# Biological Effect of Gamma Rays According to Exposure Time on Germination and Plant Growth in Wheat
## 4 Discussion
We found that the seed germination and seedling growth in wheat might be linked to the gamma radiation dose- and exposure time-related changes in free radical contents (Figures 1-3). Free radicals, such as ROS and reactive nitrogen species (RNS), are extremely reactive chemical compounds that potentially cause oxidative stress by damaging cell structures, including lipids, proteins, and DNA [46; 47]. Free radicals play crucial roles in radiation sensitivity [48]. Previous studies have shown that high gamma radiation dose rates severely damage plants and induce the formation of more free radicals [49; 50; 51]. We evaluated the free radical levels according to the radiation dose and exposure time and observed that these levels increased linearly with increasing radiation doses in the case of both short- and long-term gamma irradiation (Figure 1). In particular, the number of free radicals was larger in the case of long-term irradiation than in the case of short-term irradiation. A previous study reported that chronically irradiated colored wheat exhibited lower free radical levels compared with the controls and showed that colored wheat anthocyanins could directly influence free radical scavenging capacity during the early developmental stages of wheat [52]. Unlike the results of previous studies, the results of this study show very high free radical levels in the wheat seeds subjected to long-term exposure to gamma radiation, and plant growth was also significantly reduced. Seeds that contain antioxidants (e.g., anthocyanins), such as those in colored wheat, even when exposed to gamma radiation for a long time, eliminate free radicals via antioxidants, thereby enabling plant growth. However, in the case of non-colored varieties of wheat, such as Keumgang,the antioxidant activity is relatively low; thus, the free radical elimination ability is limited, which seems to negatively affect plant growth (Figure 3). These results suggest that when non-colored wheat seeds are continuously exposed to low doses of gamma radiation, the free radical production increases and this increase in free radicals induces oxidative stress, negatively affecting wheat seed germination and seedling growth. Chlorophyll content is a useful indicator for evaluating the physiological response to radiation [53]. A previous report indicated that exposure to 100 Gy of gamma radiation increased the content of chlorophyll in wheat [18]. In this study, short-term irradiation at 100 Gy produced the same result (Figure 4). Our results indicate that high gamma radiation doses decreased the chlorophyll content in comparison with the control. In particular, the chlorophyll content decreased significantly in the case of longer gamma radiation exposure times (Figure 4). Numerous studies have reported that high gamma radiation doses reduce photosynthetic activity, ultimately decreasing the chlorophyll content and plant growth [34; 41]. This result is consistent with those of previous studies reporting that plants exposed to gamma radiation exhibited lower chlorophyll contents than the controls. We also confirmed that the chlorophyll content in the wheat seeds exposed to gamma radiation significantly decreased in the case of long-term compared with short-term irradiation. Gamma radiation can affect the integrity of genetic information and impair genomic stability by inducing DNA damage. In contrast to animals, plants are constantly exposed to the threat of DNA damage as they cannot change locations to avoid unfavorable growth conditions. Under adverse conditions, most organisms have mechanisms, such as DNA repair, that prevent mutations caused by damaging ionizing radiation, and the DNA strand breaks induced by ionizing radiation can be repaired with fast kinetics. [PERSON] et al. [54] found that the DNA damage induced by 30 Gy of gamma radiation in tobacco was perfectly repaired within 24 h. The DNA-repair-related genes allow for the repair of ionizing-radiation-induced damage.