Introduction

Nowadays, intensive agriculture has been primarily limited by abiotic and biotic stress throughout the world (Ramegowda et al. 2020; Ranjan et al. 2021; Mousavi et al. 2022). Among abiotic stresses, salinity and drought are two main constraints in arid and semi-arid regions that negatively affect plant growth and yield, especially in wheat (Ding et al. 2020; Saddiq et al. 2019; Basirat et al. 2019). About 30% of the land in the world is affected by drought stress. Daryanto et al. (2016) and Rasool et al. (2013) reported that drought and salinity stress annually decreased plant production by 20% and 1–2%, respectively. Irrigation with saline water, improper drainage principles, and unbalanced use of chemical fertilizers are considered the main reasons for increasing soil salinity (Corwin 2021; Majeed and Muhammad 2019). Other abiotic stresses, including heavy metal contamination, cold, frost, and UV-B light, destructively affect agricultural products, but their adverse effects on plants are less than those of drought and salinity stresses (Aminiyan et al. 2022; Etesami and Jeong 2018).

Optimal plant growth depends on the uptake of a wide range of nutrients, which are divided into three groups: essential, beneficial, and toxic elements. Silicon was previously not considered an essential element for plants because the role of Si in plant metabolism had not yet been determined (Arnon and Stout 1939). However, its classification as a beneficial element has been recognized for more than 50 years (Marschner 2011; Mousavi 2022). The studies have confirmed its beneficial effects on a variety of species growing under a wide range of environmental conditions (Mousavi et al. 2018a, 2018b, 2022). This overwhelming evidence, together with studies of Si transporters in plants and the yield benefits of Si fertilization of crops, eventually led the International Plant Nutrition Institute (IPNI) to upgrade Si from complete omission to a listing as a beneficial substance in 2015 (Coskun et al. 2019). The essentiality of Si for plants has been the subject of a long debate dating back to the nineteenth century (Sreenivasan 1934; Katz et al. 2021). Because the growth of some plants depends on Si, the supply of Si leads to optimal growth and yield (Puppe and Sommer 2018; Souri et al. 2021).

Plants absorb Si in the form of silicic acid [Si(OH4)] through aquaporin channels, and Si uptake can be active or inactive. Most plants have Si carriers that increase Si uptake (Exley et al. 2020). Plants differ significantly in their ability to absorb Si. Depending on the plant species, the concentration of Si in plant tissues ranges between 0.1 and 10% of dry weight (Liang et al. 2007). In Table 1, plants are classified according to Si content in plant tissues. The concentration of Si in monocots (10–15%) is higher than that in dicots (0.5%) or less in the order of grain products ˃ grasses > vegetables > fruits > legumes (Thiagalingam et al. 1977). Generally, plants have different potentials for the uptake and accumulation of Si. Some plants, such as sugarcane, rice, and wheat, can be considered Si accumulator species, in which the rates of Si absorption have been reported 300–700 kg ha−1, 150–300 kg ha−1, and 50–150 kg ha−1, respectively (Bazilevich 1993).

Table 1 Plant classification according to the amount of silicon accumulation (Bakhat et al. 2018)
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Wheat (Triticum aestivum L.) is one of the most important cereals in terms of food security throughout the world, provides 30% of the calories and 60% of the protein consumed in the world (Grote et al. 2021). The World Bank estimates that in 2050, demand for wheat in developing countries will increase by about 60% (Akram et al. 2021). Because of a significant decrease in wheat production caused by different stresses, finding solutions to increase stress tolerance has been the focus of numerous studies over the last few decades. Wheat is a semi-sensitive crop to salinity. The salinity threshold in wheat is 6 dS.m−1, with a 7.1% yield loss, and at 13 dS.m−1, the yield loss is 50% (Asgari et al. 2011; Maas and Hoffman 1977). Depending on its severity and plant variety, drought stress can reduce the yield of wheat from less than 5% to more than 50% (Anwaar et al. 2020). When the soil moisture decreased from 80% of the field capacity to 60% (medium stress), the grain yield and shoot dry matter decreased by 21 and 13%, respectively, and when the moisture decreased to 40% of the field capacity, the grain yield and shoot dry matter decreased by 43 and 30%, respectively (Ma et al. 2014).

