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acidification of the rhizosphere by plant exudates (Doyle and Otte 1997). The
oxidation usually remobilizes the metal contaminants in the exchangeable form
(Avicennia species of mangroves) in wetland sediments (de Lacerda et al. 1993).
However, in the case of the plant Typha latifolia it is reported that, after oxidizing
the rhizosphere zone, decreased the pH within 1 cm of the roots and increased the
concentration of soluble zinc in and around the roots (Wright and Otte 1999).
Changes in sediment Eh and pH can cause changes in metal speciation, solubility,
and flux. With an increase in redox potential and pH, Pb uptake into roots and
shoots of rice plants (Oryza sativa) decreased, while Cd uptake increased with a
decrease in pH and an increase in redox potential (Reddy and Patrick 1977). Under
dry (more oxidized) soil condition better availability and uptake of Cd was seen in a
number of wetland plant species (Gambrell 1994). The wetland plants having larger
and elaborated root system may indicate better efficiency to oxidize and mobilize
metals of anoxic sediments at rhizosphere level (Ravit et al. 2003).
Mobilization and rates of uptake of metal by plants also depends upon the
different forms (“species”) of the same metal. Diverse group of bacteria present
in the sediments of marsh lands and associated with plant roots have the capacity to
reduce the very toxic form of metals to less toxic one. As for example, reduction of
highly toxic Cr(VI) to the less toxic form, Cr(III) (Pardue and Patrick 1995),
methylate arsenic into volatile (e.g., methylarsines) or nonvolatile (e.g.,
methylarsonic acid and dimethylarsinic acid [DMAA]) (Bentley and Chasteen
2002), help the plant to mobilize the same within their tissue system. Few aquatic
plants like Ceratophyllum demersum and Elatine triandra are reported to synthe-
size lipid-soluble arsenic compounds to alleviate the toxicity of the arsenic (Tamaki
and Frankenberger 1992; Zheng et al. 2003). Roots were found to be the major site
of accumulation for inorganic arsenicals, while DMAA was readily translocated to
the shoots (Carbonell-Barrachina et al. 1998). It has been observed by several
workers that roots of several wetland plants carry metal-rich (5–10 times more
than surrounding sediments) rhizoconcretions or plaques composed mostly of iron
hydroxides and other metals like manganese that are mobilized and precipitated on
the root surface. These plaques are thought to act like a barrier for some metals but
cooperative for few others (Mendelssohn and Postek 1982; Vale et al. 1990; Sundby
et al. 1998; Ye et al. 1998; Weis and Weis 2004).

7.9 Role of Microbial Association/Symbiosis with Plant Root

Microbial association and symbiosis at the root zone or rhizosphere of the wetland
plants play an important role in the accumulation of metals. Many interesting studies
have been done in this aspect. It was reported that, when rhizosphere bacteria were
inhibited with antibiotics, plants accumulated lower concentration of metals; on the
contrary when grown axenically with added bacteria, accumulated more of these
metals than axenic controls (de Souza et al., 1999; Stout et al., 2010). Plants like
Scirpus robustus and Polypogon monspeliensis were found to accumulate lower