Synergy of Selenium and Silicon to Mitigate Abiotic Stresses: a Review

Abstract

It is evident the increase in the occurrence of different stresses that impact agriculture and so there has been an increase in research to study stress mitigators including silicon (Si) and selenium (Se). However, the great challenge to be answered would be to assess whether it is possible to maximize these benefits by combining these two elements. Therefore, this review focused on discussing the feasibility of combining Se and Si in mitigating abiotic stresses and also measuring gains in yield and quality of agricultural products. These are the main challenges of plant mineral nutrition with these two elements for sustainable cultivation, ensuring food security with the possibility of improving human health. As the mode of application of an element can change absorption and assimilation processes and consequently the plant’s response, it is important to consider research with supply of these elements via the foliar and root route. Thus, we highlighted the potential of the combined application of Se and Si and whether or not they are relevant to overcome the individual application in stress mitigation or even in plants without stress. In addition, we pointed out new directions for research on this topic in order to reinforce the combined use of stress relievers and their potential benefit to crop plants.

Introduction

Agriculture and climate change occurring in different regions of the world are the main cause of abiotic and biotic stresses in plants, which can affect food production and food security (Raza et al. 2019). In addition, the number of areas with soil contaminated with potentially toxic heavy metals that is a risk to human health has increased (Ali et al. 2019) and it still has advanced agricultural areas with salinity problems. Soil salinity is the main global abiotic stress that limits crop production with an annual increment of 10% (Sattar et al. 2017). Allied to this, drought is one of the serious environmental stresses that limit the yield of agricultural crops worldwide (El-Metwally et al. 2009; Saudy and El-Bagoury 2014; El-Bially et al. 2018; Sattar et al. 2019; Saudy and El-Metwally 2022).

The main effect of abiotic stresses in plants is the excessive production of reactive oxygen species (ROS) which at low concentrations act as signaling agents but at high concentrations can cause various damages such as the destruction of the cell membrane and leakage of cell contentes (Foyer 2020). In addition, abiotic stresses can also trigger physiological and biochemical changes in plants (Takahashi et al. 2020; El-Metwally et al. 2021, 2022; Ferrari et al. 2022a, b; Makhlouf et al. 2022; Saudy et al. 2022a, b).

The projected world population in 2050 will be 9.7 to 10 billion people. Due to population growth, food production is expected to increase by 70% by 2050 to meet people’s demand for food (Anyaoha et al. 2018). Due to population growth, new strategies should be developed with the objective of mitigating abiotic stresses in agricultural production so that people’s food security is maintained (Saudy et al. 2020; Salem et al. 2021; Taha et al. 2021; Carara et al. 2022; Campos et al. 2022; Marsala et al. 2022; El-Bially et al. 2022a).

An important strategy to mitigate abiotic stress would be with the nutritional balance of the plant from the use of silicon, which is widely reported by many authors and there are many reviews on the subject (Cocker et al. 1998; Ma and Yamaji 2008; Saudy and Mubarak 2014; Luyckx et al. 2017; Souri et al. 2021; Salem et al. 2022). Another option would be the use of selenium (Se) which is an important stress reliever (Läuchli 1993; Germ et al. 2007; Lanza and Dos Reis 2021). In these studies, different topics involving physiology, biochemistry, enzymatic antioxidant metabolism and abiotic stresses are studied (Fig. 1).

Fig. 1.

Se and Si are related to relieving oxidative stress in plants. Data were collected from searches in journals indexed in the web of Science from 2016 to 2022

However, most of these researches with these elements were studied individually. Studies involving the interaction of Si and other chemical elements predominate for plant nutrients and began almost 100 years ago with a study with N, P and K, making it clear that Si interacts with these elements mitigating its deficiency (Brenchley et al. 1927; Saudy and Mubarak 2015; Saudy et al. 2022c). While studies involving the relationship between Si and beneficial elements such as Se are more recent and may be a good option to reduce the negative effects of abiotic stresses on plants (Taha et al. 2021; Huang et al. 2021), in addition to being important for agricultural production and human health (Huang et al. 2021).

Se is able to reduce the negative impacts of abiotic stresses on plants by modulating carbohydrate and nitrogen metabolism (Shahid et al. 2019), increase in chlorophyll biosynthesis, antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), flavonoids and decrease in ROS levels (Lanza and Dos Reis 2021; Cunha et al. 2022). In plants, Si increases photosynthesis, activity of antioxidant enzymes, reduces oxidative stress and increases the ability of plants to tolerate abiotic stresses (Abdelaal et al. 2020; Sales et al. 2021). Recently there are new reports indicating that Si improves C:N:P homeostasis contributing to mitigate crop stress (Frazão et al. 2020; De Souza Júnior et al. 2022a; Teixeira et al. 2022).