The studies have shown that the soil and foliar application of Si under drought and salinity conditions not only improves the plant’s resistance but also results in significantly increased growth and yield in wheat (Alzahrani et al. 2018; Taha et al. 2021). Some important mechanisms involved in plant resistance to drought and salinity stresses affected by Si nutrition are as follows: enhancing gas exchange, membrane stability, potassium (K) adsorption, improving the activity of antioxidant enzymes, reducing the content of sodium (Na) and malondialdehyde, and reducing electrolyte leakage (Alzahrani et al. 2018; Coskun et al. 2016; Verma et al. 2020). The purpose of this review is to look into various aspects of wheat plant responses to Si nutrition under drought and salinity stress conditions.

The authors made an attempt to analyze the different areas of silicon research published through the Web of Science, Google Scholar, Springer Link, and Wiley Blackwell databases using keywords such as silicon, wheat, drought, salinity, and nutrient management. The results showed that more than 316,000 references on the subject of silicon were published from 2010 to 2021. We finally identified 120 research papers for our review manuscript, covering 30 review papers and 90 research papers. Through these literature reviews, we were able to identify the gap in Si research, which focuses on the effect of Si on improving the growth and yield of wheat under environmental constraints like drought and salinity stresses.

The Sources of Si for Use in Agriculture

The ideal Si fertilizers have a high amount of available Si, the facility of mechanized application, high amounts of calcium (Ca), magnesium (Salim et al. 2013; Cheraghi et al. 2020, 2022), and K, and low concentrations of heavy metals (Bocharnikova and Matichenkov 2014; Salim et al. 2013) (Table 2). Powder silicates are usually uniformly mixed with the soil, while granules are used linearly with NPK fertilizers. Calcium silicate is a by-product obtained by industrial methods and is one of the most popular Si fertilizers. Despite its high price, potassium silicate, because of its considerable solubility, is used in hydroponic systems and foliar spraying (Rao and Pusarla 2018).

Table 2 Some of the common silicon fertilizers in agriculture and research
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In addition to chemical (mineral) sources, Si can also be supplied from organic sources. Rice husks, rice husk ash, rice straw, and biochar are organic sources of Si (Rizwan et al. 2019; Sahebi et al. 2015). The average Si content in rice straw and husk is between 4.20 and 9.26%, respectively (Rao and Pusarla 2018). Silicon-solubilizing bacteria (SSB), which generally belong to the genus Bacillus, can also be used as biological fertilizers (Rezakhani et al. 2020). These bacteria dissolve silica and make it available to the plant, increasing its resistance to biotic and abiotic stresses. For more comprehensive information on silicon and silicon fertilizers, reviews published by Rao and Pusarla (2018), Savant et al. (1999), and Bocharnikova and Matichenkov (2014) are recommended.

The Role of Si in Wheat Tolerance to Drought and Salinity Stress

Drought Stress

Drought stress is a multidimensional stress that causes changes in the physiological, morphological, biochemical, and molecular traits of wheat (Nezhadahmadi et al. 2013; Belay et al. 2021) that can occur at any stage of plant growth and development (Basirat and Mousavi 2022; Mousavi 2022). Depending on the growth stage, intensity, and duration of the stress, drought stress causes a decrease in RWC, chlorophyll degradation, a decrease in leaves’ size, a lower aperture, a decrease in the number of stomata, cell wall thickening, cutinization of the leaf surface, stomatal closure, increased osmolytes, and growth inhibition in wheat, ultimately leading to yield loss and productivity (Sharifi and Mohammadkhani 2016). The effect of drought stress on grain yield and other agronomic components was highly pronounced at the tillering stage (72% reduction), flowering stage (37%), and grain filling (17.1%) as compared to the well-watered treatment (Ashinie and Kindie 2011). Silicon application can increase drought resistance and yield of wheat (Ahmad et al. 2016; Alzahrani et al. 2018; Valizadeh-rad et al. 2022a). Under drought stress, Si increases wheat resistance by increasing leaf membrane stability, relative leaf water content, photosynthesis, osmotic regulation, reducing oxidative stress, modifying gene expression, synthesizing phytohormones, and increasing epicuticular wax production, which is discussed in the following sections (Rafi et al. 2020; Rizwan et al. 2015).

