PIPs (plasma membrane intrinsic proteins) also regulate Si levels in the cell under
abiotic stress conditions in roots of Sorghum plants (Liu et al. 2014, 2015). Salinity
stress often disrupts the hydraulic conductance of roots. Recent studies on sorghum
reveal that hydraulic conductance can be restored by exogenous application of Si
(Liu et al. 2014, 2015). Morphologically, Si application revived suppressed lateral
root growth and enhances the mechanical strength of the primary root (Liu et al.
2019).
Some other major consequences of salinity stress include reduction in photosynthetic efficiency, diminished stomatal conductance and level of transpiration, and
lastly, damaged photosynthetic apparatus (photosystem I and II) (Hetherington and
Woodward 2003; Gupta and Huang 2014; Yang et al. 2015). Recent studies in wide
varieties of plants such as sorghum, maize, tomato, tobacco, and pumpkin have
shown that application of Si not only improves stomatal conductance in plants but
also elevates the capacity of leaves to fix CO 2 (Parveen and Ashraf 2010; Nabati
et al. 2013; Hajiboland and Cheraghvareh 2014; Hu et al. 2014; Li et al. 2015).
Stomatal malfunction under salt stress conditions leads to a loss in levels of reduced
CO 2 and disrupts the process of gaseous exchange. Thus, exogenously applied Si
repairs stomatal conductance (Hetherington and Woodward 2003; Abbas et al. 2015;
Parveen and Ashraf 2010). Similarly, in salt-stressed leaves of Capsicum annuum,
stomata remained open when supplemented with exogenous Si (Manivannan et al.
2016). Recent reports also show that salinity is deleterious to photosystem I and II of
tomato plants. PS I and II can be revived by foliar application of Si (Mateos-Naranjo
et al. 2015). Similarly, Si application also improves pigment quality and efficiency in
PS II of C4 grass, Spartina densiflora under salt stress (Gorbe and Calatayd 2012;
Oukarroum et al. 2015; Mateos-Naranjo et al. 2015). Si exhibits the same mitigating
effects as 24-Epibrassinolide on Brassica juncea under salt stress (Siddiqui et al.
2018). Si (Na 2 SiO 3 ) has also been noted to carry out biofortification and reduce
water loss in salt-tolerant and -sensitive cultivars of rice. The impact of Si application
has been found to vary with varying sensitivities of salt (Das et al. 2018).
2.4.3 Heavy Metal Stress
Silicon has the ability to restrict and ameliorate heavy metal toxicity by several
mechanisms. It increases the rate of chelation in cells via stimulation of plant root
exudates which play a role in limiting uptake of heavy metals (Adrees et al. 2015). It
can quench the free heavy metallic ions from its apoplastic region which results in
reduced translocation (Adrees et al. 2015). Biosilicification is another siliconmediated tolerance mechanism wherein silicic acid undergoes polymerization in
the apoplast and a barrier of amorphous Si is formed which prevents penetration
of toxic heavy metals such as aluminum (Al), manganese (Mn), cadmium (Cd), zinc
(Zn), arsenic (As), and sodium (Na) into symplast or water transportation stream
(Ma et al. 2015; Exley 2015; Guerriero et al. 2016). Another mechanism of
counteracting heavy metals is via lignification. Lignified cell walls are good metal
binders and therefore prevent metal movement from roots to plant aerial tissue
26
S. Mehta et al.
abiotic stress conditions in roots of Sorghum plants (Liu et al. 2014, 2015). Salinity
stress often disrupts the hydraulic conductance of roots. Recent studies on sorghum
reveal that hydraulic conductance can be restored by exogenous application of Si
(Liu et al. 2014, 2015). Morphologically, Si application revived suppressed lateral
root growth and enhances the mechanical strength of the primary root (Liu et al.
2019).
Some other major consequences of salinity stress include reduction in photosynthetic efficiency, diminished stomatal conductance and level of transpiration, and
lastly, damaged photosynthetic apparatus (photosystem I and II) (Hetherington and
Woodward 2003; Gupta and Huang 2014; Yang et al. 2015). Recent studies in wide
varieties of plants such as sorghum, maize, tomato, tobacco, and pumpkin have
shown that application of Si not only improves stomatal conductance in plants but
also elevates the capacity of leaves to fix CO 2 (Parveen and Ashraf 2010; Nabati
et al. 2013; Hajiboland and Cheraghvareh 2014; Hu et al. 2014; Li et al. 2015).
Stomatal malfunction under salt stress conditions leads to a loss in levels of reduced
CO 2 and disrupts the process of gaseous exchange. Thus, exogenously applied Si
repairs stomatal conductance (Hetherington and Woodward 2003; Abbas et al. 2015;
Parveen and Ashraf 2010). Similarly, in salt-stressed leaves of Capsicum annuum,
stomata remained open when supplemented with exogenous Si (Manivannan et al.
2016). Recent reports also show that salinity is deleterious to photosystem I and II of
tomato plants. PS I and II can be revived by foliar application of Si (Mateos-Naranjo
et al. 2015). Similarly, Si application also improves pigment quality and efficiency in
PS II of C4 grass, Spartina densiflora under salt stress (Gorbe and Calatayd 2012;
Oukarroum et al. 2015; Mateos-Naranjo et al. 2015). Si exhibits the same mitigating
effects as 24-Epibrassinolide on Brassica juncea under salt stress (Siddiqui et al.
2018). Si (Na 2 SiO 3 ) has also been noted to carry out biofortification and reduce
water loss in salt-tolerant and -sensitive cultivars of rice. The impact of Si application
has been found to vary with varying sensitivities of salt (Das et al. 2018).
2.4.3 Heavy Metal Stress
Silicon has the ability to restrict and ameliorate heavy metal toxicity by several
mechanisms. It increases the rate of chelation in cells via stimulation of plant root
exudates which play a role in limiting uptake of heavy metals (Adrees et al. 2015). It
can quench the free heavy metallic ions from its apoplastic region which results in
reduced translocation (Adrees et al. 2015). Biosilicification is another siliconmediated tolerance mechanism wherein silicic acid undergoes polymerization in
the apoplast and a barrier of amorphous Si is formed which prevents penetration
of toxic heavy metals such as aluminum (Al), manganese (Mn), cadmium (Cd), zinc
(Zn), arsenic (As), and sodium (Na) into symplast or water transportation stream
(Ma et al. 2015; Exley 2015; Guerriero et al. 2016). Another mechanism of
counteracting heavy metals is via lignification. Lignified cell walls are good metal
binders and therefore prevent metal movement from roots to plant aerial tissue
26
S. Mehta et al.