16.6.1 Salinity

The role of silicon in alleviating salt stress in plants has been studied widely in many plant species including T. aestivum (Ahmad et al., 1992; Saqib et al., 2008; Hashemi et al., 2010), Z. mays (Liang et al., 2003), L. esculentum (Al-Aghabary et al., 2004), O. sativa (Gong et al., 2006), C. sativus (Zhu et al., 2004), B. napus (Hashemi et al., 2010), S. officinarum (Ashraf et al., 2010a,b), and H. vulgare (Liang et al., 2003). In Spartina densiflora, exogenous Si (500 μM) improved growth parameters, relative growth rate, and leaf elongation rate under salinity (680 mM NaCl), which was associated with higher net photosynthetic rate, greater water-use efficiency, and balanced nutrient concentrations (Mateos-Naranjo et al., 2013). Wheat plants supplemented with 0.25 and 0.50 mM Na₂SiO₃ and subsequently exposed to 100 mM NaCl showed amelioration of the negative effects of salinity by improvements in dry matter, chl content, and Pro content (Tuna et al., 2008). Shi et al. (2013) showed that Si (3 mM) could modulate various physiological processes that improved the growth of O. sativa under saline conditions (NaCl 50 mM). In canola seedlings (cv. Okapi), Si (2 and 4 mM K₂SiO₃) application increased leaf area, leaf fresh weight, and seed yield during salt stress conditions (100, 200, and 300 mM NaCl) (Bybordi, 2012).

Al-Aghabary et al. (2004) suggested that Si enhanced the photochemical efficiency of tomato plants under salinity stress. Silicon could also improve plant defense systems and help to detoxify ROS induced by salt stress; this helps to increase chl and enhances the photochemical efficiency of photosystem II (PSII). At 150 mM NaCl, exogenously applied Si levels (0.8, 1.6, and 2.8 mM) in the rooting medium improved net CO₂ assimilation rate, g_s, transpiration, and leaf substomatal CO₂ in Z. mays, which was positively correlated with plant photosynthetic and growth attributes (Parveen and Ashraf, 2010). Barley root tonoplasts maintained integrity, stability, and functions following Si mediation of salt stress (Liang et al., 2005). In Spartina densiflora, Si (500 μM) application improved photosynthesis under 680 mM NaCl stress, which was confirmed by higher net photosynthetic rate and greater water-use efficiency. Si also exerted beneficial effects on the photochemical (PSII) apparatus and chl concentrations. Moreover, when compared to salt stress alone, Si supplementation of plants exposed to NaCl resulted in higher g_s that maintained higher intercellular CO₂. Silicon addition alleviated the adverse effects of salinity on quantum efficiency of PSII (Mateos-Naranjo et al., 2013). Addition of 3 mM silicate under 50 mM NaCl stress improved net photosynthetic rate, g_s, and transpiration in rice (Shi et al., 2013).

Motomura et al. (2002) indicated that the undesirable effects of Na during salt stress could be reduced by increasing the K/Na ratio in plants. In salt-stressed H. vulgare, Si was reported to activate the root plasma membrane H-ATPase pump, which helped to increase K conductivity and to improve the K/Na ratio (Liang et al., 2006a). Silicon also reduced the concentration of Na and Cl in H. vulgare (Inal et al., 2009) and grapevine (Soylemezoglu et al., 2009). One of the ways to maintain optimum cytosolic K⁺/Na⁺ ratio is via restriction of Na⁺ influx into root cells or into the xylem stream (Chinnusamy et al., 2005). Silicate crystals deposited in the epidermal cells create a barrier to ion movement and their balance is managed (Romero-Aranda et al., 2006; Ashraf et al., 2010a,b). In the endodermis and rhizodermis, deposition and polymerization of Si block Na⁺ influx through the apoplasmic pathway, as documented in the roots of O. sativa (Yeo et al., 1999). Silicate crystals deposited in the epidermal cells create a barrier to water loss through cuticles, which improves water relations in plant tissue in salt-stressed plants (Romero-Aranda et al., 2006). Supplementation of Si (1.0 mM) to the salt solution (120 mM NaCl) increased plasma membrane H⁺-ATPase activity and restored membrane fluidity levels in H. vulgare (Liang et al., 2006a). The activity of the plasma membrane H-ATPase was improved by Si, which significantly increased K⁺ uptake in barley under salt stress (Liang et al., 2003). Shoot K⁺/Na⁺ ratios in sugarcane were increased by 150 to 266% following Si treatment of saline growth medium, which boosted its tolerance to a great extent (Ashraf et al., 2010a). According to Mateos-Naranjo et al. (2013), Si may ameliorate nutrient imbalances under salinity stress conditions by maintaining higher concentrations of minerals in the tissues, specifically P. The K/Na ratio of leaves of Spartina densiflora was greater in Si-treated plants and these plants also had higher levels of essential nutrients (Si, Al, Cu, Fe, K, and P) in their tissues (Mateos-Naranjo et al., 2013).

Silicon was effective in improving the activity of antioxidative enzymes in different plants under salinity stress. In B. napus, Si (2 and 4 mM) increased growth and yield by improving photosynthesis, chl content, and enzyme activities including ascorbate peroxidase (APX) and nitrate reductase (NR) under salt stress (100, 200, and 300 mM NaCl) (Bybordi, 2012). Silicon improved Vitis vinifera L. shoot growth and reduced oxidative damage caused by salinity, as indicated by reduced membrane damage and H₂O₂ levels with associated elevation of catalase (CAT) and superoxide dismutase (SOD) activities. Silicon also decreased Pro content (Soylemezoglu et al., 2009). Addition of Si (1.0 mM Si) to the salt treatment (120 mM NaCl) increased the glutathione (GSH) concentration by 20% in a salt-tolerant (Jian 4) H. vulgare cultivar compared to 50% in a salt-sensitive (Kepin No. 7) cultivar, which supposedly maintained better membrane properties (Liang et al., 2006b). Grapevine (V. vinifera rootstocks: 41 B, 1103 P) treated with Si (4 mM Si) significantly reduced MDA: by 20% in 41 B, by 23% in 1103 P, due to increased modulation of CAT, SOD, and APX enzyme activities and Pro content (Soylemezoglu et al., 2009).