Water Relations and Photosynthesis

Maintaining water relations in a stable and optimal condition is vital for the growth and development of wheat under drought stress (Belay et al. 2021). A reduction or inhibition of photosynthesis is one of the main effects of drought in higher plants (Keyvan 2010). Drought stress in wheat decreases the leaf chlorophyll content, the number and surface size of leaves, and consequently the number of stomata, as a result of which the stomatal conductance decreases and the stomata close (Salehi-Lisar and Bakhshayeshan-Agdam 2016). Carbon dioxide limitations due to prolonged stomatal closure lead to the accumulation of reduced photosynthetic electron transport components. The accumulation of these compounds can reduce molecular oxygen and give rise to the production of reactive oxygen species (ROS) such as superoxide and hydroxyl radicals as well as H2O2, thus causing oxidative damage in chloroplasts (Nezhadahmadi et al. 2013; Sehar et al. 2021; Ullah et al. 2021). ROS can damage the photosynthetic apparatus, including thylakoid membranes, photosynthetic pigments, and enzymes (Sehar et al. 2021). Silicon can retain water relationships and increase water use efficiency in wheat by various mechanisms (Xu et al. 2017). Applying Si to wheat plants exposed to drought stress reduces water loss through stomata, increases relative leaf water content, and produces more extensive and thicker leaves, thereby reducing water loss through transpiration (Rafi et al. 2020). Under drought stress, the application of Si leads to an increase in K content in leaves, stems, and grains of wheat, which helps maintain water balance in the plant and increases the plant’s resistance to drought stress (Ahmad et al. 2016). Subsequently, K, through controlling the opening and closing of stomata, reduces transpiration and prevents plant water loss (Ahmad et al. 2018).

Drought stress significantly reduces photosynthetic pigments in wheat. The studies show that under drought conditions, Si significantly increases photosynthetic pigments and photosynthesis in wheat (Bukhari et al. 2020), due to increased activity of ribulose bisphosphate carboxylase, glyceraldehyde-3-phosphate dehydrogenase dependent on NADP−, photosynthetic enzymes, and chlorophyll content (Gong et al. 2005). Silicon deposition in plant tissues increases the plant’s tolerance to water-deficit stress by reducing transpiration and keeping the leaf blade upright (Salman et al. 2012). The previous studies showed that foliar application of Si increased photosynthetic pigments (chlorophyll a, b, total chlorophyll, and carotenoids) in four wheat cultivars under drought and non-stress conditions, and the rate of increase was higher under stress conditions (Maghsoudi et al. 2015). Furthermore, the positive effect of Si on photosynthesis through different physiological and biochemical mechanisms has been reported in previous studies (Gong and Chen 2012; Maghsoudi et al. 2018).

Relative Water Content (RWC)

When wheat is exposed to drought stress, the water content and water potential of the leaves are significantly reduced (Farooq et al. 2009). Under these conditions, Si can improve the water status of the plant (Othmani et al. 2021). In another study, it was reported that foliar application of Si increased the relative leaf water content in four wheat cultivars (Sirvan and.Chamran, as relatively drought-tolerant, and Shiraz and Marvdasht, as drought sensitive) under drought stress (Maghsoudi et al. 2015). Stomatal conductivity and the rate of transpiration largely determine the relative water content of the leaves, and an accumulation of Si in the epidermal tissues as a thin membrane of siliceous cellulose can protect plants from water loss through the cuticular perspiration by forming a double layer of silica cuticle in the epidermis of the leaf tissue (Farooq et al. 2009; Mvondo-She and Marais 2019).

Silicon reduces stomatal and cuticle transpiration (Vandegeer et al. 2021). Accumulation of monosilicic acid or polymerization of silicic acid in the cell wall of the epidermis may form hydrogen bonds between water and hydrated silica (SiO2–nH2O); in this way, water molecules are not able to transpire quickly from the leaf surface (Liang et al. 2008; Vandegeer et al. 2021). When wheat and other crops become dehydrated, their first response is to close the pores to prevent water loss through transpiration (Chaves et al. 2002). Closure of the pores is one of the main factors in reducing photosynthesis because it reduces the amount of CO2 entering the leaf (Blanke and Cooke 2004). Silicon precipitates around the protective cells of the stomata and in the cell wall of these cells and prevents the pores from closing completely, thus reducing perspiration from the stomata and maintaining photosynthesis (Maghsoudi et al. 2016; Vandegeer et al. 2021).