Considering that the combined effect of chemical elements that are known to mitigate stress could be an important strategy to increase their benefits in plants. Thus, the objective of this review was to discuss the importance of the Se and Si combination, whether supplied via foliar or root to potentiate the mitigation of abiotic stress of plants in relation to supply in the individual form and the mechanisms involved and the future perspectives of research on this theme.

Advances in Abiotic Stresses in Agriculture

Climate change is seen as a global phenomenon and can occur in different intensities around the world (Rivero et al. 2021). In the last century, CO₂ levels and global temperature have increased significantly and the climate is predicted to continue with even more extreme changes affecting agriculture on a global scale (Islam et al. 2020).

By the year 2050, the world population is expected to reach 9.7 billion people, which would increase the pressure on agricultural land to meet the population’s demand for food (Ferrari et al. 2022a). Climate change is expected to influence rainfall and temperature patterns, causing losses in food production (Karimi et al. 2018). Crop productivity is expected to be reduced under future climatic conditions and recent studies indicate that productivity is already being affected (Shahin et al. 2018; Tan et al. 2021). The effects of climate change on agricultural crops are multiple and can result in morphological, physiological and biochemical changes in plants (Takahashi et al. 2020; Saudy et al. 2021; El-Bially et al. 2022a).

Under normal conditions, plants produce ROS as part of their metabolism that act as signaling molecules (Mittler 2017). However, under stress conditions such as drought, nutrient deficiency and high temperatures, there is excessive production of ROS that can cause the destruction of the plasma membrane, causing the leakage of cellular contents and the death of cells (Foyer 2020). Hence, reduction in crop productivity owing to abiotic stresses occured (Saudy et al. 2018; El-Metwally and Saudy 2021).

Salinity, drought and excess light can lead to the closing of stomata, reducing the concentration of carbon dioxide inside the cells, favoring the formation of singlet oxygen that can damage photosystems I and II, reducing photosynthesis in plants (Das and Roychoudhury 2014; Abd El-Mageed et al. 2022). These stresses can also reduce growth (Ors et al. 2021), chlorophyll fluorescence (Ali et al. 2022), nitrogen status (Hunter et al. 2021), nutrient uptake (Saudy and El-Metwally 2019; Mubarak et al. 2021) as well as and crop yield quantity and quality (Zörb et al. 2019; Abd-Elrahman et al. 2022).

In addition to climate change affecting agriculture, in recent years the concentration of heavy metals in agricultural soils has increased considerably, posing a threat to human food security and the environment (Sall et al. 2020). The presence of heavy metals in soils is a threat to human health due to their ability to accumulate in the edible parts of plants (Zhang et al. 2018; Yang et al. 2020) especially cadmium (Cd) and lead (Pb) (Barakat 2011; Khan et al. 2011). In plants, Cd can damage photosynthesis, relative water content, transpiration rate, stomatal conduction, electrolyte leakage and increase ROS production and consequently decrease crop yield (El Rasafi et al. 2022) in addition to increasing the accumulation of this heavy metal in the roots, shoots and grains (Abbas et al. 2017). Pb can affect the water and nutritional relationships of plants, cause oxidative damage and affect enzymatic activities, causing cell membrane damage and stomatal closure, reducing chlorophyllase synthesis delaying carbon metabolism and decreasing crop yield (Zulfiqar et al. 2019).

In general, there are many abiotic stresses that can harm agriculture and its sustainability. In this way, new technologies must be developed so that plants maintain their physiological, biochemical and yield traits, maintaining people’s food safety (Ferrari et al. 2022b). Within this context, plant nutrition with Se and Si is a good alternative to increase plant tolerance to abiotic stresses.

Properties of Se and Si

Se and Si are reported in the literature as elements that increase the tolerance of plants grown under stress conditions (Rady et al. 2020; Mostofa et al. 2021). The concentration of these elements in the solution of agricultural soils around the world is considered low (Sommer et al. 2006; Fordyce 2013). One of the ways to increase the levels of Se and Si in soils to benefit plants is through fertilization using these elements.