Several research results have shown that Se at low concentration provided protection to different plant species against salt stress. Kong et al. (2005) reported that low concentrations (1–5 μM) of Se stimulated growth and enhanced antioxidant enzyme (SOD and POD) activities in leaves of sorrel (R. patientia × R. tianshanicus) seedlings under salt stress. In contrast, at higher concentrations (10–30 μM), Se showed fewer beneficial effects. In C. sativus leaves, Se treatments (5–10 μM) increased the growth, synthesis of photosynthetic pigments, and Pro levels under salt stress (Hawrylak-Nowak, 2009). In our study, we observed beneficial effects of exogenous Se (25 μM Na₂SeO₄) in B. napus seedlings exposed to salt stress (100 and 200 mM NaCl; Hasanuzzaman et al., 2011; Tables 16.6 and 16.7). Selenium treatment had a synergistic effect: in salt-stressed seedlings, it increased the AsA and GSH contents; and the activities of CAT, APX, MDHAR, DHAR, GR, GST, and GPX, which in turn reduced levels of H₂O₂ and MDA when compared to plants exposed to salt stress alone (Tables 16.6 and 16.7). The phenotypic appearance was also better in the Se-treated seedlings (Figure 16.6). These results suggested that exogenous application at low concentration rendered the plants more tolerant to salt stress-induced oxidative damage by enhancing their antioxidant defense systems (Hasanuzzaman et al., 2011).

16.6.2 Drought

For drought tolerance, one of the most important and basic points is conservation of water within the plant body. Silicon could affect this strategy precisely and differently in various cases. Silicon deposition in cuticles or endodermal cells can reduce water loss. In stems and leaves, Si is deposited as hydrated silica (SiO₂•nH₂O) by following the evapotranspiration path (Sangster et al., 2001); thus, irrigation with Si reduces evapotranspiration (Gao et al., 2006). In O. sativa, Si deposits are 0.1 μM thick (Ma and Takahashi, 2002) and Si could reduce the transpiration rate by 30% (Ma, 2004). Polar monosilicic acid and polymerized Si(OH)₄ accumulated mainly in the epidermal cell walls and formed H-bonds between H₂O and SiO₂·nH₂O, which might prevent water loss (Liang et al., 2008). Kaya et al. (2006) found that Si (2 mM Na₂SiO₃) increased leaf relative water content (RWC) by 26.5% in water-stressed corn (50% of FC). In Z. mays, Si decreased leaf transpiration under drought, thereby improving leaf water status (Gao et al., 2006). Chen et al. (2011) found that applying 1.5 mM Si to rice under drought-stress conditions increased transpiration rate by 19% in a drought-susceptible line and 53% in a drought-resistant line.

Generally, compared with normal plants, drought-adapted plants have well-developed root systems through which they uptake even the slightest amount of water from deeper soils. In S. bicolor, Si fertilization significantly improved the drought tolerance by promoting the formation of a good root system and water uptake (Sonobe et al., 2011). Proline, a key solute for maintaining osmotic adjustment under drought stress, has been increased by Si application (Gunes et al., 2008; Crusciol et al., 2009). Crusciol et al. (2009) report that the Si increased potato plant Pro levels. Kaya et al. (2006) found that 2 mM Na₂SiO₃ decreased 43% of Pro content relative to non-treated drought plants. Pulz et al. (2008) found that the use of calcium and magnesium silicates markedly decreased stem lodging and increased potato plant height and tuber yield in drought conditions (−0.05 MPa water potential). Shen et al. (2010) observed a reduced Pro level in soybean plants subjected to PEG stress (−0.5 MPa). Sorghum plants treated with Si (200 ml L⁻¹) showed reduced drought damage and increases in leaf area index of 65%, specific leaf weight of 12%, leaf dry weight of 164%, shoot dry weight of 80%, root dry weight of 70%, and total dry weight of 75% (Ahmed et al., 2011). Silicon treatment under water deficit conditions resulted in 18, 17, 20, and 30% increases in dry mass in Z. mays, T. aestivum, G. max, and O. sativa (Janislampi, 2012).

Besides the positive effects on plant growth and development under drought, Si was effective in improving a number of physiological processes including photosynthesis, g_s, water relations, and nutrient translocation. Si (1.70 mM) addition to drought (−0.5 MPa) affected soybean increased net photosynthetic rate by 21% and g_s by 38%, while reducing transpiration by 29% after a week of drought, when compared to drought stressed plants. Increases in photosynthesis are supposedly associated with increases in activities of photosynthetic enzymes, chl content, and anthocyanin content observed in response to added Si (Shen et al., 2010). Silicon addition to S. bicolor increased g_s and transpiration rate and alleviated the photosynthetic reduction caused by drought (Hattori et al., 2005). In S. bicolor, drought caused significant damage to different growth parameters of roots, shoots, leaves, and overall growth, but addition of Si (200 ml L⁻¹) alleviated these negative effects by increasing chl contents by 52%, transpiration rate by 80%, and net photosynthetic rate by 62%; together, these changes resulted in a 75% higher total dry weight (Ahmed et al., 2011). Under drought stress, Si upholds the nutrient balance when compared to drought-stressed plants without Si (Gunes et al., 2008). Calcium and magnesium silicate fertilizer application increased P and Mn content and reduced N content in drought-stressed potato (Pulz et al., 2008). Sonobe et al. (2010) found that 1.78 mM Si (SiO₂) increased sorghum root amino acid content and decreased tissue Ca content under drought imposed for 23 days by 15% PEG-6000 (−0.6 MPa) (Janislampi, 2012).