Nutrient Uptake

Silicon plays a vital role in the balanced uptake, distribution, and transport of nutrients in plants under drought stress. Basirat and Mousavi (2022) reported that under water stress, Si application increases K, Ca, and Mg uptake. Numerous studies have only stated that under drought stress, the application of Si leads to an increase in Mg uptake in wheat, but they have not investigated the mechanism of its effect (Bukhari et al. 2015; Basirat and Mousavi (2022). Greger et al. (2018) reported that the addition of Si increased Mg uptake and accumulation in the shoots of several species grown in solution culture with optimal nutrient supply. To date, the effect of Si on the expression of Mg transporters has not been demonstrated (Pavlovic et al. 2021). There are only two studies that have investigated the effect of Si on Mg-deficient plants. Buchelt et al. (2020) reported Si-mediated alleviation of Mg stress in forage crops but attributed it to increased Mg use efficiency rather than increased Mg uptake. In another study, Hosseini et al. (2019) showed that the Si application had no influence on the uptake or translocation of Mg. Ahmad et al. (2016) reported that Si has a significant effect on increasing K concentrations in shoots and grains of wheat under drought. Increasing Ca and K uptake can be attributed to the effect of Si on reducing the permeability of the plasma membrane and increasing plasma membrane H+-ATP activity (Liang et al. 2006). K and Ca play a vital role in expressing genes that are activated in response to osmotic stress (Al-Bahrany and Al-Khayri 2012). Some of the mechanisms by which silicon alleviates nutrient deficiency, toxic-induced drought, and salinity stress are shown in Fig. 1.

Fig. 1
figure 1

A summary of mechanisms by which Si alleviates nutrient deficiency affected by drought and salinity stresses in wheat. For details, see the text (Sects. 3–1-3 and 3–2-6) and also these references (Liang et al. 2006; Kaya et al. 2006; Eneji et al. 2008; Ali et al. 2009; Frantz et al. 2011; Salim et al. 2013; Gurmani et al. 2013; Chen et al. 2014; Rizwan et al. 2015; Ahmad et al. 2016; Daoud et al. 2018; Zhang et al. 2018; Huang and Ma 2020; Vandegeer et al. 2021; Akhter et al. 2021; Basirat and Mousavi 2022; Mousavi 2022; Valizadeh-rad et al. 2022b)

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Growth and Yield

The effects of Si on plant growth under stressed or non-stressed conditions depend on plant species and the dose of Si application (Basirat and Mousavi 2022; Rezakhani et al. 2020). Silicon increases photosynthesis by increasing the activity of the Rubisco enzyme and the amount of chlorophyll in leaves (Saud et al. 2016). They increase the content of photosynthetic pigments (chlorophyll a, b, and carotenoid) in the plant, affect cell division and growth, improve morphological and physiological properties, and finally increase vegetative growth by increasing the number of leaves (Gunes et al. 2007). Increasing the leaf number results in more leaf surface and a light interception to produce dry matter more quickly (Sivanesan et al. 2010). Overall, the application of Si increases photosynthesis by increasing the amount of chlorophyll, Rubisco enzyme activity, and the number and area of leaves as a result, the amount of carbohydrates and photosynthetic reserves increases (Savvas and Ntatsi 2015). Silicon can also increase the synthesis of soluble proteins (Bharwana et al. 2013; Pei et al. 2010; Xu et al. 2017). The effect of Si on improving shoot growth under water deficit ultimately increases the grain yield (Walsh et al. 2018). Soratto et al. (2012) showed that Si fertilizers increased the yield of wheat and oats by 27 and 34%, respectively, compared to non-Si applications.

Behboudi et al. (2018) found that the foliar and soil application of Si nanoparticles under drought stress increased wheat yield by 25 and 18%, respectively. Silicon fertilization also increased fertile tillers/m2, plant height, spike, and spike length by 18%, 5%, 6%, and 9%, respectively, compared with the treatments without Si (Ahmad et al. 2016).

Effect of Si on Reducing Oxidative Stress

The antioxidant defense system in the wheat and other plants' cells includes both enzymatic [such as superoxide dismutase (SOD), catalase (CAT), POD peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), etc.] and nonenzymatic constituents [such as cysteine (Cys), reduced glutathione (GSH), ascorbic acid (Asc), etc.] (Gong et al. 2005). Under drought stress, high activities of antioxidant enzymes and high contents of nonenzymatic constituents are important for wheat to tolerate stress. Therefore, the influence of Si on each of these compounds can lead to the positive response of the wheat to oxidative stress induced by drought (Table 3). Silicon can reduce oxidative damage to wheat by enhancing the activities of superoxide dismutase, glutathione reductase, and catalase and reducing the amount of hydrogen peroxide and phospholipase activity (Mushtaq et al. 2020). Application of sodium metasilicate under drought stress increased the activity of antioxidant enzymes such as APX, POD, and CAT; the foliar application had the highest efficiency on wheat resistance to drought stress compared to fertigation and seed priming (Bukhari et al. 2015).

