CROP PHISIOLOGY

Growth of spinach plants (Spinacia oleracea L.) exposed to excess zinc and manganese

Crecimiento de plantas de espinaca (Spinacia oleracea L.) expuestas a exceso de zinc y manganeso

Fánor Casierra-Posada¹, Christian Ulrichs² and Cristian Pérez³

Received for publication: 24 January, 2012. Accepted for publication: 30 October, 2012.

ABSTRACT

Increased anthropogenic inputs of heavy metals in agricultural soils lead to worrisome effects on the water, soil, and plants. A greenhouse study was conducted in Tunja, Colombia to determine the effects of excess Zn and Mn on leaf area, root length, dry matter production and partitioning, root to shoot ratio, specific leaf weight, water uptake, and agronomic water use efficiency in spinach seedlings (Spinacia oleracea L. hyb. Marimba). Seedlings were grown floating on a complete nutrient solution in 4 L glass containers. Concentrations of 40 mg L^-1 Mn, 40 mg L^-1 Zn, 80 mg L^-1 Mn, 80 mg L^-1 Zn and combinations thereof were added to the solution, with a control treatment receiving no excess Zn or Mn. Zn at 40 mg L^-1 reduced leaf area by 78.82% in relation to the controls; 40 mg L^-1 Mn increased leaf area by 35.23%. Plants exposed to 80 mg L^-1 Zn with 80 mg L^-1 Mn increased allocation of biomass to leaves by 45.05% as compared to the control plants. Addition of 80 mg L^-1 Zn and 40 mg L^-1 Mn + 80 mg L^-1 Zn led to an increase of 9.24 and 29.75%, respectively, in dry matter allocation to stem + petioles. Roots were affected the most by excess Mn and Zn, alone and in combination. While addition of 40 or 80 mg L^-1 of Mn reduced total root length by 45.06 and 81.64%, respectively; while Zn concentrations of 40 or 80 mg L^-1 reduced total root length by 88.78 and 98.07%, respectively.

RESUMEN

El aporte antrópico de metales pesados en suelos agrícolas causa efectos preocupantes en agua, suelos y plantas. Se realizó un estudio en invernadero en Tunja, Colombia para determinar el efecto del exceso de Mn y Zn sobre el área foliar, la longitud total de la raíz, la producción y distribución de materia seca, la relación de raíz: vástago, el peso foliar específico, el consumo de agua y la eficiencia agronómica en el uso del agua, en plantas de espinaca (Spinacia oleracea L. hyb. Marimba). Las plantas se cultivaron flotando sobre 4 L de una solución nutritiva completa. A algunos contenedores se les adicionó 40 u 80 mg L^-1 de Mn, 40 u 80 mg L^-1 de Zn mg L^-1, o las combinaciones entre estas concentraciones, con un control sin exceso de Zn o Mn. A 40 mg L^-1 Zn redujo 78.82% el área foliar en relación con los controles; con 40 mg L^-1 Mn aumentó el área foliar 35.23%. Las plantas expuestas a 80 mg L^-1 Zn + 80 mg L^-1 Mn aumentaron 45.05% la asignación de biomasa a las hojas. La adición de 80 mg L^-1 Zn y 40 mg L^-1 Mn + 80 mg L^-1 Zn condujo a un aumento 9.24 y 29.75%, respectivamente, de la asignación de materia seca a tallo + pecíolos. Las raíces fueron las más afectadas por el exceso de Mn y Zn, solos y combinados. La adición de 40 u 80 mg L^-1 de Mn redujo la longitud total de las raíces 45.06 y 81.64%, respectivamente; Zn a 40 u 80 mg L^-1 la disminuyó 88.78 y 98.07%.

Palabras clave: toma de agua, distribución de la masa seca, longitud radical, metales pesados.

