The Role of Silicon as a Defense Modulator in Abiotic Stress

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The accumulation of Si in leaves is advantageous not only for UV-B irradiation defense, but also for cooling leaves in heat stress conditions.

In this situation, bio silicified structures present in epidermal cells are effective in cooling plant leaves by the mechanism of efficient mid-IR thermal radiation; thus, silicon creates a physical mechanism against heat stress (Wang et al. 2005). 

High-temperature stress limits the growth, metabolism, and productivity of plants. Due to heat stress, plant impairment is represented by oxidative stress (increased ROS production), cellular damage, membrane damage, photosynthesis inhibition, and so forth (Tan et al. 2011; Hasanuzzaman et al. 2013).

In an experiment with Agrostis palustris growing at 35°C–40°C, the temperature of the leaves decreased 3°C to 4.14°C following Si treatment in comparison to untreated control plants. 

Also, Si present in soil substrate reduced heat and was effective in the cooling of plant roots (Wang et al. 2005).

Another study suggested that Si influences the thermal stability of cell membranes of plants during heat stress (Agarie et al. 1998). In this study, electrolyte leakage caused by high temperature (42.5°C) decreased in the leaves of plants grown with Si, but not in those without Si. Further studies dealing with high-temperature stress and Si interaction found an increased level of antioxidant enzymes (SOD, APX, and glutathione peroxidase [GPX]) (Soundararajan et al. 2014). 

Silicon supplementation also significantly influenced the protein pattern and total protein content during high-temperature stress in plants. 

Overall, Si positively affected plant growth and played a vital role against high-temperature stress (Soundararajan et al. 2014). The importance of Si application under high-temperature stress was also evident in the case of alleviating fertility reduction (Wu et al. 2014). This field study revealed that silicon in various concentrations effectively increased the germinated pollen number, the number of pollen grains per stigma, the pollen germination rate, and other fertility parameters in high-temperature-sensitive and -tolerant rice hybrids (Wu et al. 2014).

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