(Ma and Yamaji 2006; Ye et al. 2012). Silicon holds the unique ability to form
complexes with metal ions in the cell wall and eventually, forming a precipitate of
metal ions as co-factors (Pontigo et al. 2017). Silicon can react to form silicates and
oxides with heavy metals (Exley 2015) keeping the toxicants out of any plant
metabolic process (Exley 2015). Application of silicon to the soil is beneficial
since it balances the disrupted soil and immobilizes heavy metals like Cd making
them unavailable to plants (Wu et al. 2013).
2.4.3.1 Cd Toxicity
Presence of Cd in soil inhibits root growth of rice plants. The toxicity can be
identified by the appearance of black spots in the cortex and pericycle of roots
(Kim et al. 2014). In wheat and maize crops, it affects seed germination, nutrient
content, and lowers shoot and root length. (Ma et al. 2015). In barley, photosynthetic
apparatus, pigments, and lipids are affected by Cd-induced toxicity (Hodson et al.
2005). Heavy metal stress studies have revealed that silicon has the ability to
decrease cadmium uptake and further limits its translocation to plant aerial tissue
like shoots. Cd and Mn are often precipitated on the epidermis of the shoot or leaf
blade by forming amorphous silica (Ma et al. 2015). Cd is often compartmentalized
in root cell walls by Si, leading to its lowered accumulation in shoots of rice (Bhat
et al. 2019). In maize plants, Cd forms colloidal silicon embedded in the cell walls to
prevent its uptake or transport to the aerial parts (Bhat et al. 2019). Similarly, the
application of Si to Poa annua seedlings imparts tolerance to cadmium toxicity
(Zama et al. 2018).
2.4.3.2 As Toxicity
Arsenic toxicity is majorly seen in rice and spinach plants. In rice plants, it is
overcome by competition with other heavy metal ions at the point of entry/site of
uptake in roots. Dry biomass of leaves is regulated positively upon the application of
Si to spinach plants. A subsequent increase in levels of glutaredoxin (GRX) is also
noted (Dubey et al. 2018). Si-biochars are components composed by coupling
bamboo with Si. The element has been used to reduce bioaccumulation of arsenic
in spinach leaves by ~38% (Li et al. 2017) (Table 2.1).
2.4.3.3 Al Toxicity
Recent studies have revealed that silicon has the ability to regulate malic and formic
acid formation in plants. The formation of these cellular byproducts is helpful in
regulating uptake of aluminum (Pontigo et al. 2017). Phenolic compounds of maize
have also been investigated in relation to their ability to reduce Al-uptake (Adrees
et al. 2015). Si often complexes with Al to form Si–Al or aluminum silicate localized
in the plant cell wall, primarily in epidermis and hypodermis. Complex formation
makes toxic Al unavailable to the plants (Horst et al. 2010; Liu et al. 2013). Another
mechanism to combat Al toxicity is to form hydroaluminosilicates in root apoplast,
thereby, reducing mobility of noxious Al (Rogalla and Romheld 2002).
2 Silicon: A Plant Nutritional “Non-Entity” for Mitigating Abiotic
27
complexes with metal ions in the cell wall and eventually, forming a precipitate of
metal ions as co-factors (Pontigo et al. 2017). Silicon can react to form silicates and
oxides with heavy metals (Exley 2015) keeping the toxicants out of any plant
metabolic process (Exley 2015). Application of silicon to the soil is beneficial
since it balances the disrupted soil and immobilizes heavy metals like Cd making
them unavailable to plants (Wu et al. 2013).
2.4.3.1 Cd Toxicity
Presence of Cd in soil inhibits root growth of rice plants. The toxicity can be
identified by the appearance of black spots in the cortex and pericycle of roots
(Kim et al. 2014). In wheat and maize crops, it affects seed germination, nutrient
content, and lowers shoot and root length. (Ma et al. 2015). In barley, photosynthetic
apparatus, pigments, and lipids are affected by Cd-induced toxicity (Hodson et al.
2005). Heavy metal stress studies have revealed that silicon has the ability to
decrease cadmium uptake and further limits its translocation to plant aerial tissue
like shoots. Cd and Mn are often precipitated on the epidermis of the shoot or leaf
blade by forming amorphous silica (Ma et al. 2015). Cd is often compartmentalized
in root cell walls by Si, leading to its lowered accumulation in shoots of rice (Bhat
et al. 2019). In maize plants, Cd forms colloidal silicon embedded in the cell walls to
prevent its uptake or transport to the aerial parts (Bhat et al. 2019). Similarly, the
application of Si to Poa annua seedlings imparts tolerance to cadmium toxicity
(Zama et al. 2018).
2.4.3.2 As Toxicity
Arsenic toxicity is majorly seen in rice and spinach plants. In rice plants, it is
overcome by competition with other heavy metal ions at the point of entry/site of
uptake in roots. Dry biomass of leaves is regulated positively upon the application of
Si to spinach plants. A subsequent increase in levels of glutaredoxin (GRX) is also
noted (Dubey et al. 2018). Si-biochars are components composed by coupling
bamboo with Si. The element has been used to reduce bioaccumulation of arsenic
in spinach leaves by ~38% (Li et al. 2017) (Table 2.1).
2.4.3.3 Al Toxicity
Recent studies have revealed that silicon has the ability to regulate malic and formic
acid formation in plants. The formation of these cellular byproducts is helpful in
regulating uptake of aluminum (Pontigo et al. 2017). Phenolic compounds of maize
have also been investigated in relation to their ability to reduce Al-uptake (Adrees
et al. 2015). Si often complexes with Al to form Si–Al or aluminum silicate localized
in the plant cell wall, primarily in epidermis and hypodermis. Complex formation
makes toxic Al unavailable to the plants (Horst et al. 2010; Liu et al. 2013). Another
mechanism to combat Al toxicity is to form hydroaluminosilicates in root apoplast,
thereby, reducing mobility of noxious Al (Rogalla and Romheld 2002).
2 Silicon: A Plant Nutritional “Non-Entity” for Mitigating Abiotic
27