Heavy metals are ubiquitous in the environment as a result of both natural and anthropogenic activities; humans, plants, and animals are exposed to them through various pathways (Wilson and Pyatt, 2007). Wastewater irrigation, solid waste disposal, sludge applications, vehicular exhaust and industrial activities are major sources of soil contamination by heavy metals, and increased metal uptake by food crops grown on such contaminated soils is often observed (Singh et al., 2004; Chen et al., 2005). Excessive contents of metal ions in plants can cause various stress responses manifested outwardly with visible symptoms such as growth inhibition, root damage, chlorosis, larger stomata, and thinning of wax deposition. Rapid physiological and slow morphological processes cause changes in plant metabolism, directly or indirectly related to plant "management" of metal stress (Schützendübel and Polle, 2002; Rai et al., 2005; Sharma and Dietz, 2006; Casierra-Posada et al., 2010). Mn toxicity is more common than Mn deficiency in some regions such as the humid tropics, where most soils are highly weathered and acidic (Hue et al., 2001).

Municipal solid waste management in Bogota, Colombia is carried out as a public service that includes collection, transport, and disposal. The waste is disposed of in the sanitary landfill Doña Juana located south of the city. It receives an average 6,000 t d^-1 of municipal solid waste; the Bogota Capital District is in charge of the management of the landfill. There is no formal recycling system for the city, a fact that has social, environmental, and economic consequences (Hincapié and Mantilla, 2007). Thus, the waste disposed of in the sanitary landfills becomes an environmental problem and contaminates soil and water with heavy metals, among other compounds.

In horticultural crops established in the Bogota Plateau, farmers irrigate with water from the Bogota river, through the "La Ramada" irrigation district , which covers 6400 ha of land. The use of this water has generated a contamination problem in these crops, causing quality deterioration of the produce, which is mainly consumed fresh. In a study carried out in plantations of lettuce, celery, cabbage, and broccoli in Soacha (Colombia) , heavy metals such as lead, cadmium, arsenic and mercury in water, soil, and edible plant parts were analyzed. Lettuce and celery plants were contaminated with Cd exceeding the limits of the European Union norms. In all evaluated vegetables, Pb contents exceeded the maximum allowed concentration in foods for breastfeeding infants and young children. Lettuce plants accumulated higher levels of heavy metals than the other three vegetable species (Miranda et al., 2008).

Although in the vegetable-growing area of Soacha, Colombia it is not common practice to apply municipal waste compost to crops, much of the scientific work done on heavy metal toxicity in plants involves application of such compost. Toxic effects of zinc on plant growth have been reported for spinach (Pavlíková et al., 2008). However, metal-enriched spinach plants have a detoxification mechanism and grow well on organic matter-enriched soil contaminated with toxic metals (Sinha et al., 2007). Acid soils have a larger pool of available Mn than neutral or alkaline soils; the addition of municipal solid waste compost to an acidic sandy loam increased Mn in blueberry leaves (Warman et al., 2004). Increased spinach uptake of Mn due to application of municipal solid waste compost has also been observed in calcareous soil (Maftoun et al., 2004). Gallardo-Lara et al. (2006) reported an increase in lettuce Mn and reduced Mn in barley in calcareous soil amended with the same compost. Swiss chard and basil Mn tissue concentrations decreased with municipal solid waste compost additions. That is to say, that the highest Mn uptake was recorded at the lowest application rate and the lowest uptake at the highest application rate (Zheljazkov and Warman, 2004b).

Hargreaves et al. (2008) have done a review of relevant agricultural studies as well as recommendations for improving municipal solid waste compost quality. Its safe use in agriculture can be ensured with source separation (or sorting of municipal solid waste to be composted) as well as the development and implementation of comprehensive industry standards. Despite this, the composts used in some studies have very high concentrations of Cu and Zn. Zinc uptake by potatoes, swiss chard, and basil grown in soil treated with municipal solid waste compost has been reported on (Sebastiao et al., 2000; Zheljazkov and Warman, 2004a). However, despite increases in available soil Zn, the Zn concentration of tomato and squash fruit, as well as grapevines, did not increase with municipal solid waste compost additions (Ozores-Hampton and Hanlon, 1997; Pinamonti et al., 1999).

The largest portion of Mn in soil treated with municipal solid waste compost was found to be bound in the iron-manganese fraction, which is unavailable to plants (Zheljazkov and Warman, 2004a). Interactions between Fe and Mn availability were also reported (Maftoun et al., 2004). There is usually a decreased plant availability of Mn as a result of municipal solid waste compost addition because of the increase in soil pH associated with municipal solid waste compost application (Warman et al., 2004).

