key: cord-0060322-tan0xxk9
authors: Verma, Krishan K.; Song, Xiu-Peng; Lin, Bo; Guo, Dao-Jun; Singh, Munna; Rajput, Vishnu D.; Singh, Rajesh Kumar; Singh, Pratiksha; Sharma, Anjney; Malviya, Mukesh Kumar; Chen, Gan-Lin; Li, Yang-Rui
title: Silicon Induced Drought Tolerance in Crop Plants: Physiological Adaptation Strategies
date: 2021-03-22
journal: Silicon
DOI: 10.1007/s12633-021-01071-x
sha: 37ebf8945460d9aed09c2d71705ce71151ff58a2
doc_id: 60322
cord_uid: tan0xxk9

nan

Water deficiency is a serious abiotic stress that sharply inhibits plant performance of agricultural crops in arid and semi-arid areas globally [1] [2] [3] . The water-stressed areas are getting expanded @ 30% per annum for various reasons viz., by global warming. Thus, increasing crop sufferance to climatic variables are a major factor to overcome declining agricultural foodstuffs producing system and to meet the requirement of nutriment supply for ever-enhancing global populae [3] [4] [5] . Globally, it has been evaluated that around 51-82% of production loss found directly associated with environmental stresses [6] .

Silicon (Si) is a ubiquitary constituent and the next most abundant after O 2 in soil. It is a major constituent (28% on dry weight basis) of the planet, forming the silicate minerals, i.e. silicate or aluminum silicate, may be differentially absorbed by the plants [7] [8] [9] . The significance of Si for agro-ecosystems has been the topic of discussion [10] [11] [12] [13] . Since the Pioneer workers as Sprengel and van Liebig in the initially to middle-1800s [14] , and the rectified by Arnon and Stout [15] , plant physiologist/ ecologist have updated the rejection of Si from the catalog of key mineral nutrients for vascular plants [16, 17] . The application of silicate on plants initiated in 1950s in Japan, and by now it is frequently used in various regions of the globe [18] [19] [20] . The plants have been categories on the base of their Si accumulation levels, and recognized as minimum, medium and maximum accumulators [9, 21] .

The International Conference on Silicon in Agriculture (ICSA) is only silicon conference of its kind that is created to discuss/ presented updated developments, significant issues of the application of Silicon in agro-ecosystems. The ICSA is held on a triennial basis around the globe. Originating in Florida, United States (1999), then Tsuruoka, Japan (2002), Uberlandia, Brazil (2005) , Durban, South Africa (2008), Beijing, China (2011), Stockholm, Sweden (2014) and Bangaluru, India (2017) have been successfully held to understand the role of Si application linked with plant productivity under various environmental stresses. Further, ICSA was proposed in New Orleans, Louisiana, USA (2020), cancelled due to coronavirus outbreak, and rescheduled in May 23-26, 2022. In this chapter, the recent and updated development of silicon role in plant resilience to drought stress and the underlying mechanisms and/or functions have been compiled to reveal the role of Si to enhance crop productivity under arid and semi-arid agroclimatic cropping zones (Fig. 1) .

to power the biochemical activities of plants [22, 23] . Growth and development of plants are mostly depends on photosynthetic efficiency. Photosynthetic capacity is down-regulated under stress conditions [2, 24] , and exogenous application of Si has been found to improve this reduction and thus sustain normal growth and development of plants subjected to drought-stressed plants. Gong et al. [25] and Verma et al. [2] noted that under limited irrigation, Si in Triticum and Saccharum spp. had conferred maximum photosynthetic efficiency. The similar findings were assessed in Sorghum [26, 27] and rice plants [28] . The photosynthesis of sorghum and sugarcane in water stress conditions were improved, incase Si applied which resulted in highest dry biomass exposed to stress conditions [3, 26] . The stomatal and non-stomatal limitations have been due to the reduction in photosynthetic responses during water stress [29] . Stomatal closure is the initial reaction of plants under extreme drought stress, and is commonly considered to be the key limiting factor for the exchange of leaf gas [30] [31] [32] . Although stomatal closure normally occurs during unfavorable atmospheric variables, nonstomatal limitations, such as decreased carbon fixation ability by chloroplast can down-regulate photosynthetic efficiency under stress as well.

