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Putative role in elemental allelopathy, defense mechanism against herbivory and pathogens

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Nutrition and toxicology

Flux of metals through food chain is of human health concern

FIGURE 25.3 Metal hyperaccumulators were later believed to have limited potential in the area of phytore-mediation, owing to their slow growth and low biomass production, which limit the speed of metal removal. By definition, a hyperaccumulator must accumulate at least 100 |J,g g-1 (0.01% dry wt.); Cd, As, and some other trace metals, 1000 |lg g-1 (0.1 dry wt.); Co, Cu, Cr, Ni, and Pb and 10,000 |lg g-1 (1% dry wt.); Mn and Ni. Plants that hyperaccumulate metals have other applications and implications. The most important applications are phytoremediation and biogeochemical prospecting. The other implications are elemental allelopathy and nutrition and toxicology, which are human health-related subjects.

FIGURE 25.4 Several factors would accelerate phytoremediation technology. The prime is genetic engineering and production of transgenics having tolerance and metal accumulation ability for use in phytoremediation, facilitating the factors that would influence the metal bioaccumulation coefficient; in turn, this will depend upon heavy metal availability in the soil, absorption, transport and sequestration, etc., as well as development of low-cost technologies for chelate-induced hyperaccumulation.

FIGURE 25.4 Several factors would accelerate phytoremediation technology. The prime is genetic engineering and production of transgenics having tolerance and metal accumulation ability for use in phytoremediation, facilitating the factors that would influence the metal bioaccumulation coefficient; in turn, this will depend upon heavy metal availability in the soil, absorption, transport and sequestration, etc., as well as development of low-cost technologies for chelate-induced hyperaccumulation.

dry wt.) of Cd, As, and some other trace metals; 1000 |g g-1 (0.1% dry wt.) of Co, Cu, Cr, Ni, and Pb; and 10,000 |g g-1 (1% dry wt.) of Mn and Ni [19,31].

The following technical terms are connected with phytoremediaton of metals in the environment:

• Phytoremediation: use of plants to remediate contaminated soil, water, and air

• Phytoaccumulation: the uptake and concentration of contaminants (metals or organics) within the roots or above-ground portion of plants

• Phytoaccumulation coefficient: metal concentration in plant dry matter/extractable metal in soil

• Phytoextraction: the use of plants at waste sites to accumulate metals into the harvestable, above-ground portion of the plant and, thus, to decontaminate soils

• Phytoextraction coefficient: the ratio of extractable metal concentration in the plant tissues (g metal/g dry weight tissue) to the soil concentration of the metal (g metal/g dry weight soil)

• Phytomining: use of plants to extract inorganic substances from mine ore

• Phytostabilization: plants tolerant to the element in question used to reduce the mobility of elements and thus stabilized in the substrate or roots

• Phytovolatilization: the uptake and transpiration of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant

• Rhizofiltration/Phytofiltration: process in which roots or whole plants of element-accumulating plants absorb the element from polluted effluents and are later harvested to diminish the metals in the effluents

• Rhizosecretion: a subset of molecular farming, designed to produce and secrete

25.2 BIODIVERSITY PROSPECTING FOR PHYTOREMEDIATION OF METALS IN THE ENVIRONMENT

Biodiversity prospecting" offers several opportunities, of which the most important is to save as much as possible of the world's immense variety of ecosystems. Biodiversity prospecting would lead to the discovery of a wild plant that could clean polluted environments of the world. This subject is in its infancy, with a great hope of commercial hype. The desire to capitalize on this new idea needs to provide strong incentives for conserving nature [32].

Potentially toxic trace elements are increasing in all compartments of the biosphere, including air, water, and soil, as a result of anthropogenic processes. For example, the metal concentration in river water and sediments has been increased several thousand-fold by effluents from industrial and mining wastes [33]. Aquatic plants in freshwater, marine, and estuarine systems act as receptacles for several metals [34-39]. Published literature indicates that an array of bioresources (biodiversity) has been tested in field and laboratory (Table 25.1). Remediation programs relying on these materials may be successful [40-42,51] (Figure 25.5).

The most successful monitoring methods for metals in the environment are based on gene-based and protein-based bacterial heavy metal biosensors [42]. Mosses, liverworts, and ferns are also capable of growing on metal-enriched substrates. These plants possess anatomical and physiological characteristics enabling them to occupy unique ecological niches in natural metalliferous and manmade environments. For example, groups of specialized bryophytes are found on Cu-enriched substrates — so-called "copper mosses" — and come from widely separated taxonomic groups. Other bryophytes are associated with lead- and zinc-enriched substrates. However, the information about bryophytes growing on serpentine soils is rather scanty. Pteridophytes (ferns) are associated with serpentine substrates in various parts of the world. Brake fern (Pteris vittata), a fast growing plant, is reported to tolerate soils contaminated with as much as 1500 ppm arsenic and its fronds concentrate the toxic metal to 22,630 ppm in 6 weeks [43]. Among angiosperms, about 450 metal hyperaccumulators have been identified, which would serve as a reservoir for biotechnological application [44] (Figure 25.6).

25.3 METAL-TOLERANT PLANTS FOR PHYTOREMEDIATION

Mine reclamation and biogeochemical prospecting depend upon the correct selection of plant species and sampling. The selection of heavy metal-tolerant species is a reliable tool to achieve success in phytoremediation. Table 25.2 shows 163 plant taxa belonging to 45 families found to be metal tolerant and capable of growing on elevated concentrations of toxic metals. The use of metal-tolerant species and their metal indicator and accumulation is a function of immense use for biogeochemical prospecting [45-47].

Brassicaceae had the highest number of taxa, i.e., 11 genera and 87 species that are established for hyperaccumulation of metals (Figure 25.7). In Brassicaceae, Ni hyperaccumulation is reported in seven genera and 72 species [48,49], and Zn in three genera and 20 species (Figure 25.8 and Figure 25.9). Different genera of Brassicaceae are known to accumulate metals (Figure 25.10).

Considerable progress had been achieved recently in unraveling the genetic secrets of metal-eating plants. Genes responsible for metal hyperaccumulation in plant tissues have been identified and cloned [50]. These findings are expected to identify new nonconventional crops, metallocrops, that can decontaminate metals in the environment [51-53]. The fundamental aspects of microbe/plant stress responses to different doses of metals coupled with breakthrough research innovations in biotechnology would successfully provide answers such as how to apply the biodiversity for advancing phytoremediation technology

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