Farming lands are started polluting by industrial waste, heavy metals, chemical inorganic fertilizer and pesticides. That leads to convert fertile land into barren land. Most of the pollutants contains heavy metals and they are non-biodegradable (Alharbi et al. 2018; Burakova et al. 2018). Metal toxicity has a greater negative impact and relevance not only to plant kingdom but also they affects the surrounding ecosystem, in which plant forms a major integral component (Liang et al. 2007). Heavy metals affects the normal physiological activities by binding with protein sulphydryl groups, distressing the other metal transporters activities, and disturbing cellular homeostasis (Hossain et al. 2012).
Whereas some heavy metals like Zn, Cu, and Ni are essential micronutrients and they serve as cofactor in various enzymatic activities when they are present in trace. Interestingly, plants are natural non-selective bio-accumulators and they uptake heavy metals with other nutrition’s (Ozturk et al. 2017). While Cd and Ld are supplied to the plants through pesticides as ingredients but they do not have any beneficial role. But they become more toxic when applied in excess amount with pesticides (Ali et al., 2017b). Heavy metal stress fuels superoxide radicals (O−2), hydroxyl radical (-OH), hydrogen peroxide (H2O2) and singlet oxygen (1O2) like ROS production, which have strong harmful influence on photosynthesis, respiration, plasma membrane and fatty acids integrity (Stohs and Bagchi 1995). Plants have enzymatic antioxidants to protect the plant cell by ROS. Ascopate peroxide (APX), catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), as well as non-enzymatic constituents such as ascorbic acid (AsA), carotenoid (Car) and gluthione (GSH) are more prominent antioxidants which regulates the ROS level in plant cell (Tiryakioglu et al. 2006). The rate of heavy metal absorption, transportation and accumulation in various organs varies from species to species. Excess accumulation of heavy metals may shows toxic symptoms like stunted growth, chlorosis, root browning, decline and death (Ozturk et al. 2015).
At cellular level, heavy metals severely damages the intra- and inter- DNA and protein molecules cross linkage, DNA bases deletion, modification, rearrangements, strand breakage and depurination. Heavy metals produces promutagenic adduct 8-0xoG (7,8-dihydro8-oxoguanine) that miss pairs with adenine and leads to transversion of C to T (Cunningham 1997; Kasprzak 1995). Cation diffusion facilitator (CDF) and macrophage protein family (Nramp) family play important role in homeostasis but also draw stratagem for heavy metal tolerance (Williams et al. 2000). ZIP gene family transporters transport Cd, Fe, Mn and Zn, so regulation of ZIP genes expression may control the accumulation of these metals (Guerinot 2000). Usually, A ZIP family proteins has eight transmembrane domains and all domains has C and N terminus and they are exposed to apoplast. ZIP1 and ZIP3 expression mainly found in roots and their activities reach maximum level when plants under Zn stress condition (Manara 2012). NRAMP metals transporters present in plasma membrane and tonoplast membrane regulates Cd and Fe transportation (Thomine et al. 2000). The Cu transporters (CTR) also located in the cell membrane and they express when plants need Cu (Puig and Thiele 2002). The smooth metal transporters like HMAs and CPx-type ATPase act as efflux pumps as they remove excess metal ions deposited in cells and same time they involved in uploading Cd and Zn in xylem from the surrounding tissues. Overexpression of AtHMA4 increases Cd and Zn stress tolerance in plants (Verret et al. 2004). Another family of transporters MATE involved in multidrug and poisonous compounds transportation from the cell (Schaaf et al. 2004).