The present study was conducted to compare the effect of heavy metals (zinc and manganese and their interactions) on growth parameters of spinach plants grown in bottles filled with a nutrient solution to which high contents of zinc and manganese were added.

Materials and methods

Plant growth conditions

The study was conducted in a greenhouse at the Universidad Pedagógica y Tecnológica de Colombia (UPTC) in Tunja, Colombia (5°33'16.25" N; 73°21'9.14" O) at 2,790 m a.s.l. Seeds of Spinacia oleracea L. hybrid Marimba (Seminis, Spain) were sown in a mixture 1:1 of soil and peat. Average growth conditions inside the greenhouse were: photosynthesis photon flux 782.2 µmol m^-2 s^-1, 15.8°C temperature and 72% relative humidity. Four weeks after germination, 90 single seedlings (ten plants for each treatment) were transplanted to styrofoam cubes floating on a nutrient solution in 4 L glass containers. The solution's content in mg L^-1 was: NO3 40.3; NH3 4.0; P 20.4; K 50.6; Ca 28.8; Mg 11.4; S 1.0; Fe 1.12; Mn 0.112; Cu 0.012; Zn 0.0264; B 0.106; Mo 0.0012 and Cu 0.00036. The average pH and EC values of the solution were 5.8 and 2.0 mmhos cm^-1 respectly. The nutrient solution was prepared adding 1 cm³ of Nutriponic® (Walco S.A.) per liter of water and was aerated with an aquarium air pump during the entire growth cycle of the plants. After transplant, when the seedlings reestablished growth, Zn and Mn from ZnSO47H2O or MnSO4H2O (Merck®), respectively, were added to the nutrient solution.

Growth measurements

Plants were harvested 50 d after transplant. The agronomic efficiency of water use (g L^-1) was calculated as the amount of water needed to produce one gram of dry matter. Water consumption (L) was taken as the amount of water added periodically to the bottles. Leaf area was measured using an LI-3000. A analyzer (LI-COR, Lincoln, NE, USA). For determination of total root length (m), roots were dipped in formaldehyde and blue methylene to preserve them and make viewing easier; roots were sectioned and formed into a line and measured as if they were a single long root. Then the plants were dried to determine total dry weight and dry weight of each plant organ (roots, petioles + stem and leaves), by drying them in an oven at 70°C for 48 h. The specific leaf weight (mg cm^-2) was obtained as a ratio between the total dry weight of the leaves and the leaf area of the same plant. Finally, the root to shoot ratio was calculated as the relation between the root dry weight and dry weight of the aerial organs (leaves, petioles and stems) of each plant.

Statistical analysis

All factors were evaluated in a completely randomized design with nine treatments: 0 Mn + 0 Zn (controls); 40 mg L^-1 Mn + 0 mg L^-1 Zn; 0 Mn + 40 mg L^-1 Zn; 80 mg L^-1 Mn + 0 Zn; 0 Mn + 80 mg L^-1 Zn; 80 mg L^-1 Mn + 40 mg L^-1 Zn; 40 mg L^-1 Mn + 80 mg L^-1 Zn; 40 mg L^-1 Mn + 40 mg L^-1 Zn and 80 mg L^-1 Mn + 80 mg L^-1 Zn. For measurement of the growth parameters, all 10 seedlings of each treatment were used. Each seedling was treated as a repetition. All the data sets obtained from the experiment were subjected to two way analysis of variance (ANOVA) using SPSS 17.0.0, taking P≤0.01 as significant, followed by a post hoc Tukey test.

Results and discussion

The results presented in Fig.1 can be explained by the antagonistic interaction between Zn and Mn. Some reviews cite ion antagonism in plants between Ca and Zn; Fe with Ca, Zn, Pb, Cd, Mn, or Cu; Mn with Zn or Ca; K with Na, Ca, Mg, Cu, Mn, Ni, Zn, or Fe; Ni with Zn, Fe, or Mn; and Cu with Ca (Rashed, 1995).

