Silicon application can minimize cadmium accumulation in wheat

Si is beneficial in healthy growth and development of plant. Different studies have proved that plant growth significantly increased by Si application. Beneficial effects of Si are prominent when plants subjected to multiple stresses (biotic and abiotic stress). It reduces toxicity of heavy metals, accumulation and uptake in plants. Si makes plant resistant against diseases caused mostly due to fungi and bacteria. It optimises the hormones and enzymatic balance in plants which helps in metabolism
WHEAT IS an important staple crop of Pakistan. Last few decades were days of wheat yield and quality declined due to soil contamination. In Pakistan yield of wheat is decreasing since 2001 due to decreasing availability of fresh water for irrigation. Water shortage and energy crisis are also there, so farmers prefer to use waste water due to ease of availability and high concentration of nutrients, that waste water is a chief cause of heavy metals accumulation in bread wheat.
Cadmium (Cd) is one of the most toxic metals. Sine 1990s worldwide annual release of Cd reached 22,000 tons and is distributed in water and soil. Major sources of exposure to Cd are natural activities such as volcanic eruptions, mining and smelting operations. From anthropogenic activities major source is smoking which have major effect on present population. Along with lead (Pb) and mercury (Hg), Cd is of great concern. Cd is only a heavy metal whose guide values published for foods. For world population from all sources the daily ingestion is 25-75 microgram per day. After seven years periods results based on market basket survey, United States Food and Drugs Administration shows that average intake of 16-10 years old males are 38.5 micrograms.
Cd is also known as human carcinogen and plant poison. When the concentration of Cd in human body reaches more than 200mg kg-1 then it exerts toxic effects on kidney and respiratory system as well as it cause skeletal disorders. In the mid of 1950s a disease named itai-itai was reported in Japan. Cd badly affects plant metabolism it accumulates in different plant organs and parts due to its high mobility and assimilability after absorption from roots it is translocated to aerial parts of plant in ionic form. During vegetative phase concentration of Cd in grains relates with the concentration of shoots. Cd cause hormones imbalance in pants and disturb whole plant growth.
Si is second most abundant element in soil found in different forms even in highly purified water there is also small concentration of Si present. Si has beneficial effects on plant in alleviating heavy metals stress especially Cd. Different varieties have different ability to uptake a specific amount of Si depending upon the source. Plants uptake Si mostly in the form of mono-silicic solutions which are very less mobile in plant and concentration ranges from 0.1 to 0.6mmol L-1 in soil solution, plants require a continuous supply of Si for optimum growth and development during growth stages. Si is of major concern due to its importance. Si have unique role in plants.
Si is beneficial in healthy growth and development of plant. Different studies have proved that plant growth significantly increased by Si application. Beneficial effects of Si are prominent when plants subjected to multiple stresses (biotic and abiotic stress). It reduces toxicity of heavy metals, accumulation and uptake in plants. Si makes plant resistant against diseases caused mostly due to fungi and bacteria. It optimises the hormones and enzymatic balance in plants which helps in metabolism.
Role of Si in plants was firstly explained by (Lovering and Engel 1959) who calculates that one hectare of forest can extract about 5000 tons of Si in 5000 years. Si chelates heavy metal ions with chelates and also by compartmentation of heavy metals in cell walls of plants. It stimulates enzymatic and non-enzymatic antioxidants by which it enhances resistance in plants. Silicic acid is a form of Si that roots uptake from roots, and then translocated. When silicic acid absorb in plant, hydroxyl group of silicic acid interacts with hydrophilic compounds and forms -SiO(OH) subunits.
Si transport in plants and when hydrolysis occur silica gel will be formed and precipitated. If the concentration exceeds 2mmol L-1 silicic acid polymerized to form silicate silica. In wheat xylem sap silicic acid is more than 2mmol L-1. When Si applied in soil it increases pH of soil. Due to increase in pH there is presence of more number of negative charges it bounds Cd and Cd becomes no freer for uptake by plants. Not only this by Si application roots becomes stronger and become resistant against Cd uptake by avoidance mechanism.
Silicate modulates activity of metals transport in plants. Reactive oxygen species produces due to heavy metals stress which is harmful to photosynthesis, fatty acids and plasma membrane. Plants have self defence against reactive oxygen species and protect themselves through enzymatic antioxidants i.e. superoxide dismutase as well as no enzymatic antioxidants i.e. ascorbic acid. Under abiotic stress Si enhances the enzymatic and non-enzymatic antioxidants demonstrated under toxicity. Under Cd stress in wheat, Si increases activities of enzymatic antioxidants superoxide dismutase, peroxidase, catalase, ascorbate peroxidase and glutathione reductase. In some experiments it is seen that there is different effects of Si depending on tissues and cultivars. Main objective of this study is to check effect of Si in minimizing Cd uptake and accumulation in bread wheat.

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