Silicon (Si) is the second most abundant element in soil, where its availability to plants can exhilarate to 10% of total dry weight of the plant. Si accumulation/transport occurs in the upward direction, and has been identified in several crop plants. Si application has been known to ameliorate plant growth and development during normal and stressful conditions over past two-decades. During abiotic (salinity, drought, thermal, and heavy metal etc) stress, one of the immediate responses by plant is the generation of reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide (O2−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH), which cause severe damage to the cell structure, organelles, and functions. To alleviate and repair this damage, plants have developed a complex antioxidant system to maintain homeostasis through non-enzymatic (carotenoids, tocopherols, ascorbate, and glutathione) and enzymatic antioxidants [superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)]. To this end, the exogenous application of Si has been found to induce stress tolerance by regulating the generation of ROS, reducing electrolytic leakage and malondialdehyde (MDA) contents, and immobilizing and reducing the uptake of toxic ions like Na, under stressful conditions. However, the interaction of Si and plant antioxidant enzyme system remains poorly understood, and further in-depth analyses at the transcriptomic level are needed to understand the mechanisms responsible for the Si-mediated regulation of stress responses.

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Silicon Regulates Antioxidant Activities of Crop Plants under Abiotic-Induced Oxidative Stress: A Review

Silicon (Si) has been known to augment plant defense against biotic and abiotic pressures. Maize (Zea maize L.) is classified as a Si accumulator and is relatively susceptible to alkaline stress. In this study, seeds of maize were grown in pots and exposed to various concentrations of Na2CO3 (0, 25, 50, and 75 mM) with or without 1.5 mM Si in the form of sodium metasilicate Na2O3Si.5H2O for 25 days. Alkaline-stressed plants showed a decrease in growth parameters, leaf relative water content (LRWC), and the contents of photosynthetic pigments, soluble sugars, total phenols and potassium ion (K(+)), as well as potassium/sodium ion (K(+)/Na(+)) ratio. By contrast, alkaline stress increased the contents of soluble proteins, total free amino acids, proline, Na(+) and malondialdehyde (MDA), as well as the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in stressed plants. On the other hand, application of Si by seed-priming improved growth of stressed plants, which was accompanied by the enhancement in LRWC, and levels of photosynthetic pigments, soluble sugars, soluble proteins, total free amino acids and K(+), as well as activities of SOD, CAT, and POD enzymes. Furthermore, Si supplement resulted in a decrease in the contents of proline, MDA and Na(+), which together with enhanced K(+) level led to a favorable adjustment of K(+)/Na(+) ratio, in stressed plants relative to plants treated with alkaline stress alone. Taken together, these results indicate that Si plays a pivotal role in alleviating the negative effects of alkaline stress on maize growth by improving water status, enhancing photosynthetic pigments, accumulating osmoprotectants rather than proline, activating the antioxidant machinery, and maintaining the balance of K(+)/Na(+) ratio. Thus, our findings demonstrate that seed-priming with Si is an efficient strategy that can be used to boost tolerance of maize plants to alkaline stress.

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Impacts of Priming with Silicon on the Growth and Tolerance of Maize Plants to Alkaline Stress.

1. Hundreds of single species studies have demonstrated the facility of silicon (Si) to alleviate diverse abiotic stresses in plants. Understanding of the mechanisms of Si-mediated stress alleviation is progressing, and several reviews have brought information together. A quantitative assessment of the alleviative capacity of Si, however, which could elucidate plant Si function more broadly, was lacking.
2. We combined the results of 145 experiments, predominantly on agricultural species, in a meta-analysis to statistically assess the responses of stressed plants to Si supply across multiple plant families and abiotic stresses. We interrogated our database to determine whether stressed plants increased in dry mass and net assimilation rate, oxidative stress markers were reduced, antioxidant responses were increased and whether element uptake showed consistent changes when supplied with Si.
3. We demonstrated that across plant families and stress types, Si increases dry weight, assimilation rate and chlorophyll biosynthesis and alleviates oxidative damage in stressed plants. In general, results indicated that plant family (as a proxy for accumulator type) and stress type had significant explanatory power for variation in responses. The consistent reduction in oxidative damage was not mirrored by consistent increases in antioxidant production, indicative of the several different stress alleviation mechanisms in which Si is involved. Silicon addition increased K in shoots, decreased As and Cd in roots and Na and Cd in shoots. Silicon addition did not affect Al, Ca or Mn concentration in shoots and roots of stressed plants. Plants had significantly lower concentrations of Si accumulated in shoots but not in roots when stressed.
4. Meta-analyses showed consistent alleviation by Si of oxidative damage caused by a range of abiotic stresses across diverse species. Our findings indicate that Si is likely to be a useful fertilizer for many crops facing a spectrum of abiotic stresses. Similarities in responses across families provide strong support for a role of Si in the alleviation of abiotic stress in natural systems, where it has barely been explored. We suggest this role may become more important under a changing climate and more experiments using non-agricultural species are now needed.

