Cadmium Toxicity in Plants

Abstract Cadmium is an important poiiutant in the environment, toxic to most organisms and a potential threat to human heaith Crops and other plants take up Cd from the soil or water and may enrich it in their roots and shoots. In this review, we suimnarize natural and anthropogenic reasons for the occurrence of Cd toxicity, and evaluate the observed phytotoxic effects of plants growing in Cd-supplemented sou or nutrient solution. Cd-induced effects include oxidative stress, genotoxicity, inhibition of the photosynthetic apparatus, and inhibition of root metabolism. We explain proposed and possible interactimis between these modes of toxicity. WhUe discussing recent and older studies, we further emphasize the environmental relevance of the experiments and the physiological response of the plant. 

Fachbtaeich Biologic, Univeisitat Kraistanz, D-78457 Konstanz, Germany e-mail hcaidrik.kuepper nni-k(mstanz.de 

Sigel (eds.). Cadmium. From Toxicity to Essentiality, Metal Ions in Life Sciences 11, DOI 10.1007/978-94-007-5179-8 13, 

Keywords cadmium cytotoxicity genotoxocity oxidative stress photosynthesis inhibition substitution of essential metals 

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Introduction Environmental Relevance of Cadmium Toxicity in Plants... 

Cadmium toxicity in plants has been a focus of intense research for several decades. Many mechanisms of Cd-induced damage to plants have been described, and those which seem most important according to our present state of knowledge have been reviewed in this chapter. As discussed in detail in the previous sectirais of this review, however, the environmental relevance of many of these proposed mechanisms of Cd-induced damage remains unclear, because they were investigated under conditions (especially high Cd concentrations) that never or only very rarely occur in the environment. Therefore, future studies should establish time and Cd concentration thresholds of these mechanisms under environmentally relevant growth conditions. 

Heavy metal toxicity in plants is infrequent (143). In many cases, metal concentrations in plant parts show poor correlation with soil concentrations of the element (147). Plants tend to exclude certain elements and readily accept or concentrate others. Lisk (148) reported natural plant soil concentration ratios of 0.05 or less for As, Be, Cr, Ga, Hg, Ni, and V. Cadmium appears to be actively concentrated and selenium appears to be easily exchangeable. Indicator plants are capable of markedly concentrating specific elements, e.g., Astragalus spp. for selenium (138) and Hybanthus floribundus for nickel (149). Plants growing on mine wastes have been shown to evolve populations which exhibit metal-specific tolerances (150). 

Fuhrer, J., 1982. Ethylene biosynthesis and cadmium toxicity in leaf tissue of beans (Phaseoulus vulgaris L.). Plant Physiol. 70, 162-167. 

Cadmium has been found to be a micronutrient for an ecotype of Thalassiosira weissflogii, a marine alga [1] and many other heavy metals such as copper, nickel and zinc are well-known for a long time already as essential trace elements for plants. While general aspects of the entry of Cd into the environment are dealt with in detail in Chapter 2 of this book [2], we will summarize here in Sections 1.1 and 1.2 a few plant-specific aspects before discussing mechanisms of Cd toxicity in plants. 

Hi) Poly amines. In many respects the role of poly amines in plant functioning is still mysterious after many years work. They are almost certainly involved in the control of growth and development through their interactions with nucleic acids and membranes (Smith, 1985). There is increasing circumstantial evidence for their involvement, especially of putrescine, in plant responses to a wide range of stresses including pH, Mg deficiency, osmotic shock, cold, SO2 pollution, and cadmium and ammonium toxicity (Smith, 1985). It remains to be determined, however, how, and indeed whether, putrescine accumulation in response to these diverse stresses is beneficial. 

Standards imposed to the industrial waste streams charged in heavy metals are more and more drastic in accordance with the updated knowledges of the toxicity of mercury, cadmium, lead, chromium... when they enter the human food chain after accumulating in plants and animals (Forster Wittmann, 1983). Nowadays, the use of biosorbents (Volesky, 1990) is more and more considered to complete conventional (physical and chemical) methods of removal that have shown their limits and/or are prohibitively expensive for metal concentrations typically below 100 mg.l-i. 

Heavy metals in the environment, especially their accumulation in soils, is a serious environmental problem which the whole world faces (Du et al. 2005). The farmland soils are an important media of the ecological cycle of Cadmium, and its harm to human health can t be neglected (Wu et al. 2004). Heavy metal migration, transformation and toxicity to plants in soil are directly influenced by the quantity proportions of various forms (Zhu et al. 2002). The toxicity of water-extractable and adsorbed and exchangeable metals are the greatest, and residual metals is the lowest (Liu etal. 2002). Different forms have different bioavailability thus their influences on the environment and human health are different. It is critical to have a good understanding of Cadmium forms in soil. This paper describes the Cadmium forms in the acid soils of eastern China. 

The green phosphoric acid from a phosphoric acid plant (PAP) contains 25-30% H3PO4. The acid is heavily entrained with impurity cations, among which arsenic, cadmium, and uranium are the most toxic. In addition, anions, like chloride, fluoride, and sulfate must be considered. Selective extraction of phosphoric acid [18] as an adduct complex with TBP according to the block diagram in Fig. 14.12 has been used to produce acid of food grade. 

