Zinc Phytotoxicity Research Articles

Zinc phytotoxicity and cationic micronutrients contents in quinoa as influenced by high Zn levels and soil amendments

Contamination of soils with heavy metals (HMs) is a serious concern due to the non-biodegradability and accumulation of these metals in living organism. The aim of this study was to evaluate the effectiveness of fish scale powder, chitin, and chitosan in reducing zinc (Zn) accumulation of quinoa grown on Zn-polluted soil. A 3*4 factorial greenhouse trial was conducted with three levels of Zn contamination (0, 100, and 200 mg kg−1) and four types of amendment (control, and 0.15% of each fish scale powder, chitin, and chitosan). The quinoa seeds were sown in soil treated with Zn contamination and amendments. Results showed that application of amendments especially chitosan increased shoot and root dry weight and greenness index of quinoa leaf. While high soil Zn levels decreased cationic micronutrients uptake in quinoa shoot, chitosan addition desirably enhanced cationic micronutrients uptake in quinoa shoot. Overall, amendment addition significantly decreased Zn concentration and uptake in plant; the highest decrease was observed following the application of chitosan. Although the application of high Zn levels significantly increased the values of phytoextraction efficiency, application of chitin and chitosan significantly decreased it. The value of translocation factor was lower than one, demonstrating that a large amount of Zn was stabilized in the root tissues, while a small amount of this metal was transferred to aboveground parts of quinoa. Results of this study revealed that fish scales derivatives were effective in preventing Zn toxicity and at the same time increasing cationic micronutrients contents of quinoa grown on HMs-polluted soil.

Response of Bacillus cereus on Zea mays under different doses of zinc sulphate

Anthropogenic activities have added a large amount of heavy metals to the environment. Heavy metal contaminants affect the physiological and biological properties of soil and plant health. Zinc (Zn) is an essential micronutrient and it promotes plant growth and development but a higher concentration of the metal causes reduction in plant growth. The present study was aimed to evaluate the response of Bacillus cereus on maize plants at different concentrations of ZnSO4 (20, 40 and 60 mg kg–1) amended in the soil under pot experiment conditions. The experiment was conducted by using complete randomized design (CRD) with three replications. Higher doses of ZnSO4 inhibited maize growth and nutrient uptake. However, inoculation of maize seeds with Bacillus cereus at 20 mg kg–1 concentration of ZnSO4 increased seed germination about 39% and plant height by 15%. Moreover, 17% increase in leaf length and a 7% increase in leaf number were observed as compared to control at 20 mg kg–1 concentrations of ZnSO4. Reductions in all growth parameters were observed with 60 mg kg–1 concentration of ZnSO4. The Zn uptake was 75% higher in treatment T8 (uninoculated seeds with 60 mg kg–1 concentration of ZnSO4) as compared to treatments which were inoculated and grown under different zinc concentrations. The results suggest that Bacillus cereus has good potential to remediate Zn from soil as well as to reduce the phyto-availibility and phytotoxicity of zinc.

Lead, Cadmium and Zinc Phytotoxicity Alter DNA Methylation Levels to Confer Heavy Metal Tolerance in Wheat.

Being a staple food, wheat (Triticum aestivum) nutritionally fulfills all requirements of human health and also serves as a significant link in the food chain for the ingestion of pollutants by humans and animals. Therefore, the presence of the heavy metals such as lead (Pb) and cadmium (Cd) in soil is not only responsible for the reduction of wheat crop yield but also the potential threat for human and animal health. However, the link between DNA methylation and heavy metal stress tolerance in wheat has not been investigated yet. In this study, eight high yielding wheat varieties were screened based on their phenotype in response to Pb stress. Out of these, Pirsabak 2004 and Fakhar-e-sarhad were identified as Pb resistant and sensitive varieties, respectively. In addition, Pirsabak 2004 and Fakhar-e-sarhad varieties were also found resistant and sensitive to Cd and Zinc (Zn) stress, respectively. Antioxidant activity was decreased in Fakhar-e-sarhad compared with control in response to Pb/Cd/Zn stresses, but Fakhar-e-sarhad and Pirsabak 2004 accumulated similar levels of Pb, Cd and Zn in their roots. The expression of Heavy Metal ATPase 2 (TaHMA2) and ATP-Binding Cassette (TaABCC2/3/4) metal detoxification transporters are significantly upregulated in Pirsabak 2004 compared with Fakhar-e-sarhad and non-treated controls in response to Pb, Cd and Zn metal stresses. Consistent with upregulation of metal detoxification transporters, CG DNA hypomethylation was also found at the promoter region of these transporters in Pirsabak 2004 compared with Fakhar-e-sarhad and non-treated control, which indicates that DNA methylation regulates the expression of metal detoxification transporters to confer resistance against metal toxicity in wheat. This study recommends the farmers to cultivate Pirsabak 2004 variety in metal contaminated soils and also highlights that DNA methylation is associated with metal stress tolerance in wheat.

