# Biological Effect of Gamma Rays According to Exposure Time on Germination and Plant Growth in Wheat
## 4 Discussion  
Mutation breeding has been used as a tool to improve the genetic diversity of plants and develop desired traits. Gamma rays are a source of mutagens that can induce mutations in plants. Several breeders use gamma rays to select desired traits by securing genetic diversity. Various studies have shown the effects of gamma irradiation on seed germination, plant growth, chlorophyll content, oxidative stress, and secondary metabolite  
Figure 8: Different gamma radiation dose and exposure time effects on the (**A**) total phenolic content and (**B**) DPPH free radical scavenging activity. Each bar represents the mean \(\pm\) SD (\(n=3\)). Values with different letters are significantly different using [PERSON]’s multiple range test (\(p<0.05\)). Figure 7: Short- and long-term gamma irradiation effects on (**A**) APX, (**B**) CAT, (**C**) SOD, and (**D**) POD antioxidant enzyme activities. Each bar represents the mean \(\pm\) SD (\(n=3\)). Values with different letters are significantly different using [PERSON]’s multiple range test (\(p<0.05\)). production [7; 33]. In general, high-dose gamma irradiation is known to negatively influence the physiological and biochemical traits of plants [9; 34; 35]. However, radiosensitivity might reveal different levels of susceptibility depending on the plant variety due to DNA-content-, repair-process-, antioxidant-reaction-, and cell cycle kinetics-related differences [36; 37; 38; 39]. In order to increase the mutation breeding efficiency, the selection of an appropriate radiation dose that could increase the mutation frequency without significantly affecting the plant's survival and reproduction is important. Most studies focused on the assessment of biological responses to different radiation doses, whereas relatively few studies have examined the different effects of gamma radiation exposure times [9; 40; 41]. Here, we report the main results of our study on the dose- and time-dependent effects of gamma irradiation, focusing on germination rate, plant growth, chlorophyll content, oxidative stress, gene expression, and antioxidant activity changes. Long-term exposure to gamma rays had a significantly negative effect on the seed germination percentage. We observed no differences in the final germination percentage between control and irradiated seeds. However, there was a significant difference in the germination pattern between short-term and long-term irradiation in the early germination stages (DAG1 and DAG2) (Figure 2). Germination-related indicators showed a tendency to decrease as the gamma radiation dose and exposure time increased (Table 1). Long-term gamma irradiation significantly reduced the GRI compared with short-term irradiation. We confirmed that the MGT increased as the exposure time increased, even at the same dose of gamma radiation (Table 1). Numerous studies have demonstrated that high gamma radiation doses cause physiological changes, such as a delay in seed germination and a reduction in the survival rate and plant growth, in wheat [19; 20; 42]. These results are consistent with those of previous researchers who reported that the seed germination potential decreased as the radiation dose increased. In addition, the germination speed was confirmed to be reduced with prolonged exposure to gamma rays even at the same radiation doses. Growth inhibition and retardation are known to be caused by cell cycle arrest in the G2 phase during somatic cell division upon exposure to high-dose gamma radiation [43; 44]. Moreover, a gamma-radiation-mediated reduction in auxin and DNA synthesis could lead to a reduced mitotic frequency in the apical menistematic tissues, resulting in reduced plant growth and development [35; 36; 45]. This study on wheat's response to gamma radiation showed that the gamma-radiation-related reduction in germination speed and plant growth could be attributed to the cell cycle arrest and changes in plant hormones and DNA synthesis.