Mutation induction with ion beam irradiation of lateral buds of chrysanthemum and analysis of chimeric structure of induced mutants

We compared the effects of ion beam and gamma ray irradiation on mutation induction in axillary buds of chrysanthemum, and analyzed the chimeric structure of the resulting mutants. Axillary buds were irradiated with carbon ions at 2 Gy (mean linear energy transfer 122 keV/μm), helium ions at 10 Gy (mean 9 keV/μm), and gamma rays at 80 Gy, all of which have the similar effects on survival. The lower five nodes of the shoots elongating from the irradiated buds were cut one by one, and new shoots were allowed to grow from the axillary buds. This procedure was repeated twice, and flower color mutation was investigated. Chimeric structure was analyzed by comparing the flower color of mutants to that of plants regenerated from the roots. Flower color mutants emerged at a high frequency (17.4–28.8%), and there were no significant differences in the mutation frequency between the treatments. All the flower color mutants induced with gamma rays were periclinal chimeras. In contrast, some mutants obtained with ion beams had the same flower color as that of the plants derived from the roots. This suggests that they were solid mutants, where both LI and LIII tissues were derived from the same mutated cell. Solid mutants were also obtained when irradiated with 5 Gy of helium ions, which had less effect on survival and mutation than other treatments. Factors for obtaining solid mutants only with ion beams are also discussed.