5.5 Plant Mechanisms for Metal Detoxification
Plants, even hyper-accumulators, cannot grow well in metal-enriched soils if there is
no internal detoxification. The primary condition for the enrichment of HMs by
plants is that the root and stem cells of these plants can tolerate high concentrations
of corresponding elements, called detoxification. One of the major mechanisms of
detoxification is the compartment effect of vacuoles and their complexation on HMs.
It has been confirmed that Zn in the vacuoles of Thlaspi caerulescens leaves was
higher than that of the apoplast, and the same results are verified in Thlaspi
caerulescens roots. Biotransformation, another mechanism of hyper-tolerance,
reduces the toxicity of HMs through chemical reduction in plants or in combination
with organic compounds. A report by Brooks et al. (1981) stated that organic acids
were present at a higher concentration level in the plant Alyssum serpyllifolium when
compared to other species. The cadmium that is accumulated by plants can be
detoxified by a process of binding with thiol (SH)-rich peptides, phytochelatins
(PC) (Vogeli-Lange et al. 1990; Salt et al. 1995a, b; Griller et al. 1989). Moreover,
plants can eliminate the reactive oxide species brought by HM stress, shielding cells
from toxicity. Also, hyper-accumulators require higher metal concentrations for
normal growth than do other plants.
5.6 Plant Limitations
There are no plant species that can grow well in all environment conditions, even in
sufficiently highly metal-enriched soils. Thus, the detoxification by plants is limited.
Depending upon the content and type of metal at a specific site, 15 years or even
more time is needed for successful remediation; which limits its practical applications. Actually, the absorbed dose of some hyper-accumulator plants is not that great
because of their lesser biomass. In addition, soil pollution is sometimes a combination of organic and inorganic pollutants, so hyper-accumulators that have a role in a
particular pollutant cannot meet the remediation requirements. For contamination
under deep soils (deeper than 50 cm), phytoremediation seems to be useless. There is
still a long way to go for researchers to spend time on finding strengthening measures
that can enhance the bioavailability of HMs in soils and optimal hyper-accumulator
plants.
5.7 Improving Phytoremediating Plants
The traditional methods in practice limit the usage of these plants as such on a large
scale, so it is necessary to improve the plant species to make it more efficient for the
remediation process. One report states many modern tools in the field of chemical,
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
93
Plants, even hyper-accumulators, cannot grow well in metal-enriched soils if there is
no internal detoxification. The primary condition for the enrichment of HMs by
plants is that the root and stem cells of these plants can tolerate high concentrations
of corresponding elements, called detoxification. One of the major mechanisms of
detoxification is the compartment effect of vacuoles and their complexation on HMs.
It has been confirmed that Zn in the vacuoles of Thlaspi caerulescens leaves was
higher than that of the apoplast, and the same results are verified in Thlaspi
caerulescens roots. Biotransformation, another mechanism of hyper-tolerance,
reduces the toxicity of HMs through chemical reduction in plants or in combination
with organic compounds. A report by Brooks et al. (1981) stated that organic acids
were present at a higher concentration level in the plant Alyssum serpyllifolium when
compared to other species. The cadmium that is accumulated by plants can be
detoxified by a process of binding with thiol (SH)-rich peptides, phytochelatins
(PC) (Vogeli-Lange et al. 1990; Salt et al. 1995a, b; Griller et al. 1989). Moreover,
plants can eliminate the reactive oxide species brought by HM stress, shielding cells
from toxicity. Also, hyper-accumulators require higher metal concentrations for
normal growth than do other plants.
5.6 Plant Limitations
There are no plant species that can grow well in all environment conditions, even in
sufficiently highly metal-enriched soils. Thus, the detoxification by plants is limited.
Depending upon the content and type of metal at a specific site, 15 years or even
more time is needed for successful remediation; which limits its practical applications. Actually, the absorbed dose of some hyper-accumulator plants is not that great
because of their lesser biomass. In addition, soil pollution is sometimes a combination of organic and inorganic pollutants, so hyper-accumulators that have a role in a
particular pollutant cannot meet the remediation requirements. For contamination
under deep soils (deeper than 50 cm), phytoremediation seems to be useless. There is
still a long way to go for researchers to spend time on finding strengthening measures
that can enhance the bioavailability of HMs in soils and optimal hyper-accumulator
plants.
5.7 Improving Phytoremediating Plants
The traditional methods in practice limit the usage of these plants as such on a large
scale, so it is necessary to improve the plant species to make it more efficient for the
remediation process. One report states many modern tools in the field of chemical,
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
93