EFFECTS OF LOW, CHRONIC DOSES OF IONISING RADIATIONS ON ANTIOXYDANT ENZYMES AND G6PDH ACTIVITIES IN STIPA CAPILLATA (POACEAE)

Stipa capillata (Poaceae) seeds were harvested on a control area (0.23 µSv.h-1) and 2 contaminated areas (5.4 and 25 µSv.h-1) on the Semipalatinsk nuclear test site (SNTS) in Kazakhstan. The plants were grown during 124 days in greenhouse under controlled conditions and exposed to low, chronic doses of ionising radiations. The first treatment consisted in chronic exposure to external irradiation (E) from a sealed 137Cs source; the second treatment (E+I) combined the external exposure with an internal contamination of the plants by 134Cs and 85Sr taken up from soil. The external dose rate amounted to 66 µSv.h-1 in both treatments while root uptake of 134Cs and 85Sr lead to a respective internal contamination at harvest of 100 and 5 Bq.g-1 of plant dry weight. At harvest, the activity of SOD, APX, GR, PER, CAT, G6PDH and MDHAR enzymes was measured in plant leaves. APX and PER showed significantly higher activities in plants originating from contaminated zones on SNTS. Treatment E induced an enhanced activity of all enzymes except MDHAR); this increase was proportional to the ? rate in the environment of origin on the SNTS. Compared to the control and treatment E, treatment E+I had no effect on PER and CAT activities; in plants grown from seeds collected in contaminated areas of the SNTS it induced a decrease of APX, MDHAR and GR activities and a significant increase in G6PDH activity. In all plants, treatment E+I provoked a significant increase of SOD activity, however greater in plants originating from contaminated areas. This suggest that exposure to low dose rate of ionising radiation during 50 years in their original environment has led to a natural selection of the most competitive genotypes and/or to an efficient induction of anti-oxidant enzyme activities, specially SOD, involved in plant protection against active oxygen species.