Chen et al. [28] indicated that Si-mediated enhancement of leaf gas exchange ability in Oryza sativa plants during water-stressed was correlated with stomatal and nonstomatal variables. Gong and Chen [33] and Verma et al. [34] demonstrated the diurnal variations of photosynthetic traits in wheat and sugarcane plants subjected to water stressed conditions with loss in stomatal conductance. However, Si enhanced the photosynthetic efficiency during day time. In the daily cycle, Si amendment also enhanced the stomatal conductance during water stress. These observations suggested that stomatal and non-stomatal variables were associated in the maintenance of photosynthetic CO 2 assimilation by Si. Ming et al. [34] and Verma et al. [2] demonstrated that Oryza and Saccharum spp. both enhanced transpiration incase Si application safe and subjecting to drought stress. Several other results on water-stressed plants have been reported by Si amendment to be consistent with increased leaf transpiration [3, 26-28, 33, 34, 36-42] Verma et al., 2021) (Figs. 1 and 3; Table 2 ).

Photosynthetic pigments play an important role in the ability to photosynthesize and are responsible for complex light harvesting. Lobato et al. [43] and Verma et al. [2, 41] explored that the use of Si could increase the quality of chlorophyll in Capsicum annuum and Saccharum spp. during drought, indicating that Si may mitigate stress-induced damage to the photosynthetic machinery and thus sustains the leaf gas exchange. According to Yin et al. [44] , Si-mediated increase in polyamine synthesis and enhancement of chlorophyll content was noted, indicating that polyamines are involved in improving the overall photosynthetic pigment content and thus delaying leaf senescence. Applied Si not only improved the content of chlorophyll, but also enhanced the ratio of chlorophyll a/b, showing the level of thylakoid stacking [9, 45] . Leaf chlorophyll fluorescence variables may provide useful tools for plant photosystem II (PS II) activity, and PSII photochemistry's minimum and maximum quantum efficiency are linked to photosynthetic capacity [3, 46] . Amendment of Si not only enhanced the chlorophyll content but also enhanced ration of Fv /F 0 and Fv /fm in Oryza sativa and Saccharum spp. during water stressed [2, 28, 41] . The Si amendment increased the activity of RuBisCO in hydroponically grown cucumbers [47] . Gong and Chen [33] observed that in water-stressed plants, the concentration of inorganic phosphorus in Triticum aestivum leaves was reduced while it was upregulated by applying Si. Improving the inorganic phosphorus content will promote the synthesis of ATP needed in the CO 2 assimilation cycle [48] . In photosynthetic reactions, more work is needed to disclose the significant of Si because very little is known on Si′s role in diurnal variations in plant photosynthetic efficiency [34] .

In addition to improved photosynthetic assimilation rate, the improvement/maintenance of plant growth and development through the application of Si during abiotic stresses may be linked to nutrient uptake. Drought stress and other abiotic stresses limit the absorption by plant roots of essential nutrients and their transport to leaf shoots and thus reduce the supply of nutrients [2, 31, 49] . In improving/maintaining the absorption, transport and allocation of mineral nutrients in stressed plants (Fig. 3) . Silicon also plays an important role and thus increases plant output in unfavorable environmental variables. The Si amendment was observed in the Zea mays leaves to increase the Ca and K content [49] . Improved uptake of Ca and K during abiotic stresses could also lead to stress tolerance. The reduction in plasma membrane permeability and up-regulation of plasma membrane H + -ATP activity in Si-applied plants may extend plant performance [32, 49, 50] .

Detmann et al. [51] demonstrated that Si could enhance the productivity of rice grains and the efficiency of N use. More scientific research is therefore needed to determine how Si controls the absorption of P and N and what factors influence the efficacy of Si. Nutrient uptake is associated with root growth, morphological traits such as diameter, area, volume, root dry bulk, total and main root length [52] [53] [54] . The significant role of Si on root development during stress have been noted in few studies because stress may condition, reduce shoot/root ratio and maximum accumulation of root dry biomass in sorghum [26] . The root growth stimulation by Si application may be correlated with root elongation by increasing the extensibility of cell walls in the sorghum growing area [55] .