Table 3 The beneficial effect of silicon in reducing the negative effects of drought stress on various parameters of wheat
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In the plant cell, the acid phospholipase (AP) is a kind of hydrolysis enzyme of phospholipids, and its activity can be taken as an indicator of de-esterification of phospholipids (Cuyas et al. 2022). Under drought stress, the application of Si prevents the increase of AP activity and alleviates the de-esterification of phospholipids in wheat plants (Gong et al. 2008). Silicon enhances plasma membrane H+-ATPase activity by reducing oxidative damage to proteins, as demonstrated in drought-tolerant wheat (Gong et al. 2005, 2008; Valizadeh-rad et al. 2022a). The amount of nonenzymatic antioxidants is also increased by using Si (Gunes et al. 2007). One of the several non-protein thiols is glutathione, which acts primarily as an antioxidant in plant cells. Valizadeh-rad et al. (2022b) observed that the addition of Si increased glutathione reductase activity in wheat under water-deficit stress. Also, previous studies showed that with increasing K in plants, the activity of NADPH oxidases (nicotinamide adenine dinucleotide phosphate oxidase) decreased, which in turn led to a reduction in the production of reactive oxygen species, indicating that plant K protects from drought stress (Cakmak 2005). Under drought, the application of Si increases K concentration in wheat plants (Bukhari et al. 2015; Gharineh and Karmollachaab 2013).

Osmotic Adjustment

Drought stress increases osmotic stress in plants, and osmotic regulation is an essential mechanism for tolerating drought stress. The previous studies showed that the use of Si under drought stress increased proline levels in wheat (Ahmad et al. 2016; Maghsoudi et al. 2018). Proline is one of the crucial osmolytes that help cells regulate osmosis, and its accumulation in response to osmotic stress has been widely reported (Koenigshofer and Loeppert 2019; Siddique et al. 2018). Zhang et al. (2017) observed that Si increased the concentration of proline, sugars, and soluble proteins in plants under drought and salinity stresses. Si can also improve water uptake by the roots through the accumulation of amino acids and soluble sugars when plants are prone to water deficiency (Sonobe et al. 2010).

Effect of Si on Plant Hormones

When wheat plants are subjected to drought stress, they use many physiological, morphological, and biochemical mechanisms to resist the drought. These processes are controlled by numerous phytohormones [such as abscisic acid (ABA), auxin, gibberellic acid, cytokinins (CKs), brassinosteroids, jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and strigolactone], which are the basic mediators to tolerate or avoid the negative effects of water deficit (Ullah et al. 2018; Salvi et al. 2021). These phytohormones perform as chemical messengers in response to drought and abiotic stresses. After stress signal perception, phytohormones are released, which activate various plant physiological and developmental processes including stomatal closure, root growth stimulation, and the accumulation of osmolytes to avoid drought conditions (Daszkowska-Golec and Szarejko 2013).

Under drought stress, the application of Si increases the synthesis of some phytohormones such as ABA and JA in wheat plants (Dolatabadian et al. 2009; Xu et al. 2017). Abscisic acid reduces the adverse effects of oxidative stress. Also, by regulating the entry of K into the stomatal guard cells and regulating the opening and closing of the pores, abscisic acid regulates the conduction of the pores and ensures that the plants do not lose more than a certain amount of water under stress (Kim et al. 2014). Xie et al. (2003) stated that under drought stress, indole acetic acid in wheat decreases. Also, they reported that the biosynthesis of indole acetic acid was reduced, which indicates that auxin biosynthesis may be suppressed by drought stress (Du et al. 2013). Xu et al. (2017) showed that Si application reduces the synthesis of jasmonic acid and indole acetic acid and increases the synthesis of abscisic acid in wheat plants. This regulation of phytohormones is associated with improved physiological factors and increased resistance to drought stress by increasing the activity of antioxidant enzymes. Table 3 shows the beneficial effects of silicon on reducing the negative effects of drought stress on wheat.

