Effect of Silicon on the Growth of Soybean Plants in a Solution Culture

Soybean plants were cultured in solution with and without silicon application (silicon-supplied plants and silicon-free plants). Both silicon-free and silicon-supplied (l00 ppm SiO2) plants grew normally in the earlier growth stages. At the flowering stage, however, the newly-developed leaves (7th and 8th leaves) of the silicon-free plants showed malformations such as curling and curving to the outside, while the silicon-supplied plants grew normally. In severe cases, numerous necrotic spots were observed on the leaves of the plants. The growth of the silicon-free plants was markedly inferior to that of the silicon-supplied plants. The pollen fertility rate of the silicon-free plants was lower than that of the silicon-supplied plants. As the Ca content of the soybean plant was much higher than the SiO2 content, the soybean plant is rated as a Ca-type plant like the tomato plant. However, unlike the tomato plant in which most of Si is retained in the roots, the soybean plant translocates Si freely from the roots to the tops. Therefore the mode of Si uptake of the soybean plant is not of the rejective type like in the tomato plant but of the intermediate type as in the case of cucumber plants which stand between the active uptake type and the rejective type.