Soil Bacteria Override Speciation Effects on Zinc Phytotoxicity in Zinc-Contaminated Soils

The effects of zinc (Zn) speciation on plant growth in Zn-contaminated soil in the presence of bacteria are unknown but are critical to our understanding of metal biodynamics in the rhizosphere where bacteria are abundant. A 6-week pot experiment investigated the effects of two plant growth promoting bacteria (PGPB), Rhizobium leguminosarum and Pseudomonas brassicacearum, on Zn accumulation and speciation in Brassica juncea grown in soil amended with 600 mg kg-1 elemental Zn as three Zn species: soluble ZnSO4 and nanoparticles of ZnO and ZnS. Measures of plant growth were higher across all Zn treatments inoculated with PGPB compared to uninoculated controls, but Zn species effects were not significant. Transmission electron microscopy identified dense particles in the epidermis and intracellular spaces in roots, suggesting Zn uptake in both dissolved and particulate forms. X-ray absorption near-edge structure (XANES) analysis of roots revealed differences in Zn speciation between treatments. Uninoculated plants exposed to ZnSO4 contained Zn predominantly in the form of Zn phytate (35%) and Zn polygalacturonate (30%), whereas Zn cysteine (57%) and Zn polygalacturonate (37%) dominated in roots exposed to ZnO nanoparticles. Inoculation with PGPB increased (>50%) the proportion of Zn cysteine under all Zn treatments, suggesting Zn coordination with cysteine as the predominant mechanism of Zn toxicity reduction by PGPB. Using this approach, we show, for the first time, that although speciation is important, the presence of rhizospheric bacteria completely overrides speciation effects such that most of the Zn in plant tissue exists as complexes other than the original form.

Mobility of Zinc in Heavy Metal Contaminated Soil after Compost Amendment and <i>Brassica juncea</i> Harvest

Pot experiments were performed to investigate the effect of compost amendment on the mobility of zinc through analysis of Zn fractions in heavy metal contaminated soil. The results showed that the total Zn concentration decreased 8.11%, 10.15%, 16.15%, 20.05%, 7.28% and 5.02% after the amendment of 0, 20, 40, 60, 80, 100 g/kg compost to soil and Brassica juncea harvest, respectively. Zn was mostly concentrated in the residual fraction and Fe-Mn oxides fraction in soil. The percentage of Zn in water-soluble fraction, organic fraction and residual fraction had no correlation with the amount of compost amendment. The percentage of Zn in the exchangeable fraction decreased and the percentage of Zn in Fe-Mn oxides fractions increased obviously. Furthermore, the mobility factor of Zn decreased significantly from 19.20% without compost amendment to 19.09%, 18.70%, 18.15%, 16.45% and 16.12% after the amendment of 0, 20, 40, 60, 80, 100 g/kg compost to soil, the compost amendment could lowered the mobility and phytotoxicity of zinc through bound to Fe-Mn oxides.

Phytotoxicity of zinc and manganese to seedlings grown in soil contaminated by zinc smelting

Historic emissions from two zinc smelters have injured the forest on Blue Mountain near Palmerton, Pennsylvania, USA. Seedlings of soybeans and five tree species were grown in a greenhouse in a series of mixtures of smelter-contaminated and reference soils and then phytotoxic thresholds were calculated. As little as 10% Palmerton soil mixed with reference soil killed or greatly stunted seedlings of most species. Zinc was the principal cause of the phytotoxicity to the tree seedlings, although Mn and Cd may also have been phytotoxic in the most contaminated soil mixtures. Calcium deficiency seemed to play a role in the observed phytotoxicity. Exposed soybeans showed symptoms of Mn toxicity. A test of the effect of liming on remediation of the Zn and Mn phytotoxicity caused a striking decrease in Sr-nitrate extractable metals in soils and demonstrated that liming was critical to remediation and restoration.