Salinity Stress

Salinity reduces germination, the number of tillers, the size and number of leaves of wheat plants (Grieve et al. 2001), the shelf life of tillers and florets in the spike (Ranjbar 2010), and the number and size of grains with premature aging of the wheat plant (Nadeem et al. 2020). It was reported that salinity stress, depending on the level of salinity, can reduce the yield of wheat plants by 18–80% (Kale Celik 2022). A summary of the adverse effects of salinity on wheat was reported by Sabagh et al. (2021). However, the use of Si can increase salinity tolerance in many critical agricultural products, including wheat (Tibbitts 2018), barley (Akhter et al. 2021), rice (Kim et al. 2014), corn (Ali et al. 2021), tomatoes (Korkmaz et al. 2018), and other agricultural products.

Photosynthesis, Growth, and Yield of Wheat

The cessation or decrease in plant growth caused by salinity is mainly due to the suppression of the photosynthetic system and the decrease in the content of photosynthetic pigments. Under salinity, Si supply can increase leaf chlorophyll a and b and photosynthesis at all stages of growth (Daoud et al. 2018). Jasim and Abood (2018) reported that foliar application of Si significantly increased plant height, leaf area, spike length, and chlorophyll content in six wheat cultivars under saline conditions. In addition, Si-induced reduction of oxidative stress caused by reactive oxygen species leads to a significant increase in chlorophyll a compared to chlorophyll b, leading to an increase in photosynthesis and plant growth (Soratto et al. 2012). Plant growth and metabolic processes are suppressed in salinity due to the overproduction of reactive oxygen species with disrupted plasma membranes and ion imbalance (Liu et al. 2019). In contrast, Si supply reduces the destructive effect of ROS by increasing the activity of antioxidant enzymes such as catalase and superoxide dismutase and thus improving plant growth. For instance, Tahir et al. (2011) applied three Si contents (0, 75, and 150 μg g– 1 Si using potassium silicate) to both salt-sensitive and salt-resistant wheat varieties at two levels of salinity (0 and 60 mM NaCl). The results showed that at 60 mM NaCl with 150 μg Si g–1, the reduction of grain yield changed from 62 to 33% for the sensitive varieties and from 44 to 20% for the resistant varieties. Also, Ahmad (2014) reported that the application of 2 and 4 mol Si l−1 at 100 mol NaCl l−1 increased wheat grain yield by 13.8 and 24.2%, respectively, compared to the non-Si application.

Root System

One of the adverse effects of salinity stress in plants is the unbalanced growth of the root system relative to the shoot (Acosta-Motos et al. 2017). Under salinity and drought stresses, the plant faces a water shortage and would spend more energy to increase the volume and surface of the roots, which leads to a sharp decrease in shoot growth and an increase in the root-to-shoot ratio. Studies have found that silica supply improves root growth parameters in wheat under salinity and drought stresses (Daoud et al. 2018; Hameed et al. 2013). Silicon-reinforced root growth can be attributed to the stretching of the cell wall in the area of the root extension (Hattori et al. 2003). Jasim and Abood (2018) reported that Si increased root dry weight in six wheat genotypes, likely by increasing root thickness. Silicon may also increase photosynthesis and accelerate the proliferation of top root cells (Hattori et al. 2003). Some studies showed that Si increases the absorption of water and nutrients by plants by improving the hydraulic conductivity of the roots (Shi et al. 2016), thus increasing the efficiency and activity of the roots (Chen et al. 2011).

Reduction of Sodium (Na) Toxicity

Under salinity, there is competition between Na and K ions which reduces the ratio of K to Na in plants (Zhang et al. 2018). In the presence of Si, a polymer gel is produced in the endoderm and exoderm of the roots of the wheat and rice plants, which reduces the apoplastic transfer of Na ions from the roots to the shoots (Gong et al. 2003; Yeo et al. 1999). Plants take up Si in the form of silicic acid, which is transported to the shoot, and after the loss of water, it is polymerized as silica gel on the surface of leaves and stems (Ma et al. 2001). Also, when the concentration of Si increases in the root, excessive Na is bound in hydrophilic, salicious gel, so both Si and Na are unable to be released in the xylem for upward translocation (Ahmad et al. 1992). Yeo et al. (1999) stated that the deposition of silica in the endodermis and rhizodermis and the polymerization of silicate via colloidal silica to silica gel throughout the root apoplast are possible mechanisms by which Si could physically block the transpirational bypass flow across the roots. Silica gel is formed by acidifying aqueous silicic acid solutions, a process that may be aided by the activity of the root P-ATPase. This could account for the reduction in Na uptake caused by Si that is not accounted for by the effects on transpiration.