Silicon can enhance the antioxidative machinery through effects on both its enzymatic and non-enzymatic components, thereby reducing drought-induced oxidative stress. Silicon enhanced the stability of lipids and protected the structural and functional integrity of cell membranes in rice plants against drought (Agarie et al., 1998). Supplementation of drought-stressed Cicer arietinum with Si significantly increased activity of some antioxidant enzymes (SOD, CAT, APX) in shoots as well as some non-enzymatic antioxidant activity, which reduced lipid peroxidation (MDA), lipoxygenase (LOX) activity, and Pro and H₂O₂ accumulation (Gunes et al., 2007a). Silicon (1.7 mM Si) application significantly reduced the H₂O₂ levels and thus reduced membrane damage as indicated by reduced lipid peroxidation and osmolyte leakage in drought (−0.5 MPa)-affected G. max plants. The probable reason for this alleviation of oxidative damage was modulation of SOD, CAT, and peroxidase (POD) activity induced by Si (Shen et al., 2010). Gong et al. (2005) also observed Si (2.11 mM Na₂SiO₃) increased SOD and CAT activity in wheat under drought, which alleviated oxidative damage and improved other indicators of physiological status. According to Gunes et al. (2008), Si applied to the soil reduced sunflower tissue H₂O₂ content.

Several plant studies that focused on the protective effects of Se in under drought stress indicated that the effects of Se are due to its ability to regulate the water status of plants under water deficit. Kuznetsov et al. (2003) reported that the addition of 0.1 or 0.25 mM Se caused a 2–6% increase in leaf water content, thereby increasing the drought resistance. The Se-induced improvement in leaf tissue water status was accompanied by a sharp (two- to four-fold) inhibition of stress-induced accumulation of Pro and a significant inhibition of POX activity (Kuznetsov et al., 2003). These results support the suggestion that Se exerts an antioxidant effect via a decreased concentration of intracellular ROS, which induces the de novo biosynthesis of Pro and POX production. The Se-induced effect on the content of intracellular water was also less pronounced at the optimal rate of water supply. In buckwheat (Fagopyrum esculentum), Tadina et al. (2007) observed that plants under water deficit exhibited significantly lower g_s, while Se supplementation significantly improved g_s. Selenium also improved the efficiency of PSII, which was attributed to the improvement in plant water status. Yao et al. (2009) suggested that low concentrations of Se (1.0, 2.0, and 3.0 mg Se kg⁻¹) favored the growth of T. aestivum seedlings under drought. In addition, Se supplementation increased the Pro, carotenoid, and chl content and the activities of antioxidant enzymes (namely, POD and CAT), which in turn decreased the MDA content. In B. napus affected by late season drought stress, foliar application of Se had significant and additive effects on vegetative and reproductive parameters. It increased plant height and improved pollen survival and fertilization, which resulted in higher numbers of pods and seeds; ultimately, the seed yield, biological yield, harvest index, and oil yield improved (Zahedi et al., 2009). While studying T. aestivum seedlings under drought conditions, Xiaoqin et al. (2009a) observed 12, 15, and 12% increases in the fresh weight of the shoot, root, and total biomass, respectively, following supplementation with exogenous Se (0.5 mg kg⁻¹). Selenium-induced drought tolerance was also indicated by the decrease in the shoot/root ratio and enhanced activities of POD and CAT (Xiaoqin et al., 2009a). In another study, Xiaoqin et al. (2009b) showed that exogenous Se (1.0 and 2.0 mg kg⁻¹) increased root activity, Pro content, and POD and CAT activities and reduced the MDA content of T. aestivum seedlings. Selenium application at 1.0 and 2.0 mg kg⁻¹ increased the chl (a+b) content by 26 and 32%, carotenoid content by 6 and 9%, and total biomass by 12 and 36%, respectively. Valadabadi et al. (2010) observed that drought stress markedly reduced the physiological and growth indices (namely, dry weight, leaf area index, relative growth rate, and crop growth rate) of B. napus, while application of Se (30 g L⁻¹) improved those indices and mitigated the stress-induced damage.

Wang et al. (2011) examined the effect of Se (5 μM Na₂SeO₄) on the AsA–GSH cycle in Trifolium repens seedlings subjected to PEG-induced water deficit. They observed that Se application decreased the contents of H₂O₂, TBARS, DHA, and GSSG; increased the levels of GSH and AsA; and inhibited the decreases of AsA/DHA and GSH/GSSG ratios when compared to control plants. Selenium supplementation significantly increased the activities of MDHAR, DHAR, and GR. Among the enzymes, GR showed the highest increase in activity compared to DHAR and MDHAR. In our laboratory, we studied the beneficial role of Se pretreatment (25 μM Na₂SeO₄, 48 h) in B. napus seedlings under drought stress (10 and 20% PEG-6000) (Hasanuzzaman and Fujita, 2011b). The seedlings exposed to drought stress showed significant increases in GSH and GSSG content; however, the AsA content increased only under moderate stress (Table 16.8). The MDHAR and GR activity increased only under moderate stress (10% PEG). The activities of DHAR, GST, and GPX significantly increased at all levels of drought, while CAT activity decreased (Table 16.9). Drought stress resulted in a marked increase in the levels of H₂O₂ and MDA (Table 16.8). In contrast, Se-pretreated seedlings exposed to drought stress showed a rise in AsA and GSH content, and up-regulated activities of CAT, APX, DHAR, MDHAR, GR, GST, and GPX when compared with the drought-stressed seedlings without Se. In turn, the Se-treated seedlings showed a considerable decrease in the levels of H₂O₂ and MDA and considerable alleviation of oxidative stress (Tables 16.8 and 16.9; Hasanuzzaman and Fujita, 2011b). Nawaz et al. (2013) found beneficial role of Se priming in conferring drought stress tolerance. In their experiment, seeds of T. aestivum were soaked in distilled water or Na₂SeO₄ solutions (25, 50, 75, and 100 μM) for 30 or 60 min, followed by re-drying and subsequent sowing. Priming with Se significantly increased root length, stress tolerance index, and total biomass of germinated seedlings.