Phytotoxicity of zinc, chromium (VI) and cadmium in purging nut (Jatropha curcas) seedlings grown in hydroponics

This study investigated physiological, morphological, biochemical and metal uptake patterns in purging nut (Jatropha curcas L.) seedlings exposed to zinc (Zn), chromium (Cr) and cadmium (Cd) in a hydroponic system. Uptake of metals in all parts of the seedlings was found to be highest for Zn, followed by Cd and Cr. Catalase (CAT) and superoxide dismutase (SOD) activities were increased on exposure to heavy metals, but 5 mM Cd and Cr significantly inhibited SOD activity compared to Zn. This may reflect damage to the plant due to lipid per-oxidation as shown by a decrease in the % membrane stability index at higher metal concentrations. With increase in concentration of all the heavy metals tested, there was a significant decrease in chlorophyll content, however there was a stimulatory effect at low Zn concentrations (up to 0.5 mM). Root length and surface area were both significantly decreased with increasing metal concentration.

Effects of two sludge application on fractionation and phytotoxicity of zinc and copper in soil

The potential harm of heavy metals is a primary concern in application of sludge to the agricultural land. A pot experiment was conducted to evaluate the effect of two sludges on fractionation of Zn and Cu in soil and their phytotoxicity to pakchoi. The loamy soil was mixed with 0%, 20%, 40%, 60% and 80% (by weight) of digested sewage sludge (SS) and composted sludge (SC). The additions of the both sludges caused a significant raise in all fractions, resulting in that exchangeable (EXCH) and organic matter (OM) became predominance for Zn and organic bound Cu occupied the largest portion. There was more available amount for Zn and Cu in SS treatments than SC treatments. During the pot experiment, the concentration of Zn in EXCH, carbonate (CAR) and OM and Cu in EXCH and OM fractions decreased in all treatments, so their bioavailability reduced. Germination rate and plant biomass decreased when the addition rate was high and the best yield appeared in 20% mixtures at the harvest of pakchoi. The two sludges increased tissue contents of Zn and Cu especially in the SS treatments. Zn in pakchoi was not only in relationship to ΔEXCH and ΔCAR forms but also in ΔOM forms in the sludge-soil mixtures. Tissue content of Cu in pakchoi grown on SC-soils could not be predicted by ΔEXCH. These correlation rates between Zn and Cu accumulation in pakchoi and variation of different fractions increased with time, which might indicate that sludges represented stronger impacts on the plant in long-term land application.

Changes in nitrate reductase activity and oxidative stress response in the moss Polytrichum commune subjected to chromium, copper and zinc phytotoxicity

The main aim of this paper was to investigate the effect of chromium (Cr), copper (Cu) and zinc (Zn) on nitrate reductase (NR) activity and oxidative stress responses in the moss Polytrichum commune. Cr, Cu and Zn resulted in the inhibition of NR activity. A decline in total chlorophyll content was observed after 24 and 48 h of metal treatment. Accumulation of the metals showed a dose and time dependent increase. High accumulation of Cu, Cr and Zn were seen in moss shoots after 24 and 48 h of treatment. Treatment of Cr, Cu and Zn for 24 or 48 h resulted in the increase of malondialdehyde (MDA) content in moss shoots. The highest increase was observed in shoots under Cu treatment followed by Cr and Zn. The MDA content was significantly higher after 48h. Antioxidant enzymes viz., catalase (CAT), guaiacol peroxidase (GPx), glutathione reductase (GR) and superoxide dismutase (SOD) were affected by elevated concentrations of the three metals. Increase in the activity of CAT, GR and SOD was seen after 24 and 48 h of treatment. GPx activity declined under Cr treatment. However, under Cu and Zn, an increase in GPx was seen after 24 h and 48 h of treatment. For Zn, the antioxidant efficiency was less affected as compared to Cr and Cu. The response of Polytrichum commune to toxic concentrations of Cr, Cu and Zn appears to induce oxidative damage as observed by the increase in MDA content and antioxidant metabolism.