Se was first reported in 1817 by a Swedish chemist named Jons Jacons and was initially thought to be an element that could cause toxicity (Shahid et al. 2018), but at higher concentrations it can result in strong toxicity in plants (Da Cruz Ferreira et al. 2020). There are six forms of Se isotope with atomic masses ranging from 74 to 82 g mol⁻¹ (Stüeken et al. 2013). The forms of Se available for plant uptake are selenate, elemental Se, thioselenate, selenite (Se(IV)) and selenite (Se(VI)), the last two being the most abundant forms and absorbed by plants (Shahid et al. 2018). At low concentrations, Se improves plant growth and survival, decreases ROS, increases proteins, amino acids, sugars, antioxidant enzymes, nitrogen metabolism, chlorophyll biosynthesis, yield and photosynthesis (Ríos et al. 2010; Lanza and Dos Reis 2021). In addition, Se can mitigate sulfur (S) deficiency resulting in greater uptake of S by plants (Tian et al. 2017).

Si is a metalloid with an atomic mass of around 28.08 g mol⁻¹, being the second most abundant element in the Earth’s crust. There are three naturally occurring isotopic forms of Si (Souri et al. 2021). In soils, Si is present in the forms of silica, silica gel and silicate (Souri et al. 2021). Most Si compounds are insoluble in soil solution and are not available for plant uptake (Richmond and Sussman 2003). Silicic acid or Si(OH)4, which are the available forms of Si (Currie and Perry 2007) and absorbed by plants, being the soluble form in the soil at pH lower than 9 (Mandlik et al. 2020). The sources of Si available for soil application are naturally occurring calcium silicate based on rock or agro-industry residues (Prado et al. 2003). There are other sources of Si that can be supplied directly in the soil or in solution in fluid form, which are potassium silicate, sodium silicate, nanosilica, monosilicic acid and other derived sources (De Oliveira et al. 2019; Dos Santos et al. 2020; De Souza Júnior et al. 2022b). In plants, Si can attenuate the effects of several abiotic stresses including salinity, drought, excess solar radiation and the presence of heavy metals (Frew et al. 2018; Rocha et al. 2021). Some research indicates that Si can increase photosynthesis, lignin, transpiration, antioxidant enzyme activity, yield and mitigate plant stress (Alvarez et al. 2018; Abdelaal et al. 2020; Sales et al. 2021).

Although there are several studies on the individual effects of each element on growth, antioxidant metabolism, production of sugars, amino acids, photosynthesis proteins, oxidative stress, nitrogen metabolism and crop productivity, few studies have verified the combined effects of Si and Se.

In plant mineral nutrition, the interaction between two elements can be synergistic (positive), antagonism (negative) or there is no influence of one in the presence of the other (Rietra et al. 2017). Synergism is when the yield of the combined application of two elements is greater than the yield of the elements applied individually. The antagonistic relationship occurs when the combined application yield of two nutrients is less than the individual application yield of each nutrient (Rietra et al. 2017). When there is no relationship between two nutrients, it occurs when the yield of the combination of two nutrients is equal to the yield of the isolated application of each nutriente (Rietra et al. 2017).

To verify the relationship between Se and Si, the mode of application of these elements was respected as they can affect the response of the plant, so the research with the application of these elements via the foliar and then the root route will be discussed.

Synergy Ae and Ai Applied Via Foliar in the Mitigation of Stress in Plants

Some researches have reported that Se and Si have a synergistic effect in combating oxidative stress and relieving plants from abiotic stress. A study on wheat plants showed that salinity caused biological damage to the plant by decreasing the chlorophyll content, the activity of antioxidant enzymes, total sugars, soluble proteins, proline and impaired photosynthetic processes and consequently the production of matter. root and shoot dryness (Sattar et al. 2017). However, the authors found that the combined supply of Se and Si in relation to the isolated application was more efficient in alleviating the toxic effects of salinity as it increased the efficiency of antioxidant metabolism, accumulation of osmoprotectants, chlorophyll content and dry weight production.

In wheat plants grown under salt stress, Taha et al. (2021) demonstrated that salt stress increased lipid peroxidation and damages the physiological, biochemical, biometric and productive traits of plants. On the other hand, the authors reported that spraying with Se and Si was more efficient in reducing the harmful effects of stress by increasing growth, antioxidant enzymes, non-enzymatic components causing higher plant yield compared to the isolated application of each element.