Ali et al. (2012) found that saline-tolerant wheat genotypes (SARC-5) showed better performance in response to Si compared to susceptible genotypes (Auqab-200). Under salinity, Si leads to the binding of Na to the cell wall of wheat leaf, which reduces the concentration of free Na in leaf sap compared to the control treatment (Saqib et al. 2008). Hajiboland et al. (2017) reported that Si decreases the concentration of Na in cell sap and increases its attachment to leaf cell walls, indicating Si detoxification of Na (Ahmad et al. 1992). Silicon can also reduce the transfer of Na to the shoot by forming a Si–Na complex in the root. Silicon application also causes Na accumulation in the roots of the wheat plants and, to a much lesser extent, in the shoots than in the non-Si control (Tuna et al. 2008). Although Si supply reduces the absorption and transportation of Na and maintains a balanced ratio of Na to K in the wheat plant under salinity stress, little is known about whether Si leads to a uniform distribution of Na in the plant or the excretion of Na through exudates and secretions from the roots. There is surprisingly little historical evidence that Si can form a complex with Na. Further studies are also required to provide an optimal level of Si under stress conditions for the wheat plant.

Effect of Si on Salinity-Induced Oxidative Stress

Like drought stress, salinity can disrupt the growth and yield of wheat and other crops by overproducing reactive oxygen species (ROS) (Fig. 2). Application of Si in nutrient solutions increased the activity and production of antioxidant enzymes such as superoxide dismutase and catalase, thus reducing oxidative damage due to salinity stress (Ali et al. 2019). Daoud et al. (2018) applied different amounts of Si to wheat plants under salinity stress and found that Si significantly increased the superoxide dismutase and catalase activity in the leaves of the plants in the start-up stage in relation to a significant reduction in the H2O2 content.

Fig. 2
figure 2

The involved mechanisms by which Si alleviate oxidative damage affected by drought and salinity stresses in wheat. For details, see the text (Sects. 3–1-5 and 3–2-4) and also these references (Gong et al. 2003, 2005; Gunes et al. 2007; Li et al. 2007; Gong et al. 2008; Ahmad and Haddad 2011; Bharwana et al. 2013; Kim et al. 2014; Ma et al. 2016; Zhang et al. 2017; Taffouo et al. 2017; Ali et al. 2019; Shams et al. 2019; Mushtaq et al. 2020; Verma et al. 2020; Sabagh et al. 2021; Taha et al. 2021)

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Osmotic Adjustment

Salinity stress causes a reduction of leaf water content (RWC), which appears in wheat after a few days of exposure to salinity (Saddiq et al. 2019). The reduction in plant growth under salinity is mainly due to low osmotic potential (Hmidi et al. 2018). Silicon can reduce osmotic stress to some extent by increasing the accumulation of Na in the roots of wheat and preventing its transfer to the leaves, whereas no such effect is observed in plants grown under non-stress conditions (Tuna et al. 2008). Similar results have been reported for sorghum, tomato, and Portulaca oleracea (Kafi and Rahimi 2011; Liu et al. 2015). These studies confirm that Si increases plant tolerance to salinity stress by reducing osmotic pressure.

Most plants under salinity stress accumulate some osmolytes in their tissues in addition to antioxidants. These compounds mainly include proline, glycine betaine (GB), carbohydrates, and polyols (Chen and Jiang 2010; Singh et al. 2015). Studies have shown that treating wheat with Si under salinity stress increases soluble sugars, proteins, and free amino acids, especially proline (Alzahrani et al. 2018; Chen and Jiang 2010). Proline is a non-toxic and protective osmolyte produced under osmotic stress and is often involved in osmotic protection (Szabados and Savouré 2010). Hajiboland et al. (2017) observed that under salinity stress, the proline concentration significantly increased in the leaves of Si-treated wheat. It has also been reported that under salinity stress, proline concentrations decreased with the application of Si in sorghum (Yin et al. 2013), wheat (Tuna et al. 2008), and barley (Gunes et al. 2007). Therefore, there are conflicting reports regarding proline accumulation and the effect of Si on proline content under salinity stress conditions that need further investigation. Saleh et al. (2017) reported that the application of Si (higher than 50 mg kg−1 soil) significantly led to an increase in Glycine betaine (GB) concentration in wheat grown under salinity stress. GB accumulation in salt-stressed plants lowers leaf water potential, resulting in improved water uptake by the cells (Ahmad and Haddad 2011). It has also been found that foliar application of nano-silicon fertilizer leads to an increase in the concentration of GB, water-soluble carbohydrates, and free amino acids in wheat plants grown under drought stress (Hajihashemi and Kazemi 2022). The observed increase in the free amino acids in the nano-silicon application can be attributed to their antioxidant power to scavenge free radicals, and osmotic adjustment potential to maintain the cell's osmotic pressure higher than the outer medium to induce water absorbance under stress conditions (Abdel-Haliem et al. 2017; Hajihashemi and Kazemi 2022). Some of the beneficial effects of silicon in reducing the destructive effects of salinity-induced osmotic stress are shown in Fig. 3.

