1. For establishing the radiation breeding technique of soybean (Glycine max (L.) Merrill), we examined the varietal differences of radiosensitivity with such seed characters as 1000 grains weight, total carbohydrate percentage, crude fat content and crude protein content of fully dried seed, irradiated by γ-ray from Cs^137 at dose rate of 20, 40, 60 and 80 Kr. 2. We could estimate the varietal differences of radiosensitivity of irradiated seed by the survival rate on 30 days after sowing at 40 Kr dose, or elongation rate of hypocotyl on 15 days after sowing, although we could not discriminate the varietal differences according to the germination rate, or survival rate on 30 days after sowing at those dose rates as 20 or 80 Kr. According to the above mentioned criteria we classified 11 varieties into 6 groups, that is, very weak, weak, medium weak, medium strong, strong and very strong. 3. Within the same variety. the heavier grain (large seed) seemed to be pretty more sensitive than the lighter one (small seed), and between varieties we could classify into three groups, namely, (1) large seed group (heavier than 250 g of 1000 seeds weight) showing low survival rate, (2) small seed group with high survival rate and (3) small seed group with low survival rate. 4. We found negative correlation between survival rate and total carbohydrate percentage, that is, varieties with high percentage of total carbohydrate showed high irradiation sensitivity. 5. Allowing two exceptions, positive correlation was generally observed between survival rate and crude fat content of seed. 6. Although correlation between survival rate and crude protein content in seed was not strong, positive correlation was observed. 7. We pointed out that further studies must be continued to determine varietal differences of radiosensitivity of soybean plant from seed characters.
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