In an attempt to detoxify the cells, metal ions are transported into the vacuoles present in the cells. Where they are stored as complex compounds in neutral activities condition. The tonoplasts present in the cell contains number of highly specific transporters which regulate the metals transportation. While, ABC family transporters chelate and sequestering the excess heavy metals present in the cells and then transport to vacuoles (Ortiz et al. 1995). CDF and MTP transport family transport heavy metals to endoplasmic reticulum or apoplast and also they act as sensors for metals present (Maser et al. 2001). MHX and CAX members of CaCA family are involved in homeostasis of metals. They antiport the Mg2+ and Zn 2+/H+ from xylem cells and also involved in Cadmium transport (Manara 2012). Other than these transport family proteins, various other biomolecules and structural component present in the cells regulate the homeostasis of heavy metals.
In vitro study of Si-mediated metal, precipitation provides the sufficient information, in which Si decrease the availability of toxic metals to plants (Fu et al. 2012) by precipitation (Liang et al. 2007). In some circumstances, Si reduces the bioavailability of metals to plants (Liang et al. 2005). In some circumstances, Si induces use of stored immobilized metals as a source of plant nutrients when deficiency condition occurred by Si induced reversible processes (Garcia-Mina et al. 2013). When comes to soil, Si increases the pH of the rhizosphere region of the plants by forming silicate precipitates that decrease the heavy metal phyto-availibility (Cocker et al. 1998). Higher Si accumulators like rice, external Si application increases plant growth even under multi metal contaminated acidic soil (Gu et al. 2011). Exogenous Si reduces the lipid peroxidation and fatty acid desaturation in plants and improves the plant physiological activities under heavy metal stress (Nagajyoti et al. 2010). Most widely accepted Si mediated heavy metal detoxification mechanism are metal immobilization in soil before absorption from plant roots and stimulation of enzymatic and non-enzymatic antioxidants, co-precipitation of metals, metal ion chelation, and compartmentation. The possible mechanism for reducing heavy metals can be explained that Si strongly form complexes with cell wall molecules and alter the structural integrity which leads to the blockage of the apoplastic transport and directly or indirectly restricts the entry of these heavy metals (Meharg and Meharg 2015). It was widely accepted that, Si has beneficial to plant growth and development by mitigating multifarious biotic and abiotic stresses including heavy metal stress (figure 1.). Below, we discussed in detail about the mechanism of Si-induced mitigation of heavy metal toxicity in plants.
Arsenic (As) being considered as non-essential element and it may cause severe damage to soil health, plant growth and reproduction and ultimately affect human health after consumption of contaminated plant produce (Zhao et al. 2010). Due to excess accumulation of As in soil reduces the plants nutrient absorption ability which resulting in impaired plant physiological activities (Sanglard et al. 2016). K being analogous to As and compete for some carriers present in the plasma lemma by providing same ionic strength. This result in reduction in As influx (Hasanuzzaman et al. 2015). As is a semi-metallic element and form the organic and inorganic arsenicals by reacting with other elements. In these, inorganic As compounds are more lethal to plants as compared to the others. Inorganic As species largely occurs in arsenit Fe (AsIII) in a reduced form and arsenate (Asv) an oxidized form (Finnegan et al. 2012). Nodulin 26-like intrinsic protein (NIP) is a major entry point for arsenite, whereas arsenate is taken by plants roots from rhizosphere via phosphate transporters (Wu et al. 2011). It is well known that As accumulation leds to oxidative stress by generating reactive oxygen species (ROS). To combat oxidative stress, plants produces enzymatic and non-enzymatic biomolecules. However, response of all the detoxification machineries influenced by As bio-availibity, toxicity and mobility and presence of other ligands (Violante et al. 2010).