16.6.3 High temperature

Reports on the effects of Si on HT stress are scare. However, some findings have indicated that Si exerts a beneficial effect under heat stress. Agarie et al. (1998) observed that relative to O. sativa plants subjected to heat stress alone, plants grown in 100 ppm SiO₂ showed significantly reduced heat stress symptoms, determined by a 2.5-fold reduction in stress-induced electrolyte leakage. This result suggested that Si might have beneficial effects in maintaining thermal stability of lipids in cell membranes, thereby helping to maintain structural and functional integrity of cell membranes. Besides decreasing the electrolyte leakage, Si addition also increased the levels of polysaccharide in the cell walls, indicated by an increased ratio of cell wall polysaccharide to total carbohydrate. Thus, Si also helped to improve the cell wall structure (Agarie et al., 1998). In O. sativa, Si supplementation increased the number and diameter of pollen grains relative to heat treatment (39°C) alone (Li et al., 2005). Silicon application resulted in a dramatic increase in partial dehiscence (135% higher) of the anthers of rice flowers relative to heat stress alone. This consequently developed into a marked increase in fully dehiscence anthers, which was 111% higher in plants in the Si-fertilized treatment when compared to plants exposed to heat stress alone. Anther cracking rate was increased by 130% and the stigma pollination probability was increased by 66%, which enhanced the fertilization of rice flowers (Li et al., 2005). In Euphorbia pulcherrima Willd. “Ichiban,” Si-treated (50 mg L⁻¹ Si, either as K₂SiO₃, Na₂SiO₃, or CaSiO₃) plants were more tolerant to HT (35±1°C) compared to control plants (Son et al., 2011).

Very few reports have also appeared regarding the protective role of Se under HT. Djanaguiraman et al. (2010) investigated the effects of Se foliar spray (75 mg L⁻¹) on leaf photosynthesis, membrane stability, antioxidant enzyme activity, grain yield, and yield components in S. bicolor plants grown under HT stress (40°C /30°C). They observed that HT stress decreased chl content, chl a fluorescence, photosynthetic rate, and activities of antioxidant enzymes. Heat stress also promoted oxidative stress and membrane damage, which ultimately affected the grain yield negatively. However, Se supplementation prevented membrane damage and improved the antioxidant defense system, which helped in attaining a higher grain yield. Overall, Se application significantly increased photosynthetic rate, g_s, and transpiration rate by 13.2, 12.4, and 8.11%, respectively, compared with the unsprayed control. In addition, foliar spray of Se significantly reduced O₂^•− content, H₂O₂ content, MDA level, and membrane injury by 11.5, 35.4, 28.4, and 17.6%, respectively, compared with control plants. Moreover, Se application increased CAT activity in both control and HT stress; however, the maximum increase was observed in HT stress. Across the days of observation, Se application increased CAT and POX enzyme activity by 25.9 and 23.6%, respectively, under HT stress, and 9.2 and 3.3%, respectively, at the control temperature. As a result, an Se spray significantly increased filled seed weight and seed size by 26.3 and 10.7%, respectively, over the untreated controls (Djanaguiraman et al., 2010).

16.6.4 Low temperature

Many reports have indicated a protective role for Si under low temperature (LT) or chilling stress. Silicon at both 0.1 and 1.0 mM improved the shoot dry weight and water status of wheat leaves under a freezing stress of −5°C (Liang et al., 2008). Silicon-treated plants O. sativa, Z. mays, C. sativus, H. annuus, and Benincasa hispida grown hydroponically showed enhanced tolerance to LT (0–4°C) in terms of root nutrient absorbing capability and prevention of wilting (Liang et al., 2006b). Lemon fruits dipped in 0, 50, 150, and 250 mg L⁻¹ solutions of K₂SiO₃ for 30 min showed no chilling injury symptoms when stored at −0.5°C for 28 d. Only 27% of the fruits showed chilling injury when dipped at 50 mg L⁻¹ K₂SiO₃, whereas 97% of untreated fruits were injured and showed significant weight loss. Furthermore, treatment with 50 mg L⁻¹ reduced the occurrence of chilling injury symptoms (Mditshwa et al., 2013).

The homeostasis of soluble sugar concentration is considered to be one of the osmosis-regulated conditions in plant tissues that confer tolerance in freezing-stressed plants. Compared with the non-Si-amended freezing treatment, the Si-amended freezing treatment decreased the content of soluble sugar significantly in wheat but enhanced freezing tolerance (Zhu et al., 2006; Liang et al., 2008). Silicon also maintained balanced Pro levels for conferring higher chilling tolerance in T. aestivum (Zhu et al., 2006; Liang et al., 2008). Under LT, Si improved the photosynthesis and its related parameters. In the wheat leaf, photosynthesis and water-use efficiency were significantly inhibited under freezing stress but these processes were restored by Si (Zhu et al., 2006). Treatment of T. aestivum with exogenous Si increased the net photosynthetic rate, g_s, transpiration rate, chl content, and chl a/b ratio and decreased the soluble sugar content (Zhu et al., 2006).

