Soybean Takagi [10] identified two major genes, which control radio-sensitivity, in some soybean varieties. When the 50% reduction rate (RD50) of root length was determined with acute irradiation to the seeds or the chronic irradiation to the plants for the entire growth period, radio-sensitivity of a sensitive cultivar, “Shinmejiro,” is more than twice that of the resistant variety, “Tachisuzunari.” The differences in radio-sensitivity between the varieties to the chronic irradiation in the Gamma Field were controlled by a single recessive gene, rs1 . Besides, the second recessive gene rs2, which was discovered in “Goishishirobana,” whose activity is only expressed following acute seed radiation.