Water and essential mineral elements are absorb by the plant roots, water stressed plants can have a harmful effect on plant growth, development by proper root development [41, 56] . An increment in the plant root's morphological traits provides additional exposed areas for the absorption of diffusible ions [56, 57] . Increased root growth caused by the application of Si has also been documented in various drought studies [53, 54, 58] . The up-regulated root/shoot ratio in Si-applied plants suggests that Si-mediated root ultrastructure modifications can also account for Siapplied plants' increased water uptake capability [36] (Figs. 1 and 2).

During favorable situation, water is absorbed by the plant roots and reduced from the plant leaves, and plants maintain natural water balance by constantly changing these processes [32, 59] . Plants have a particular technique to improve and/or balance of water when conditions are unfavorable [60] . Drought stressed conditions, the first reaction of the plants to prevent low water potential by changing its water balance between root water absorption in leaf water [9, 61, 62] . Plants may decrease loss of leaf water by regulating the transpiration rate and also by reducing their leaf area-expansion. The Si amendment may enhance/ balance water status in stressed plants. Silicon could enhance the water potential of leaves in plants subjected to water-stressed [27, 63, 64] . The rate of transpiration may affect plant water relationships [2, 31] . Plants mainly transpired by leaves, via the cuticle and stomata. The significant role of Si in plant development and water retention has long been linked to the mitigation of abiotic stresses. Several studies have well recorded the improvement of leaf stomatal conductance, transpiration rate, leaf water content (LWC) and root and whole plant hydraulic conductance by plants irrigated with Si [3, 65, 66] (Figs. 2 and 3; Table 2 ). The hydraulic conductivity of the roots mainly depends on the capacity of water absorption and root anatomypermeability and driving force [67] . The status of water in leaf is determined by the absorption and uptake of water, i.e. the loss of transpirational water [68] . The variation in transpiration rate is different mechanism by which plants may regulate water status [32, 69] .

The findings indicated that Si was correlated with stomatal movement control. The decrease in transpiration from the cuticles or stomata may be associated with the conditions of plant species/cultivars or culture. However, application of Si does not always reduce the rate of plant transpiration. Hattori et al. [26] and Verma et al. [2] evaluated Si application during water-stressed conditions to improve stomatal conductance

and transpiration rate of plant leaves in sorghum and sugarcane. Stressed wheat [25, 63] , rice [28] and sugarcane [41] were also found to have similar findings. In order to understand how and if Si regulates stomatal movement and what potential factors are involved, further studies are required. Another potential reason may be correlated with growth conditions, such as soil or solution culture, for the differences in transpiration rate [70] . Plants typically enhance root length and/or root area to achieve better access to water. The roots are still in contact with water, and plants need to adjust by increasing their internal hydraulic conductivity to drought stress [9, 28, 70] .

In addition to leaf transpiration, the close relationship between transpiration rate and Si suggests that there may be some mechanisms that linked to Si-mediated avoidance to abiotic stresses. Water root uptake is a very critical process that improves the balance of water in crop plants. Silicon can influence the growth and development of roots and thus regulate water relations. The amendment of Si enhanced the root development during stress [2, 28] . Si-mediated increment in root area zone have also been monitored in sorghum and sugarcane subjected to water-stressed plants [3, 44] . The development of root was correlated with root plasticity modulation, which has been regulated by Si-mediated enhance in polyamine and reduce ethylene contents. The improved root area may increase the absorption of water, which helps to enhance the avoidance of water stress by plants. The induced aquaporin activity inhibition. The role of higher root hydraulic conductance enhanced the uptake and transport of water, which promote to maintain maximum leaf photosynthetic efficiency and improve plant tolerance to stressed condition. The proposed model mechanism was conceived based from the following literature [2, 32, 42, 66, 72, 89, 90] . Dotted and line arrows indicate down-regulation and upregulation of genes (with Si application). SbPIP -Sorghum Plasma membrane intrinsic protein significant observation was noted in shoot development [23, 71] .

