Studies on the Utility of Artificial Mutations in Plant Breeding.: X. Effects of Heat-shock Treatment on Radiosensitivity and Mutation Frequency in Rice

Abstract

Effects of heat-shock treatment following irradiation at an extremely low temperature were examined on radiosensitivity, mutation frequency and response to selection of quantitative traits. Seeds of rice variety Gimbozu were irradiated with various doses of γ- or X-rays at about -70°C, followed by immediate soaking into the water of which temperature was adjusted to 0, 30 and 60°C, for 2 sec. to 6 min. Striking decrease of radiation-induced injuries was observed, as measured in survival rate, seedling height and seed fertility in the M₁ generation. Within the ranges of temperature and time for shock in this experiment, the radio-protection effect increased as temperature and time increased, LD₅₀ in 60°C-2min. -shock being about 2. 5 times as high as that in irradiation at room temperature. These effects of heat-shock treatment may be resulted Lrom the restricted mobility of free radicals due to extremely low temperature during irradiation, the restricted recombination among long-lived radicals and the accelerated metabolic activity at raised temperature immediately after irradiation at low temperature. Higher dose irradiation followed by the heat-shock treatment brought about a marked increase of the obtainable highest mutation frequency in three quantitative characters, heading date, culm length and panicle length, and also in qualitative characters except seed fertility : the frequencies of mutants per R unit in all these characters did not differ from those in usual irradiation. In contrast with these results, the frequency of mutants in seed fertility was clearly decreased. Both genetic gain and heritability in heading date and culm length were also proved to be significantly increased by the heat-shock treatment. From these results, it may be concluded that the heat-shock treatment is an effective method for enhancing the relative frequency of genic mutations to that of gross chromosomal aberrations.