@article{oai:repository.naro.go.jp:00001479,
 author = {山口, 博康 and YAMAGUCHI, Hiroyasu},
 journal = {花き研究所研究報告, Bulletin of the National Institute of Floricultural Science},
 month = {Dec},
 note = {Mutation breeding is a useful method for crop improvement. Its success, however, depends on the type of mutagenic treatment and the treatment methods used. Ion beams show promise as a new mutagen which induces mutations with high frequency and produces novel mutants, because ion beams have high linear energy transfer (LET) and thus have greater biological effects compared with low LET radiations such as gamma rays and X-rays. Therefore, the mutagenic characteristics of ion beams for mutation breeding were investigated. The practical use of ion beams requires information about effective and efficient doses. In gamma rays, there are no experiment-based reports on the optimum irradiation dose, although gamma rays have been commonly used. In addition, there are few data regarding the usefulness of the gamma ray irradiation with a low dose rate to prevent radiation damage. Therefore, optimum irradiation methods of ion beams and gamma rays for mutation breeding were investigated. 1. Characteristics of ion beams as mutagens for mutation breeding 1) Efficiency Efficiency, which is defined as the ratio of specific, desirable mutagenic changes to plant damage caused by mutagen treatment, was compared between ion beams and gamma rays using rice (Oryza sativa) and chrysanthemum (Chrysanthemum morifolium). Rice seeds were irradiated with 220 MeV carbon ions (mean LET 107 keV・μm^-1), 320 MeV carbon ions (mean LET 76 keV・μm^-1), 100 MeV helium ions (mean LET 9 keV・μm^-1) and gamma rays. Fertility was examined in the M_1 generation, and the frequency of the chlorophyll mutations was examined in the M_2 generation. The relationship between the mutation frequency per M_2 plant and fertility indicated that the mutation frequency based on fertility induced by ion beams equaled or exceeded that induced by gamma rays.In an irradiation experiment of leaf explants of chrysanthemum carried out using the same ion beams, the effects of irradiation treatments were investigated using mutation frequency in flower color and nuclear DNA content as indices of radiation damage. Mutation frequency with the 220 MeV carbon ion beam was high compared to the other ion beams and gamma rays, while there was little reduction in the nuclear DNA content as the irradiation dose increased. In contrast, when the irradiation dose of the 100 MeV helium ion beam increased, the nuclear DNA content decreased, although the mutation frequency did not increase compared to the other treatments. Thus, the efficiency, which was determined as the relationship between the frequency of flower color mutation and the reduction in nuclear DNA content, differed according to the type of ion beam used, and the efficiency with 100 MeV helium ions was lower than that with gamma rays. In conclusion, the efficiencies of ion beams generally appear to be higher than those of gamma rays ; however, some types of ion beams (e.g., 100 MeV helium ions) produced lower efficiencies than those produced by gamma rays. 2) Mutated sector size A wide mutated sector is desirable to establish mutants efficiently. Therefore, the width of mutated sectors with ion beams was compared to that with gamma rays using rice and chrysanthemum. The results of segregation frequency of chlorophyll mutants in the M_2 generation of rice showed that ion beams produced wider mutated sectors than did gamma rays. In chrysanthemum, flower color mutants were produced by irradiation of ion beams and gamma rays to lateral buds and by subsequent release of mutants from chimeric status with cutting back twice. Analysis of the chimeric structure made by comparing the flower color of the mutants to that of plants regenerated from the roots of mutants indicated that some of the mutants obtained with ion beams were solid mutants, where both LI and LIII layers were derived from the same mutated cell. No such solid mutants were obtained with gamma rays. Thus, the expansion of mutated sectors through all the layers of shoot apex occurred only with ion beam irradiation. In conclusion, it appears that ion beams produced wider mutated sectors compared with gamma rays.3) Mutation spectra New mutagens are expected to have a different spectrum from the mutagens presently used. In a comparison of the relative frequency of each type of chlorophyll mutation (albina, xantha, viridis and others) in rice, no significant differences between ion beams and gamma rays were found. In chrysanthemum as well, the spectra did not seem to differ between ion beams and gamma rays because neither ion-beam- nor gamma-ray-specific mutants were observed, and there was no difference in the relative frequencies of each flower color mutation. From these results, it appears that there was no difference in mutation spectra between ion beams and gamma rays. 2. Optimum irradiation methods for mutation breeding I examined suitable irradiation doses to obtain the maximum number of mutant lines from rice seeds sown after irradiation by both ion beams and gamma rays. As a result, it was found that the number of mutated lines per irradiated seed was highest at the shoulder dose in the survival curves. Irradiation at the shoulder dose did not affect survival markedly, giving approximately 90% survival. This result demonstrated that irradiation with such low doses was enough to efficiently produce mutants. Also in chrysanthemum and rose, mutants were obtained even at low doses of ion beam irradiation that did not affect shoot regeneration or survival, respectively. To clarify the usefulness of the gamma ray irradiation with low dose rate for reducing radiation damage, I investigated the effect of total irradiation dose and dose rate on flower color mutation and nuclear DNA content as indices of radiation damage in chrysanthemum. Chrysanthemum plants were gamma-irradiated with various total doses at rates of 0.5, 1, and 2 Gy・h^-1. The frequency of flower color mutation and the nuclear DNA content were investigated in regenerated plants. The dose rate of gamma rays influenced the nuclear DNA content but did not affect mutation frequency, and the same mutation frequencies were obtained without large reductions in nuclear DNA content by 0.5 Gy・h^-1, when compared with 2 Gy・h^-1. In conclusion, gamma ray irradiations with high total doses at low dose rates efficiently induced mutations with less radiation damage in chrysanthemum.},
 pages = {47--83},
 title = {イオンビームおよびガンマ線による突然変異育種に関する研究},
 volume = {12},
 year = {2012},
 yomi = {ヤマグチ, ヒロヤス}
}