Plant Absorption of Silicon Tissue analyses from a wide variety of plants showed that silicon concentrations range from 1 to 100 g Si kg-1 of dry weight, depending on plant species (7). Comparison of these values with those for elements such as phosphorus, nitrogen, calcium, and others shows silicon to be present in amounts equivalent to those of macronutrients (Figure 19.1). Plants absorb silicon from the soil solution in the form of monosilicic acid, also called orthosilicic acid [H4SiO4] (8,9). The largest amounts of silicon are adsorbed by sugarcane (300–700 kg of Si ha-1), rice (150–300 kg of Si ha-1), and wheat (50–150 kg of Si ha-1) (10). On an average, plants absorb from 50 to 200 kg of Si ha-1. Such values of silicon absorbed cannot be fully explained by passive absorption (such as diffusion or mass flow) because the upper 20 cm soil layer contains only an average of 0.1 to 1.6 kg Si ha-1 as monosilicic acid (11–13). Some results have shown that rice roots possess specific ability to concentrate silicon from the external solution (14).

Forms of Silicon in Plants
Basically, silicon is absorbed by plants as monosilicic acid or its anion (9). In the plant, silicon is transported from the root to the shoot by means of the transportation stream in the xylem. Soluble monosilicic acid may penetrate through cell membranes passively (15). Active transport of monosilicic acid in plants has received little study.



After root adsorption, monosilicic acid is translocated rapidly into the leaves of the plant in the transpiration stream (16). Silicon is concentrated in the epidermal tissue as a fine layer of silicon– cellulose membrane and is associated with pectin and calcium ions (17). By this means, the double-cuticular layer can protect and mechanically strengthen plant structures (18). With increasing silicon concentration in the plant sap, monosilicic acid is polymerized (8). The chemical nature of polymerized silicon has been identified as silicon gel or biogenic opal, amorphous SiO₂, which is hydrated with various numbers of water molecules (9,19). Monosilicic acid polymerization is assigned to the type of condensable polymerization with gradual dehydration of monosilicic acid and then polysilicic acid (20,21):


Plants synthesize silicon-rich structures of nanometric (molecular), microscopic (ultrastructural), and macroscopic (bulk) dimensions (22). Ninety percent of absorbed silicon is transformed into various types of phytoliths or silicon–cellulose structures, represented by amorphous silica (18). Partly biogenic silica is generated as unique cell or inter-cell structures at the nanometer level (23). The chemical composition of oat (Avena sativa L.) phytoliths (solid particles of SiO₂) was shown to be amorphous silica (82–86%) and varying amounts of sodium, potassium, calcium, and iron (24). Phytoliths are highly diversified, and one plant can synthesize several forms (25,26). A change in plant-silicon nutrition has an influence on phytolith forms (27).


Biochemical Reactions with Silicon
Soluble silicon compounds, such as monosilicic acid and polysilicic acid, affect many chemical and physical-chemical soil properties. Monosilicic acid possesses high chemical activity (21,28). Monosilicic acid can react with aluminum, iron, and manganese with the formation of slightly soluble silicates (29,30):




Monosilicic acid under different concentrations is able to combine with heavy metals (Cd, Pb, Zn, Hg, and others), forming soluble complex compounds if monosilicic acid concentration is less (31), and slightly soluble heavy metal silicates when the concentration of monosilicic acid is greater in the system (28,32).



Silicon may play a prominent part in the effects of aluminum on biological systems (33). Significant amelioration of aluminum toxicity by silicon has been noted by different groups and in different species (34). The main mechanism of the effect of silicon on aluminum toxicity is probably connected with the formation of nontoxic hydroxyaluminosilicate complexes (35). The anion of monosilicic acid [Si(OH)₃]^- can replace the phosphate anion [HPO₄]^2- from calcium, magnesium, aluminum, and iron phosphates (12). Silicon may replace phosphate from DNA and RNA molecules. As a result, proper silicon nutrition is responsible for increasing the stability of DNA and RNA molecules (36–38).


Silicon has also been shown to result in higher concentrations of chlorophyll per unit area of leaf tissue (39). This action may mean that a plant can tolerate either low or high light levels by using light more efficiently. Moreover, supplemental levels of soluble silicon are responsible for producing higher concentrations of the enzyme ribulose bisphosphate carboxylase in leaf tissue (39). This enzyme regulates the metabolism of CO₂ and promotes more efficient use of CO₂ by plants.


The increase in the content of sugar in sugar beets (Beta vulgaris L.) (3,40) and sugar cane (41,42) as a result of silicon fertilizer application may be assessed as a biochemical influence of silicon as well. The optimization of silicon nutrition for orange resulted in a significant increase in fruit sugar (brix) (43).



There have been few investigations of the role and functions of polysilicic acid and phytoliths in higher plants.


In spite of numerous investigations and observed effects of silicon on plants and the considerable uptake and accumulation of silicon by plants, no evidence yet shows that silicon takes part directly in the metabolism of higher plants.