We investigated the radiation damages caused by two types of proton beam and gamma ray in the Cymbidium hybrid RB001 [(C. sinensis × C. goeringii) × Cymbidium spp.] to characterize proton beam as a new mutagen for Cymbidium mutation breeding. The protocorm-like bodies (PLBs) of Cymbidium hybrid were irradiated with a 45 MeV proton beam [mean linear energy transfer (LET) = 1.461 keV· m -1 ], a 100 MeV proton beam (LET = 0.7306 keV· m -1 ), and gamma ray (LET = 0.2 keV· m -1). The PLBs treated with radiation doses of 0-100 Gy were analyzed using the comet assay and their physiological responses as indices of radiation damage. In the comet assay, the 45 MeV proton beam caused significant damage to the DNA integrity, but the 100 MeV proton beam and gamma ray showed relatively little radiation damage between the untreated control and treated PLBs. Malondialdehyde (MDA) content, an index of the indirect effects of ionizing radiation, was increased slightly by the 45 MeV proton beam with higher-LET, but highly increased by the 100 MeV proton beam with lower-LET. These results suggested that the 100 MeV proton beam caused extreme oxidative stress. Therefore, proton beam have unique characteristics in DNA mutagenesis pattern according to its LETs. Based on these results, proton beam is expected to be a useful tool for developing new mutant varieties of Cymbidium.Additional key words: comet assay, gamma ray, linear energy transfer, physiological responses Hort. Environ. Biotechnol. 56(2):240-246. 2015.

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