Silicon is the second-most abundant element in the earth’s crust, and the percentage of Si in the dry matter of plants is between 0.1% and 10%. This quantity is equivalent to those of other macronutrients such as calcium (Ca), magnesium, and phosphorus, and in some herbaceous plants, Si is present in levels as high as other inorganic constituents (Epstein, 1999; Rafi and Epstein, 1999). After studying the Si content in 500 plant species, Ma and Takahashi (2002) suggested a classification based on their Si contents and Si:Ca ratios (Table 1). However, this classification is complex. On the one hand, the groups are somewhat arbitrary, and the Si concentration may be better considered as a continuous spectrum (Cooke and Leishman, 2011). On the other hand, there are considerable differences among genotypes within the same species (Deren, 2001; Hodson et al., 2005; Ma and Takahashi, 2002). In addition, a given genotype of a plant growing under different Si concentrations can absorb different amounts of Si (Henriet et al., 2006; Ma and Takahashi, 2002). Thus, even though Si accumulation is a phylogenetic feature according to Guntzer et al. (2012), the availability of Si determines the amount of Si absorbed by plants.
Criteria to distinguish silicon (Si) accumulators from non-accumulators (Ma and Takahashi, 2002).
Generally, dicotyledons present low Si concentrations in tissues, on the order of 0.1% by dry weight (Jones and Handreck, 1967). However, plants belonging to the orders Cucurbitales and Urticales have intermediate contents (2% to 4%), and the species belonging to the Commelinaceae, Poaceae, Equisetaceae, and Cyperaceae families generally have high Si contents (>4%) (Hodson et al., 2005) with notable differences in concentration between them; dryland grasses (such as oats and rye) have ≈1% to 3% Si, and the “wetland” Gramineae (paddy-grown rice) and Cyperaceae have 10% to 15% or even higher levels of Si (Jones and Handreck, 1967).
Most Si present in the soil is in an insoluble form, so it is not available for plants (Takahashi and Hino, 1978). In soils, the prevailing form of Si is monosilicic acid, Si(OH)4, an unionized form, in solutions with pH values below 9 (Fig. 1). On average, the Si concentration in soils is 14 to 20 mg Si/L (ranging between 3.5 and 40 mg) with a tendency to decrease at high pH values (7) when large amounts of sesquioxides are present in soils and anion adsorption is prevalent (Jones and Handreck, 1965).
Plants absorb Si by their roots in the form of monosilicic acid [nSi(OH)4], which is transported through the plant via the xylem (Epstein, 1999), condenses into solid silica (Prychid et al., 2003), and is deposited as amorphous silica, SiO2·nH2O, [referred to as opal, silica gel, or phytoliths in higher plants (Richmond and Sussman, 2003)] mainly in the epidermis and in the sheath cells of vascular bundles. Solid silica is deposited in cell walls, cell lumens, the intercellular matrix (Prychid et al., 2003), and a layer under the wax cuticle (Kim et al., 2002; Yoshida, 1981). The silica in silica cells is deposited after the protoplast is degraded, and silica is deposited to a higher degree in wounded cells and in older cells (Blackman and Parry, 1968). Silicon also can be found in the forms of monosilicic acid, colloidal silicic acid, or organosilicon compounds in plant tissues (Datnoff and Rodrigues, 2005).
Recent studies have yielded a better understanding of the transport, structure, and function of Si in higher plants (Bauer et al., 2011; Ma et al., 2011); the role it plays against a wide range of biotic and abiotic stress (Balakhnina and Borkowska, 2013; Van-Bockhaven et al., 2013); and the ecological importance of the biomineralization of Si by plants (He et al., 2014) and its possible applications in modern agriculture (Haynes, 2014). Although the beneficial effects of Si absorption are different between species and, in general, can only be observed under conditions of biotic or abiotic stress, it is difficult to develop an integral understanding of the biological function of Si and its role in the plants’ health.
Researching the possible nutritional role of Si has proven to be challenging due to its various beneficial effects on monocotyledons and dicotyledons as well as the subsequent problems that arise in studies focused solely on one genetic model (Richmond and Sussman, 2003). Although there are apparent differences between the beneficial effects caused by Si in high-accumulator and nonaccumulator plants, published enzymatic assays and elemental analysis results show that both groups responded to supplemental Si (Frantz et al., 2011). In addition, these beneficial effects manifest at multiple levels, ranging from physiological changes to altered gene expression (Khandekar and Leisner, 2011; Li et al., 2008), at least in the response of plants to copper toxicity.
Consequently, the beneficial effects of Si fertilization have the potential to mitigate the depletion of soil nutrients (Guntzer et al., 2012), so Si fertilization is an alternative to the extensive use of N–P–K fertilizers that could potentially increase plants’ resistance to diseases, pathogens, (Gurr and Kvedaras, 2010; Van-Bockhaven et al., 2013), viruses (Zellner et al., 2011), salinity, and hydric stress (Stamatakis et al., 2003; Zhu and Gong, 2014); increase their tolerance to heavy metals (Li et al., 2008; Neumann and Zur Nieden, 2001); and improve the quality and efficiency of plants (Korndorfer and Lepsch, 2001; Toresano-Sánchez et al., 2010 and 2012). Additionally, excess Si does not damage plants (Ma et al., 2001) and is an essential nutrient of great importance for human biology. Furthermore, it has been observed that Si is necessary for bones, cartilages, and connective tissues (Bissé et al., 2005).
Guntzer (2010) concluded that Si fertilization is a sustainable alternative to the intensive use of fertilizers and pesticides.
Recent studies have discussed the possibility of including Si as one of the macronutrients necessary for the nutrition of higher plants (Frantz et al., 2011; Kernan and Marx, 2000). Currently, Si is considered to be a beneficial rather than an essential nutrient for plants, and the effects of Si fertigation have frequently been demonstrated in Si-accumulator crops such as rice and sugarcane (Haynes, 2014). Silicon is not considered to be an essential element according to the classic criteria of Arnon and Stout (1939) because many plants can complete their cycles without it (Marschner, 2012), but some authors disagree with this assertion because it is very difficult to eliminate Si from the environment (Epstein, 1999). Silicon has been called “quasi-essential” or “semi-essential” by certain authors (Epstein, 1999; Ma et al., 2007; Rafi et al., 1997; Savvas et al., 2009) as well as a “nonessential beneficial plant nutrient” (Richmond and Sussman, 2003). It is not normally included in the basic composition of standard nutrient solutions (Arnon and Hoagland, 1940; Cooper, 1996; Hewit, 1966), although some authors, including Sonneveld and Straver (1994), consider it a part of the basic nutrient solutions used for cucumber, melon, and lettuce plants, especially in soilless culture.
The aim of this study is to assess the effect of adding Si to the nutrient solution used for fertigating the seedlings of five vegetable species, in three different families (Solanaceae, Cucurbitaceae, and Asteraceae) with different capacities to accumulate Si, and to investigate their vegetative growth, cuticle development, and resistance to the illness caused by B. cinerea.
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