Silicon has potential to abate As toxicity and improves photosynthesis, carbohydrate accumulation. In some tomato cultivars, Si application significantly increases seed germination with inhibiting As accumulation in plants (Marmiroli et al. 2014). Under As stress condition, varioustransporters helps the movement of Si from root epidermis into root steel, and then shoot via xylem sap. Furthermore, study conducted on Lsi1 and Lsi2 transporter genes under arsenite treatment revals that these Si transporters effectively serve as major entry path for arsenite transport.Upregulation of OsLsi1, OsLsi2 and OsLsi6 recorded in As (III) + Si as compared to As (III) alone treated plants, indicating that the concentration of OsLsi family genes expression might not be sufficient to accrue As in the presence of Si (Sanglard et al. 2016). Increased activity of GPX and GST observed under As stress due to activation of GSH-dependent peroxide scavenging mechanism, which helps in the reduction of oxidative damage, and prevent membrane damage with the help of Si. Till now Si- dependent amelioration of As toxicity from germination not studied in horticulture crops in detail.
Aluminum toxicity is the one of the limiting factor for crop growth in acidic soil (Von Uexkull and Mutert, 1995). Al normally form insoluble oxides and complex aluminosilicates (AS) at pH valve higher than 5.0, whereas at lower pH (acidic condition) Al is solubilized to the monomeric form (Al+3) which becomes available to plants, thereby affecting a wide range of physiological processes with a consequent reduction in plant growth (Singh et al. 2017). Al+3 affects the cell wall functions (Horst et al. 2010), nutrient homeostasis (Delhaize and Ryan, 1995), plasma membrane properties (Yamamoto et al. 2001) and signal transduction pathways (Goodwin and Sutter 2009). To cope with the deleterious effects of Al, plants adopted Al exclusion and /or internal tolerance mechanisms (Poschenrieder et al. 2008). Several membrane transorter genes are involved in regulating efflux of organic acid anions, including membrane of the ALMT aluminum-activated malate and MATE families (Ryan et al. 2011).
Si has been used in horticulture crops especially in vegetable crops to alleviate the Al toxic effect by forming hydroxylaluminium silicate (Hodson and Evans 1995). For the first time in horticulture crops, Baylis et al. (1994) showed the alleviation of Al toxicity by Si recorded in soybean crops. Later similar result was observed in barley (Hammond et al. 1995), wheat (Zsoldos et al. 2003), rice (Singh et al. 2011), and some coniferous crops (Hodson and Sangster 1999). Cocker et al. (1998a) suggested that Si can regulate the Al toxicity by three ways: (i) by increasing the pH of the solution, (ii) reduce the Al availability to plants by forming complexes of hydroxyaluminosilicates (HAS) in the external solution and (iii) improves the Al detoxification mechanism in the plants. Bityutskii et al. (2017) proved that, Si down regulate the Al toxicity under acidic conditions in cucumber. Recently Dorneles et al. (2019) highlighted that Si remove the physiological damage caused by Al in the root system in potato via elevating activity of antioxidant enzymes. When maize seeds treated with Si just before sowing leds to higher exudation of Al chelating catechin and quercetin as well as malic acid observed (Kidd et al. 2001). It has been reported that Al availability to plants in the presence of Si can be minimized by forming hydroxyaluminosilicate (HAS) like Al-Si complexes at root zone (Hodson et al. 1994).