The cucumber plants has also been monitored for similar results [70] . Drought stress enhanced the root hydraulic resistance to the flow of water, which was significantly reduced by the application of silicon in sorghum [72] . During drought stress, osmotic adaptation plays a major role in the uptake of root water [73] . The Si amendment decreased the osmotic potential of the roots and increased the relative content of leaf water, indicating that the role of osmotic adjustment in the absorption of root water in Si-applied plants [36] . In Oryza sativa, Si-induced improved water status and accumulation of soluble sugars in plant roots [74, 75] . Proline is one of the major compatible solutes normally accumulated during stress and plays a significant role in osmotic adjustment [64, 76, 77] . Compatible solutes or osmolytes increase primarily proline, glycine betaine and polyols in plants during stress, i.e. drought [64, [77] [78] [79] . By stabilizing proteins and their complexes, as well as membranes during abiotic stresses, the above compounds can mitigate the limiting factor of raising the maximum ion content of enzyme activities [8] .

There are various scientific reports that Si fertilizer may also boost plant resistance to water deficit by changing the content of solutes, i.e. proline [2, 65, 80] , glycine betaine [81] , carbohydrates [35] , polyols, antioxidant compounds such as total phenolics [82] , soluble sugars and free amino acids [36, 83] ( Table 1) .

Pei et al. [64] and Verma et al. [2] showed that the proline level was enhanced during drought in Triticum aestivum and Saccharum spp. leaves, while the use of Si reduced its accumulation, implying that proline accumulation was a sign of injury to stress. The improvement of osmotic adjustment in terms of osmolytes enhancement through the application of Si [32, 37] . Aquaporins are significant facilitators of water transport in plants for root water absorption [84, 85] . Aquaporins can be subdivided into various subfamilies such as PIPs -intrinsic plasma membrane proteins, TIPs -intrinsic tonoplast proteins, SIPs -small basic intrinsic proteins, NIPs -intrinsic nodulin26-like proteins and XIPs -uncharacterized X intrinsic proteins [86] . PIPs and TIPs represent the core pathway of water transport between and within cells among these aquaporins [9, 32, 84] . The Si amendment could boost root hydraulic conductance and induce the enhancement of PIP gene expressions during drought in sorghum plant, suggesting that Si regulating water uptake during stressed ( Fig. 3 ; Table 1 ). Silicon may influence the formation of casparian bands in plant roots [32, 87, 88] . Little information is available on the relationship between Si and the metabolism of compatible solutes and the absorption of water in plants, and further research is expected in the near future.

The primary targets of abiotic stresses include plant cell membranes such as plasma and endomembrane [9, 91] . The balance/maintenance of membrane cell integrity is essential Copper transporter Transporter gene [102] for the growth of plants in unfavorable environmental variables. The leakage (EC) of electrolytes from the cell has long been used as a symbol of membrane damage. The Si amendment indicated to reduce the EC level in Oryza sativa, Glycine max and Triticum aestivum during water-deficit conditions [64, 91, 92] , showing the defensive role of Si against membrane damage. Silicon effects the changes of cell morphology in wheat during water-stress [93] . The thylakoids were swollen in chloroplast, and the matrix lamella seemed to be degraded/damaged; this resulted in a decrease in plasmolysis when Si was fed to the plants and upgraded the ultrastructure of chloroplast. The leaf ultrastructure was also upgraded in the Si-applied plants (Figs. 2 and 3 ) cultivated during water stress [2, 35] . For the function of the cell membranes, fluidity is a simple and important parameter. Huang and Yang [94] demonstrated that the balance fluidity in the mitochondrial membrane was important to maintain its structure. However, little information is available on the role of Si in the fluidity of the membrane in plants during stress. Membrane fluidity is depends on various factors, i.e. interactions of proteins, lipids and lipid composition [9, 32, 94, 95] . Si was shown in strawberry plants to increase the amount of membrane lipids during control conditions with Si application [96] . Therefore, there is a possibility that the amended Si enhanced the contents of phospholipids and proteins both, with a greater increase of the latter. A downregulation in fatty acid unsaturation is generally considered to result in a decrease in membrane fluidity [97] . Si enhanced the unsaturation of fatty acids, which was reduced during water-deficit condition [25] . The upregulation of plasma membrane H + -ATPase activity by Si can also lead to enhance uptake of nutrients during stress as discussed previously [28, 49, 64, 90] .

