## 3 Uptake, Translocation and Accumulation of Si in Plants  
The uptake of Si from the soil depends mainly on the plant species and the chemical form of the Si in the soil. The only form of Si that can be absorbed by plants is monosilicic acid (chemical formula: Si(OH)\({}_{4}\)). [PERSON] et al. [32] reported that Si uptake in rice, cucumber and tomato is from soil to root, from root to cortical cells and then to xylem vessels. However, the Si concentration in the symplast is higher than that in the soil. Additionally, Si accumulation in rice is three to five times higher than in cucumber or tomato. The uptake and transportation of Si differ among various plants. In addition, it has been reported that the radial transport of Si is mediated by a type of transporter with a \(K_{\text{m}}\) value of 0.15 mM in all species. However, the different \(V_{\text{max}}\) values have the following order: rice > cucumber > tomato. It has been reported that the uptake of Si in rice is an energy-dependent process that works even at low temperatures or even in the presence of metabolic inhibitors [32]. The Si concentration in the xylem sap of rice is much higher than those in cucumber and tomato. Furthermore, the xylem loading of Si is mediated by a transporter in rice, whereas in cucumber and tomato Si xylem loading occurs by diffusion. Thus, the Si transporter in rice shoots is the most decisive factor affecting its high-level accumulation. However, the lower levels of Si accumulations in cucumber and tomato may be due to lower densities of Si transporters to take up Si from external solutions and the lack of, or defective, xylem loading transport. [PERSON] et al. [27] suggested that the low Si accumulation in tomato results from the low efficiency of Si export from the endodermis, resulting in extremely low root-to-shoot translocation of Si and subsequent low accumulation in the shoots. In the xylem loading process, after taking up Si from the external solution, it is transported to the xylem. At present, the specific mechanism of xylem loading of Si remains unclear [33]. It has been suggested that Si is transferred to the shoot through the xylem, and when the silicic acid concentration exceeds 2 mM, the silicic acid polymerizes to form silica gel (SiO\({}_{2}\).nH\({}_{2}\)O). Whereas the Si concentration in xylem sap is usually greater than 5.0 mM, it remains unclear how high concentrations of silicic acid remain unpolymerized in xylem sap. There are no silicate complexes in rice xylem sap that may prevent Si polymerization in xylem tissues [32; 34].  
[PERSON] et al. [31] reported that, after Si was transferred from the root, silica in the form of hydrated polymers was deposited in the cell walls of stems, leaves and fruit shells. In the shoot, silicic acid is further concentrated and polymerized by transpiration. As the concentration of silicic acid increases, it is eventually converted to silica gel. The distribution and accumulation of Si in the shoots depends on the transpiration rate, and their deposition occurs below 0.1 mm of the cuticle layer, forming a cuticle-Si double layer. Silica-motor cells and silica cells are two forms of silicified cells present in rice leaves, and they form different shapes after accumulating in specific cells [35].  
In addition to being applied to the soil, Si is also applied to the foliage. Different types of Si-containing compounds are used as foliar sprays, such as silicates, stabilized silicic acid and Si nanoparticles (Si-NPs) [36]. Positive effects of foliar Si applications have been observed in accumulating and non-accumulating Si plant species [37; 38; 39; 40]. Despite the observed benefits of foliar applications, the mechanisms of Si uptake and transport in leaf cells are not well understood. Further research is needed to study the Si transport mechanism in the shoots at the molecular level.