In recent years, Nickel (Ni) also considered as an essential element at a low concentration for normal physiological activities of the plant (Eskew et al. 1984). But when concentration crossed 0.05-5 µg g−1 dry weight by plants (Rizwan et al. 2017), it may become severely toxic to plants and reduce the normal growth and yield (Matraszek and Hawry-lak-Nowak, 2010). It act as cofactor for several enzymes which are involved in glyoxalases, hydrogenase, methyl-CoM reductase, urease, superoxide dismutase and peptide deformylases. It also has crucial rule in hydrogen metabolism, ureolysis, methane biogenesis and acidogensis as well as regulate the stress tolerance by maintain cellular redox potential (Vatansever et al. 2017). Fluctuation of net photosynthetic rate, stomatal conductance, carbon dioxide concentration, and transpiration rate in plant under high Ni content is due to the interference in normal stomatal activities (Chen et al. 2009; Yusuf et al. 2011). The increase of Ni to toxic levels is due to excess fertilizer application, sewage incineration, smelting, mining and burning of fossil fuel and other electronic compounds (Aziz et al. 2015; Rizwan et al. 2018). Available form of Ni compounds could be transported by Fe2+, Mg2+, Cu2+ and Zn2+ transporters by forming chelates such as citric acid, histidine and nicotianamine. Even proteins such as metallothionein, metallochaperones and permeases involved in Ni transportation (Vatansever et al. 2017). Plants grown in soil contain excess Ni produces symptoms like necrosis, chlorosis, and nutrients deficiency (Yadav 2010). Ni at high concentration stimulate the excess production of ROS (Turan et al. 2018) and cytotoxic α,β-dicarbonyl aldehyde compounds called ‘methylglyoxals’ (MG) (Kaur et al. 2014), disturb the normal activities of phtotsynthesis reaction centers (P680 for PSII, and P700 for PSI) and reduces the electron transport (Sirhindi et al. 2016). An increase in uptake of Ni leads to reduced root and shoot growth, decrease in K, Ca, and Mg content in different tissues of plants. Elsayed F. Abd Allah (2019) revels that excess accumulation of Ni in mustard plants significantly reduces plant growth (34.46%), total chlorophyll content (57.63%), LRWC (24.34%), and enhances the H2O2 (3.23 fold), MDA (2.07 fold) and methylglyoxal (MG by 3.23%) content in plant cells.
One of the suitable premium approaches is the use of Si in alleviation of Ni induced stress. Several resechers suggest that Si reduces the endogenous Ni level in plants by creating reducing environment. When Ni stressed mustard plants where treated with Si at 10-5 concentration, drastically ameliorated the negative effect of Ni in plants. Si improves the dry matter content in tomato plants even by Si-Ni precipitation in root zone (Ashraf et al. 2013). Foliar application of Si reduces the electrolyte leakage and proline contents in periwinkle (Catharanthus roseus L.) with restoring normal growth process and enhanced the alkaloid content (Indrees et al. 2013). Si plays a prominent role in ROS metabolism and MG detoxification under Ni stress by activating numerous defence systems mainly, AsA-GSH cycle in plants. Increased phenols and flavonoids production and accumulation was observed to protect plants from oxidative damage as marked by decreased MDA content has been reported in Brassica spp, tomato (Marmiroli et al. 2017) and gladiolus (Zaheer et al. 2017).
Among the listed soil contains heavy metals, lead (Pb) is one of the most toxic pollutants of the environment and it has lethal effects on both plants and animal health (Ashraf et al., 2015). The toxicity effect of Pb depends on the amount of Pb uptake by plant and soil type (Reddy et al. 2005). Once entered into the plant cells, Pb changes hormonal activities, cell membrane integrity, inhibiting sulfhydryl group containing enzymes activity and reduce the water and mineral content in the cells. At physiological level, it adversely affect the photosynthesis pathways, blocks the chlorophyll and carotenoid like essential pigments, inhibit Calvin cycle and electron transport chain and also downregulate stomatal movement (Sharma and Dubey 2005). It was observed that Ld accumulate large amount in roots followed by petiole and leaf tissues (Malar et al. 2014).
Application of Si in Pb contaminated soil markedly reduced the uptake and translocation of Pb, electrolyte leakage, limited hydrogen peroxide (H2O2) and malondialdehyde (MDA) content and increased biomass yield with the lowest amount of Pb contamination in seed (Bharwana et al. 2013). Shi et al. (2005) reported that Si minimize the Cd toxicity by reducing Cd absorption from soil. Similarly, Shim et al. (2014) have recorded the externally applied Si immobile the excess soil Pb by forming Pb-silicate in the soil.