On the other hand, Lidon (2002) reports that in shoots of 15-d-old rice plants, increasing Mn concentrations triggered an antagonistic effect with the contents of Fe, Cu, and Zn, but a synergistic tendency was found thereafter. During all the experimental periods, net Mn and Cu uptake increased until the last treatment, but the absorption of Zn and Fe increased only 21 and 28 d after germination. Total shoot accumulation of these micronutrients displayed similar patterns from the 21st d onwards. After this experimental period, the translocation rates also showed a synergistic increase for all metals, but in 15-d-old rice plants an antagonism was found between Mn and Zn. It was concluded that before and after the end of the mobilization of seed reserves, rice adaptation to high Mn concentrations is associated with significant changes of micronutrient accumulation in the tissues (Lidon, 2002).

In the present work we can also observe Zn/Mn antagonism. In the results for leaf area in plants exposed simultaneously to Zn and Mn, Zn induced greater toxicity than Mn, reflected by a smaller leaf area in plants exposed to high Zn contents. Nevertheless, when both metals were added to the nutrient solution, Zn toxicity symptoms were attenuated by Mn.

Exposure of plants to high Mn content can not only reduce leaf area, but even cause defoliation, although in the present study such a reduction was not observed. In watermelon, Mn toxicity is so dramatic that it is referred to as "sudden crash" syndrome. The crop looks quite healthy and grows rapidly from transplant or sowing to flowering, or sometimes to the early fruit-filling stage, and then the older leaves suddenly wilt, dry up, and drop in a matter of days. Loss of leaves reduces the ability of plants to absorb sunlight for photosynthesis, resulting in reduced fruit set, fruit size, and quality (Cooperative Extension Service, 1998).

In the present experiment, exposure of spinach plants to Zn and/or Mn changed patterns of dry matter allocation in different organs of the plant. The dry matter accumulated in leaves, in roots, and in stems + petioles showed statistically significant differences between the treatments (P≤0.01). Except for the treatment with 80 mg L^-1 Mn + 40 mg L^-1 Zn, the remaining treatments presented values of biomass allocation to the leaves greater than in the control plants. Even the addition of 80 mg L^-1 Zn and 80 mg L^-1 Mn induced 45.05% more allocation of biomass to the leaves than in the controls (Fig.2). The addition of 80 mg L^-1 Zn and 40 mg L^-1 Mn + Zn 80 mg L^-1 led to an increase of 9.24 and 29.75%, respectively, in dry matter allocation to the stem + petioles. All other treatments induced a reduction in dry matter allocated to the petioles + stem. Accumulated dry matter in the roots of treated plants was lower in all cases than that accumulated in roots of the control plants, with statistically significant results (P≤0.01).

In similar experiments, Casierra-Posada et al. (2010) found that broccoli plants exposed to high Zn concentrations reduced dry matter allocation to the roots in an inversely proportional relation to Zn soil concentration, as this metal principally affects roots. In the present study, in plants exposed to high concentrations of Zn and Mn together, it was found that the Mn softened the toxic effect of Zn on the roots (as it did with leaf area) due to an antagonistic relation between the two elements.

In the present study, the roots were the organ most affected by both excess Mn and Zn, as well as by the interaction of both metals, added to the nutrient solution. All treatments showed a significant effect (P≤0.01) on root length. While the addition of 40 and 80 mg L^-1 of Mn reduced the total root length by 45.06 and 81.64%, respectively, as compared with the controls, Zn concentrations of 40 and 80 mg L^-1 reduced root length by 88.78 and 98.07%, respectively. All mixtures of Zn and Mn together in the nutrient solution also decreased this variable, over 90% (Fig.3).

