aboveground parts. The metals from the aboveground biomass of the plant are
crucial, feasible, desirable, and effective when compared to its root counterpart
(Ali et al. 2013).
5.2 What Is a Natural Hyper-accumulator Species?
Plants with a greater ability for absorbing a high content of metals from the
contaminated environment and accumulating it in their aboveground biomass
through their effective root systems are said to be natural hyper-accumulators. The
process of absorption and accumulation of metals into the tissues of the aboveground
biomass is also found be higher in rate when compared with other plant species
(Wu et al. 2010).
5.3 How Do Plants Tolerate High Metal Concentration
in Soil?
To grow in a metal-contaminated ecosystem, plants must possess two indispensable
and inherent properties, avoidance and tolerance (Baker 1987). Avoidance means
that the plants prevent the absorption of a high concentration of heavy metals from
soils by some kind of external mechanism; in some, this is achieved by the prevention of uptake of toxic metals by the root systems. The root of these plants may
produce organic compounds, and also, along with rhizosphere bacteria, transform
toxic metals into unavailable forms. Otherwise, the selective permeability of the
plasma membrane prevents the accumulation process. This phenomenon often
happens in tolerant populations. However, accumulators (especially hyperaccumulators) usually do not prevent contaminant entry into their roots, and thus
possessing tolerance to toxic metals is necessary. The tolerance depends on the
ecological and genetic characteristics of plants, so it varies from species to species.
These plants have evolved specific mechanisms that detoxify the effects of high
levels of metals that are accumulated in their cells, which allows even extremely high
levels of metal accumulation.
5.4 Mechanisms of Metals Uptake into the Root
and Translocation to Shoots
The main point of HMs in soil extracted by plants is their bioavailability, which
means the HM exists in an activated chemical form. The bioavailability of HMs in
soil is to a large extent dependent on soil factors (as listed earlier); plant root
exudates and even some microbial activities near the rhizosphere greatly influence
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
91
crucial, feasible, desirable, and effective when compared to its root counterpart
(Ali et al. 2013).
5.2 What Is a Natural Hyper-accumulator Species?
Plants with a greater ability for absorbing a high content of metals from the
contaminated environment and accumulating it in their aboveground biomass
through their effective root systems are said to be natural hyper-accumulators. The
process of absorption and accumulation of metals into the tissues of the aboveground
biomass is also found be higher in rate when compared with other plant species
(Wu et al. 2010).
5.3 How Do Plants Tolerate High Metal Concentration
in Soil?
To grow in a metal-contaminated ecosystem, plants must possess two indispensable
and inherent properties, avoidance and tolerance (Baker 1987). Avoidance means
that the plants prevent the absorption of a high concentration of heavy metals from
soils by some kind of external mechanism; in some, this is achieved by the prevention of uptake of toxic metals by the root systems. The root of these plants may
produce organic compounds, and also, along with rhizosphere bacteria, transform
toxic metals into unavailable forms. Otherwise, the selective permeability of the
plasma membrane prevents the accumulation process. This phenomenon often
happens in tolerant populations. However, accumulators (especially hyperaccumulators) usually do not prevent contaminant entry into their roots, and thus
possessing tolerance to toxic metals is necessary. The tolerance depends on the
ecological and genetic characteristics of plants, so it varies from species to species.
These plants have evolved specific mechanisms that detoxify the effects of high
levels of metals that are accumulated in their cells, which allows even extremely high
levels of metal accumulation.
5.4 Mechanisms of Metals Uptake into the Root
and Translocation to Shoots
The main point of HMs in soil extracted by plants is their bioavailability, which
means the HM exists in an activated chemical form. The bioavailability of HMs in
soil is to a large extent dependent on soil factors (as listed earlier); plant root
exudates and even some microbial activities near the rhizosphere greatly influence
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
91