The combined application of Se and Si is also important to alleviate data caused by water deficit. Mahdavi Ardakani et al. (2021) reported that water deficit in oregano plants caused damage to physiological attributes and reduced yield. The authors also verified that the combined application Se and Si compared to the isolated application was more efficient in minimizing the effects of water deficit by improving physiological processes and increasing plant yield. Sattar et al. (2019) indicated that water deficit caused biological damage in wheat seedlings with a reduction in physiological and biochemical traits, dry weight and shoot and root length. The authors reported that the combined application of Se and Si was more efficient than the isolated application in relieving the stresses caused by drought by increasing the activity of antioxidant enzymes, photosynthetic attributes, chlorophyll content, transpiration rate and water relations resulting in stimulation in the growth of seedlings. In a study carried out with rice plants grown under water deficit, it was clear a decrease in photosynthetic pigments and losses in physiological processes (Ghouri et al. 2021). However, the combined application of Se and Si compared to the isolated application was more efficient in alleviating the effects of drought as it reduced water loss and increased the chlorophyll content and consequently the yield.

In strawberry plants grown under water deficit, Zahedi et al. (2020) found that the application of a solution containing Se and Si in relation to the individual in the form of nanoparticles increased biochemical parameters such as anthocyanin, total phenolic compounds, vitamin C and antioxidant activity of the fruits. In addition, an improvement in the nutritional quality of the fruits, a reduction in the levels of malondialdehyde and hydrogen peroxide, an increase in the activity of antioxidant enzymes resulting in greater accumulation of dry weight was also reported. According to these authors, the combination of Se and Si is promising to reduce the harmful effects of drought not only on strawberry plants but also on other agricultural crops.

In a research carried out with rice plants grown in contaminated soil (total content Cd = −0.8 and Pb = 86.1 mg kg⁻¹ of soil), Hussain et al. (2020) found that heavy metals harmed plant development with damage to growth and yield. The authors reported that the combined application of Se and Si via foliar in relation to individual application in the form of nanoparticles was more efficient in reducing the toxicity of heavy metals, resulting in higher biomass production, yield and Se content in the grains and lower levels of these. metals in the grains.

In a soil contaminated with heavy metals, a study was carried out and observed that the combined application of Se and Si in the form of nanoparticles in relation to individual ones increased the expression of genes and key proteins related to photosynthesis, the chlorophyll a content and decreased the Pb and Cd contents in the leaves but it was not enough to increase the yield although it increased the Se content in the rice grains (Wang et al. 2020). In this case, the combination Se and Si had little physiological effect to alleviate the toxicity of these metals, but the fact of reducing the content of these metals in the grain is relevant to prevent these metals from reaching the human food chain.

Some studies reported that the combination of foliar Se and Si did not outperform the individual application especially in crops under the presence of heavy metals. Thus, Gao et al. (2018) studied rice plants cultivated with the presence of cadmium and observed that the combined application of Se and Si compared to individual application was not more efficient in promoting stress relief in plants, as these plants had higher accumulation of Cd and less stomatal conductance. Demin et al. (2022) also found that the combined application of Se and Si did not have a synergistic effect as it was not sufficient to decrease the Cd content in the plant and mitigate the stresses of this metal in rice plants. The reason would be that the use of the combined application of Se and Si increased the colloidal aggregates and formed relatively large colloidal particles that impede the passage through the cellulose of the cell wall, diminishing the beneficial effects of these elements in decreasing the absorption of Cd and mitigating this stress. In cabbage plants grown under the presence of Cd, Wu et al. (2018) reported that the combined addition of Si and Se facilitates the activities of glutathione reductase, CAT, DHAR in leaves and roots but not enough to increase dry weight production and overcome the individual application of these elements.

In the studies discussed that evaluated the application of Se via foliar, they were carried out using only a relatively low dose or concentration, ranging from 0.08; 0.25; 0.38; 0.5; 1.27; 30 and 40 mmol L⁻¹ of Se (Table 1). The same applies to Si where concentrations ranged from 0.35; 0.53; 0.78; 1.5;1.7;2.5; 30 and 40 mmol L⁻¹ of Si (Table 1). It is observed that the variation in the concentrations of these elements is relatively high, indicating the need for further studies. The greatest concern in the use of very high concentrations is only for Se because it has a risk of toxicity in plants, which does not occur with Si because there are no reports of toxicity of this element in plants.

Table 1.

