Table 2.

Effect of exogenous Si on plant stress tolerance mechanisms in various plant species under salinity stress.

Plant name	Source of silicon	Proposed Si-mediated tolerance mechanisms	Reference
Triticumaestivum L.	Potassium silicate	The results suggest that Si application hinders the uptake of Na+ and reduces the accumulation of proline, which could be due to the interaction of Si with Na+ uptake and proline accumulation. Hence, Si regulates the uptake of micro- and micronutrients under salinity stress.	Ibrahim et al. (2016)
Lycopersicon esculentum	Potassium silicate	The higher water levels in Si-treated plants could explain the higher plant growth and could be related to salt dilution within the plant and the consequent mitigation of salt toxicity effects.	Romero-Aranda et al. (2006)
Cucumis sativus L.	Sodium metasilicate	Supplementation of exogenous Si increases the accumulation of polyamines such as spermidine and spermine in cucumber plants. The enhanced polyamine accumulation with silicon application might play a role in modulating the antioxidant defense system and reducing oxidative stress, thus increasing the salt tolerance of cucumber plants.	Yin et al. (2019)
Puccinellia distans	Sodium metasilicate	The results suggest that Si application increases the levels of osmoregulatory organic solutes and reduces Na+ in sensitive tissue. Furthermore, Si improves plasma membrane activity via lower electrolyte leakage possibly through greater H+-ATPase activity, which could assist in Na+ secretion and exclusion from sensitive tissues. Si also increases the biosynthesis of lignin and cellulose levels, which could also facilitate Na+ secretion and exclusion.	Soleimannejad et al. (2019)
Triticum aestivum L.	Sodium metasilicate	In this study, the authors propose that improved growth in Si-treated plants can be attributed to reduced Na+ uptake, its restricted translocation to the shoots, and enhanced K+ uptake.	Tahir et al. (2011)
Helianthus	Sodium metasilicate	To alleviate the negative effects, silicon positively affects the uptake of nitrogen and antioxidant enzymes.	Conceição et al. (2019)
Foeniculum vulgar mill.	Sodium metasilicate	Silicon treatment improves the translocation of minerals, and the higher tolerance of salinity is believed to be associated with lower sodium concentrations and higher potassium concentrations.	Rahimi et al. (2012)
Rosa hybrida	Potassium silicate	Si increases tolerance by augmenting root hairs, which increase water uptake and consequently mitigates the osmotic imbalance. Si also hinders the uptake of Na+. In addition, Si boosts the antioxidant machinery, which could also be a reason for the increased tolerance in Si-treated plants.	Soundararajan et al. (2018)
Triticum aestivum cv.	Sodium metasilicate	The suppression effect of salinity stress was alleviated by exogenous Si by increasing the activity of antioxidant enzymes and by restoring the nutrient balance and osmotic potential.	Saleh et al. (2017)
maize	Metasilicic acid	The author suggests that silicon treatment improves growth mainly because of changes in ion accumulation, the enhancement of photosynthesis, and the regulation of antioxidant defense systems enzymes.	Khan et al. (2018)
Cicer arietinum L.	Potassium silicate	Exogenous application of Si hinders the uptake of Na+ and significantly improves the K+/Na+ ratio.	Garg and Bhandari (2016)
Cucumis sativus L.	Sodium silicate	Silicon improves transpiration rates and leaf water levels by maintaining the water balance. The study also suggests that silicon-mediated changes in root morphology may also account for the increased water uptake of silicon-treated plants.	Wang et al. (2015b)
Solanum lycopersicum	Metasilicic acid	Exogenous Si reduces the uptake of Na+ and Cl- and boosts the antioxidant machinery in the roots of tomato, which facilitates root growth and hydraulic conductance, and thus improves the water status in the leaves.	Li et al. (2015)
Wheat	Calcium silicate	Si reduces the concentration of Na+ in wheat leaves. Hence, hindering Na+ uptake is a good indicator of salt tolerance in plants.	Ali et al. (2009)
Glycine max L.	Sodium metasilicate	Exogenous Si hinders the uptake of Na ions. Furthermore, the study demonstrates the interaction of Si with plant stress-related hormones. In this study, exogenous Si enhances the biosynthesis of ABA while reducing jasmonic acid biosynthesis. The regulation of these hormones under salinity stress is a possible reason for Si-based tolerance.