Fig. 3
figure 3

The involved mechanisms by which Si alleviate osmotic stress affected by drought and salinity stress in wheat. For details, see the text (Sects. 3–1-6 and 3–2-5) and also these references (Tahir et al. 2006; Chen and Jiang 2010; Ahmad and Haddad 2011; Ali et al. 2012; Al-Bahrany and Al-Khayri 2012; Yin et al. 2013; Ahmad 2014; Liu et al. 2015; Singh et al. 2015; Taffouo et al. 2017; Zhang et al. 2017; Daoud et al. 2018; Walsh et al. 2018; Shams et al. 2019; Koenigshofer and Loeppert 2019; Thorne et al. 2021; Vandegeer et al. 2021; Akhter et al. 2021)

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Balance of Nutrients

Salinity stress causes an imbalance of nutrients and the disruption of water uptake by plants. Silicon not only increases the absorption and transfer of K and reduces the absorption and transfer of Na and chlorine (Cl) from the roots to the shoots but also increases the absorption of P, N, Ca, and other essential nutrients (Pavlovic et al. 2021). The uptake of nutrients is related to the properties of the roots, including length and surface area; as root surface area increases, more space is provided for the uptake of dispersible ions (Vuyyuru et al. 2018). Many studies have reported a Si-induced increase in root growth under salinity stress (Basirat and Mousavi 2022; Mousavi 2022; Mushtaq et al. 2020, 2019), while other studies have shown that Si increases nutrient uptake by increasing root efficiency for water uptake (Basirat and Mousavi 2022; Mousavi 2022). The stimulating effect of Si on root growth in wheat and sorghum can be attributed to increased root elongation due to increased cell wall dilation in the growing area (Daoud et al. 2018; Hattori et al. 2003). Increased nutrient uptake may also be related to increased plasma membrane proton pump (H+-ATPase) activity. For example, Kaya et al. (2006) have reported an increase in K and Ca absorption. There have been conflicting reports on the relationship between Si, P, and Zn uptake. Gao et al. (2005) found that the application of Si significantly reduced P concentration in the sapwood of maize. Eneji et al. (2008) found that Si increased P uptake by increasing P concentration in soil solution. Table 4 summarizes the different effects of Si on improving various parameters of wheat under salinity stress.

Table 4 The beneficial effect of silicon on reducing the negative effect of salinity stress on various parameters of wheat
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Conclusion and Suggestions

Since Si was recognized as a quasi-essential element for plants, many studies have investigated the effects of Si on various plants. Although most studies have shown positive roles for Si under stress conditions, there is some evidence that Si can improve the growth and yield of a wide range of plants under non-stress conditions. To date, there is little information about the effect of Si on plant metabolism. In many regions, Si has not been considered a fertilizer, even though it is beneficial to many crops, especially rice and wheat. In summary, more research work is warranted for the Si roles in the aspects listed below:

  1. a)

    Most studies about the positive effects of Si on physiological and biochemical processes that contribute to wheat resistance to salinity and drought stresses have been done in greenhouses and controlled growth chambers. It is essential to investigate the effects of Si on crops under field conditions.

  2. b)

    More attention is needed to evaluate the effectiveness of the Si-fertilizer formulation and its combination with other nutrients, as well as the best time to use it.

  3. c)

    Knowledge is still limited about the role of Si in plant metabolic activity under different stresses, especially in the molecular aspects.

  4. d)

    Few studies are reporting the potential of organic, biologic (Si-soluble bacteria), and nanoparticle (Si-NPs) sources of Si to alleviate drought and salinity stresses.