Results of the interaction of Si and Se in different agricultural crops

Crop	Treatment	Conditions	Application	Results	Citation
Crops with stress application
Fragaria ×ananassa Duch	Nanoparticles of Se/SiO2 (100 mg L−1) (or 1.7 mM Se and 1.7 mM Si) + drought stress	Green house	Foliar	Synergy between Se and Si was due to preservation in photosynthetic pigments and improvement in levels of carbohydrate, proline, water use efficiency, membrane stability index, relative water content, CAT, APX, GPX, SOD and dry weight	Zahedi et al. (2020)
Origanum vulgare	30 mg L−1 Se + 15 mg L−1 Si (or 0.38 mM Se + 0.53 mM Si) + drought stress	Green house	Foliar	Synergy between Se and Si was due to improvement in physiological characteristics and increase in plant yield	Mahdavi Ardakani et al. (2021)
Oryza sativa L	6 µg mL−1 (or 0.08 mM) Se (nanoparticles) + 22 µg mL−1 (0.78 mM) Si (nanoparticles) + heavy metal stress	Pot	Foliar	Absence of synergism between Se and Si	Wang et al. (2020)
Oryza sativa L	20 mg L−1 (0.25 mM) Se (nanoparticles) + 10 mg L−1 (0.35 mM) Si (nanoparticles) + heavy metal stress	Field	Foliar	The synergy between Se and Si was due to a reduction in the levels of heavy metals in the grains and an increase in dry weight production, yield and Se content in the grains	Hussain et al. (2020)
Oryza sativa L	40 mg L−1 (0.5 mM) Se (Na2SeO3) + 2.5 mmol L−1 Si (C8H20O4Si) + heavy metal stress	Pot and field	Foliar	Abscence of synergism between Se and Si	Demin et al. (2022)
Oryza sativa L	+40 mg L−1 (0.5 mM) Se (Na2SeO3) + 2.5 mmol L−1 Si (C8H20O4Si) + heavy metal stress	Field	Foliar	Abscence of synergism between Se and Si	Gao et al. (2018)
Oryza sativa L	0.5 mM Se + 1.5 mM Si + drought stressa	Green house	Foliar	The synergism between Se and Si was due to the reduction in water loss and increase in chlorophyll levels, growth and rice yield	Ghouri et al. (2021)
Triticum aestivum L	40 mM Se + 40 mM Si + salinity stressa	Green house	Foliar	The synergism between Se and Si was due to the improvement in growth, water relations, photosynthetic characteristics, respiration rate, chlorophyll contents, antioxidant enzymes, proline, total soluble sugars and dry weight	Sattar et al. (2017)
Triticum aestivum L	40 mM Se + 40 mM Si + drought stressa	Green house	Foliar	The synergism between Se and Si occurred due to the increase in the activity of antioxidant enzymes, photosynthetic attributes, chlorophyll content, transpiration rate and water relations, resulting in stimulation of seedling growth	Sattar et al. (2019)
Triticum aestivumL	30 mM Se (Na2SeO4) + 30 mM Si (Na2SiO3) + salinity stress	Field	Foliar	The synergism between Se and Si was due to the increase in plant growth, number of tillers, shoot dry weight, performance, membrane stability index, relative water content, total soluble sugars, CAT, SOD and APX activity, anatomical traits of leaves and stems and plant yield	Taha et al. (2021)
Boehmeria nivea (L.) Gaud	1 µM Se (Na2SeO3) + 1 mM Si (Na2SiO) + heavy metal stress	Hydroponic	Nutrient solution	The synergism between Se and Si was due to a reduction in the levels of malandialdehyde and an increase in the activity of antioxidant enzymes and dry weight in shoots and roots	Tang et al. (2015)
Oryza sativa L	3 µM Se (Na2SeO3) + 1.5 mM Si (Na2SiO3·9H2O) + + heavy metal stress	Hydroponic	Nutrient solution	The synergism between Se and Si was due to the increase in glutathione, shoot dry weight and reduction in the accumulation of Cd in the roots	Huang et al. (2021)
Zea mays L	0.5 μM Se (Na2SeO3) + 1 μM Si (silicic acid) + salinity stress	Hydroponic	Nutrient solution	The synergism between Se and Si was due to the increase in the net efflux of Na+ and dry weight of the plants, in addition to reducing the influx of Na+ in the roots and positively regulating the genes responsive to saline stress. (ZmNHX e ZmSOS2)	Xu et al. (2021)
Crops without stress application
Cucumis sativus L	0.025 mM Se (Na2SeO3) + 1 mM Si (Na2SiO3)	Field	Foliar	The synergism between Se and Si was due to the increase in growth, yield and nutritional quality of the fruits	Hu et al. (2022)
Phoenix dectylifera L	0.01–0.02% Se (Na2SeO3) + 0.05–0.1% Si (potassium silicate)	Field	Foliar	The synergism between Se and Si occurred due to the increase in leaf area, chlorophyll, percentage of N, P and K, yield and nutritional quality of the fruits	El-Kareem et al. (2014)
Oryza sativa L	240 mg kg−1 Se + 60 mg kg−1 Sia	Pot and Field	Soil	The synergism between Se and Si was due to the increase in grain yield, plant dry weight, SPAD index, antioxidant enzyme activity and reduction in malondialdehyde levels	Liu et al. (2020)

