EFFECTS FROM IRRADIATION OF RICE SEED WITH GAMMA RAYS AND NEUTRONS ON SEVERAL CHARACTERISTICS OF THE R$sub 1$ GENERATION

An investigation of radiation effects on a dormant seed variety of Nato, a variety of cultivated rice, was made using Co/sup 60/ gamma rays and neutrons. Seed lots were exposed to 15, 20, 25, 30, 35, 40, and 50 Kr of gamma radiation or to neutrons for 1/4, 1/2, 1, 2, 3, and 4 hours at a flux rate of 1.4 x 10/sup 12/ thermal neutrons and 7.9 x 10/sup 10/ fast neutrons per cm/sup 2/ per hour. Shortly after irradiation, seeds were planted in flats, and seedlings were transplanted to the field about three weeks after sowing. All doses of gamma rays ranging from 15 to 50 Kr caused measureable reductions in survival of R/sub 1/ plants. The reduction with increasing dosage was linear, with a drop of about 10% for each increase of 10 Kr. Dormant seed of rice proved highly sensitive to neutron radiation. Consequently, neutron radiation cannot be recommended for induction of mutations in rice. All treatments of both gamma rays and neutrons caused marked reductions in fertility. However, the dosagefertility curve was not linear for gamma rays as there was a distinct tendency for fertility to reach a minimum and level off before the highestmore » dosage was reached. The same so- called saturation effect was found for the gamma ray treatments on frequency of translocations. The translocation frequency reached a maximum at about 35--40 Kr and did not increase further from 50 Kr. Several lines of evidence indicated that damage to R/sub 1/ plants by irradiation of dormant seed was not caused primarily by chromosome breakage. Because of major differences in results obtained here with rice and those reported with barley, it is apparent that general conclusions concerning response of various economic plants to irradiation of dormant seed cannot be drawn from research with a single or a fewv species. Due to a relatively high frequency of rings of 6 chromosomes compared to 2 rings of 4 chromosomes, it was concluded that certain of the 12 pairs of rice chromosomes undergo breakage much more frequently than do others. The results indicated that gamma ray dosages of 40-50 Kr can be utilized with dormant seed of rice, provided the moisture content of the seed is within the range 13-15%. (auth)« less