# Biological Effect of Gamma Rays According to Exposure Time on Germination and Plant Growth in Wheat
## 4 Discussion
Previous reports indicated that the DNA-repair-related genes, such as _PnLIG4_, _PnLUT0_, _PnXRCC4_, _PnPCNA_, and _PnRAD51_, from Lombardy poplar (_Populus nigra_) were upregulated upon gamma irradiation [55]. In addition, the _AtRAD51_, _AtLIG4_, and _AtXRCC4_ transcripts were upregulated upon gamma irradiation treatment [56]. We assessed DNA-repair-related gene expression patterns at various gamma radiation exposure times and doses (Figure 5). We observed that the expression of most DNA-repair-related genes (_XRCC_, _KU70_, _DuMT2_, and _MSH6_) was downregulated by gamma radiation in a dose- and time-dependent manner except for the case of short-term irradiation at 100 Gy. DNA-repair-related gene transcript levels in the case of long-term irradiation were lower than in the case of short-term irradiation. We demonstrated that long-term exposure to gamma radiation affected wheat growth more seriously than short-term exposure through DNA-repair-related gene expression levels. To avoid gamma-radiation-induced oxidative stress, plants use antioxidant defense systems comprised of enzymatic antioxidants (such as APX, CAT, SOD, GR, GPX, MDHAR, and DHAR) and non-enzymatic antioxidants (such as ascorbic acid (AA), reduced glutathione (GSH), \(\alpha\)-tocopherol, carotenoids, and flavonoids) [47]. The antioxidant defense systems are used to maintain intracellular homeostasis by regulating cellular ROS levels, resulting in the protection of plants against various stress conditions [57]. To determine the antioxidant activity in wheat plants at different gamma radiation doses and exposure times, antioxidant-related gene expressions and antioxidant enzyme activities were measured (Figures 6 and 7). We found that antioxidant-related gene transcripts showed lower expression levels in the case of long-term irradiation compared with the control and the case of short-term-irradiation. The antioxidant enzyme activities did not show the same pattern as the antioxidant-related gene expressions. This result indicates that the gamma-radiation-generated excess ROSs were not eliminated to an appropriate extent due to the reduced gene expression and antioxidant enzyme activity, thereby negatively affecting wheat plant growth. In this study, the _SOD_, _CAT_, and _APX_ gene transcript profiles displayed no direct correlation with the corresponding enzymatic activities, potentially due to the existence of several different antioxidant enzyme isoforms [58]. In this study, no direct relationship appeared to exist between gene expression and antioxidant enzyme activity. However, elucidating the antioxidant-related gene expression patterns could help us to better understand the molecular mechanisms underlying how different gamma radiation doses and exposure times affect wheat. As most investigations on the effects of gamma irradiation were performed under short-term irradiation conditions, the findings of this study provide direct information on various effects of gamma irradiation on wheat mutagenesis through the comparison of short- and long-term exposure to gamma radiation. Based on previous research, 100 Gy of gamma radiation can be considered an optimal dose for mutagenesis, and the effective dose for generating a high degree of genetic diversity and a small amount of physical damage is 200-400 Gy. Furthermore, 200 Gy of gamma radiation is an appropriate dose to obtain a drought-resistant wheat mutant line [59; 60; 61]. However, the optimal dose of gamma radiation for mutagenesis differs depending on the cultivar or desired agricultural trait. The results of our study investigating the gamma irradiation conditions-related correlation between plant oxidative stress and plant growth response in non-colored wheat provide useful information for the gamma-radiation-assisted mutation breeding of wheat.