Benefits of Silicon
Silicon: Promotes greater photosynthesis, cell wall strength, plant rigidity, root development, water efficiency and enhanced responses to abiotic and biotic stressors.
Silicon is present mainly in the form of mineral silicates, alumino-silicates and silicon dioxide (SiO2), but most of these forms are inaccessible to the plants.
Plants can absorb silicon only in the form of monosilicic acid (H4SiO4) which is naturally present in the soil, but the concentration depends on the soil texture, properties, pH, organic matter, minerals present etc. (Tubana and Heckman, 2015). A study conducted by Rutgers on New Jersey soils revealed an average of 18 ppm of monosilicic acid content as compared to 100 ppm required for optimum plant health.
Fertilization of soil with silica brings about various physical, biochemical and molecular changes in plants, that confer tolerance to different abiotic stresses (salinity, heavy metal, drought etc.) and biotic stresses (bacteria, fungi, viruses, insects, herbivores) (Artyszak, 2018; Eneji et al., 2008; Etesami and Jeong, 2018). Silicon reduces the uptake of heavy metals, the effect of ROS mediated membrane lipid peroxidation and protects the photosynthetic machinery, under the effect of different abiotic and biotic stresses (Imtiaz et al., 2016; Santa-María and Rubio, 2018). At the same time, it plays a crucial role in plant pathogen interactions (Fauteux et al., 2005).
Nanosilica has emerged as a key player in orchestrating plant growth and conferring tolerance to various abiotic and biotic stresses. Nanosilica has increased absorptivity that accounts for an increased uptake of silica, although the exact mechanism is not fully understood. Nanosilica uptake in the roots and leaves reduces the accumulation of reactive oxygen species (ROS) and membrane lipid peroxidation. It is known to restrict the entry of sodium ions and other heavy metals in plants. Concurrently, nanosilica deposition in the leaf tissue enhances the plant defense against pathogens.
Several fertilizers consist of silicates but it is mainly present in the form of calcium silicate which is not readily soluble in water. As a result of its insolubility, the availability of the silicates to the plants decreases and therefore requires to be used in greater concentration to obtain the desired effect (Cuong et al., 2017; Laane, 2018; Meena et al., 2014).
Several studies have reviewed nanomaterials for growth, disease protection and mitigation of stress in crop plants (Aslani et al., 2014; Khan and Rizvi, 2014; Khot et al., 2012; Prasad et al., 2017; Zhao et al., 2020).
‘Nanosilica’ is is the subject of accelerated pace of research in the last five years to explore their superiority over the use of bulk silica for plant growth.
Nanosilica are synthesized from bulk silica, generally having a size below 100 nm which enhances the uptake of silica in plants and at the same time allows them to act as a carrier for other essential nutrients (Jeelani et al., 2019; Yuvakkumar et al., 2011).
The uptake of nanosilica by plant roots have been proposed to follow the apoplastic route mainly, as the silicon transporters are less sensitive to nanosilica (Nazaralian et al., 2017).
Though the uptake mechanism is not fully understood it is comparatively absorbed at a higher rate than the other silicates (Asgari et al., 2018; Schaller et al., 2013).
Abiotic stress in plants is manifested by many factors like salinity, drought, overlogging, high temperature, freezing, heavy metals, UV rays etc., which account for considerable losses in crop yield. Application of nanosilica to stressed plants has been reported to bring about marked improvements in the morphological status of the plants. Also, it is known to alleviate the harmful effects of these stresses by improving the physiological and biochemical status of the plant.
Nanosilica plays a significant role in Biotic stress management due to their antifungal and antibacterial activity (Rajwade et al., 2020). Nanosilica application to maize plants led to increased resistance against Aspergillus and Fusarium (Suriyaprabha et al., 2014).
This increased resistance was attributed to increased concentration of phenols and defense enzymes (Suriyaprabha et al., 2014). Hamza et al. (2013) also reported increased activities of peroxidases and polyphenol oxidases in maize.).
Only Aquaritin delivers bioavailable silicon in a micronized spray to boost the absorption of minerals.