INCREASING MUTAGENIC EFFICIENCY OF GAMMA-RADIATION IN RICE BY IRRADIATION OF SEEDS AT EXTREMELY LOW-TEMPERATURE

Dry seeds of rice, O. sativa L., containing about 13% moisture, were exposed to 60Co .gamma.-rays at extremely low temperature (-196.degree. C). M1 [1st mutant generation] damage on survival rate, seedling height and seed set became much smaller in the cooling treatment than in the not-cooled control. Chlorophyll and heading date mutation frequencies in the M2 were not affected by the decreased M1 damage caused by the cooling treatment, and the mutation frequencies could be significantly increased by applying higher doses combined with the cooling treatment. Frequency of sterile or semi-sterile mutants in the M2 was significantly lower in the cooling treatment than in the not-cooled control. Extremely low temperatures during irradiation apparently could reduce the amount of physiological damage and gross chromosome aberrations. This results in an increased efficiency of producing minute deletions and/or point mutations.