| Summary: | Ionizing radiation combined with in vitro tissue culture has been used for development of new cultivars in diverse crops. The effects of ionizing radiation on mutation induction have been analyzed on several orchid species, including <i>Cymbidium</i>. Limited information is available on the comparison of mutation frequency and spectrum based on phenotypes in <i>Cymbidium</i> species. In addition, the stability of induced chimera mutants in <i>Cymbidium</i> is unknown. In this study, we analyzed the radiation sensitivity, mutation frequency, and spectrum of mutants induced by diverse γ-ray treatments, and analyzed the stability of induced chimera mutants in the <i>Cymbidium</i> hybrid cultivars RB003 and RB012. The optimal γ-irradiation conditions of each cultivar differed as follows: RB003, mutation frequency of 4.06% (under 35 Gy/4 h); RB012, 1.51% (20 Gy/1 h). Re-irradiation of γ-rays broadened the mutation spectrum observed in RB012. The stability of leaf-color chimera mutants was higher than that of leaf-shape chimeras, and stability was dependent on the chimera type and location of a mutation in the cell layers of the shoot apical meristem. These results indicated that short-term γ-irradiation was more effective to induce mutations in <i>Cymbidium</i>. Information on the stability of chimera mutants will be useful for mutation breeding of diverse ornamental plants.
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