Factors Affecting Isoflavone Content in Soybean Seeds: Changes in Isoflavones, Saponins, and Composition of Fatty Acids at Different Temperatures during Seed Development

Factors affecting the isoflavone contents of soybean seeds were studied. Isoflavone contents of seeds of varieties grown at different locations, on different planting dates, and under different temperatures during seed development were determined by HPLC analysis. Fatty acid composition and contents of DDMP-conjugated saponin were also analyzed. The isoflavone content, together with the ratio of linoleic plus linolenic acid to total fatty acid, significantly decreased in the seeds harvested after growth at a high temperature for all soybean varieties tested. A general decrease was observed for all isoflavones, rather than a decrease restricted to a single molecular species. Of the total seed isoflavones, 80-90% were located in cotyledons, with the remainder in the hypocotyls. The hypocotyls had a higher concentration of isoflavones on a weight basis compared with cotyledons. While the isoflavone content of cotyledons exhibited large changes in response to high temperature during seed development, the isoflavone content remained high in the hypocotyls. As previously reported for other saponins, the DDMP-conjugated saponin content of seeds remained stable in response to elevated temperatures during seed development. These studies provide a basis for attempts to improve seed quality by the reduction of isoflavone content.