to the other) between days 8 and 22 were calculated. Results show that Zn was not translocated from one root section to its connected root section. Uptake rates of Cd increased as solution Zn concentrations increased. Cd translocation from one root section to the other decreased significantly when the Zn concentration in either pot was greater than 1 μM. These results show the potential of Zn to inhibit movement of Cd via the phloem, and suggests that providing adequate Zn levels may limit phloem loading of Cd into wheat grain. Increasing the rhizosphere activity of Zn2+ in Cd-containing soils may therefore result in reduced Cd accumulation in grain even while net Cd uptake is slightly enhanced.