Page 102 - Plant-Based Remediation Processes

5 Impact of Metal/Metalloid-Contaminated Areas on Plant Growth 91

shoots (100-fold greater level than those typically measured in common non-
accumulator plants), i.e. more than 10,000 mg kg 1 dw for Ni, Zn, and Mn,
1,000 mg kg 1 dw for As, Pb, Cu, Ni, and Co, 100 mg kg 1 dw for Cd, 10 mg kg 1
dw for Hg, and 1 mg kg 1 for Au (Wei et al. 2008). To enhance metal uptake by
plants, chelating agents and genetic manipulation are also used. Some hyperaccu-
mulators have very unique ecophysiological behaviour and have the capacity to
accumulate significant amounts of metals and compartmentalise them efficiently
in the cell wall, vacuole, and to the specific subcompartment and/or compartments
of the cytosol in order to render them innoxious or nontoxic and keep them away
from active metabolic sites in plant cells (Memon and Schro ¨der 2009).
Approximately 450 plant species from at least 45 plant families have been
reported to hyperaccumulate metals (As, Cd, Co, Cu, Mn, Ni, Pb, Sb, Se, Tl, and
Zn). However, new reports of this kind of plants continue to accrue (Robinson et al.
2006; Sun et al. 2006; Venkatachalam et al. 2009; Rascioa and Navari-Izzo 2011),
so it is conceivable that many yet unidentified hyperaccumulators may occur in the
environment. As regards the many plants useful in decontamination of polluted
areas and their different traits, it is important to present three selected plant groups
(1) hyperaccumulators, (2) non-hyperaccumulators, and (3) transgenic plants.

5.4.1 Hyperaccumulators

5.4.1.1 Thlaspi caerulescens (Brassicaceae)

Possibly, Thlaspi caerulescens is the best-known metal hyperaccumulator.
T. caerulescens (alpine pennycress) can accumulate Cd, Ni, Zn, and Pb. As a
hyperaccumulator of Cd and Zn it could remove as much as 60 kg Zn ha 1 and
8.4 kg Cd ha 1 (Robinson et al. 1998). This plant species has an unusual ability not
only to accumulate Pb in its roots but also to translocate Zn and Cd to the
harvestable parts, as well as high rates of elements’ uptake and translocation, and,
what is more, the Pb ion is less accumulated in the shoots than in the roots. The
species can be used for phytostabilization of Pb-contaminated sites (El Kheir et al.
2008). T. caerulescens has also been shown to develop a dense root system with a
large proportion of fine roots, which may also contribute to enhanced uptake of
metals (Keller et al. 2003; Wenzel 2009). What is more, T. caerulescens is useful
for moderately Zn- and Cd-contaminated soils but would take far too long on highly
contaminated ones. It also appears that season length, method of sowing seeds, and
soil pH have effects on the capacity of T. caerulescens for extraction of Zn and Cd
from the polluted soil (McGrath et al. 2006; Yanai et al. 2006). The efficiency of
phytoextraction, besides biomass production, depends on the metal bio-
concentration factor (BCF – the plant to soil concentration ratio) and for Zn and
Cd it decreases log-linearly with soil metal concentration (Zhao et al. 2003).
Moreover, the phytoremediation potential differs between different populations
of T. caerulescens. The southern French ecotype showed a higher ability to