The first visible damage in plants due to excess zinc is commonly observed on root growth (Harmens et al., 1993), which corresponds with the present study's observations in spinach. Powell et al. (1986) reported that elongation of the longest root was much more inhibited by increasing Zn concentration in a non-tolerant (S59) cultivar than in tolerant (Merlin) red fescue plants. The mitotic index in the root meristem was reduced to a greater extent by increasing Zn concentration in S59 than in Merlin, while the cell doubling time was increased by Zn much more in S59 than in Merlin. Treatments with 0.1 and 0.2 mg Zn L^-1 increased the length of the cell cycle by 40 and 132% respectively (compared with the zero Zn control treatment) in S59, but only by 6 and 16% respectively in Merlin. The increase in cell cycle length was due mainly to an increase in the duration of G1 in both cultivars; Zn had little effect on the duration of the other phases of the cell cycle. The growth fraction was progressively reduced by increasing Zn concentration in S59 but was increased with 0.1 mg Zn L^-1 in Merlin and then reduced to the control level at the higher Zn concentration. Based on these results, we can infer that the reduction in root growth found in the present study is due to a reduction in cell division caused by excess Zn.

The results of the present study for water uptake, agronomic efficiency of water use, fresh weight, dry weight, and root to shoot ratio, showed statistically significant differences (P≤0.01). The addition of 40 mg L^-1 of Mn resulted in an increase in values of these variables by 21.24; 0.43; 7.79; 22.0 and 13.71%, respectively, compared with the controls. The remaining treatments in which Zn and/ or Mn was added, produced a decrease (Tab.1). On the other hand, the specific leaf weight showed significant differences (P≤0.01) in some but not all treatments, as compared to the control.

It has been found in Citrus volkameria, that excess Mn is tied up by plants in the form of increased chloroplasts (Papadakis et al., 2007). If such a mechanism also exists in spinach, it would explain many results in the present study. Increased chloroplasts would increase photosynthetic capacity, thus augmenting factors such as leaf area and dry weight. Roots, on the other hand, would not possess such a photosynthetic mechanism for detoxifying excess manganese, and hence would display the reduced growth seen in the present study.

Although manganese is not a common pollutant in soils, various soil conditions often present in acid and volcanic soils, or flooding conditions, can lead to Mn reduction or increase and create Mn toxicity in many natural and agricultural systems (Foy et al., 1978). The deleterious effect of Mn toxicity is often observed in the shoot as stunted growth, chlorosis, crinkled leaves, and brown lesions (Marschner, 1995), although such effects were not observed in the present study. In the current study, as a result of the reduction in leaf area and root length caused by the toxicity of both metals, the plants took up less water and thus the agronomic efficiency of water use was negatively affected. In a study conducted to evaluate the nutrient composition of biomass and conductive tissue in the habanero pepper, the order of concentrations in mg plant^-1 was: Fe> Zn> Mn> Cu at four tested growth stages, with Mn slightly exceeding Zn after 75 d of plant development. The dynamics for Zn and Mn showed an increase in the early stages of development, and declined with plant age, especially for Zn (Noh-Medina et al., 2010). Based on this result we can infer that plants show a differential response to uptake of heavy metals according to the growth stage and also have a different sensitivity to each metal that may present a toxicity risk. All these factors negatively affect the amount of dry matter produced by each plant, due to the alterations caused by these metals on the agronomic efficiency of water use. On the other hand, the root to shoot ratio is affected as dry matter partitioning to the various plant organs changes.

Conclusions

In general, the addition of excess zinc was highly toxic to the plants, while excess manganese showed less drastic effects as compared to the control treatment. In some cases (leaf area and dry weight, for example), excess Mn even increased growth parameters, while with Zn such increases were limited to dry matter allocation to the stems and petioles and specific leaf weight. In some cases, the presence of excess Mn seemed to attenuate the effects of excess Zn on the plant, though this tendency was not consistent and was often anecdotal. The results of the present study are relevant for areas such as the southern Bogota plain, where horticultural production occurs near municipal waste disposal sites. The highly toxic effects of excess Zn and Mn on spinach in the present study point to the importance of monitoring heavy metal content in soil and groundwater at such sites, to assure good plant growth. In addition, the absence or subtlety of Mn effects on plants is a warning against relying solely on visual indicators to detect heavy metal presence; according to the present study, it would be possible in certain cases to obtain satisfactory yields and products of normal appearance that may in fact contain levels of heavy metals that are dangerous for human consumption.

Acknowledgments

The team gratefully acknowledges the generous support of the Research Department (Dirección de Investigaciones – DIN) of the Pedagogical and Technological University of Colombia (UPTC) for providing us with the funding and opportunity to conduct this research project. We also gratefully acknowledge matching support from the members of the Research Group in Plant Ecophysiology (Grupo de Ecofisiología Vegetal) of the Faculty of Agricultural Sciences of the UPTC.