Heat Transfer The movement and dispersion of heat by conduction, convection, or radiation. Heavy Metals High-density metallic elements generally toxic to plant and animal life in low concentrations (e.g. mercury, chromium, cadmium, arsenic, and lead). 

The metals of most concern are the heavy metals, especially cadmium, lead, and mercury. Although it is a metalloid with characteristics of both metals and nonmetals, arsenic is commonly classified as a heavy metal for a discussion of its toxicity. Though not particularly toxic, zinc is abundant and may reach toxic levels in some cases. For example, zinc accumulates in sewage sludge and crop productivity has been lowered on land fertilized with sludge because of zinc accumulation. Copper may be toxic to plants. Aluminum, a natural constituent of soil, may be leached from soil by polluted acidic rainwater and reach levels that are toxic to plants. Other metals that may be of concern because of their toxicides include chromium, cobalt, iron, nickel, and vanadium. Radium, a radioactive alpha particle-emitting metal, can be very toxic at even very low levels in water or food. 

Organization guidelines for drinking water quality (World Health Organization, 1984) include limit values for the organic and toxic substances (see Table 11.6), based on acceptable daily intakes (ADI). These can be adopted direcdy for ground-water protection purposes, but in view of the possible accumulation of certain toxic elements in plants (e.g. cadmium and selenium), the intake of toxic materials... 

Some of the effects of toxic chemical mixtures on soil pollution are predictable. Acidic soils dissolve otherwise insoluble metal oxides and salts, thereby increasing available metal concentrations and toxicity to flora and fauna. Available copper content is inversely proportional to increased pH of soiU4 Earthworm mortality in soil polluted by lead increases as pH decreases. I15l The addition of ethylenediaminetetraacetic acid (EDTA) and its disodium salt to soil contaminated with cadmium, lead, and zinc increases the availability of these metals to plants and results in significant increases in the uptake of these in plants. I25l... 

Fertile soils supply plants with all of the trace elements essential for growth, believed at the present time to be Fe, Mn, Zn, B, Cu, Mo, and CL These seven elements are called the micronutrients, a term that indicates the small quantities needed by plants but not necessarily the concentrations found in soils. Deficiencies can occur in soils either because they contain extremely low concentrations of these elements or because the elements are present in very unavailable (insoluble) forms. Conversely, many trace elements, including ail of the micronutrients, can reach concentrations in soils that are toxic to plants and microoiganisms. Some of the most toxic are mercury (Hg), lead (Pb), cadmium (Cd), copper (Cu), nickel (Ni), and cobalt (Co). The first three are particularly toxic to higher animals. The last three are more toxic to plants than animals and are termed phytotoxic. From the standpoint of potential hazard to human health, an extended Ust of priority metals has been established. This list presently consists of ... 

Cadmium (Cd) is a highly toxic heavy metal element, that in plants may cause oxidation damage or even death.1 Research indicated salicylic acid (SA) and lanthanum, may adjust plant physiology pathways or change ion channels to reduce Cd absorption and repair damage caused by Cd stress. SA has been reported to induce a number of defense responses to abiotic stress,2 and La3+ can decrease the accumulation of Cd in crops,3however, whether the mixture of SA and La3+ could alleviate Cd stress is still unclear, and this is the focus of these studies. 

Not only have toxic metals been implicated notably in cancer, but crops have been found to absorb these chemicals from the soil. Thus, lettuce absorbs cadmium like a sponge (Wilson, 2001, p. 251). During the controversy, a Colorado soil professor, John Mortvedt, called Mr. Micronutrient, was consulted, who admitted that plants picked up various heavy metal compounds, notably cadmium, especially in acidic soils (Wilson, 2001, p. 172ff). His findings were watered down in subsequent hearings, when he waffled by stating that very little research had been done on heavy metals in micronutrient fertilizers (Wilson, 2001, p. 215) which, Wilson notes, is precisely the point. 

Van Assche F, Cardinaels C and Clijsters H (1988) Induction of enzyme capacity in plants as a result of heavy metal toxicity dose—response relations in Phaseolus vulgaris L, treated with zinc and cadmium. Environ Pollut 52 103-115. 

Zomova, R, Vasquez, S., Esteban, E., Femandez-Pascual, M., Carpena, R., 2002. Cadmium-stress in nodulated white lupin strategies to avoid toxicity. Plant Physiol. Biochem. 40,1003-1009. 

Heavy Metals - Metallic elements that are located in the transitional series of the periodic chart of elements, These metals are not required in plant nutrition and usually are found in relatively small amounts in nature. Toxicity to plants could result with high concentrations of some heavy metals in soil, and problems in animal and human health may result if heavy metal concentrations in the diet are above certain critical levels. Examples are cadmium (Cd), chromium (Cr),-lead-(Pb),-nickel (Ni), and vanadium (V). Levels of heavy metals may be a problem in some phosphate rock deposits, sewage sludge, animal manures, and some industrial byproducts used as a source of fertilizer. 

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