Page 135 - Plant-Based Remediation Processes

7 Use of Wetland Plants in Bioaccumulation of Heavy Metals 125

they are precipitated as insoluble forms or bound within the crystalline mineral
lattice (Gambrell 1994). Therefore, the uptake of contaminants and rhizosphere
actions depend upon the state of metals and also the capability of the particular
plant and its root characteristics (Gleba et al. 1999; Salt et al. 1998; Williams 2002;
Prasad 2004). Several researchers have explored the contaminant uptake and its
method by plants which can help to optimize the factors to improve the perfor-
mance of plant uptake. In a polluted environment, plants may act both as
“accumulators” and “excluders”. Accumulators survive despite concentrating
contaminants (biodegrade or biotransform into inert forms) in their aerial tissues.
The excluders restrict contaminant uptake into their biomass (Tangahu et al. 2011).
However, for both types of the plants, the common mechanisms involved in the
uptake, translocation, and storage of toxic elements are aided by plant-produced
chelating agents and plant induced pH changes and redox reactions. The range of
known transport mechanisms or specialized proteins embedded in the plant cell
plasma membrane involved in ion uptake and translocation include proton pumps
(-ATPases that consume energy and generate electrochemical gradients), co- and
anti-transporters (proteins that use the electrochemical gradients generated by
-ATPases to drive the active uptake of ions), and channels (proteins that facilitate
the transport of ions into the cell). Each transport mechanism is likely to take up a
range of ions (Tangahu et al. 2011). However, after uptake, transportation of metal
ions to the shoots is desirable, which will help to harvest the plant biomass (Salido
et al. 2003). Avoiding metal toxicity is the interesting property of the metal-
accumulators having higher concentration stored within the body. Multiple
mechanisms are involved for this purpose like storage contaminants in the vacuole
or the process of evapotranspiration that helps in moving contaminants into the
plant shoots. Translocation and accumulation of contaminants in plant shoots are
desirable as shoots can be harvested from time to time, while leaving the original
soil undisturbed. Usually hyperaccumulators thrive in the metal infested wetlands,
require little maintenance and produce high biomass, although few plants perfectly
fulfill these requirements (Salido et al. 2003). It has been reported by Tangahu and
his coworkers that hyperaccumulator plant species can concentrate heavy metals
like Cd, Zn, Co, Mn, Ni, and Pb up to 100 or 1,000 times those taken up by
nonaccumulator (excluder) plants (Tangahu et al. 2011). However, to mobilize
metal ions and increase the bioavailable fractions to plants, microorganisms like
bacteria and fungi, living closely associated in the rhizosphere significantly con-
tribute towards this action (Tangahu et al. 2011).

7.8 Role of Rhizobium of Wetland Plants

As mentioned elsewhere, wetlands mostly contain anoxic sediments. Root zone of
many of the wetland plants have the capability to mobilize and uptake metals from
the anoxic area by either oxidizing the sediments through the movement of oxygen
downwards through aerenchyma tissue (Moorhead and Reddy 1988)orby