Silicon as a Beneficial Element and as an Essential Plant Nutrient: An Outlook (Review)

Abstract

Silicon application has been known to enhance the plant tolerance against several biotic and abiotic stresses. The mechanisms of alleviation of UV-radiation, drought, salt, and heavy metal stresses have been investigated but still definitely unknown. As a consequence, the needs for understanding the role of Si in the physiological and biochemical processes in plants are of a major importance. In most cases, the published data supported the role of Si in regulation and alleviation the adverse effects of several environmental stresses under specific conditions and for specific plant species. The physiological functions of Si in plants have been studied and showed that Si influx and efflux is regulated by specific genes in plasma membrane of plant cells. The role of these genes in Si transport and alleviation several stress conditions is still poorly understood. The occurrence of high Si content in tissues of graminaceous plants (rice, barley and maize) has positively activated physiological, physical and biochemical defense mechanisms for increasing stress balance, such as decreased lipid peroxidation, reduced membrane permeability, and stimulation of defense enzymes activities. The activities of stress defense enzymes, also called antioxidative enzymes (ascorbate peroxidase, catalase, peroxidase, and superoxide dismutase), have been found to be linked to Si nutrition especially under abiotic stress conditions. There are genotypic differences controls the accumulation of Si within a specific plant species, such as rice. These differences are originated from variations in abundance of certain transporters of Si in roots or/and shoot of plant. These genotypic differences allow the plant to facilitate high growth under abiotic stress conditions. However, the responsible genes for Si accumulation in plant varieties have not yet been well characterized. Harvesting cultivated high Si – accumulator plant species results in removal of large quantity of available Si from soil. In order to replenish the needed amount of available Si from such soil, application of Si – fertilizer would improve its status in soil and maintain appreciable amount of available Si in this soil. Fertilization of soil by the required and safe rate of Si has been found to improve the chemical, physical and biological properties of plant and soil. Tests for available Si in soil and for Si content in specific plant tissue in combination with evaluation of the quantitative relationship between both would be reliable guides for Si–fertilization. In this concern, whether Si is known as a beneficial element or as an essential plant nutrient, the use of Si compounds as a fertilizer or as an amendment has been proved to be of a major importance in the present and near future.