In agriculture, Cd content of soil is limited to 100mg/kg, if concentration crossed this limit, than the field is declared as unfit for forming (Salt et al. 1995). Cd toxicity results in chlorosis, retardation of plant growth and development, modification of enzyme activities, accumulation of excess ROS and protein denaturation (Shi et al. 2010). Even it inhibit or modify the Fe+3 reductase activity, that leds to Fe+2 deficiency. Cd also interferes with the uptake of macro and micro nutrients mainly, Ca, P, K and Mg and reduces the absorption and translocation of nitrate by inhibiting nitrate reductase. In higher plants, excess Cd level can effectively inhibit photosynthesis by downregulating carotenoids function (Prasad 1995). At cellular level, it has potential to cause nucleolus damage leading to chromosome fragmentation and aberration, reduces the respiration by decomposition of mitochondria, affects the electron transport chain by interfering with redox reactions and it replace the Ca in calamodulin which is involved in cell signaling (Rivetta et al. 1997). In Brassica napus, even low concentration of Cd (5 μM) reduced the plant growth, photosynthesis and transpiration by closing stomatal activities (Baryla et al. 2001). Similar negative result was obtained in the root structure of Cd treated rice plant. However, application of Si greatly minimized the Cd side effects and improved the plant biomass and chlorophyll content in rice plants (Yoon-Ha Kim et al. 2014). Normally Cd deposit slowly in endodermis and epidermis of the root by absorption whereas Si deposition in endodermis act as a physical barrier for the apoplastic bypass flow across the roots and limit the Cd transportation. Si increases the cell wall extensibility, enhanced water use efficiency, light use efficiency, carboxylation of ribulose-1, 5 bisphosphate carboxylase oxygenase (RuBisCO) (Chika and Alfredo 2011), and stimulate the antioxidant activity (Gangrong et al. 2010). As well as upsurge the activities of CAT, APX and SOD, but reduces the concentration of MDA and H2O2 in pakchoi (Song et al. 2009b).
To clarify the roles of Si in mitigating Cd toxicity, Nwugo and Huerta (2011) analyzed leaf proteome and they found 60 proteins ware responsible to minimize the Cd toxicity in plants when grown in Cd contaminated soil, among them over 50 proteins associated with photosynthesis, regulation of protein synthesis system, redox homeostasis, and pathogen response were differentially regulated by Si with up-regulation of a class III peroxidase activities. Previously studies in Arabidopsis thaliana confirmed that, HMAs (AtHMA1, AtHMA2, AtHMA3, and AtHMA4) detoxify the Zn present in the chloroplast organs and regulate the Cd accumulation in various cell organelles especially in vacuoles and plasma membrane. Various researchers assessed the OsHMA2 and OsHMA3 genes expression in response to Cd stress in rice plants, and they found that the expression of OsHMA2 at 1-DAT was significantly increased with Si treatment (Courbot et al. 2007).
Addition of Si in rhizosphere zone contaminated by Cd, enhances the shoot and root biomass by 43-90% and 38-50% respectively. A similar detoxification mechanism has also been observed in peanut (Arachis hypogaea L.) (Shi et al. 2010), cucumber (Cucumis sativus L.) (Feng et al. 2010), pakchoi (Brassica chinensis L.) (Song et al. 2009a) and strawberry (Fragaria x ananassa) (Treder and Cieslinski 2005). In general, Si improves the tolerance to Cd stress. In Cd stressed cucumber plants, exogenous Si alleviate blade necrosis, prevent chloroplast swelling, protect thylakoids and increase pigment contents (Feng et al. 2010). Song et al. (2009) also confirmed the role of Si in increasing concentration of GSH, AsA and non-protein thiols (NPT). The mechanism of Si-promoted plant growth under Cd stress at molecular level is still unclear in horticulture crops, while several research work shows the positive role against Cd toxicity in several cereal crops. So, Si is recommended in horticulture crops also.
