# Biological Effect of Gamma Rays According to Exposure Time on Germination and Plant Growth in Wheat
###### Abstract  
Gamma rays as a type of ionizing radiation constitute a physical mutagen that induces mutations and could be effectively used in plant breeding. To compare the effects of gamma and ionizing irradiation according to exposure time in common wheat (Keumgang, IT 213100), seeds were exposed to \({}^{60}\)Co gamma rays at different dose rates. To evaluate the amount of free radical content, we used electron spin resonance spectroscopy. Significantly more free radicals were generated in the case of long-term compared with short-term gamma-ray exposure at the same dose of radiation. Under short-term exposure, shoot and root lengths were slightly reduced compared with those of the controls, whereas long-term exposure caused severe growth inhibition. The expression of antioxidant-related and DNA-repair-related genes was significantly decreased under long-term gamma-ray exposure. Long-term exposure caused higher radiosensitivity than short-term exposure. The results of this study could help plant breeders select an effective mutagenic induction dose rate in wheat.  
Keywords:wheat; gamma rays; exposure time; free radicals; growth inhibition; mutation breeding +
Footnote †: journal: selected
DNA molecules [4; 5]. Indirect damage generates highly reactive oxygen species (ROS), such as hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), superoxide radicals (O\({}_{2}\)\({}^{-}\)), and hydroxyl radicals (OH), potentially leading to oxidative stress [6]. The reported effects of ionizing radiation on plant species include effects on the mutation rate, cytogenetic effects, and effects on biological responses [7; 8; 9]. Among the radiation sources, gamma rays are very important in mutation breeding and in vitro mutagenesis to the development of the required features in plants and increasing the genetic variability [10]. A previous report indicated that gamma rays affect changes in the phytochemical composition of plants [11]. Numerous studies on the use of gamma rays to develop new cultivars with diverse genetic backgrounds reported that gamma rays affected plant characteristics such as grain color, flower color, and plant height [12; 13; 14; 15; 16]. Approximately 63% of the mutant varieties registered in the Mutant Variety Database (a joint FAO/IAEA project) are varieties created through gamma irradiation [17]. Gamma irradiation is still considered an attractive tool for mutagenesis.  
Most previous research focused on the germination of seeds and physiological and biochemical changes in plants according to the dose of gamma radiation [18; 19; 20]. This study aimed to investigate the response in wheat according to different gamma radiation doses and exposure times. We irradiated wheat (Keumgang: a Korean wheat variety) seeds with 100, 300, and 500 Gy of gamma rays for 8 h and 14 days (the final dosages were the same, but the dose rates per hour were different after gamma irradiation). To compare the gamma irradiation effect according to the radiation dose and exposure time, we investigated oxidative stress, seed germination percentage, plant growth, chlorophyll content, antioxidant enzyme activity, and gene expression levels. Before gamma irradiation, we examined various doses and exposure times to determine which irradiation conditions could efficiently induce mutations. The results of this study could be used to select the appropriate gamma radiation dose to induce mutations in wheat mutation breeding.