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Synergy Se and Si Applied in Nutrient Solution or in Soil to Mitigate Stress in Plants

Studies that evaluated the interaction between Si and Si applied to plant roots are very limited compared to the application of these elements to leaves, with reports only on the mitigation of stress by heavy metals and salinity.

An initial study was carried out to verify the interaction of Se and Si during the process of absorption of these elements by the plant. It was found that the effect of Si on the Se concentration in the shoot and in the roots of tomato seedlings was smaller than that of P (Wang et al. 2020). The authors observed that the Se concentration in the root decreased in treatments with high Si content at pH 3, but was not affected at pH 5 and 8, probably due to the specific permeability of H₂SeO₃ mediated by Si influx transporters that are present in the tomato plants. An important fact verified by the authors is that increasing the pH of the solution, the proportion of H₂SeO₃ species gradually decreased and approached 0 at pH 4. Therefore, the addition of Si did not affect the uptake of selenite by plant roots at solution pH levels above 5. A similar result was reported by Zhao et al. (2010), who indicated that at pH 5.6, the Se concentration in rice roots did not change with increasing Si concentration in the nutrient solution. Studies are still lacking to better understand the effects of Si on the expression of genes associated with Se transporters and vice versa.

In rice plants grown under the presence of Cd, Huang et al. (2021) found that heavy metal impaired plant development due to the accumulation of Cd in the root, which caused an increase in lipid peroxidation and a decrease in the production of shoot dry weight. However, the authors reported that the combined application of Se and Si, compared to their individual application, was more efficient in attenuating stress as it decreased Cd absorption and increased glutathione and phytochelatin synthesis and consequently decreased the content of Cd. malondialdehyde and increased shoot dry weight production.

A study on Boehmeria nivea (L.) cultivated in the presence of Cd caused an increase in lipid peroxidation and a decrease in biomass production, but this was mitigated due to the associated application of Se and Si (Tang et al. 2015). This was because the joint application of these two elements in relation to the isolated application stood out as it resulted in lower levels of malondialdehyde and greater activity of superoxide dismutase, glutathione reductase resulting in greater production of dry weight in the shoots and roots of the crop.

It is possible that the application of Si may be strongly bound to plant cell walls and compose organosilicon (Si-O‑C bond) and compartmentalized in vacuoles, which can co-localize Cd and form insoluble Cd-Si complexes in the cell wall (Fan et al. 2016; Wu et al. 2018). While the application of Se can increase the production of phytochelatins favoring chelation with Cd to detoxify Cd, which is related to the synthesis of selenoenzymes (e.g. glutathione) (Rizwan et al. 2021; Hussain et al. 2020) and this may restrict Cd translocation in the plant (Huang et al. 2018). Furthermore, foliar application of Se and Si can decrease transcription of genes, for example, related to the Cd transporter (OsLCT1, OsCCX2, TaCNR2 and OsPCR1) that are downregulated to decrease Cd translocation from leaves or peduncle to brown rice (Wang et al. 2020).

An experiment in corn plants evaluated the salt stress that was responsible for the decrease in growth and dry weight production (Xu et al. 2021). The authors observed that the combination of Se and Si relative to individual application was more effective to upregulate saline stress-responsive genes, including ZmNHX and ZmSOS₂. They concluded that the synergistic effects of Si and Se against salt stress were caused by the upregulated expression of transporter genes that act in the uptake, exclusion and sequestration of sodium in the vacuoles of the roots, favoring the increase in the production of shoot dry weight of the plants.

In general, the previously discussed studies that reported the synergy of Se and Si in hydroponic studied the concentration of Se in the nutrient solution that ranged from 0.5 to 3 µM in the form of sodium selenate while that of Si ranged from 1 to 1.5 mM in the form of sodium silicate (Table 1). It should be noted that the predominance of the use of Se in the works in the form of sodium selenate in relation to sodium selenite is probably due to the possibility of lower risk of causing toxicity in plants.

