Trace element "biogeogenic cycling" in the environment is an integral function of the ecosystem (aquatic, terrestrial, and atmospheric systems). Metal enrichments in these compartments may result from natural sources or from human activities, such as smelting, mining, processing, agricultural, and waste disposal technologies. Metals are present in the Earth's crust in various quantities . Their relative abundance, however, differs greatly in regions over the globe, and the region at which a metal is found in high concentration serves as the source of the metal. Although a metal may be present in high concentration in a region, it does not pose any threat to the environment until the landmass of the region is used for agroindustry. This is because the metals present remain tightly bound to their Lewis components as sulfides, oxides, or carbonates, as the case may be , and the ore particles also remain tightly packed along with the particles of the soil, which makes them highly immobilized. It is only the mining of the ore, and subsequent uses of the extracted metals that lead to far and wide contamination of the environment. From the figures of the crustal abundance of various metals and their production per annum (Table 16.1), the magnitude of contamination or pollution by these metals as a result of anthropogenic activities may be imagined.
The concentration of a metal that existed in a region before the advent of industrial activity is termed its natural or background level. This is a result of release of the metal due to natural weathering of the metal-bearing formations in the area. The knowledge of natural contamination of a metal provides a true reference point for estimating the extent of pollution from the element and allows the contemporary situation to be seen in perspective — i.e., whether it is in excess from the point of view of its toxicity to organisms (Figure 16.1). However, the natural, or background levels of metals for some areas may be difficult to obtain because they may not exist due to human intervention; this is particularly true for lead, mercury, cadmium, and arsenic. In fact, although naturally occurring geochemical materials are the primary source of metals in the environment, not many examples are known for which the interaction between natural weathering processes and mineralized zones is completely devoid of a human contribution.
Anthropogenic activities lead to pollution of the three nonliving components of the environment — air, water, and soil — and the biosphere by metals . The magnitude of pollution depends largely upon the nature and intensity of the activities; the most important among them are mining; industrial processing of ores and metals; and the use of metals and metal components, which affect the environment in a wide variety of ways [2,4,8-10]. However, this discussion will be restricted to terrestrial contamination and the adaptive process that plants undergo to face the challenge of the presence of high levels of natural or man-induced metals around them.
Excluders prevent metal uptake into roots and avoid translocation and accumulation in shoots . They have a low potential for metal extraction, but they can be used to stabilize the soil to avoid further contamination spread due to erosion. Resistance of plants to heavy metal ions can be achieved by an avoidance mechanism, which includes mainly the mobilization of metal in root and in cell walls. Tolerance to heavy metals is based on the sequestration of heavy metal ions in vacuoles; on binding them by appropriate ligands like organic acids, proteins, and peptides; and on the presence of enzymes that can function at high levels of metallic ions (Figure 16.2 and Figure 16.3) .
The effective xylem loading of hyperaccumulators may be due to smaller sequestration of metals in the root vacuoles of hyperaccumulators . Translocation of Ni from roots to shoots may involve specific ligands in some hyperaccumulator species. Kramer et al.  showed that spraying histidine on the leaves of the nonaccumulating Alyssum montanum greatly increased Ni tolerance and capacity for Ni transport to the shoots. The detoxification of heavy metals commences only when they enter the cells and occurs in the cell by the process of chelation, compartmental-ization, or precipitation .
Metallothionein (MT)-II genes have been identified in plants [16,17]. Although detection of plant metallothioneins has been problematic, evidence suggests that they have the ability to bind heavy metals. Also, accumulation of heavy metals in plants has been shown to induce the production of phytochelatins (PCs), a family of thiol-rich peptides . The synthesis of PCs has been documented to be induced by a variety of metals. However, PCs have been shown to be primarily involved in Cd and Cu tolerance . A recent study suggested that PCs may also be involved in As detoxification .
The processes of heavy metal uptake, accumulation, distribution, and detoxification have been studied in a wide range of crop and herbaceous species . The mechanisms involved in perennials have been partially investigated and reported to be considerably tolerant . Several sequestration and detoxification strategies are reported to take place in plants exposed to elevated doses of toxic trace elements (Figure 16.4 and Figure 16.5) .
Complexation with phytochelatin peptides synthesized from glutathione has been identified as an important mechanism for detoxifying metals such as Cd, Pb, and Zn. Yet, phytochelatins do not appear to be the primary mechanism. Large increases in histidine levels and coordination of Ni with histidine have been reported in the xylem sap of Alyssum lesbiacum, suggesting that histidine is important for Ni tolerance and transport in hyperaccumulators .
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