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Consistent alleviation of abiotic stress with silicon addition: a meta-analysis.

Silicon (Si), the quasi-essential element occurs as the second most abundant element in the earth’s crust. Biological importance of Si in plant kingdom has become inevitable particularly under stressed environment. In general, plants are classified as high, medium, and low silicon accumulators based on the ability of roots to absorb Si. The uptake of Si directly influence the positive effects attributed to the plant but Si supplementation proves to mitigate stress and recover plant growth even in low accumulating plants like tomato. The application of Si in soil as well as soil-less cultivation systems have resulted in the enhancement of quantitative and qualitative traits of plants even under stressed environment. Silicon possesses several mechanisms to regulate the physiological, biochemical, and antioxidant metabolism in plants to combat abiotic and biotic stresses. Nevertheless, very few reports are available on the aspect of Si-mediated molecular regulation of genes with potential role in stress tolerance. The recent advancements in the era of genomics and transcriptomics have opened an avenue for the determination of molecular rationale associated with the Si amendment to the stress alleviation in plants. Therefore, the present endeavor has attempted to describe the recent discoveries related to the regulation of vital genes involved in photosynthesis, transcription regulation, defense, water transport, polyamine synthesis, and housekeeping genes during abiotic and biotic stress alleviation by Si. Furthermore, an overview of Si-mediated modulation of multiple genes involved in stress response pathways such as phenylpropanoid pathway, jasmonic acid pathway, ABA-dependent or independent regulatory pathway have been discussed in this review.

Silicon Regulates Potential Genes Involved in Major Physiological Processes in Plants to Combat Stress

Silicon is one of the most significant elements in plants under abiotic stress, so we investigated the role of silicon in alleviation of the detrimental effects of salinity at two concentrations (1500 and 3000 ppm sodium chloride) in sweet pepper plants in two seasons (2018 and 2019). Our results indicated that relative water content, concentrations of chlorophyll a and b, nitrogen, phosphorus and potassium contents, number of fruits plant−1, fruit fresh weight plant−1 (g) and fruit yield (ton hectare−1) significantly decreased in salt-stressed sweet pepper plants as compared to control plants. In addition, electrolyte leakage, proline, lipid peroxidation, superoxide (O2−) and hydrogen peroxide (H2O2) levels, soluble sugars, sucrose, and starch content as well as sodium content significantly increased under salinity conditions. Conversely, foliar application of silicon led to improvements in concentrations of chlorophyll a and b and mineral nutrients, water status, and fruit yield of sweet pepper plants. Furthermore, lipid peroxidation, electrolyte leakage, levels of superoxide, and hydrogen peroxide were decreased with silicon treatments.

Silicon Foliar Application Mitigates Salt Stress in Sweet Pepper Plants by Enhancing Water Status, Photosynthesis, Antioxidant Enzyme Activity and Fruit Yield.

