377
The physiological mechanisms of tolerance have been categorized as either
exclusion, blocking the movement of metals at the soil/root or root/shoot interface,
or accumulation, allowing uptake of metals into aerial parts and rendering them
nontoxic through chemical binding or intracellular sequestration (Baker 1981;
Baker and Walker 1990; Ernst et al. 1992). Combinations of all of these mechanisms in the plant species contribute to metal tolerance or hyperaccumulation.
Additionally, in most plant species, metals are predominantly accumulated in the
roots. As a consequence, the ratio shoot/root of metal concentrations (the so-called
translocation factor, TF) is generally substantially below unity in these plants.
Hyperaccumulation can be seen as a special and extreme case of the wider category
of accumulators where metal concentrations are much higher in the leaves than in
the roots, with TF abnormally high and above unity (Krämer 2010).
Only a restricted number of plant species from the local flora are able to grow in
metalliferous soils. In several studies, these plants are shown to have physiological
adaptations, such as metal tolerance or metal accumulation, that are distinct from
non-tolerant plant species of the same species that grow in uncontaminated soils,
supporting the idea of different ecotypes (Lefebvre and Vernet 1990). Reclamation
of metal-contaminated sites by plant species native to toxic areas leads to stabilization of the soil, immobilizing trace elements in the rhizosphere and, thereby, reducing the risk of dissemination of metalliferous dust by wind or water erosion.
Understanding the metal accumulation physiology of the plants through the
metal contents in the different plant components is important. Studies have shown
that metal ions from soil are absorbed by the root system and transported to the stem
tissues through the xylem prior to their sequestration in leaf tissue (Hall 2002;
Harper et al. 2002). These investigations show that such areas are characteristically
different specifically with the underlying geology and the associated type of soil
formation. With vegetation largely influenced by soil type, it can be inferred that
unique plant species occur in such area.
Among the most toxic elements emitted by mining and mining activities are
antimony, arsenic, and lead. These elements are frequently present together in ore
minerals or in mineral associations. The exploitation of such deposits for the economic ore minerals provokes a simultaneous release of such toxic elements into the
environment. None of them has a known biological function in animals or plants,
being highly toxic, especially to mammals. Inorganic arsenic species are generally
highly toxic to plant species. In humans arsenic and antimony cause a great variety
of adverse health effects, including mutagenic and carcinogenic effects, whereas Pb
mainly causes hematological, gastrointestinal, and neurological dysfunctions, with
children being particularly sensible (Winship 1987; Lockitch 1993; Mandal and
Suzuki 2002; Sundar and Chakravarty 2010). Arsenate acts as a phosphate analog
and affects phosphate metabolism, while arsenite reacts with sulfhydryl groups of
enzymes and tissue proteins, leading to inhibition of cellular function and death
(Meharg and Hartley-Whitaker 2002).
In arsenic non-hyperaccumulating plants, resistance to environmental arsenic
has been shown to involve a decreased uptake of arsenate due to suppression of the
high-affinity phosphate uptake system (Meharg and Macnair 1991, 1992). The
13 Accumulation Abilities of Endemic Plant Species from the Vicinity of an As-Sb-Tl…
The physiological mechanisms of tolerance have been categorized as either
exclusion, blocking the movement of metals at the soil/root or root/shoot interface,
or accumulation, allowing uptake of metals into aerial parts and rendering them
nontoxic through chemical binding or intracellular sequestration (Baker 1981;
Baker and Walker 1990; Ernst et al. 1992). Combinations of all of these mechanisms in the plant species contribute to metal tolerance or hyperaccumulation.
Additionally, in most plant species, metals are predominantly accumulated in the
roots. As a consequence, the ratio shoot/root of metal concentrations (the so-called
translocation factor, TF) is generally substantially below unity in these plants.
Hyperaccumulation can be seen as a special and extreme case of the wider category
of accumulators where metal concentrations are much higher in the leaves than in
the roots, with TF abnormally high and above unity (Krämer 2010).
Only a restricted number of plant species from the local flora are able to grow in
metalliferous soils. In several studies, these plants are shown to have physiological
adaptations, such as metal tolerance or metal accumulation, that are distinct from
non-tolerant plant species of the same species that grow in uncontaminated soils,
supporting the idea of different ecotypes (Lefebvre and Vernet 1990). Reclamation
of metal-contaminated sites by plant species native to toxic areas leads to stabilization of the soil, immobilizing trace elements in the rhizosphere and, thereby, reducing the risk of dissemination of metalliferous dust by wind or water erosion.
Understanding the metal accumulation physiology of the plants through the
metal contents in the different plant components is important. Studies have shown
that metal ions from soil are absorbed by the root system and transported to the stem
tissues through the xylem prior to their sequestration in leaf tissue (Hall 2002;
Harper et al. 2002). These investigations show that such areas are characteristically
different specifically with the underlying geology and the associated type of soil
formation. With vegetation largely influenced by soil type, it can be inferred that
unique plant species occur in such area.
Among the most toxic elements emitted by mining and mining activities are
antimony, arsenic, and lead. These elements are frequently present together in ore
minerals or in mineral associations. The exploitation of such deposits for the economic ore minerals provokes a simultaneous release of such toxic elements into the
environment. None of them has a known biological function in animals or plants,
being highly toxic, especially to mammals. Inorganic arsenic species are generally
highly toxic to plant species. In humans arsenic and antimony cause a great variety
of adverse health effects, including mutagenic and carcinogenic effects, whereas Pb
mainly causes hematological, gastrointestinal, and neurological dysfunctions, with
children being particularly sensible (Winship 1987; Lockitch 1993; Mandal and
Suzuki 2002; Sundar and Chakravarty 2010). Arsenate acts as a phosphate analog
and affects phosphate metabolism, while arsenite reacts with sulfhydryl groups of
enzymes and tissue proteins, leading to inhibition of cellular function and death
(Meharg and Hartley-Whitaker 2002).
In arsenic non-hyperaccumulating plants, resistance to environmental arsenic
has been shown to involve a decreased uptake of arsenate due to suppression of the
high-affinity phosphate uptake system (Meharg and Macnair 1991, 1992). The
13 Accumulation Abilities of Endemic Plant Species from the Vicinity of an As-Sb-Tl…