Information on the synergistic effects of Se and Si applied to the soil is very limited, with only one study (Se = 240 mg kg⁻¹ and Si = 60 mg kg⁻¹) (Table 1) (Liu et al. 2020). The authors concluded that these synergistic effects of these elements are due to the benefits on the plant related to the increase in grain yield, plant dry weight, SPAD index, antioxidant enzyme activity and reduction in malondialdehyde levels.

Synergy Se and Si Applied Via Foliar or Root in Plants Without Application of Stress

The effects of the interaction between Si and Se applied via foliar were also reported in plants cultivated without stress induction. Thus, Hu et al. (2022) reported that the combined application of Se and Si in relation to the isolated application in the cucumber crop was more efficient in increasing photosynthesis, yield and the content of Se and Si and amino acids in the fruits, favoring their quality.

The synergistic effect of Se and Si was also verified in the mango crop due to the increase in yield and fruit quality (Ahmed et al. 2018). El-Kareem et al. (2014) also reported a synergistic effect of Se and Si in relation to the individual application of these elements in Phoenix dectylifera L. plants by increasing the chlorophyll content, nitrogen, phosphorus and potassium contents in the leaves, resulting in an increase in yield and nutritional quality of the fruits.

The synergism between Si and Se applied via the root was also reported in plants grown under normal conditions without inducing stress. In rice cultivars, Liu et al. (2020) found that the combined application of Se and Si via soil in relation to individual application resulted in greater increases in the SPAD index, in addition to decreasing malondialdehyde levels and increasing antioxidant enzymes, resulting in greater crop yield.

In general, the research carried out to date indicates that the interaction of Si and Se is not important during the process of absorption and transport of the two elements by plants, however, their joint action is much more evident, strengthening the system of plant antioxidant defense from the increase of non-enzymatic and enzymatic compounds, enhancing their biological benefits in the plant.

The effectiveness of Si on overall plant performance depends on both Si and nutrient tissue concentrations, so that the existence of a feed-back loop between Si and nutrients is self-evident although the nature of this cross-linkage is unclear (Pavlovic et al. 2021).

Final Considerations and Future Research Challenges

With the advancement of climate change and its impacts on agriculture combined with population growth and growing demand for food by the population, new practices in plant mineral nutrition to promote plant defense against abiotic stresses should be used to ensure food security. Within this context, the combined application of Se and Si may be promising to promote an increase in the efficiency of enzymatic and non-enzymatic antioxidant metabolism in plants, resulting in greater tolerance to abiotic stresses (Fig. 2). It was verified by the review that most of the works showed that the synergy of Se and Si promoted the reduction in oxidative stress, favoring the physiological and biochemical processes and consequently the production of plants grown under stress and without stress conditions.

Fig. 2.

The combined application of Se and Si can increase the tolerance of plants against saline stress (a), drought (b), and presence of heavy metals (c)

In addition, the use of these two elements can increase the content of these elements in the grain, favoring the nutritional quality of food. The works by Hussain et al. (2020), Wang et al. (2020) and Hu et al. (2022) demonstrated that the combination of Se with Si, in addition to attenuating stress, was more efficient in increasing the Se content in grains. In times of a pandemic by COVID-19, increasing the content of Se in grains and ingestion of the element by people is extremely important to increase immunity by producing antibodies (Bae and Kim, 2020; Bermano et al. 2021) and for improving the function of cytotoxic effector cells in COVID-19 (Bae and Kim 2020) decreasing risk of death (Moghaddam et al. 2020).

An important aspect in the review is the fact that the researches were carried out with the application of Se and Si via foliar or root route. When Si and Se are applied in a combined way via foliar, there is divergence in the synergism of these elements in relieving the stress caused by heavy metals. This may have probably been influenced by the difference in the source of the elements used (Table 1). On the other hand, when Se and Si were applied in the form of nanoparticles, there was synergism between these elements in all studies found. So, the occurrence of the synergistic effect when Se and Si are applied via foliar can be influenced by the used source of the elements. Nano-Se and nano-Si are considered promising nanoparticles in agriculture due to their significant roles in biological systems (El-Ramady et al. 2018). Unlike Se and Si sprayed on leaves, when the application was performed on the roots, no studies were found that differed the results on the synergism of these elements in relieving the stress caused by heavy metal.

Thus, it was clear in the review that the benefits of this interaction occurred when provided via foliar and root in species accumulating or not of Si. Thus, this may be indicative that the beneficial effect of Si and its interaction with Se does not depend on the high absorption of Si, which could imply a physical effect of Si accumulated in the leaf, but a biochemical effect, that is, its effect on the antioxidant defense system.

