Silicon in plants

After oxygen, silicon (Si) is the second most abundant element in earth’s crust and is a constituent of most minerals. In soil solution, silicon is mainly present as monomeric silicic acid (H₄SiO₄), which is also the form in which it is taken up by plants.

Plant tissue contains silicon. The concentration of silicon varies considerably among plant species, ranging from 0.1 to 10% Si on a dry weight basis.

Silicon is an essential element for plants. Deficiencies can cause various abnormalities in plant growth. However, it has not been recognized as an essential nutrient for plant growth, mainly because it is not involved in the metabolism of the plant.

Several benefits of silicon to plants are now recognized. These benefits of silicon vary among plant species and are expressed mainly under stress conditions.

Such benefits include:

• Increased resistance to pests and diseases
• Increased photosynthesis
• Alleviation of heavy metals toxicity
• Better nutrient balance
• Increased drought and frost tolerance

Below are some examples of diseases that are suppressed or have reduced severity when silicon is applied:

• Blast disease (Magnaporthe grisea) in rice
• Fusarium wilt in banana
• Powdery mildew in barley
• Pythium in cucumber
• Pink rot in melon
• Gray leaf spot in ryegrass
• Anthracnose and angular leaf spot in beans
• Powdery mildew in grapes

Silicon can be applied to the soil or by foliar applications. Foliar applications have been reported to be effective against powdery mildew in several crops.

Enhanced resistance to diseases and pests 

The enhanced resistance to diseases is attributed to the accumulation of silicon in the epidermal system. In order to effectively infect the plant, the pathogen must penetrate some physical barriers, such as the cuticle and the epidermis. The silicon layer that is formed under the cuticle provides another physical barrier against pathogens and strengthen the plant.

In addition to the physical mechanism, silicon induces the formation and accumulation of compounds that suppresses plant diseases, such as phenolic compounds and phytoalexins, and activates defense enzymes.

Silicon can suppress various pests, such as rice leaffolder, stem borers, green leafhoppers and brown planthoppers, armyworms in corn, whitefly in beans, mites and more.

The mechanisms in which silicon enhances the resistance to pests are also comprise of physical and biochemical mechanisms. Silicon hardens and roughens plant tissue, which makes it more difficult for pests to feed and damage their mouthparts.

In addition, silicon enhances the natural defense mechanisms of the plant against pests, either direct defense or indirect defense.  For example, it was found that silicone enhances the induced production of chemicals that attract the natural enemies of the pest (indirect defense).

Tolerance to abiotic stress conditions 

Water stress – The accumulation of silicon under the cuticle can reduce transpiration rate. This may result in better tolerance of plants to drought conditions.

Temperature stress – leakage of electrolytes from plant cells is an indication of a damage made to the cell membrane. Both heat and freezing stress conditions may cause such a damage to cell membranes. There is evidence that silicon improves the stability of the membranes and alleviate the damage caused by temperature stress. In addition, silicon stimulates the production of antioxidants and heat shock proteins.

Silicon was also found to enhance plant resistance to radiation damage.

Salinity stress – Sodium uptake is one of the main causes of salinity stress. High sodium concentration adversely affects plant metabolism. Silicon reduces sodium uptake and accumulation and restricts its mobility within the plant.