Casierra-Posada, F., L.A. González, and C. Ulrichs. 2010. Crecimiento en plantas de brócoli (Brassica oleracea L. var. Itálica) afectadas por exceso por zinc. Rev. Colomb. Cienc. Hortic. 4(2), 166-174.         [ Links ]

Chen, Y., C. Wang, and Z. Wang. 2005. Residues and source identification of persistent organic pollutants in farmland soils irrigated by effluents from biological treatment plants. Environ. Intl. 31, 778-783.         [ Links ]

Cooperative Extension Service. 1998. Managing manganese toxicity in former sugarcane soils on Oahu. Soil and Crop Management. College of Tropical Agriculture and Human Resources (CTAHR), Hawaii.         [ Links ]

Foy, C.D., R.L. Chaney, and M.C. White. 1978. The physiology of metal toxicity in plants. Ann. Rev. Plant Physiol. 29, 511-566.         [ Links ]

Gallardo-Lara, F., M. Azcon, and A. Polo. 2006. Phytoavailability and fractions of iron and manganese in calcareous soil amended with composted urban waste. J. Environ. Sci. Health B 41, 1187-1201.         [ Links ]

Hargreaves, J.C., M.S. Adl, and P.R. Warman. 2008. A review of the use of composted municipal solid waste in agriculture. Agric. Ecosyst. Environ. 123, 1-14.         [ Links ]

Harmens, H., N.G.C.P.B. Gusmao; P.R. Den Hartog, J.A.C. Verkelij, and W.H.O. Ernst. 1993. Uptake and transport of zinc in zincsensitive and zinc-tolerant Silene vulgaris. J. Plant Physiol. 141, 309-315.         [ Links ]

Hernando, S., M. Lobo, and A. Polo M. 1989. Effect of the application of a municipal refuse compost on the physical and chemical properties of soil. Sci. Total Environ. 81/82, 589-596.         [ Links ]

Hincapié, I. and Z. Mantilla. 2007. Solid waste management in Bogotá, Colombia. International Waste Working Group (IWWG). Sardinia, Italy.         [ Links ]

Hue, N.V., S. Veja, and J.A. Silva. 2001. Manganese toxicity in a Hawaiian Oxisol affected by soil pH and organic amendments. Soil Sci. Soc. Amer. J. 65, 153-160.         [ Links ]

Lidon, F.C. 2002. Micronutrient uptake and translocation in Mntreated rice. J. Plant Nutr. 25(4), 757-768.         [ Links ]

Maftoun, M., F. Moshiri, N. Karimian, and A. Ronaghi. 2004. Effects of two organic wastes in combination with phosphorus on growth and chemical composition of spinach and soil properties. J. Plant Nutr. 27 (9), 1635-1651.         [ Links ]

Marschner, H. 1995. Mineral nutrition of higher plants. 2nd ed. Academic Press, Cambridge, UK.         [ Links ]

Miranda, D., C. Carranza, C.A. Rojas, C.M. Jerez, G. Fischer, and J. Zurita. 2008. Acumulación de metales pesados en suelo y plantas de cuatro cultivos hortícolas, regados con agua del río Bogotá. Rev. Colomb. Cienc. Hortíc. 2(2), 180-191.         [ Links ]

Murphy, C. and P.R. Warman. 2001. Effect of MSW compost applications on low-bush blueberry soil and leaf tissue trace elements. p. 166. In: Proceedings of the 6th International Conference on the Biogeochemistry of Trace Elements, Guelph, ON.         [ Links ]

Noh-Medina, J., L. Borges-Gómez, and M. Soria-Fregoso. 2010. Composición nutrimental de biomasa y tejidos conductores en chile habanero (Capsicum chinense Jacq.). Trop. Subtrop. Agroecosyst. 12, 219-228.         [ Links ]

Ozores-Hampton, M. and E. Hanlon. 1997. Cadmium, copper, lead, nickel and zinc concentrations in tomato and squash grown in MSW compost amended calcareous soil. Compost Sci. Util. 5(4), 40-46.         [ Links ]