Mercury doesn’t involve in any plant biochemical and physiological activities and it has no beneficial effect at all (Hameed et al. 2017). Hg exist in many forms in soil such as methyl-Hg, HgS and Hg2+ with ionic form. It can persist in soil for longer period by forming carbonate, hydroxide sulfide and phosphate chelates. Anaerobic bacterium present in the soil convert precipitated Hg into methylated form (Tangahu et al. 2011). From there, it get absorbed by plant roots with water and bind to water channel proteins and impede water movement. Later in plants, Hg affect the chloroplast and mitochondrial activities by creating oxidative stress along with membrane degradation and biomolecules oxidation occurs (Nagajyoti et al. 2010). It blocks cellular functions and normal plant growth and development (Malar et al. 2015). There is very diminutive information is available in regards to role of Hg in plants and how plants defend from Hg toxicity.
Industrial and sewage water are the major source of Cr, which causes serious contamination in soil, groundwater and sedimentation (Shanker et al. 2005). In recent years global emission of chromium (Cr) has significantly increased with industrialization (Kabata-Pendias and Mukherjee 2007). The most toxic form of Cr for a living being is Cr+4 because it can easily enter cytoplasm with no restriction from the cell membrane and affect the metabolic processes (Singh et al., 2013). Cr directly affects the photosynthesis processes by degrading photosynthesis pigments and anthocyanin (Boonyapookana et al. 2002), water balance and nitrogen metabolism in a cell with a drastic reduction in seed germination percentage (Singh et al., 2013). Cr depresses the activity of amylases during seed germination and hence sugar availibity to developing seedlings from embryo is restricted. It also affect the various enzymatic activities which are involved in the carbon fixation, electron transport, etc. (Yadav 2010). Cr increases metabolite production especially, glutathione and ascorbic acid which deleteriously affect the plant growth (Shanker et al. 2003).
From laser-induced breakdown spectroscopy (LIBS) and inductively coupled plasma atomic emission spectroscopy (ICAP-AES) study, the accumulation of Cr in plant tissue reduced after Si application and improves the nutrients (K, Ca, Mg and Na) uptake. Silicon application under chromium stress condition improves the transpiration rate and chlorophyll fluorescence efficiency and retreating leaves and root structure, swelled chloroplast, damaged thylakoid membrane, increased in plastoglobuli, damaged vacuole and disruption of the nucleus (Shafaqat et al. 2013).
Ding et al. (2013) have shown that the Si markedly decrease the exchangeable Cr content in soil by accelerating the precipitation of organic matter bound Cr fraction. The Si treated plants shows the increased plant growth and development even under Cr-stressed conditions. Hosain et al. (2002) hypothesized that exogenous application of Si cell wall extensibility. The possible mechanism for the reticence of Cr absorption in growing seedling by Si are mainly involved in the two aspects: (i) lignin deposition in cell wall and induces Cr bind to cell wall, that reduce the transportation of Cr from roots to shoot (Shi et al. 2005a; Kaya et al. 2009), (ii) the Si-Cr complex formation (Hodson and Sangster 1993). Other than these, there are several methods present in plants which reduces the Cr uptake and transportation (Nwugo and Huerta 2008). Thus it may be conclude that exogenous application of Si successfully ameliorates Cr- induced toxicity. Although the exact physiological mechanism associated with the Si in regulation of Cr toxicity have not been established in horticulture crops.
