In the last decades, the concentration of atmospheric CO2 and the average temperature have been increasing, and this trend is expected to become more severe in the near future. Moreover, environmental stresses exposure represent a threat for ecosystems and agriculture. Environmental changes negatively affect plant growth, biomass and yield production, and also enhance plant susceptibility to pests and diseases. Chemical fertilizer is gaining increasing attention and has been the center of much research which indicating complex beneficial and harmful effects. Chemical fertilizer might cause some environmental hazards to the biosphere, especially in the agricultural ecosystem. The application of silicon (Si) fertilizer in agriculture has been proved to be able to create good economic and environmental benefits. Si is the second most abundant earth crust element. Si fertilizer improves soil quality and alleviates biotic and abiotic crop stress. It is of great significance to understand the function of Si fertilizer in agricultural utilization and environmental remediation. Its use in agriculture enhances plant silicon uptake, mediates plant salt and drought stress and remediates heavy metals. Plants take up silicon from the soil in the form of monosilicic acid, which is the only plant-available form of silicon in the soil. The monosilicic acid maintain the plant protection against the effects of heavy metal contamination. However, phenomenon associated with Si fertilization, such as increased plant resistance to drought, frost and viral attacks, were not explained. New data obtained from greenhouse, laboratory and field experiments, as well as from literature sources, allowed a new hypothesis to be formed with regard to reinforcement of the plant's defense system by active Si. According to the hypothesis, plants, as well as every other living organism have unique protective mechanisms which involve in part the mobile Si compounds. It is speculated that the function of the Si constituent can provide additional synthesis of stress protection molecules, and this synthesis is carried out under genetic control but without “physical” participation of the genetic apparatus. The active forms of Si within plants are being considered as a matrix-depot for the formation of compounds which assist the organism(s) to maintain positive homeostasis under stressful conditions. This hypothesis provides the possibility of elaborating new ways to protect cultivated plants against unfavorable conditions and biotic stresses.

Keywords: plants, silicon, active silicon, stress tolerance