We investigated the usefulness of ion beams for mutation breeding in rice (Oryza sativa L.) by comparing the efficiency (i.e., the ratio of desirable mutations to plant damage such as lethality and sterility), mutation rate, spectrum, and optimum dose to that of gamma rays. Rice seeds were irradiated with carbon ions (mean linear energy transfer = 76 and 107 keV/µm), helium ions (9 keV/µm), and gamma rays, and their survival and fertility were examined in the M 1 generation. The frequency of chlorophyll mutations and their types (albina, xantha, and viridis) were examined in the M 2 generation, using the M 1 -plant progeny method. The efficiency of ion beams either equaled or exceeded that of gamma rays. In addition, the mutation rate of ion beams was higher than that of gamma rays. Thus, ion beams appeared to efficiently induce mutants with little radiation damage. No remarkable difference was observed in the relative frequencies of each type of mutation among 3 types of ion beams and gamma rays, thus suggesting that there was no difference in the spectrum. A shoulder dose, which hardly affected survival, was sufficient to efficiently obtain mutants for both types of radiation.

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We investigated the effect of total irradiation dose and dose rate on flower color mutation and nuclear DNA content as an index of radiation damage in chrysanthemum. Chrysanthemum morifolium cv. 'Taihei' plants grown by in vitro culture were gamma-irradiated with a total dose of 15, 30 and 60 Gy at a rate of 0.5, 1, 2 and 5 Gy/h. Leaf explants cut from the irradiated plants were tissue cultured, and the regeneration rates and frequency of flower color mutation were investigated. Nuclear DNA content was measured by flow cytometric analysis. The regeneration rate decreased with increase in the total dose and dose rate of irradiation. Mutation frequency did not differ significantly among dose rates, indicating that mutation frequency was independent of dose rate and was dependent mainly on total dose. Comparison of the average of the nuclear DNA content with each treatment revealed that it was influenced by both dose rate and total dose, and that the reduction in nuclear DNA content was less at low dose rates, even when total doses were high. It appears that same mutation frequencies were obtained without large reduction in nuclear DNA content by 0.5 Gy/h, when compared with 2 Gy/h. Consequently, we conclude that gamma ray irradiations of high total doses at low dose rates efficiently induce mutations with less radiation damage in chrysanthemum.