Threshold dose rates for the cytogenetic effects in crested hairgrass populations from the Semipalatinsk nuclear test site, Kazakhstan

Abstract An assessment of cytogenetic effects in crested hairgrass (Koeleria gracilis Pers.) populations was carried out within the Semipalatinsk nuclear test site (Kazakhstan) where combat radioactive substances were tested in 1953–1957. Current levels of radioactive contamination within this site are varied by orders of magnitude, while soil characteristics and heavy metal pollution are similar. The main contribution to the absorbed by plants doses at this site was caused by incorporated 90Sr. The frequency of cytogenetic alterations in crested hairgrass was investigated in a wide range of doses (10−4–13 Gy/growing season) at 100 sampling points. For the first time in the field conditions the shape of the cytogenetic effects - dose rate relationship was evaluated with acceptable accuracy and found to be nonlinear. The frequency of aberrant cells remained practically unchanged up to 49 µGy/h. Exceeding the threshold dose rate lead to a steep increase in the aberrant cells frequency from less than 2% up to 16%. The main contribution to the cytogenetic effects was made by double bridges and fragments. Breakpoints for other types of cytogenetic alterations were also evaluated (7 µGy/h for single fragments and bridges; 74 for double fragments and bridges; 81 for mitotic abnormalities).