The various strategies of Si-mediated stress mitigation, the primary stress-combating mechanisms utilized by Si is the maintenance of photosynthetic process in the stressed plants. Even though, several demonstrations have evidenced the potential role of Si on leaf gas exchange, only a little have assessed the molecular mechanism behind the expression of genes upon Si application [33, 77, 90, [98] [99] [100] [101] . The physiological improvement of photosynthetic mechanisms and loss in the degradation of photosynthetic pigments documented by various pioneer workers can be linked with the genic regulation of photosynthetic genes by Si at molecular basis (Table 1) .

Si had no effect on the plasma membrane H + -ATPase in field conditions, indicating that Si effect on membrane fluidity and enzyme activity could be secondary [103] . Si in Oryza sativa plants significantly increased the polysaccharide content in the leaves [32, 91] . Si also affects other components of the cell wall, including pectic acid, protein, polyphenols and lignin [9, 87, 104, 105] . These results indicated that Si is involved in the biosynthesis of components of the cell wall, thereby influencing the mechanical properties of the cell walls and, consequently, their water permeability. The effects of Si on the properties of the cell wall can in turn affect the properties of the membrane [32, 91] ; this needs to be confirmed by mechanistic experiments, however. The enhanced antioxidant defense mechanism is also interconnected with Simediated balance/improvement in membrane integrity and stability in stressed plants.

The degree of drought tolerance varies from species to species and also from different genotypes, whereas there will be differences in the individual plants of the same cultivars. By completely distorting the control mechanism, the plant water deficit will adversely affect the electron transport chain (ETC) in mitochondria and chloroplasts [106] . One of the immediate effects of developing reactive oxygen species (ROS) on water stressed plants. ROS is a potent oxidizing compound that can damage the functions of the plasma membrane and endomembrane [107] . Developed a complex antioxidant machinery to improve/ balance homeostasis through enzymatic and non-enzymatic antioxidants such as superoxide dismutase (SOD), peroxidase (POD), catalsae (CAT), ascorbate peroxidase (APx), dehydroascorbate reductase (DAR), guaiacol peroxidase (GPOD), glutathione reductase (GR) and carotenoids, non-protein amino acids, phenolic compounds tocopherols, ascorbate and glutathione are one of the strategies to mitigate and restore the damage caused by ROS in plants [2, 41, 58, 108] . Si can also minimize oxidative damage in plants during stress by regulating the antioxidant machinery of the plants [8, 108] . Oxidative damage during water-stressed conditions due to overproduction of ROS is also mitigated by exogenous Si application. The foliar spray of Si upregulated the SOD level in barley, tomato and sugarcane [3, 63, 68, 77] especially in low available water in soil content [109] . The amendment of Si enhanced the level of SOD and reduced POD activity in barley crop at the rain filling stage [63] of water-deficit plants. Silicon significantly enhanced the glutathione level in wheat leaves subjected to drought, presumably due to enhanced GR activity ( Table 2) .

One of the most destructive processes in living organisms is ROS-induced oxidation of functional molecules such as lipids, proteins, etc. [9, 29, 110] . Different demonstrations have shown that the avoidance of Si-mediated plants toward environmental stresses, i.e. salinity and heavy metal stress, is correlated with decreased oxidative damage of cell functional molecules. The amendment of Si enhanced the photosynthetic pigments and protein levels in stressed wheat and sugarcane Silicon Silicon plants [2, 25, 42] . In various crops, i.e. chickpea, sunflower, soybean, rice and sugarcane, the Si-mediated reduction in lipid peroxidation damage during drought [41, 74, 92, 111] . The enhancement of GR activity could lead to the upregulated level of GSH [64] . Loss of phospholipase activity in Si-applied plants during water stress indicates downregulation of phospholipid de-esterification damage in stressed plants [25, 112] . The CAT and POD activities were stimulated and the SOD activity decreased during water stress in Glycine max plants, while their activities and levels of H 2 O 2 were all reduced by Si [92] . The impacts of Si on SOD and CAT activities were plant species dependent [85, 101] . ROS is central to cell signaling and affects a broad range of essential and cascading processes, like gene expression, growth, programmed cell death and a suite of stress responses [110, 113] . Therefore, it is no surprise that down-regulation of ROS with the Si application results in different downstream changes during unfavorable situations [9, 13, 101, [114] [115] [116] . Differences in regulatory positions can be intertwined with plant cultivars and circumstances of stress.

