Enzyme polymorphism of antioxidant system in chronically irradiated Scots pine populations

Background. The Chernobyl accident caused dramatic and long-term increases in ambient radiation doses to many forest environments. Sites still exist in the Bryansk region of Russia, 25 years after the Chernobyl accident, where radioactive contamination significantly exceeds background. There is growing evidence that effects of chronic radiation exposure lead to population-level impacts. Isozymes polymorphism assessment is thought to be promising since it sensitive enough, may integrate the impacts of harmful agents in the environment, and provide meaningful information on biological damage The aims of this study were to examine whether Scots pine trees have experienced stress in areas with relatively low levels of radioactive contamination and, if so, to quantify such an impact.Materials and methods. Scots pine (Pinus sylvestris L.), the dominant tree species in North Eurasia boreal forests, was chosen as a test organism for an assessment of the possible effects from the radioactive contamination. Four study sites were chosen in the most contaminated areas of the Bryansk region of Russia. This area was significantly contaminated by Chernobyl fallout with an initial 137Cs ground deposition level more than 1 MBq/m 2 in some locations (Ramzaev et al., 2008). Sites are uniform in climate and the physical and chemical properties of soils, forest stand is homogeneous, and pine trees occupy a large part of phytocenosis. Dose rates at the study sites ranged from 0.37 to 1.21 μGy/h, compared to 0.10 μGy/h at control site. Pine cones were collected in December of 2009–2010. Specific activities of radionuclides in soil samples were measured, and doses to the pine trees’ generative organs were estimated. Antioxidant enzymes polymorphism (superoxide dismutase (SOD, KF 1.15.1.1), glutathione reductase (GR, KF 1.6.4.2), and glutathione peroxidase (GPX, KF 1.11.4.2)) was investigated by vertical electrophoresis.Results. The study sites did not differ substantially in soil properties, or chemical content of soil or pine cones (Geras'kin et al., 2011; Geras'kin et al, 2008). The main contribution to the radioactive contamination was from 137Cs with activity concentrations at the contaminated sites (averaged through 0–15 cm soil layer) ranging from 2.4 to 56.3 kBq/kg of soil. These activity concentrations exceed those at the referent site by factors of 47–1081 (Geras'kin et al., 2011). Annual absorbed doses rates changes from 7.0 to 130 mGy/year compared to 0.14 mGy/year at reference site. The average frequency of mutations in three enzymes significantly (r = 99 %, p <0.01) increases with increasing absorbed dose by generative plant organs. Indices of phenotypic diversity in the populations studied are significantly different from the control. Proportion of rare morphs dramatically increases in populations inhabiting areas contaminated with radionuclides, which is associated with the emergence of new alleles in several enzyme systems (null alleles, allele 0.80 in the GPX-1 and 0.95 in the GPX-2). The observed and expected heterozygosity in affected populations increase along with the level of contamination sites (r = 99 %, p <0.01).Conclusions. These data show that chronic, low level radiation exposure (7–130 mGy/year) results in an increase in frequency of mutations in the endosperm and the seed embryos and modify the genetic structure of Scots pine populations. These data suggest that a high level of mutations occurs in the progeny of these pine populations and that their diversity is conditioned by radiation exposure. Thus, in spite of their low values, dose rate observed can be considered as a factor capable of modifying the genetic structure of populations.

Chernobyl accident, radioactive contamination, chronic irradiation, izozymes, genetic variability, rare morphs, null-alleles