Iron (Fe) plays a crucial role in plant metabolism (Ricachenevsky et al. 2010; Vigani et al. 2013). When excess Fe accumulation occurs in the plants, increased the production of ROS via Fenton and Haber-Weiss reaction (Onaga et al. 2016), which ultimately damage the plants by degrading cell wall components (Pereira et al. 2013), increase nutritional disorders (Muller et al. 2015) and iron plaques formation in roots to immobile Fe ions (Pinto et al. 2016). Excess Fe accumulation disturb the carbon metabolism by inhibit the ATP and NADPH synthesis by lowering the efficiency of Calvin-Benson cycle (Siedlecka et al. 1997). Fe deficiency causes major stress as compared to access Fe in plants. Fe deficiency symptoms are common in calcareous soil, as the soil pH reaches 7.5-8.5 because of high bicarbonate accumulation (Romheld and Marschner 1986). Deficiency symptoms like interveinal leaf yellowing, necrosis appeared during plant growth period and ultimately reduces quality and yield. Plants developed a different anatomical and physiological strategy to minimize the Fe deficiency under high soil pH such as, increased Fe reduction power by the help of Fe (III) - chelate reductase enzyme along with an increase in Fe (II) transporters biosynthesis especially in non-graminaceous crops and phenolic compounds released from the roots to rhizosphere zone for acidification to increase the availability of Fe for a certain level (Hindt and Guerinot 2012). Same as other stress conditions, accumulation of metabolites such as glucose, fructose and organic acids observed under high Fe accumulation in plants. Which may modify cell redox balance or non-cyclic functioning of the mitochondrial tricarboxylic acid cycle (Igamberdiev and Eprintsev 2016).
Fe uptake and translation increased with the addition of Si into the soil (Fu et al. 2012). At pH7, all forms of Si like solid silica, crystalline, and amorphous dissolve completely in the water at a different rate based on the surface area and phase of solute. The equilibrium concentration of monomeric silicic acid (H4SiO4 or Si(OH)4) at neutral pH is less than 2mM. If the pH of the solution increased, silicic acid forms the polyconditionized colloidal particles (Birchall 1990). You-Qiang et al. (2012) observed that Si increase the transportation of Fe from roots to shoot by increasing Si transporters expression. Increased Fe level observed in root system after Si treatment to Fe deficient plants (Bityutskii et al. 2014), which attributed to increase in Fe level in the apoplastic pool of roots (Pavlovic et al. 2013) or it can precipitate at root external (Fu et al. 2012). Si improve the chlorophyll synthesis and stimulate the plant growth when planted in Fe deficient soil by improving Fe mobility (Pavlovic et al. 2013) and metabolism (Bityutskii et al. 2014). In some situations, Si per se did not affect the net assimilation rate in Fe toxicity affected plant (Detmann et al. 2012). In various research, the downregulation of leaf conductance in response to excess Fe was reversed by Si application. Si enhance the roots oxidizing ability and inhibit the significant amount of iron absorption by converting iron into ferric ion under Fe toxic condition (Ma and Takahashi 2002). But exact effect of Fe toxicity at genetic level remain unclear in horticulture crops, although the effect of Fe in various crops have been reported (Sanglard et al. 2014).
Zinc (Zn) is one of the essential element for plants for its normal physiological activities. It involved in various plant physiological events including auxin, carbohydrate and protein metabolism and participate in several enzymatic activities as cofactor chiefly in pollen formation (Aziz et al. 2016). Same as iron, Zn availability is limited by calcareous soil and this condition becomes more complex as the alkalinity of the soil increases (Marshner 1995). However it requires in vary lower amount and sometime, excess accumulation may cause impede plant growth even complete senescence may occurs. Plants shows Zn deficiency as concentration goes below 15-20 mg per kg of plant dry mass. When Zn concentration crosses the plant threshold limits, it produces visible symptoms like phosphorous deficiency (Lee et al. 1996; Nagajyoti et al. 2010), photosynthesis inhibition, low floral fertility with flower and fruit drop (Marschner 1995). Lower Cu/Zn superoxide dismutase (Cu-Zn-SOD) activity during Zn toxicity may downregulate the ATPs synthesis with increased production of ROS resulting. These symptoms are reversible after normalizing Zn concentration in plants. Whereas Zn toxicity effects may become permanent if stress persists. Such as, by replacing Mg2+ ions in RUBISCO (ribulose-1,5-bisphosphate-carboxylase/oxygenase), ultimately inhibit the carbon fixation which leds to plant death (Todeschini et al. 2011). Cell vacuoles act as Zn storehouse and they controls the level of Zn content by immobilization and detoxify the cell. Based on the organic acid concentration in different parts of the plant, Vacuoles of Shoot tissue considered as the storehouse of metals in the form of organic acid complexes (Sinclair and Kramer 2012).
