Significance Of Silicon in Rice

The second most abundant element in the earth's crust is silicon (Si). It is not considered an essential nutrient, but it is beneficial to crop growth, particularly in Poaceae crops. Plant shoot Si concentration varies greatly between species, ranging from 0.1 to 10% Si by dry weight.

Si is present in the soil solution as Monosilicic acid and Polysilicic acid. It combines with organic and inorganic compounds such as aluminium oxides and hydroxides to form complexes. Si is not a very mobile element for plants in soil.

Factors Affecting The Solubility Of Silicon In The Soil

1. The particle size of the silicon fertilizer

2. Soil acidity (pH)

3. Organic complexes

4. Presence of aluminium (Al), iron (Fe), and phosphate ions 

5. Dissolution reactions and soil moisture. 

Role Of Silicon In Rice

1. Si is a beneficial element for plant growth and is agronomically essential for improving and sustaining rice productivity. 

2. Si has many fold advantages of increasing nutrient availability (N, P, K, Ca, Mg, S, Zn), decreasing nutrient toxicity (Fe, P, Al), and minimizing biotic and abiotic stress in plants. Hence, the application of Si to soil or plant is practically useful in laterite-derived paddy soils, not only to increase yield but also to alleviate the Fe toxicity problems. 

3. Si increases the mechanical strength of the culm, thus reducing crop lodging (Savant et al., 1997).

Beneficial Effects Of Silicon On Rice 

1. Decreases Lodging- Si in rice shoots enhanced the thickness of the culm wall and the size of the vascular bundles which result in a reduction in lodging. 

2. Increases crop growth and yield- Si promotes growth, strengthens culms, favours early panicle formation, increases the number of spikelets per panicle and percentage of matured rice grains, and helps to maintain erect leaves which are important for the higher rate of photosynthesis.

3. Improves availability of applied nutrients- 

Phosphorus- The application of calcium silicate to highly weathered soils enhanced upland rice response to applied phosphate. The overall beneficial effect of Si may be attributed to a higher P: Mn ratio in the shoot due to the decreased manganese and iron uptake, and thus indirectly improved phosphorus utilization within the rice plants.

Potassium- The silicification of cell walls seems to be linked with potassium nutrition. According to Noguchi and Sugawara (1966), potassium deficiency reduces the accumulation of silicon in the epidermal cells of the leaf blades, thus increasing the susceptibility of the plant to rice blast. Therefore, silicon management integrated with potassium may be more important for sustaining rice yields in upland areas than in lowland areas. 

1. Decrease metal toxicities of Fe and Al:

Iron toxicity: Silicon increases the oxidizing power of roots, which converts ferrous iron into ferric iron, thereby preventing a large uptake of iron and limiting its toxicity (Ma et al., 2002). Silicon will regulate Fe uptake from acidic soils through the release of OH- by roots. 

Aluminium toxicity: Si application alleviates aluminium toxicity by creating inert aluminosilicates, stimulating phenolic exudation by roots or by sequestration in phytoliths (Guntzer et al., 2012).

2. Increases abiotic stress tolerance:

Alleviate salt stress: Si may correct salt stress in higher plants by improved photosynthetic activity, enhanced K/Na selectivity ratio, increased enzyme activity, and increased concentration of soluble substances in the xylem (Sahebi et al., 2015). 

Alleviate Drought stress: The deposition of Si in the culms, leaves, and hulls also decreases transpiration from the cuticle thus increasing resistance to drought stress. 

3. Increases biotic stress tolerance:

Pest tolerance: Soluble silicic acid (as low as 0.01 mg/ml) in the sap of the rice plant acts as an inhibitor of the sucking activity of the brown plant hopper. Si increases the resistance of plants to many insects in rice like stem borer, leaf folder, brown plant hopper, etc. 

1. Disease tolerance:Antifungal compounds like momilactones were found to accumulate in Si-treated rice plants and these acted against blast pathogens. Si has been found to decrease several diseases in rice like sheath blight, brown spot, grain discolouration, etc.

Deficiency Symptoms Of Silicon

• Si deficiency makes rice plants susceptible to pests and diseases.

• Si deficiency leads to soft and droopy leaves, reduced photosynthetic activity, reduced grain yields, increased insect pest incidence, reduced number of panicles and filled spikelets per panicle (IRRI, 2016).

Why and where it occurs?

• Si deficiency is common in areas with poor soil fertility and highly weathered soils.

• Its deficiency is also seen in organic soils with less Si reserves and also occurs in highly weathered soils.

• The critical level of Si in the soil is 40 mg kg^-1 and the critical level of Si in rice (leaf and straw) is 5%.

Silicon Fertilizers

Calcium silicate, fine silica and sodium silicate are mostly used as silicon fertilizers. Potassium silicate, though expensive, is highly soluble and can be used in hydroponic culture and also applied through foliage. Rice husk, rice husk ash and straw are organic sources of Si. Rice straw hauled away from rice fields is used for various purposes, such as animal feed, biogas production, or mushroom cultivation, and may maintain its nutrient value as a source of Si; thus the end products of these uses should be recycled. Si content in rice straw and rice husk ranges from 4-20% and 9-26% respectively. Silicon solubilising bacteria (SSB) is a bio-fertilizer which contains spores of the Bacillus mucilaginous. It is used as an effective soil inoculant.

Conclusion

Rice is a silicon accumulator, so adequate attention should be given to silicon nutrition. Highly weathered soils of the tropics and subtropics are low in available Silicon. Silicon management agenda includes silicon fertilization and recycling of silicon in rice crop residues. Silicon has manifold advantages. It is essential for the healthy growth and productive development of the rice crop. Silicon increases the efficiency of applied nutrients, increases crop yield, and increases resistance against lodging, biotic stresses, and abiotic stresses. Silicon management is essential for sustaining rice productivity in temperate, tropical, and subtropical soils.

References

Guntzer, F., Keller, C. and Meunier, J.D. (2012). Benefits of plant silicon for crops: A review. Agron. Sustain. Dev. 32: 201- 213.

IRRI (International Rice Research Institute), (2016). Silicon deficiency. Available: http://www.irri.com.

Ma, J.F., Tamai, K., Ichii, M., Wu, K. (2002). A rice mutant is defective in active Si uptake. Plant Physiol. 130: 2111-2117.

Savant, N.K., Snyder, G.H., Datnoff, L.E. (1997). Silicon management and sustainable rice production. Adv. Agron. 58: 1245-1252.

Sahebi, M., Hanafi, M.M., Akmar, A.S.N., Rafii, M.Y., Azizi, P., Tengoua, F.F., Azwa, J.N.M. (2015). Importance of Silicon and Mechanisms of Biosilica Formation in Plants. Bio. Med. Res. Int., 16.

Authors: 

Priyanka Sanwal, Shabnam, Amit Kumar, Sandip Kumar Gautam

Ph. D. Scholar, CCSHAU, Hisar

sanwalpriyanka2015@gmail.com