Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 208

of TEs in the shoots of hyperaccumulator plants
render them less palatable to pathogen and herbi-
vores, thus reducing the possibility of their attacks
and stimulating a defence against them. Despite
the numerous studies supporting this hypothesis,
more information is required since only a few taxa
and a limited number of TEs have been analysed.
One of the key factors for the success of
phytoextraction is the high uptake of TEs in the
above-ground biomass, and that is why hyperac-
cumulators have been widely studied for the
commercial development of phytoextraction
technologies. The ability of hyperaccumulators
to take up high amount of TEs depends on two
unique traits, such as the constitutive up-
regulation of transmembrane metal transporter,
which confers faster and effective root-to-shoot
translocation of TEs, and hypertolerance, which
is mostly dependent on effective detoxifi cation
and storage of TEs in leaf cell vacuoles. Another
very important characteristic of hyperaccumula-
tors relative to normal crops is a greater ability to
take up TEs from the soil.
It has been shown that hyperaccumulators
absorb metals from the same labile pool in soils
as normal plant species; however, crop plants
cannot absorb high amounts of TEs to support
phytoextraction. Despite the recent advance-
ments in understanding the physiological mech-
anisms of metal uptake and translocation to
shoot (Milner and Kochian 2008 ), no mecha-
nisms are yet known where hyperaccumulator
plants can attack the non-labile pool of metals in
soils. Centofanti et al. ( 2012 ) investigated
whether the Ni hyperaccumulator Alyssum cor-
sicum possess distinct extraction mechanisms
for different Ni species present in soils, as they
have different solubility and potential bioavail-
ability to roots. Their study showed that Ni
uptake is related to Ni solubility and plant tran-
spiration rate. The authors also suggested that
Ni uptake is driven by convection, which
depends on the initial concentration of Ni in
solution and the plant transpiration rate. Metals
enter the roots via uptake of the soil solution,
which is then transferred to the stems and leaves
and lost via transpiration. High metal concentra-
tion in the roots can result from plant water

uptake inducing metal migration via mass fl ow
(Zhao et al. 2000 ). Hyperaccumulators have the
ability to translocate the absorbed metals from
the roots to the shoot and store them in the leaf
cell vacuoles (Broadhurst et al. 2004 , 2009 ;
Tappero et al. 2007 ).
The ability of plants, and of trees in particular,
to pump large amount of water and solutes has
been used to decrease the downward movement
of solutes and leaching into the groundwater and
to stabilise and break down contaminants in soil
and groundwater. A successful example of boron
(B) phytoextraction has been reported by
Robinson et al. ( 2003a , 2007 ) where high water-
use poplar trees were used to evapotranspire
water, control leaching, and remove B from the
site by coppicing the trees that accumulated sig-
nifi cant amount of B in their leaves. Fast-growing
and metal-resistant trees (i.e. Salix spp.) have two
advantages relative to hyperaccumulator plants:
(1) extracting more metals from the soil because
of their large biomass in both above and below
ground (Pulford and Watson 2003 ) and (2) stabi-
lising the metals in the soil and reducing soil ero-
sion by wind and water.
The low biomass production is the major
limitation to the commercial development of
phytoextraction using hyperaccumulator plants.
Most hyperaccumulator species have a small size
and usually small leaf area, thus producing little
biomass compared to crop plant and trees.
Robinson et al. ( 2003b ) suggested a model to cal-
culate the time needed for phytoextraction to
lower the contaminant concentration in soil to the
level required by environmental regulation.
Phytoextraction is a time-consuming process
because it is dependent on biomass production
and ability of the plant to take up metals that is a
function of root exposure to bioavailable TEs.
Low biomass production results in low evapo-
transpiration, which affects the uptake and trans-
location of metals from the soil solution. In
addition, the distribution of TEs in the soil profi le
is heterogeneous, and the plant roots might not
have access to the TE 'hot spots'. Furthermore,
hyperaccumulator are metal specifi c and can only
extract high concentrations of one contaminant.
Therefore, remediation of a site polluted with