Silicon might play a role in maintaining moisture (Gong et al., 2003; Tesfay et al., 2011). Silicon-treated avocado fruit maintained a higher water status compared with non-Si-treated plants during post-harvest storage at 5.5°C. In fruit peel, deposition of Si has been reported to cause impregnation of the intercellular parts, which might cover the fruit stomata with an Si layer and reduce fruit respiration (Hammash and El Assi, 2007); this would concomitantly result in decreased weight loss of avocado under chilling storage of 5.5°C. This Si deposition could also affect the membrane integrity and fruit firmness (Tesfay et al., 2011). Phenolic and flavonoids are correlated with chilling tolerance. Silicon in avocado could also adjust polyphenol oxidase (PPO) activity to regulate total phenolics and increase free polyphenol concentrations in the mesocarp, thereby improving chilling tolerance and thus fruit firmness. A reduced weight loss was observed when compared to untreated fruit following exposure to 5.5°C temperature (Tesfay et al., 2011). Uninjured fruits had higher phenolic and flavonoids contents compared to injured fruit. The chilling susceptible lemon fruit contained lower phenolic and flavonoid concentrations, with higher weight loss and electrolyte leakage, when compared with chilling-resistant lemons (Mditshwa et al., 2013). This electrolyte leakage reduction might be due to Si-induced improvement of antioxidant enzyme activities involved in plant stress defense systems (Crusciol et al., 2009; Keeping and Reynolds, 2009). Low temperatures—both chilling and freezing—induce the production of ROS (Xu et al., 2008), which damages membrane lipids and other cellular and subcellular organelles and leads to cell death (Molassiotis et al., 2006) and exogenous Si application was found beneficial for improving these effects.

Exogenous Si supplementation reduced the percentage of withered cucumber leaves under chilling (15/8°C) stress, which was supposedly correlated with Si deposition in leaves. The other reasons might be improved antioxidant activities including SOD, GPX, APX, MDHAR, GR, GSH, AsA, which would lower the levels of H₂O₂, O₂^•−, and MDA and reduce the leaf withering percentage (Liu et al., 2009). The addition of Si (1.0 mM) significantly suppressed H₂O₂, free Pro, and MDA under freezing stress (−5°C) in T. aestivum, which might be due to the observed increases in SOD and CAT activities of 39 and 59%, respectively, and also to the significant augmentation of non-enzymatic antioxidants (namely, AsA and GSH) (Liang et al., 2008). In avocado fruit, K₂SiO₃ application increased the CAT activity, reduced lipid peroxidation, and improved mesocarp electrical conductivity when fruit was stored at 5.5°C for 17 d (Tesfay et al., 2011).

Under LT stress, Se also exerted beneficial effects at low concentration. In C. sativus, Se-treated seedlings avoided the membrane damage caused by LT stress (10°C/5°C, 24 h), which was attributed to Se-induced higher Pro levels and higher shoot biomass (Hawrylak-Nowak et al., 2010). In contrast, the seedlings without Se supplementation showed higher membrane damage, as indicated by higher MDA content (Hawrylak-Nowak et al., 2010). Different levels of Se (0.5, 1.0, 2.0, 3.0 mg Se kg⁻¹) were imposed on T. aestivum subjected to LT stress (4°C); the lower concentration of Se (1 mg Se kg⁻¹) was found to be more beneficial as it enhanced the content of anthocyanins, flavonoids, and phenolic compounds and activities of antioxidant enzymes (namely, POD and CAT), which reduced oxidative damage and concomitant reduction in the levels of MDA and O₂^•− (Chu et al., 2010). Soaking of T. aestivum seeds with Se (5 mg L⁻¹ for 5, 10, and 15 h) made the seedlings tolerant to LT stress (3 or 5°C). Se improved the shoot and root length, total biomass, and the contents of chl, Pro, and soluble sugars. Low temperature caused membrane damage, electrolyte leakage, and increased lipid peroxidation. Se supplementation, through enhancement of different antioxidant components like anthocyanin and AsA and promotion of enzymatic activities of CAT and PPO, significantly reduced oxidative damage. Furthermore, the Se-induced cold tolerance was confirmed by the appearance of novel protein bands (Akladious 2012). According to Djanaguiraman et al. (2005), Se was able to increase tolerance of G. max plants to LT stress by promoting antioxidant capacity and it improved growth and developmental processes of that plant under LT. Abbas (2012) found that SeO₄²⁻ at low concentrations (3 and 6 mg L⁻¹) enhanced growth, levels of chl, anthocyanin, sugar, Pro, and AsA, and enzymatic activities in S. bicolor seedlings subjected to LT stress. However, high levels of SeO₄²⁻ (12 mg L⁻¹) resulted in toxic effects. Low SeO₄²⁻ (3 and 6 mg L⁻¹) also diminished lipid peroxidation by enhancing the activities of APX and GPX.

16.6.5 Heavy metals

Among all the abiotic stresses, the effects of Si on heavy metal (HM) stress is probably the most widely studied. Silicon has been shown to reduce HM content within different plants subjected to HM stresses. Silicon-induced modification of HM binding properties of the cell walls may be an important mechanism for alleviation of HM toxicity (Ye et al., 2012). Proposed mechanisms include coprecipitation of Si and HMs in the cell wall, which would reduce the concentration of biologically active HMs (Neumann and Zur Nieden, 2001); reducing the cell wall porosity by depositing Si in endodermal cell walls; and limiting apoplasmic transport (Shi et al., 2005; da Cunha and do Nascimento, 2009). Silicon addition is documented as reducing the uptake and translocation of HMs (Shi et al., 2005; Nwugo and Huerta, 2008; Kaya et al., 2009).

