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Therefore, Si-mediated tolerance of Mn in cucumber is a consequence of stronger binding of Mn to cell walls and a lowering of the Mn concentration within the symplast. In other reports, alleviation of Mn toxicity by Si in cucumber was attributed to a significant reduction in membrane lipid peroxidation caused by excess Mn and to a significant increase in enzymatic (e.g., SOD, APX, and GR) and nonenzymatic antioxidants (e.g., ascorbate and GSH) (Shi et al. 2005a).

14.6.3.2 Aluminum Toxicity

Interactions of Al and Si has been studied in some plant species and alleviating effects of Si on Al toxicity has been reported for sorghum, barley, maize, and soybean (Liang et al. 2007). It has been established that interaction between Si and Al and reduction of the activity of toxic metal ions in the medium, is one possible external mechanism for the detoxification of Al toxicity by Si (Hiradate et al. 1998). The precipitation of subcolloidal, inert hydroxyaluminosilicate species seems to be responsible for the diminished concentration (activity) of phytotoxic Al in solution. The codeposition of Si with Al seems to occur not only in the growth media but also within plants (Liang et al. 2001). It has been reported that added Si increased the shoot Al concentration, which may arise from the formation of hydroxyaluminosilicate complexes in shoots leading to enhanced Al transport from roots to shoots (Birchall 1990). It has been also proposed that low-solubility aluminosilicates or hydroxya-luminosilicates (or both) are formed within the root cell wall (apoplastic) space, thereby reducing the concentration of free, toxic Al3+ ions (Cocker et al. 1998) . Maize plants exposed to toxic Al concentrations were less inhibited in their growth in the presence of Si that was attributed mainly to the inhibitory effect of Al uptake by the plants because of higher concentrations of malice and formic acids in the presence of added Si, thus a purely internal mechanism related to the physiological processes within plants (Corrales et al. 1997) . Silicon may have additional roles in increasing tolerance of Al by mediating the metabolism of phenolic compounds as it has been reported that silicon-treated plants release 15 times more phenolics than untreated maize plants (Kidd et al. 2001). These flavonoid-phenolics (i.e., catechin and quercetin) have a strong Al-chelating ability and may provide metal tolerance in plants.

14.6.3.3 Cadmium Toxicity

It is generally recognized that an external mechanism similar to Si-alleviated toxicity to Al applies to Si-mediated detoxification of Cd in soil/plant systems, i.e., reduction in Cd availability via Cd immobilization arising from a rise in pH. This is true when Na metasilicate, slag or alkaline Si-containing materials such as biosolids are incorporated into Cd-contaminated soils as Si sources (Chen et al. 2000). Moreover, more Cd was found to be in the form of specific adsorbed or Fe-Mn oxides-bound fraction in the Si-amended soil. These results suggest an external interaction between Si and Cd (Liang et al. 2005). Other investigations suggested an internal mechanism for detoxification of Cd by Si occurring within plants. The role of Si in minimizing uptake and root-to-shoot transport of metal ions has recently been confirmed in seedlings of rice grown with toxic Cd (Shi et al. 2005b) and with arsenate (Guo et al. 2005). It has been suggested that (Shi et al. 2005b), the heavy deposition of silica in the vicinity of the endodermis might offer a possible mechanism by which silicon did at least partially physically block the apoplast bypass flow across the roots, and restrained the apoplastic transport of Cd. Cadmium concentration in the xylem exudates was significantly decreased in the Si-amended Cd treatments and Cd was bound to the root cell walls but less to cytosols or symplast in +Si plants than in -Si plants under Cd stress, suggesting a root apoplastic role of Si in detoxification of excess Cd (Liang et al. 2007). a mechanism similar to that responsible for the Si-mediated Mn tolerance in plants (Rogalla and Romheld 2002).

14.6.3.4 Other Heavy Metals

Using electron-energy-loss spectroscopy and other techniques, Neumann and zur Nieden (2001) have reported the occurrence of codeposition of silicon and Zn in heavy metal tolerant Cardaminopsis halleri. The formation of

Zn-silicate is part of the mechanism for tolerance to heavy metals and may be responsible for the amelioration of Zn toxicity in Cardaminopsis halleri.

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Body Detox Made Easy

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