Zinc phytotoxicity to oilseed rape grown on zinc-loaded substrates consisting of Fe oxide-coated and calcite sand

The risk of zinc (Zn) phytotoxicity in soils has increased in various regions following application of different anthropogenic materials. In order to assess the relative efficiency of Fe oxide and calcite in sorbing Zn and hence alleviating Zn phytotoxicity, we grew oilseed rape for 28 days in pots containing Zn-loaded model substrates consisting of Fe oxide (ferrihydrite)-coated sand (FOCS, 0.2–0.5 mm, 0.3 m2 ferrihydrite g−1 sand) and calcium carbonate (calcite) sand (CCS, 0.2–0.5 mm, 0.3 m2 calcite g−1 sand). Five substrates containing 5, 10, 20, 40, and 80% FOCS and supplied with ZnSO4 at a rate of 30, 100, 300, and 1000 mg Zn kg−1 were used in the cropping experiment and in an in vitro study of Zn desorption for 62 days. Plants exhibited good growth and a similar dry matter yield (DMY) at the 30 and 100 mg Zn kg−1 rates. On the other hand, DMY was markedly reduced at the 300 and, especially, at the 1000 mg Zn kg−1 rate, particularly for the substrates with the higher FOCS proportions. Symptoms of phytotoxicity (viz. chlorosis, purple colouration due to P deficiency) were apparent at such rates and were accompanied by high Zn concentrations in both shoot (average values >1000 and >1500 mg Zn kg−1 dry matter for the 300 and 1000 mg Zn kg−1 rate, respectively) and root (average values >2500 and >6000 mg Zn kg−1 dry matter for the 300 and 1000 mg Zn kg−1 rate, respectively). Total Zn uptake was maximal at 300 mg Zn kg−1. The results of water extractable Zn in the substrate after cropping and the dissolved Zn concentrations measured in substrate–water systems (desorption experiment) suggest that, on a surface area basis, calcite is more effective than Fe oxide to retain Zn and thus alleviate phytotoxicity at high Zn loadings. However, the Zn-sorption capacity of the Fe oxide cannot be neglected, particularly at low Zn loadings, where Fe oxide seems to exhibit a higher affinity for Zn – but not a higher Zn-sorption capacity – than does calcite.

Poplar Trees for Phytostabilization of Abandoned Zinc-Lead Smelter

Poplar trees represent one vegetation option for phytostabilization of heavy metal-contaminated sites. The objective of this study was to investigate the suitability of deep-planted hybrid poplars for phytostabilization of a heavy metal-contaminated site. Tree survival was greatly improved when trees were planted using cattle manure as a soil amendment as compared to no amendment. Survival after 3 years with manure ranged from 18 to 56% for four poplar hybrids, while without manure, the range was 0–18%. Increases in trunk diameter and tree height were also greater with manure than without after the first growing season. Zinc, Cd, Pb concentrations were as high as 3,606, 44, and 33 mg kg\U-1\N, respectively, in leaves, and there was little influence of manure amendment or hybrid. Zinc phytotoxicity may have been a factor in tree survival. Average metal concentrations in plant tissues followed the order: leaves>bark>twigs>wood for Zn and Cd, and bar>wood>twigs≅leaves for Pb. Manured treatments supported higher rates of both photosynthesis and transpiration. Comparisons among varieties indicated that DO1 and Imperial Carolina hybrids had more than twice the rates of photosynthesis of the other two hybrids and that each variety was unique in terms of water-sue efficiency (WUE), with Imperial Carolina being higher than any other hybrid. This combination of high rates of photosynthesis and high WUE suggests a unique suitability of this hybrid for remediation of this site. More extensive characterization of the hybrids under a variety of conditions, including drought, is likely to reveal more about inherent suitability of each hybrid for site-specific remediation.

Adsorption of Zinc Ions by Scenedesmus Obliquus and S. Quadricauda and its Effect on Growth and Metabolism

Zinc adsorption by two green algae, Scenedesmus obliquus and Scenedesmus quadricauda, was investigated. The maximum adsorption capability of zinc ion obtained from the Langmuir adsorption isotherms was higher for S. obliquus (6.67) than for S. quadricauda (5.03), and S. obliquus was more tolerant to zinc phytotoxicity than S. quadricauda. Lower concentrations of zinc increased dry mass, chlorophylls a and b, carotenoids, and total amino acid contents in both algae. On the other hand, higher concentrations of zinc were inhibitory for growth and the other metabolic activities in a concentration dependent manner.