When Si was applied under Cd stress, it improved the growth parameters in maize including fresh weight, dry weight, primary seminal root, and leaf area, when compared to Cd treatment alone. Silicon might have exerted these positive effects because it increased the cell wall extensibility. Again, by increasing apoplasmic concentration of Cd, Si would considerably decrease the symplasmic content in maize shoots, which might affect other physiological processes related to plant growth (Vaculík et al., 2012). Cucumis sativus L. plants treated with Si accumulated higher biomass against Cd stress. The reasons mentioned included the possibility that Si could improve photosynthetic rate by protecting the chloroplast from disorganization and by increasing pigment contents. Application of Si also alleviated the inhibition of photosynthesis and Fv/Fm (ratio of the variable fluorescence to the maximum fluorescence) and actual quantum efficiency of PSII under Cd stress. Moreover, Si alleviated the inhibition of the enzymes of N metabolism including nitrogen reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) (Feng et al., 2010). In rice (O. sativa L.) under Cd (0, 2, and 4 μM) stress, Si (2 and 4 mM) supplementation increased shoot and root biomass by 125–171% and 100–106%, respectively (Zhang et al., 2008). Silicon can modify the development of root tissues, thereby reducing the uptake and concentration of Cd in other plant parts. Si can also bind toxic Cd ions in the apoplasmic space including the cell wall, or detoxify Cd by sequestering it in vacuoles (Vaculík et al., 2012). Si influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root, thereby helping to decrease symplasmic and increase apoplasmic concentrations of Cd in maize shoots. These apoplasmic barriers enhanced binding of Cd to the apoplasmic fraction in Z. mays shoots (Vaculík et al., 2012).

Silicon also increased growth parameters in T. aestivum and H. vulgare against boron (B) toxicity (Inal et al., 2009). For B detoxification, some assumptions were confirmed for decreases in B concentrations including the formation B–SiO₄⁴⁻ complexes in the soil and reduction of B translocation from the roots to shoots (Gunes et al., 2007b). Silicon has ameliorative effects as it irreversibly precipitates as amorphous silica (SiO₂•nH₂O) in the cell walls and lumen, which reduces the HM-induced membrane damage (Epstein, 1999). In O. sativa, Si also decreased shoot Cd concentrations by 30–50% and Cd distribution ratio in shoots by 25.3–46%, compared to the treatment without Si supply (Zhang et al., 2008). The mitigation of Cd concentration and its toxicity by Si was also predicted in other plant species including T. aestivum (Rizwan et al., 2012), B. rapa (Song et al., 2009), A. hypogaea (Shi et al., 2010), and C. sativus (Feng et al., 2010). Silicon reduced the chromium (Cr) concentration in O. sativa shoots by 41% and in roots by 30% in plants grown in Cr (100 μM)-enriched medium; these effects were due to the dramatic decrease in the translocation factor by 16% (Zeng et al., 2011).

Kidd et al. (2001) suggested that the development of aluminum (Al) tolerance in Z. mays may occur by an enhanced exudation of phenolic compounds, which leads to complexation of Al. Zinc (Zn) could be detoxified by forming bonds with Si; this was found to be localized in the intercellular space, cytoplasm, nucleus, and vacuolar vesicles of leaf mesophyll cells (Neumann and Zur Nieden, 2001; da Cunha and do Nascimento, 2009). However, Al detoxification was reported to occur by reduction of the Al³⁺ content in symplasm and the formation of hard soluble aluminosilicates or hydroxyaluminosilicates in the apoplasmic space and outer epidermal cell walls (Hodson and Sangster, 1993; Hodson and Evans, 1995; Wang et al., 2004). Prabagar et al. (2011) found that the formation of aluminosilicate complexes in the cell wall and apoplasm of Norway spruce (Picea abies) plants create a significant barrier to Al penetration and Si-induced cell damage. Chromium-induced reductions of seedling height, dry biomass, and soluble protein content were alleviated by Si application, which was due mainly to reduced Cr uptake and enhanced antioxidant activity (Zeng et al., 2011). In Norway spruce, the presence of 1.0 mM Si reduced the cell death due to Al stress. Silicon reduced the maximum percentage of cell death by 50, 45, and 35% at Al concentrations of 0.2, 0.5, and 1.0 mM, respectively, after 48 h of exposure (Prabagar et al., 2011). In H. vulgare, 2 mM Si application enhanced plant growth relative to the control, and alleviated Cr (100 μM) toxicity, as reflected by a significant increase in growth and photosynthetic parameters, such as SPAD value, net photosynthetic rate, cellular CO₂ concentration, g_s, transpiration rate, and chl fluorescence efficiency (Ali et al., 2013). In Zea mays, Si treatment prior to exposure to excess manganese (Mn) caused increased thickness of the leaves and epidermal cells when compared to the control plants without Si pretreatment. Silicon-pretreated maize plants maintained higher concentrations of chl a and b and carotenoids when compared to plants exposed to Mn stress treatment alone and Si pretreatment also helped to maintain maximum quantum efficiency of PSII photochemistry, the actual efficiency of PSII electron transport, and the rate of photosynthetic O₂ evolution (Doncheva et al., 2009).

