Functions of Silicon in Higher Plants

Abstract

Silicon (Si) is the second most abundant element, either on the basis of weight or number of atoms, in the earth’s crust. Because of its strong affinity with oxygen, in nature Si always exists as silica (Si dioxide) or silicates which are combined with various metals. Si dioxide comprises about 60% of the earth’s crust. In soil, Si dioxide accounts for more than 50% and Si concentration in soil solution is between 3.5 and 40 mg Si L-1 in the form of silicic acid (Marscner 1995). Therefore, all plants rooting in soil contain Si in their tissues.However, because of its universal existence, earlier researchers paid little attention to the impact of Si on plant growth. Sacks (1862) first asked the following question in his article on Si nutrition “… whether silicic acid is an indispensable substance for those plants that contain silica,whether it takes part in nutritional processes, and what is the relationship that exists between silicic acid and the life of the plant.” However, Sachs concluded that Si was insignificant for the nutritional process of maize when he found that water-cultured maize which contained 0.3% Si did not show any abnormality in growth compared to maize containing 9% Si. Since then, a number of studies have been carried out on the effect of Si on plant grwoth.However, until now Si has not been put in the list of essential elements for higher plants.According to the criteria proposed by Arnon and Stout (1939) for essential elements, a given plant must be unable to complete its life cycle in the absence of the element.