Sensitivity of genotypically diverse rice varieties to radiation and the related changes to antioxidant enzyme activities.

Purpose: Radiation mutagenesis, which typically involves gamma rays, is important for generating new rice germplasm resources. Determining the appropriate radiation dose range is critical for the success of radiation mutagenesis. Clarifying the sensitivity and tolerance of genotypically diverse rice varieties to gamma irradiation as well as the radiation-induced changes to reactive oxygen species (ROS) generation and antioxidant enzyme activities is crucial for increasing the utility of radiation mutagenesis in rice breeding programs.

Materials And Methods: The seeds of the following four rice varieties with different genotypes were used as test materials: Zhe 1613, glutinous Zhe 1708, Zhejing 100, and glutinous Zhenuo 65. Additionally,Co was used as the source of gamma rays. The rice seeds were irradiated with 14 doses (0, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, and 750 Gy). Non-irradiated seeds were used as the control. The seedling survival rate for each variety was recorded at 3, 7, 14, and 28 days after sowing. Moreover, the median lethal dose (LD) and critical dose (LD) were calculated according to the seedling survival rates at 28 days after sowing. The seedling superoxide anion (O), hydrogen peroxide (HO), and malondialdehyde (MDA) contents and the superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) activities were analyzed at 7 days after sowing.

Results: As the radiation dose increased, the seedling survival rate decreased. The seedling survival rate also decreased significantly as the number of days after sowing increased. Among the rice genotypes, the rank-order of the radiation tolerance was as follows: Zhe 1613 > glutinous Zhe 1708 >  Zhejing 100 > glutinous Zhenuo 65. The LD values were 426.7 Gy for Zhe 1613, 329.2 Gy for Zhe 1708, 318.3 Gy for Zhejing 100, and 316.6 Gy for Zhenuo 65. Increases in the radiation dose resulted in significant increases in the seedling O and HO contents, but only up to a certain point. Further increases in the radiation dose caused the seedling O and HO contents to decrease. The HO content for each variety peaked when the radiation dose was very close to the LD. We propose that the radiation dose associated with the highest HO content (±50 Gy) should be used as the recommended dose for the gamma irradiation of rice. The radiation dose that resulted in peak seedling O contents in the analyzed rice varieties was very close to the LD. In all rice varieties, the MDA content increased as the radiation dose increased. The SOD, CAT, POD, and APX activities increased as the radiation dose increased within a certain range (less than 600 Gy for Zhe 1613 and 400 Gy for the other varieties), but there were slight differences among the rice varieties.

Conclusions: Genotypically diverse rice varieties vary regarding their sensitivity to gamma irradiation. Our findings suggest that ROS generation and antioxidant enzyme activities are important factors associated with the radiation mutagenesis of rice. The close relationship between the activities of key antioxidant enzymes, such as SOD, POD, APX, and CAT, and the LD and LD may be exploited to enhance radiation mutagenesis through the use of plant growth regulators.