In general, the interaction of Si and Se in plants with or without stress can be more evident when optimal concentrations of these two elements are used, as it guarantees adequate plant nutrition, a fundamental fact to favor the benefits of the interaction. This is important because it has some implications because at high concentrations of Si in solution, although it cannot cause toxicity in plants, it can induce Si polymerization and decrease the absorption of this element, decreasing its beneficial effects. While high concentrations of Se occur the opposite of Si, that is, it can cause severe toxicity, harming its benefits in plants. There are reports that concentrations above 8 µM of Se in the nutrient solution cause oxidative stress and chlorophyll degradation, limit the carboxylase activity of ribulose biphosphate, stomatal closure, impairing the photosynthetic rate of the plant (Da Cruz Ferreira et al. 2020).

Thus, new research should emerge to consolidate the synergism between Se and Si and to improve the knowledge of the factors that influence this phenomenon in order to increase plant resistance to environmental stresses or enhance plant growth under stress-free cultivation.

Thus, considering the current information, we can suggest new approaches to research on the interaction between Si and Se in plants. (1) Additional studies involving Se and Si interaction in crops with nutritional disorders of a third element. For example, this interaction may be much more evident in crops with S deficiency, which is common in sandy textured soils with low organic matter content, as according to Barreto et al. (2022) Si is an important mitigator of S deficiency in different species. This is important because the similar role of Se in S metabolism is known (White 2018). (2) Expand the studies involving the application of these elements in plants in the nutrient solution or especially in the soil, since most studies were carried out with application via foliar (3) Research using different doses or concentrations of Se and Si is essential, since most studies were carried out with only one concentration or dose. This is fundamental for a better understanding and potentiation of the interaction of elements in the biological processes of the plant (4) It is essential to expand studies on plants subjected to other abiotic stresses, as current studies restrict only three stresses (saline, water deficit and toxicity of Cd and Pb metals), but there are many others that are harming agriculture. In addition, it is known that in the field, different stresses often occur at the same time, so studies involving multistresses are opportune. (5) It is also possible to advance studies on these elements in stress-free crops as the information is incipient, but there are indications that it is possible to increase the productivity and quality of crop production. (6) The associated effect of Si and Se in food biofortification is still little explored, but it is very important because they are two elements considered essential for humans and animals. There are studies with individual application of Si (Dos Santos et al. 2020; Garcia Neto et al. 2022) and Se (Silva et al. 2020) indicating the feasibility of applying these elements for biofortification (7) Current studies have contributed little to elucidate the effects of these elements at genomic, metabolomic and proteomic levels. This should improve the understanding of the effects of Se and Si using themselves at low concentrations that normally occur with foliar applications but sufficient to improve plant growth or yield in stressed or even non-stressed crops (8) expand the studies in non-Si accumulating plants since most of the studies in the review were carried out in Si accumulating plants as it can affect the interaction of Se and Si. Thus, it is necessary to evaluate whether the mechanisms of Si to attenuate the stresses seen in accumulating plants could be or not the same in non-accumulating species. (9) increase studies at the subcellular level to investigate the distribution of Si and Se in organelles as it could improve the understanding of the effects of these elements in increasing plant tolerance to abiotic stress. (10) Studies involving the dynamics of the Se and Si interaction in the soil are still unknown. It is possible that these elements can benefit the soil microbiota by contributing to attenuate different abiotic and biotic stresses, but this needs to be proven.

In general, the advancement of knowledge on the interaction of Se and Si will be able to more accurately predict their benefits and the responses of different crops and may be a strategy to mitigate abiotic stress and at the same time enhance crop growth and yield. Even the use of these two elements in solution, whether for foliar application and especially in fertigation in field crops and even in crops in a protected environment from hydroponic crops, should require relatively low doses per area of Se and Si when compared to macronutrients, a fact that could favor an excellent benefit/cost ratio of its use in agriculture. This is important because the combination of stress relievers is a strategy that should be further studied to increase the sustainability of crops and at the same time produce better quality food, benefiting food security and human health.

Matheus Luís Oliveira Cunha

is agronomist and Master’s Degree student at São Paulo State University (UNESP). Her research focus is related to plant nutrition and physiology to mitigate abiotic stresses.

Conflict of interest

M. Luís Oliveira Cunha and R. de Mello Prado declare that they have no competing interests.