Bioremoval of zinc ions by Scenedesmus obliquus and Scenedesmus quadricauda and its effect on growth and metabolism

Zinc biosorption by two green algae, Scenedesmus obliquus and Scenedesmus quadricauda, was investigated. Data show that, at concentrates between 0.5 and 8 ppm , zinc adsorption by algae increased with increasing metal ion concentration. The maximum specific adsorptive capacity of zinc ions obtained from the Langmuir adsorption isotherms was higher for S. obliquus (6.67) and lower for S. quadricauda (5.03). These S. obliquus appears to be more efficient than S. quadricauda for removing zinc ions. However, the affinity constant of zinc on the biomass of both algae were 0.19 and 0.27 indicating that S. obliquus is more tolerant to zinc phytotoxicity than S. quadricauda. Algal growth, pigment fractions, total amino acids, and the activities of some phosphatases enzyme (acid phosphatase, alkaline phosphatase, and ATPase) in the algal cells were measured in the exponential growth phase after exposure to various concentrations of zinc. Lower concentrations of zinc increased dry weight, chlorophyll a, b, carotenoids, and total amino acid contents in both algae. On the other hand, higher concentrations of zinc were inhibitory for growth and the other metabolic activities.

Peroxidases in roots and primary leaves of Phaseolus vulgaris Copper and Zinc Phytotoxicity: a comparison

Summary The contribution of peroxidases was studied in roots and primary leaves of bean seedlings as a part of the defence mechanism against copper and zinc imposed oxidative stress. Based on the metal content of the different plant organs, copper was mainly «stored» in the roots, whereas zinc was readily transported to the upper ground parts. In the roots copper had an increasing effect on the capacities of guaiacol and syringaldazine peroxidases. A possible role for lignification in establishing an apoplastic transport barrier in roots is hypothesised. Zinc, on the other hand, had a limited impact on the root metabolism. Future studies should focus on other protection mechanisms in this plant organ. In the primary leaves, both elements had a pronounced (increasing) effect on the peroxidase activities. The data obtained in this study show that peroxidases contribute to the antioxidative defence of the primary leaves of 10 days old bean seedlings after application of both metals and that an interorgan signal transduction pathway is involved. It is pointed out that additional experimental approaches are necessary before the protection mechanism against different metals can be fully compared.

ZINC PHYTOTOXICITY

We studied zinc phytotoxicity with lettuce seedlings in solution culture. Zinc absorption in lettuce (Lactuca sativa L. cv. Climax) consisted of two distinct uptake phases; a slow uptake phase below 5 μM and a rapid uptake phase above 5 μM of solution Zn. In both phases the absorption rate, as measured by Zn concentration in the tissue, increased with solution concentration, but the rate of increase was much faster in the second phase. The rate of change of Zn absorption during the first phase was not materially affected by the Ca concentration of the nutrient solution. The second phase of Zn absorption (the potentially phytotoxic range) was substantially decreased as solution Ca concentration was increased. Zinc tissue concentrations showed relatively little variation between normal tissue and those clearly showing initial symptoms of Zn phytotoxicity. Zinc phytotoxicity as expressed by reduced growth appeared to be a direct result of the accumulation of Zn in tissues to toxic levels with the average tissue concentration being 500 mg L−1 Zn at threshold toxicity. This accumulation of Zn was associated with the second uptake phase. The time required to accumulate toxic levels of tissue Zn is determined by the uptake rate, which was a function of a number of parameters, including both Zn and Ca concentrations. If the threshold of latent toxicity is defined as the lowest solution concentration that has the potential to cause toxicity with extended exposure, then for lettuce this threshold is 5 μM Zn in the external solution, the concentration associated with the initiation of the second uptake phase.

