Effects of low chronic doses of ionizing radiation on antioxidant enzymes and G₆PDH activities in Stipa capillata (Poaceae)

Stipa capillata (Poaceae) seeds were harvested from a control area (displaying a γ dose rate of 0.23 μSv h^-1) (C plants) and from two contaminated areas (5.4 and 25 μSv h^-1) on the Semipalatinsk nuclear test site (SNTS) in Kazakhstan. The plants were grown for 124 d in a greenhouse under controlled conditions and exposed to three different treatments: (0) control; (E) external γ irradiation delivered by a sealed ¹³⁷Cs source with a dose rate of 66 μSv h^-1; (E+I) E treatment combined with internal β irradiation due to contamination by ¹³⁴Cs and ⁸⁵Sr via root uptake from the soil. The root uptake led to a contamination of 100 Bq g^-1 for ⁸⁵Sr and 5 Bq g^-1 for ¹³⁴Cs (of plant dry weight) as measured at harvest. The activity of SOD, APX, GR, POD, CAT, G₆PDH, and MDHAR enzymes was measured in leaves. Under (0) treatment, all enzymes showed similar activities, except POD, which had higher activity in plants originating from contaminated areas. Treatment (E) induced an enhancement of POD, CAT, GR, SOD, and G₆PDH activities in plants originating from contaminated areas. Only control plants showed any stimulation of APX activity. Treatment (E+I) had no significant effect on APX, GR, CAT, and POD activities, but MDHAR activity was significantly reduced while SOD and G₆PDH activities were significantly increased. The increase occurred in plants from all origins for SOD, with a greater magnitude as a function of their origin, and it occurred only in plants from the more contaminated populations for G₆PDH. This suggests that exposure to a low dose rate of ionizing radiation for almost a half century in the original environment of Stipa has led to natural selection of the most adapted genotypes characterized by an efficient induction of anti-oxidant enzyme activities, especially SOD and G₆PDH, involved in plant protection against reactive oxygen species.