Papadakis, I., A. Giannakoula, I. Therios, A. Bosabalidis, M. Moustakas, and A. Nastou. 2007. Mn-induced changes in leaf structure and chloroplast ultrastructure of Citrus volkameriana (L.) plants. J. Plant Physiol. 164, 100-103.         [ Links ]

Pavlíková, D., M. Pavlík, L. Staszková, V. Motyka, J. Száková, P. Tlustosa, and J. Balíka. 2008. Glutamate kinase as a potential biomarker of heavy metal stress in plants. Ecotoxicol. Environ. Saf. 70, 223-230.         [ Links ]

Pinamonti, F., G. Nicolini, A. Dalpiaz, G. Stringari, and G. Zorzi. 1999. Compost use in viticulture: effects on heavy metal levels in soil and plants. Commun. Soil Sci. Plan. 30(9-10), 1531-1549.         [ Links ]

Powell, M.J., M.S. Davies, and D. Francis. 1986. The influence of zinc on the cell cycle in the root meristem of a zinc-tolerant and a non-tolerant cultivar of Festuca rubra L. New Phytol. 102, 419-428.         [ Links ]

Rashed, M.N. 1995. Trace elements in some wild plants from the shores of the high dam lake and the adjacent desert, as determined by atomic absorption spectroscopy. J. Arid Environ. 29(2), 185-197.         [ Links ]

Rai, V., S. Khatoon, S.S. Bisht, and S. Mehrotra. 2005. Effect of cadmium on growth, ultramorphology of leaf and secondary metabolites of Phyllanthus amarus Schum. and Thonn. Chemosphere 61, 1644-1650.         [ Links ]

Sebastiao, M., A. Queda, and L. Campos. 2000. Effect of municipal solid waste compost on potato production and heavy metal contamination in different types of soil. pp. 760-772. In: Warman, P.R. and B. Taylor (eds.). Proceedings of the International Composting Symposium. CBA Press Inc. (Pubs.), Halifax/Dartmouth, Nova Scotia, Canada.         [ Links ]

Schützendübel, A. and A. Polle. 2002. Plant responses to abiotic stresses: heavy metal-induced oxidative stress and protection by mycorrhization. J. Exp. Bot. 53, 1351-1365.         [ Links ]

Sharma, S.S. and K.J. Dietz. 2006. The significance of amino acids and amino acid-derived molecules in plant responses and adaptation to heavy metal stress. J. Exp. Bot. 57, 711-726.         [ Links ]

Singh, K.P., D. Mohan, S. Sinha, and R. Dalwani. 2004. Impact assessment of treated/untreated wastewater toxicants discharged by sewage treatment plants on health, agricultural, and environmental quality in the wastewater disposal area. Chemosphere 55, 227-255.         [ Links ]

Sinha, S., S. Mallick, R.K. Misra, S. Singh, A. Basant, and A.K. Gupta. 2007. Uptake and translocation of metals in Spinacia oleracea L. grown on tannery sludge-amended and contaminated soils: Effect on lipid peroxidation, morpho-anatomical changes and antioxidants. Chemosphere 67, 176-187.         [ Links ]

Warman, P.R., C. Murphy, J. Burnham, and L. Eaton. 2004. Soil and plant response to MSW compost applications on lowbush blueberry fields in 2000 and 2001. Small Fruit Rev. 3(1/2), 19-31.         [ Links ]

Wilson, B. and F.B. Pyatt. 2007. Heavy metal dispersion, persistence, and bioaccumulation around an ancient copper mine situated in Anglesey. UK. Ecotoxicol. Environ. Saf. 66, 224-231.         [ Links ]

Zheljazkov, V. and P.R. Warman. 2004a. Source-separated municipal soil waste compost application to Swiss chard and basil. J. Environ. Qual. 33, 542-552.         [ Links ]

Zheljazkov, V. and P.R. Warman. 2004b. Phytoavailability and fractionation of copper, manganese, and zinc in soil following application of two composts to four crops. Environ. Pollut. 131, 187-195.         [ Links ]