An investigation into the phytotoxicity of zinc, copper and nickel using sewage sludge of controlled metal content

A replicated factorial pot trial with perennial ryegrass and a sandy loam soil (pH 7·0) investigated the relative phytotoxicities of Zn, Cu and Ni, and the nature of their interaction. Sewage sludge of controlled metal content made from enriched sewage was used for the soil treatment so that each pot received the same amount of sludge dry solids whilst quantities of metals added were adjustable as required. Upper critical concentrations (total) of the three elements in soil were found to be 319 mg Znkg −1, 105 mg Cukg −1 and 221 mg Nikg −1. The relative toxicities of Zn/Cu/Ni were 1·0/2·6/1·0 on a loading rate basis. The comparatively low toxicity of Ni was attributed to the pH value (7·0) of the soil used. At subcritical levels in the soil phytotoxic effects of mixtures of the three elements were independent but some additivity was seen when one or more element was already at a hypercritical level. At subcritical levels the element present in highest concentration relative to its critical concentration determined the phytotoxic effect on ryegrass yield of mixtures of Zn, Cu and Ni in soil. But where two or more elements exceeded their critical soil concentrations their combined phytotoxic effect could be taken as being additive. It was concluded that the three elements could be dealth with separately in guidelines for sludge utilisation on agricultural land.

Metal Complexation in Xylem Fluid

Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to calculate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes.Program CHELATE was used to calculate the distribution of six metals in 0- to 1-hour exudate from soybean (Glycine max L. Merr.) and tomato (Lycopersicon esculentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. The results indicated that Fe is bound by citric acid, and Cu is bound by several amino acids in the normal-Zn exudate. Most of the Cu in soybean exudate is bound to asparagine and histidine. In tomato, Cu is bound to histidine, glutamine, and asparagine. Zinc, Mn, Ca, and Mg are bound primarily by citric acid and malic acid in both species; the per cent bound for these metals is lower than that for Fe and Cu. Zinc phytotoxicity caused equilibrium concentration shifts and resulted in the formation of several additional metal complexes not found in the normal-Zn exudate.

Metal Complexation in Xylem Fluid

Xylem fluid was analyzed for numerous solutes to characterize chemically the sap as a medium for forming and transporting metal complexes. The stem exudate was collected hourly for 8 hours from topped 31-day-old soybean (Glycine max L. Merr.) and 46-day-old tomato (Lycopersicon esculentum Mill.) plants grown in normal (0.5 micromolar) and Za-phytotoxic nutrient solutions. Soybean plants were grown in the normal and high-Zn solutions for 24 days; tomato plants were grown for 32 days. The exudate was analyzed for seven organic acids, 22 amino acids, eight inorganic solutes, apparent ionic strength, and pH. Significant changes in many solutes occurred over the 8-hour sampling period. These fluctuations depended on plant species, individual solute, and Zn treatment, and demonstrated that extrapolation of xylem-fluid analyses to whole-plant xylem sap is valid only for sap samples collected shortly after topping a plant. Exudate pH decreased over the 8-hour period for both species; exudate ionic strength increased for tomato and decreased for soybean. At the normal-Zn treatment (0 to 1 hour), the highest acid micromolar concentrations in soybean exudate were: asparagine, 2,583; citric, 1,706; malic, 890; and malonic, 264. Under the same conditions, the highest acid micromolar concentrations in tomato exudate were: maleic, 1,206; malic, 628; glutamine, 522; citric, 301; and asparagine, 242. Cysteine and methionine were above detection limits only in soybean exudate. Zinc phytotoxicity caused significant changes in many solutes. The analyses reported here provide a comprehensive data base for further studies on metal-complex equilibria in xylem fluid.

Phytotoxicity of zinc, nickel, cadmium, lead, copper and chromium in three pasture plant species supplied with graduated amounts from the soil.

In pot trials, (a) Plantago lanceolata: (b) white clover and (c) perennial ryegrass were grown on an acid sandy soil with added levels of Zn, Ni, Cd, Pb, Cu and Cr as single test elements. The metals were classified for inhibitive metal conc. in the shoots of the plants and in the soil. Ni, Cd and Cu were found to cover a 5-fold range with one exception: Cd in (a) was inhibitive at a 10-fold higher tissue conc. than in (b) and (c). Zn was more efficiently absorbed, but tolerated at a higher soil conc. because it was much less toxic to the tissue. Cr was tolerated at low shoot conc. but at high conc. in the soil because its uptake by the shoots was disproportionally smaller. Differences between plant spp. in response to the soil test levels were illustrated in terms of relative uptake and shoot conc. for toxicity. (Abstract retrieved from CAB Abstracts by CABI’s permission)