Glycine max L.	Silicic acid	The results suggest that Si can increase the level of endogenous gibberellin and jasmonic acid while reducing salicylic acid. Hence, it is clear from this study that exogenous Si improves the tolerance of plants by regulating the biosynthesis of stress-related phytohormones.	Hamayun et al. (2010)
Poa pratensis L.	Sodium metasilicate	Silicon enhances leaf erection, which facilitates light penetration and promotes photosynthesis by significantly lowering the production of ethylene, which destroys chlorophyll and reduces plasma permeability.	Bae et al. (2012)
Abelmoschus esculentus L.	Silicic acid	Silicon confers salt tolerance on okra, possibly by enhancing the water status, improving antioxidant activity, and enhancing nitrogen metabolism.	Abbas et al. (2017)
Triticum aestivum L.	Calcium silicate.	The application of Si helps wheat plants to absorb high amounts of K+ and hinder the uptake of Na+ or its translocation.	Tahir et al. (2006)
Oriza Sativa L.	Sodium silicate	Silicon effectively reduces sodium ion transportation within the plant. It is also found that the reduction in silicon occurs not via transpiration but from reduced soil transport.	Yeo et al. (1999)
Physalis peruviana L.	Silicic acid	Silicon can act by increasing the capture of CO2 and maintaining the photosynthetic rate by increasing the stomatal density of the leaf. Silicon promotes the increase of this variable, indicating that it contributes to the reestablishment of stomata, reaching a number similar to the control.	Rezende et al. (2018)
Acacia gerrardii Benth	Potassium silicate	Silicon application improves the tolerance of Acacia gerrardii to salinity stress by improving the activity of both the enzymatic and non-enzymatic antioxidant defense systems. Si also reduces lipid peroxidation by enhancing the production of proline and glycine betaine.	Al-Huqail et al. (2017)
Borago officinalis L.	Sodium silicate	The addition of Si improves stress tolerance via various mechanisms such as improving the water status and efficiency of photosynthesis, increasing the production of proline while reducing that of glycine betaine, improving the antioxidant machinery, and reducing the uptake, transportation, and accumulation of sodium ions in sensitive tissue.	Torabi et al. (2015)
Cucurbita pepo L.	Potassium silicate	Exogenous Si application improves plant growth parameters by improving net photosynthesis by specifically hindering Na+ and Cl- uptake and translocation to sensitive plant tissues, hence enhancing tolerance to salinity.	Savvas et al. (2009)
Hordeum vulgare L.	Potassium silicate	The presence of Si reduces the uptake of Na+ ions from the roots to shoots. Thus, Si-enhanced salt tolerance is associated with the selective uptake and transport of potassium and sodium by plants.	Zhu and Gong (2014)
Ajuga multiflora	Silicic acid	The addition of Si to the shoot induction medium significantly increases shoot induction. Thus, Si appears to promote shoot regeneration by altering the activity of antioxidant enzymes.	Sivanesan and Jeong (2014)
Oryza sativa L.	Sodium silicate	Exogenous Si improves tolerance by decreasing the sodium ion concentration in leaves.	Gong et al. (2006)
Vicia faba cv.	Sodium silicate	Si salt enhances stress tolerance by reducing Na+ translocation and decreasing transpiration under salinity.	Shahzad et al. (2013)
Saccharum officinarum L.	Calcium silicate	The results conclude that Si selectively interacts with Na+, and thus reduces Na+ uptake and translocation from the roots to shoots.	Ashraf et al. (2010a)