Silicon can reduce HM-induced oxidative damage. Silicon improved growth characteristics of Vitis vinifera rootstock under B as indicated by decreases in stomatal resistance, lipid peroxidation, membrane damage, and MDA and H₂O₂ levels and increases in CAT and SOD activity (Soylemezoglu et al., 2009). Similarly, LOX (224%) and APX (61%) activity were increased in H. vulgare by Si (280 mg kg⁻¹) under sodic-B toxicity conditions, which were correlated with decreased peroxidation of lipid (MDA) by 42%, H₂O₂ by 42%, and membrane permeability by 21% and increased Pro content (53%). Together, these changes could result in increases in fresh (35%) and dry weight (42%) in H. vulgare plants. Silicon also decreased the B (18%) and Na (8%) concentration (Gunes et al., 2007a). Silicon alleviates Cr toxicity in rice plants by inhibiting the uptake and translocation of Cr, together with enhanced activities of antioxidant enzymes, SOD, POD, CAT, and APX, which help to reduce TBARS content (Zeng et al., 2011). Alleviation mechanisms against Mn toxicity by Si include Mn adsorption by cell walls, active removal of excess Mn by soluble Si in the apoplasm, and enhanced enzymatic and non-enzymatic antioxidant levels (Horst et al., 1999; Iwasaki et al., 2002a,b; Rogalla and Römhled, 2002; Shi et al., 2005). Song et al. (2011) proved that Si improved antioxidant defense capacity and membrane integrity to prevent Zn toxicity in O. sativa. Silicon promoted the expression of free radical metabolizing enzymes, which amplified plant biomass parameters and decreased copper (Cu) uptake and leaf chlorosis (Nowakowski and Nowakowska, 1997; Li et al., 2008; Khandekar and Leisner, 2011).

Selenium has been documented to reduce metal toxicity in several research studies. The modes of action were varied and are still unclear; however, some suggested reasons included improvement of the antioxidant defense system, reduction of metal uptake, formation of non-toxic Se–metal complexes, and phytochelatin activity (Vorobets and Mykiyevich, 2000; Sun et al., 2010). Moreover, Se is effective at sustaining physiological activities, growth, and developmental processes even in HM toxic environments (Pedrero et al., 2008; Cartes et al., 2010). In B. oleracea, Cd phytotoxicity resulted in elevated MDA level and decreased photosynthetic pigment and tocopherol concentrations, but Se treatment effectively alleviated these adverse effects (Pedrero et al., 2008). In B. napus, Se (2 μM) conferred tolerance to Cd (400 and 600 μM) stress by reducing lipid unsaturation and peroxidation, modulating the activity of antioxidative enzymes (SOD, CAT, APX, GPX), and preventing Cd-induced changes in the DNA methylation pattern (Filek et al., 2008). Sun et al. (2010) summarized the mechanisms for the positive role of Se on Cd toxicity as: (1) removal of Cd from metabolically active cellular sites, (2) induction of Se to scavenge the Cd-induced ROS generated, and (3) the regulation phytochelatin synthesis-associated enzymes induced by Se. These three actions mitigated the effects of Cd on A. sativum. In our recent study, we observed that rapeseed seedlings grown under Cd stress (0.5 and 1.0 mM CdCl₂) showed substantial increases in MDA and H₂O₂ levels. The AsA content of the seedlings decreased significantly upon exposure to Cd stress. The amount of GSH increased only at 0.5 mM CdCl₂, while GSSG increased at any level of Cd with concomitant decreases in the GSH/GSSG ratio. The activities of antioxidant enzymes also reduced under Cd stress. Importantly, Se-pretreated seedlings exposed to Cd showed increases in the AsA and GSH contents; GSH/GSSG ratio; and the activities of APX, MDHAR, DHAR, GR, GPX, and CAT. However, in most of the cases, pretreatment with 50 μM Se showed better results compared to 100 μM Se. These results indicated that the exogenous application of Se at low concentration increased the tolerance of the plants to Cd-induced oxidative damage by enhancing their antioxidant defenses. The phenotypes of the Se-supplemented seedlings were also superior to the seedlings subjected to Cd stress without Se (Figure 16.7).

The effect of Se (5 or 10 μM of Na₂SeO₄) on Pteris vittata plant was studied under arsenic (As, 150 or 300 μM of Na₂HAsO₄) stress. Selenium was effective at increasing the levels of thiols and GSH (increased by 24%) and decreasing lipid peroxidation (by 26–42%) in a dose-dependent manner (Srivastava et al., 2009). In ryegrass, Cartes et al. (2010) proposed two probable mechanisms for Se-induced alleviation of Al toxicity: (1) the improved dismutation of O₂^•− to H₂O₂, and (2) the activation of antioxidant enzymes (POD, a H₂O₂ scavenger). In addition, a low concentration of Se (2 μM) also improved root growth and decreased lipid peroxidation in plants grown under Al stress (0.2 mM Al). In contrast, a higher concentration of Se (>2 μM) caused phytotoxicity. Ribeiro et al. (2011) observed that the growth of seedlings of Stylosanthes humilis was inhibited by Al³⁺, while elongation was recovered with Na₂SeO₄ at 1.0 μM. Methyl viologen and H₂O₂, which are ROS-generating compounds, also inhibited seedling elongation and again growth was restored by SeO₄²⁻. Selenium-induced enhanced oxidizing ability, resistance to oxidation, and reduced membrane lipid peroxidation of rice roots under iron (Fe) stress was reported by Qi et al. (2004) and Peng et al. (2002).