Overall, it can be concluded that Si can reduce oxidative damage in crop plants by regulating enzymatic and nonenzymatic antioxidant protection systems on the basis of the available findings [8, 41, 108] . Therefore, understanding the mechanisms that regulate stressed ROS signaling at the cellular level will provide a more successful strategy for improving resistance to extreme water-stressed conditions. To explain how silicon starts these responses, further studies are also needed. In addition, different results are obtained from studies of the floating water system, and thus further demonstrations are needed during the cultivation of agro-farming systems. [64] also demonstrated that Si enhanced ABA level in Triticum aestivum leaves cultivated under drought stressed condition ( Fig. 1 and Table 1 ). Silicon may increase the plant adaptation to water deficit by balancing the plant hormones levels [101, 120] . Silicon has maintained/enhanced the water stress resistance of sorghum by altering polyamine and 1-aminocyclopropane-1-carboxylic acid (ACC) synthesis [44] . Similar findings have been observed regarding the function of Si in promoting cytokinin biosynthesis and its involvement in leaf delay in arabidopsis and sorghum plants [116, 121] . Yin et al. [65] showed that Si increased the resistance of water-stress in sorghum plants by interfering with the equilibrium between polyamines and the content of ethylene. The upregulated content of polyamines in favor of low output of ethylene and decreased ACC level in sorghum plants, thereby delaying the process of leaf senescence and improving water deficit avoidance [44, 77] .

In summary, the published reports point to silicon's role in stimulating the endogenous content of some plant hormones during environmental stresses. The research results mentioned are still limited, however, and evidence of a direct involvement between Si and biosynthesis of plant hormones is currently unavailable. Further research is therefore required to resolve the potential effects of Si on further phytohormones, while also concentrating on the physio-biochemical role of Si, hormone biosynthesis, and hormone signaling of plants under water stress.

The application of silicon appears potentially significant to enhance crop productivity linked with agro-industries may be an enormous challenge for near future to feed the people globally with ecofriendly ecosystem with improve sustainable agricultural crop efficiency/ carbon transformation into the precisions food for future under limited water availability in arid and semi-arid agro-climatic zones worldwide.

Acknowledgments We are grateful to the Guangxi Academy of Agricultural Sciences, Nanning, Guangxi, China for providing the necessary facilities for this experiment.

Availability of Data All the supporting data/ findings of this study is available within the article. 

Conflict of Interest None of the authors has any financial or other relationships that could lead to a conflict of interest.

Consent for Publication Not applicable.

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Effects of silicon on H + -ATPase and H + -PPase activity, fatty acid composition and fluidity of tonoplast vesicles from roots of salt-stressed barley (Hordeum vulgare L.)

Silicon alleviates salt stress and increases antioxidant enzymes activity in leaves of salt-stressed cucumber (Cucumis sativus L.)

Silicon promotes cytokinin biosynthesis and delays senescence in Arabidopsis and sorghum

Hormone balance and abiotic stress tolerance in crop plants

A new perspective of phytohormones in salinity tolerance: regulation of proline metabolism

Phytohormones and plant responses to salinity stress: a review

Regulation of jasmonic acid biosynthesis by silicon application during physical injury to Oryza sativa L

The controversies of silicon's role in plant biology

Augmenting drought tolerance in sorghum by silicon nutrition

Effect of silicon on growth and development of strawberry under water deficit conditions

Silicon induced improvement in morpho-physiological traits of maize (Zea Mays L.) under water deficit

Influence of silicon fertilization on maize performance under limited water supply

Influence of foliar application of silicon on chlorophyll fluorescence, photosynthetic pigments, and growth in water-stressed wheat cultivars differing in drought tolerance

Effects of silicon and drought stress on biochemical characteristics of leaves of upland rice cultivars

Influence of silicon on sunflower cultivars under drought stress, I: growth, antioxidant mechanisms, and lipid peroxidation

Silicon supplementation improves drought tolerance in canola plants

Growth enhancement of rye by silicon application under two different soil water regimes

Regulation and physiological role of silicon in alleviating drought stress of mango

Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses

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