In this study, effects of silicon (Si) on the growth and activities of major antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and guaiacol peroxidase (GPX), and protein profile under high temperature stress were investigated. Salvia splendens ‘Vista Red’ and ‘Sizzler Red’ were grown in a glasshouse and potassium silicate (0 or 100 mg·L−1) was supplemented hydroponically. Overall important growth parameters, chlorophyll content, and biomass were significantly increased by Si application. In both cultivars, more Si deposition occurred in the root, followed by leaf and flower. In both normal and high temperature conditions, supplemented Si increased the activity of SOD, APX, and GPX, while it decreased the CAT activity. In the SDS-PAGE protein profile, three bands responding differentially to the Si treatment were observed. Especially in ‘Vista Red’ treated with Si, a protein band, approximately 46 kDa, was expressed strongly under the temperature stress. Results of this experiment showed that Si not only promoted the growth of salvia, but also played a vital role against temperature stress.

Influence of silicon supplementation on the growth and tolerance to high temperature in Salvia splendens

Objective
To determine the effects of different cytokinins at various concentrations on in vitro shoot multiplication of an important medicinal plant.

Effect of cytokinins on in vitro multiplication of Sophora tonkinensis.

This study was carried out to investigate the effect of potassium silicate on the growth and leaf epidermal characteristics of horticultural crops viz., begonia (Begonia semperflorens Link et Otto) ‘Super Olympia Red’ and ‘Super Olympia Rose’ and pansy (Viola × wittrockiana Hort.) ‘Matrix White Blotch’ and ‘Matrix Yellow Blotch’ in vitro. Seeds after germination were grown on a quarter strength MS medium supplemented with potassium silicate (K₂SiO₃) at 0, 100, 200, or 300 mg ? L -1 and were maintained under a photoperiod of 16 hours at 25℃. Growth parameters such as plant height, root length, chlorophyll content, fresh, and dry weights have been recorded after a growth period of 58 days for begonia and 94 days for pansy. In begonia, fresh weight was significantly greatest in the 200 mg ? L -1 K₂SiO₃ treatment in both ‘Super Olympia Red’ and ‘Super Olympia Rose’. In both pansy cultivars, fresh weight was the greatest in the 200 mg ? L -1 K₂SiO₃ treatment than other treatments. Chlorophyll content was significantly greater in the 100 mg ? L -1 K₂SiO₃ treatment for both the cultivars of begonia. Leaf area significantly increased with the higher concentrations of K₂SiO₃ treatment in both cultivars of pansy. Stomatal structures on the leaf epidermis were observed with scanning electron microscopy (SEM). In begonia ‘Super Olympia Rose’, the structure of stomata were more compact in size in the 300 mg ? L -1 K₂SiO₃ treatment than in the control. Similarly, in pansy ‘Matrix White Blotch’ the surface of stomata appeared to be smoother in the 300 mg ? L -1 K₂SiO₃ treatment than those wrinkled appearance in the control. The surface of the leaf epidermis appeared to be compact due to Si deposition, and thus results indicated that Si positively affected the growth and biomass production of these species. Our data show that the effect of Si on growth parameters is strongly dependent on cultivar of the plant species tested.

Effect of Potassium Silicate on Growth and Leaf Epidermal Characteristics of Begonia and Pansy Grown in Vitro

Silicon: the most under-appreciated element in horticultural crops.

AnefficientmethodhasbeendevelopedforIn vitro regenerationofshootandmicrocormfromcormexpla ntsof� Crocus vernus .�Cormswerecutinto�0.5�1.0�cmlongsegmentsand� culturedontheSHmediumsupplementedwith�0.5,�1. 0,� 2.0,�or�4.0�mgL �1� 2�isopentyladenine�(2�iP),�N 6 �benzyladenine�(BA),� andN 6 �furfuryladenine�(kinetin,�Kin)�aloneor� combinationwith�0.5�or�1.0�mgL �1� α�naphthaleneaceticacid�(NAA)�forshootregenerat ion.�Ofthethreecytokininstested,� BAwasfoundtobethemosteffectivecytokininfor

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Papers

Influence of silicon supplementation on the growth and tolerance to high temperature in Salvia splendens

Effect of cytokinins on in vitro multiplication of Sophora tonkinensis.

Effect of Potassium Silicate on Growth and Leaf Epidermal Characteristics of Begonia and Pansy Grown in Vitro

Silicon: the most under-appreciated element in horticultural crops.

In Vitro Shoot Regeneration and Microcorm Development in Crocus vernus (L.) Hill