Several experiments were conducted to know the exact impact of Si application on Zn distribution in plants (Gu et al. 2012; Bityutskii et al. 2017). After Si application, the deposition of cell wall bond Zn increased in roots, stem, and leaves at the seedling stage (Gu et al. 2012). Even in dicotyledonous Minuartia Verna plant, Zn silicates were detected in the leaf epidermal cell wall, which explains its Zn tolerance after application of Si (Neumann et al. 1997). Citrate level in plant increased when plants are treated with Si under Fe deficiency condition (Bityutskii et al. 2014), same phenomenon could occurs during Zn distribution in the plant. When Si applied under the absence of Zn to cucumber seedlings.
Copper (Cu) is an essential element and which can plays various key biochemical activities including photosynthesis, carbon assimilation and ATP synthesis. Cu is an important constituents of cytochrome and plastocyanins and cytochrome oxidase, both of which are vital components of both respiration and photosynthetic systems (Yadav 2010). In rhizosphere region, 1-20% of Cu present in an available form and remaining bond to organic matter. It is considered as nearly immobile in plants, so initially, fresh leaves and reproductive parts shows Cu deficiency symptoms. At the physiological level, symptoms of Cu deficiency include reduced respiration, impaired photosynthetic electron transport, and stunted growth symptoms that appeared after Cu deficiency (Marschner 1995). Neelima and Reddy (2002) studied the effect of Cu on Solanum melongena seeds and found that, excess Cu can adversely affect the germination and seedling establishment.
There are fewer studies conducted to know the Si–Cu interaction in plants (Frantz et al. 2011). Cu toxicity causes chlorosis on leaves and root biomass reduction in Arabidopsis thaliana (Khandekar and Leisner 2011) and Triticum aestivum (Nowakowski and Nowakowska 1997), after application of Si, these Cu toxic symptoms were diminished by Cu binding with deposited Si on cell wall (Rogalla and Ro¨mheld 2002) but the level of Cu content was not significantly changed in leaf even after external application of Si (Li et al. 2008).
At low concentration, Mn is essential for normal enzymatic activities. Plant species like peach, wheat and soybean are very susceptible to Mn deficiency whereas maize and rye are much less vulnerable (Reuter et al. 1988). Significantly yield reduction observed mainly in winter crops when Mn availability is below the critical level. Mn deficiency symptoms like decreased dry matter accumulation, decline in photosynthesis and chlorophyll content were normally observed in plants when grown in Mn deficient soil. (Papadakis et al. 2007). In dicotyledonous, interveinal chlorosis of younger leaves observed whereas, in cereals, a gray speck are major common symptoms. Observed in plants when grown in sandy and calcareous soils.
Positive interaction observed in-between Manganese and Si in rice (Okuda and Takahashi 1962), barley (Horiguchi and Morita 1987), trichomes base (Iwasaki and Matsumura 1999), cowpea (Iwasaki et al. 2002) cucumber and bean (Shi et al. 2005a), and pumpkin (Iwasaki and Matsumura 1999). Exogenous Si application improves the root oxidation capacity in root rhizosphere region and also precipitate the Mn outside the plant roots. These deposited Mn oxides are used when plants feel deficiency for Mn. In pumpkin, When cucumber plants are treated with Si presented that almost 90% of Mn bond to the cell wall, in which Mn distributed in symplast and apoplast in equal quantity (Rogalla and Ro¨mheld 2002). However, the effect of Si in plants grown under Mn toxic conditions has been scarcely studied. For that reason, further studies should be done in depth to know the exact mechanism involved in plummeting Mn toxicity in plants by exogenous Si application.