Environmental Engineering Reference
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including (i) the excretion of metal-chelating
molecules (phytosiderophores) that solubilise
metals, (ii) the plasma membrane-bound metal
reductases that reduce metal ions and (iii) the
excretion of protons (Salt et al.
1995
; Meers
2005
). Solubilised metal ions may enter the roots
by extracellular (apoplastic) or intracellular
(symplastic) pathways. To enter the xylem, met-
als must cross the Casparian strip that divides the
endodermis from the epidermis. Metals must
move symplastically to cross this strip. The sym-
plastic transport within the endodermis is a rate-
limiting step. Xylem cell walls have a high cation
exchange capacity which would severely retard
the movement of metal cations to the shoot.
Metals in the xylem and phloem may be trans-
ported chelated to organic acids, phytochelatins
or metallothioneins (Raven et al.
1992
; Salt et al.
1995
).
Non-linear removal kinetics of heavy metals
have been reported, suggesting that rooted
aquatic macrophytes use several removal mecha-
nisms. Sorption by the root is probably the fastest
component of metal removal including processes
such as physical adsorption and chemisorption
(chelation and ion exchange). Biological pro-
cesses including intracellular uptake and translo-
cation to the shoots and root-mediated
precipitation are probably responsible for the
slower components of metal removal (Salt et al.
1995
; Maine et al.
2004
). Studies in many natural
and constructed wetlands have demonstrated
similar allocation patterns of metals in rooted
emergent macrophyte species, with most plants
having higher concentrations of metals in their
belowground biomass than in their shoot tissues
(Weis and Weis
2004
). Restricted translocation
of metals to the shoots is at the basis of this
finding.
merged and floating aquatic plants, in particular
about the relative importance of shoot/root
uptake. Roots of submerged aquatic macrophytes
were initially thought to act as holdfasts, whereas
element uptake was mainly through the leaves.
This was suggested by early studies which
showed a good correlation between metal con-
centrations in the leaves and the surrounding
water. For aquatic macrophytes that have roots
but do not have a close association with the sedi-
ment, the water is probably the principal source
of elements. However, the situation for sub-
merged macrophytes with a well-developed root-
rhizome system and submerged foliage such as
Myriophyllum
and
Potamogeton
is much more
complex (Guilizzoni
1991
). Studies with radio-
active tracers showed that elements are mainly
taken up by the roots from the sediment (Jackson
1998
). Guilizzoni (
1991
) hypothesised that the
water is the dominant source of elements when
dissolved metal concentrations in the water are
high or when metals are not readily available in
the sediment. This hypothesis was supported by
Maine et al. (
2004
) who studied the causes of the
increase of the Cr concentration in the shoots of
the floating macrophytes
Pistia stratiotes
and
Salvinia herzogii
, when exposed to
Cr-contaminated solutions. In an attempt to
address whether (i) translocation from the roots
to the shoots or (ii) direct uptake by the shoots
through contact with the solution was the respon-
sible mechanism, the experimental set-up
included plants of which the aerial parts were
exposed to the water and plants of which the aer-
ial parts were separated from the water by means
of thin sheets of polystyrene. The increase of the
Cr concentration in the shoots was mainly attrib-
uted to the direct contact between the shoots and
the water and not by translocation. Guilizzoni
(
1991
) suggested that the uptake of ions by sub-
merged leaves involves a binding step to the cell
membrane and a transfer inside. Cations enter the
abaxial epidermis by means of a multiple mecha-
nism of passive (diffusion) or active uptake. After
uptake, metals are translocated in different plant
parts. It still remains unclear what factors govern
root or shoot absorption and by what mechanisms
the translocation occurs.
4.3.2 Uptake of Heavy Metals
by the Shoots
For floating or submerged macrophytes, plants of
which the shoots are in close contact with the
water or are completely inundated, roots are not
the sole entry points of metals. According to
Guilizzoni (
1991
) and Keskinkan (
2005
), there is
scarce study about heavy metal uptake by sub-
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