16.6.6 UV radiation

Although studies are scarce, Si has been proved to be beneficial for improving different growth, developmental, physiological, and biochemical parameters in different plants growing under ultraviolet radiation stress. In G. max seedling, Si (1.70 mM) application increased photosynthesis by 21.0 and 21.5% under UV-B radiation (UV 290–320 nm) and the combination of UV-B and drought treatment, respectively, with increases seen in shoot dry matter of 23%, root dry matter of 42%, and root-to-shoot ratio of 16%. Si could also improve the relative water content (RWC) by 19% after a week of Si application in a combined treatment of UV and drought (Shen et al., 2010). In O. sativa, Si increased plant height and leaf dry matter relative to treatment without Si. More significant results were obtained for total leaf area and specific leaf area both at the boot and milk ripe stage. The increased values for these two parameters were 208 and 113% for the boot stage and those for the milk stage were 51 and 25%, respectively (Goto et al., 2003). Exogenous Si application could modulate the plant’s internal Si and uptake of other nutrients and their contents in soybean plant. Increases in leaf N and P of 9 and 16%, respectively, and decreases in leaf Mg content of 9% and Ca of 24% were seen in rice plants under UV stress. UV-B radiation also increased the allocation of P, potassium (K), and Ca to roots compared with stem and leaves, presumably to ensure sustained nutrient uptake under stress. Silicon application improved the uptake of P and Mg by 11%, which favored the partitioning of dry mass to shoots under UV-B radiation and the allocation of tissue P and Ca to roots (Shen et al., 2009). Gao et al. (2003) also reported that P, Ca, Na, and Si uptake was increased in rice leaves by exogenous Si application under UV stress. The Si-treated rice plants also showed lower cinnamyl alcohol dehydrogenase (CAD) levels in the cell walls of sclerenchyma, vascular bundle sheath, and metaxylem vessel cells, which might the reason why Si-treated rice plants have a lower UV absorbance of around 280 and 320 nm in the leaf blades compared with plants without Si treatment (Goto et al., 2003). After a week of Si application in UV-B stress conditions, net photosynthetic rate in G. max seedlings was significantly increased by 18%, which might be associated with increased g_s (by 15%), and transpiration (by 4%) and significant increases in photosynthetic pigments like chl and anthocyanin in G. max seedlings grown under UV-B stress (Shen et al., 2010).

Within the plant, a protective accumulation of anthocyanins and other UV-absorbing compounds like flavonoids and total phenols occurs in response to UV radiation; these compounds may act as solar screens in the leaf (Alexieva et al., 2001; Li et al., 2004; Shen et al., 2010). Phenolics and flavonoids are effective in reducing UV radiation-induced damage as they prevent the accumulation of ROS, as was seen in Arabidopsis (Bieza and Lois, 2001). In the rice leaf surface, more specifically in the epidermal cells, phenolic compounds are effective protectors against UV-induced damage as they absorb high amounts of UV radiation. Some insoluble compounds of Si are deposited in the leaf epidermis and act as promoters of constitutive phenols (Goto et al., 2003; Li et al., 2004). UV radiation promoted Si deposition in rice leaves (IRRI, 1991). In the rice leaf epidermis, soluble and insoluble UV-absorbing compounds were increased by 21 and 67%, respectively, due to Si treatment (compared to Si-untreated leaves). Moreover, the cell walls and cell lumina of the epidermis contained more Si-rich insoluble UV-absorbing compounds. This led Li et al. (2004) to conclude that Si increased phenolic compounds in the epidermis, which rendered UV tolerance as evidenced by healthy leaves free from the brown spots and strips of UV damage symptoms present in leaves not treated with Si. At the mature stage, rice glumes consist of about 20% silica on a dry matter basis (Okuda and Takahashi, 1961), which was considered as vital for protecting the kernel from severe damage from UV radiation (Ebata et al., 1989). In G. max, UV-B caused substantial membrane damage, as assessed by lipid peroxidation and osmolyte leakage, but Si application significantly reduced the H₂O₂, lipid peroxidation, and membrane damage, and ultimately reduced the leakage of electrolyte. This might have been due to modulation of antioxidant enzymes, phenols, and anthocyanins in relation to other physiological parameters (Shen et al., 2010). Silicate application can increase silica deposits and decrease CAD activity and ferulic and p-coumaric acid contents in rice plants, and these responses are closely related to alterations in the UV defense system (Goto et al., 2003).

Selenium improves plant growth and survival under UV radiation, as reported in several studies. Valkama et al. (2003) investigated the possible ameliorative effects of Se addition (0.1 and 1 mg kg⁻¹) to soil on the detrimental effects of enhanced UV-B radiation in strawberry (Fragaria × ananassa) and barley (H. vulgare) plants. In contrast, higher concentrations of Se had no protective effects and even increased the sensitivity to UV-B radiation. In buckwheat, Se improved the effective quantum yield of PSII under UV-B radiation; thus, plant height and biomass production were increased (Breznik et al., 2005). Selenium might play positive roles in recovery of the photosynthetic and respiratory machinery, as documented in Euglena gracilis. During exposure to different UV radiation (UV-A, 320–400 nm, of 1.02 W m⁻² plus UV-B, 280–320 nm, of 0.73 W m⁻²), no effect was seen for Se (10⁻⁷, 10⁻⁸, 10⁻⁹, and 10⁻¹⁰ M, Na₂SeO₃.5H₂O) in E. gracilis. However, after a 24 h recovery period, Se increased photosynthesis and light-enhanced dark respiration by its protective effects against photodamage and oxidative stress (Ekelund and Danilov, 2001). Se (0.01 and 0.05 mg kg⁻¹ soil) improved the antioxidative capacity, protected chloroplast enzymes, and increased shoot yield in Lactuca sativa under combined UV-B and UV-C stress (Pennanen et al., 2002). Significant increases in the activities of POD and SOD, together with reduced MDA and O₂^•− levels, were documented in T. aestivum under UV-B radiation. Selenium also increased root activity and flavonoid and Pro contents in this plant (Yao et al., 2010a,b). Selenium also showed positive effects and increased yield in ryegrass (Hartikainen et al., 2000), lettuce (Xue et al., 2001), potato (Turakainen et al., 2004), and pumpkin (Germ et al., 2005) exposed to UV-B radiation.