Effects of Acute and Chronic Gamma Irradiation on the Cell Biology and Physiology of Rice Plants

Abstract

The response to gamma irradiation varies among plant species and is affected by the total irradiation dose and dose rate. In this study, we examined the immediate and ensuing responses to acute and chronic gamma irradiation in rice (Oryza sativa L.). Rice plants at the tillering stage were exposed to gamma rays for 8 h (acute irradiation) or 10 days (chronic irradiation), with a total irradiation dose of 100, 200, or 300 Gy. Plants exposed to gamma irradiation were then analyzed for DNA damage, oxidative stress indicators including free radical content and lipid peroxidation, radical scavenging, and antioxidant activity. The results showed that all stress indices increased immediately after exposure to both acute and chronic irradiation in a dose-dependent manner, and acute irradiation had a greater effect on plants than chronic irradiation. The photosynthetic efficiency and growth of plants measured at 10, 20, and 30 days post-irradiation decreased in irradiated plants, i.e., these two parameters were more severely affected by acute irradiation than by chronic irradiation. In contrast, acutely irradiated plants produced seeds with dramatically decreased fertility rate, and chronically irradiated plants failed to produce fertile seeds, i.e., reproduction was more severely affected by chronic irradiation than by acute irradiation. Overall, our findings suggest that acute gamma irradiation causes instantaneous and greater damage to plant physiology, whereas chronic gamma irradiation causes long-term damage, leading to reproductive failure.

Keywords: DNA damage; acute and chronic irradiation; antioxidant activity; growth and reproduction; oxidative stress; photosynthetic efficiency.

Figure 1

DNA damage induced by acute and chronic gamma irradiation in rice plants. (a) Acute irradiation treatments; (b) chronic irradiation treatments. Data represent mean ± standard deviation (SD). Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, *** p < 0.001).

Figure 2

Determination of the free radical content of gamma-irradiated rice plants using the electron spin resonance (ESR) method. (a,b) ESR intensity in rice plants subjected to acute (a) and chronic (b) gamma irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 3

Quantification of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents of rice plants subjected to acute and chronic gamma irradiation. (a,b) H₂O₂ contents of plants subjected to acute (a) and chronic (b) gamma irradiation. (c,d) MDA contents of plants subjected to acute (c) and chronic (d) gamma irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 4

Estimation of superoxide radical (O₂^•−) scavenging activity in gamma-irradiated rice plants by measuring half maximal inhibitory concentration (IC₅₀) values. (a,b) IC₅₀ values of plants subjected to acute (a) and chronic (b) gamma irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05).

Figure 5

Measurement of antioxidant enzyme activities in rice plants. (a–h) Activities of ascorbate peroxidase (APX) (a,b), catalase (CAT) (c,d), peroxidase (POD) (e,f), and superoxide dismutase (SOD) (g,h) in plants subjected to acute (a,c,e,g) and chronic (b,d,f,h) gamma irradiation. Data represent mean ± SD. Asterisks indicate significant differences between the control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 6

Phenylalanine ammonia-lyase (PAL) activity, phenolic compound content, and ascorbic acid (AsA) content of rice plants subjected to gamma irradiation. (a–f) PAL activity (a,b), phenolic compound content (c,d), and AsA content (e,f) of rice plants subjected to acute (a,c,e) and chronic (b,d,f) gamma irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 7

Photosynthetic efficiency of rice plants subjected to gamma irradiation. (a–d) Maximum quantum yield of PSII (F_v/F_m) (a,b) and quantum yield of PSII electron transport (ΦPSII) (c,d) in rice plants subjected to acute (a,c) and chronic (b,d) gamma irradiation. DAI, days after irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).

Figure 8

Growth analysis of rice plants exposed to gamma irradiation. (a) Photographs of rice plants treated with acute and chronic gamma irradiation. (b–e) Plant height (b,c) and tiller number (d,e) of rice plants subjected to acute (b,d) and chronic (c,e) gamma irradiation. DAI, days after irradiation. Data represent mean ± SD. Asterisks indicate significant differences between control (0 Gy) and other treatments (* p < 0.05, ** p < 0.01, *** p < 0.001).