The radiomimetic effect of bleomycin (BLM) at concentrations of 5 and
10 mg/l on root meristem cells was studied. The roots of germinating s
eeds of Crepis capillaris were exposed to BLM 16 h after the seeds wer
e soaked. At this time, the root meristem cells were at the S or late
G(1) phase of the cell cycle. The mutagenic effect of BLM was found wh
en the cells were fixed at the 29th, 32nd, 36th, 39th, 42nd, or 45th h
our (i.e., the early germinating fraction). The aberrations induced in
cluded chromosome and chromatid rearrangements. At late fixations, cel
ls were observed, which had, in addition to definite double-hit aberra
tions, multiple chromosomal breaks and pronounced chromosomal fragment
ation. A hypothesis is advanced that BLM causes inducibIe repair that
is similar to SOS-repair. This repair is prone to DNA synthesis errors
, which account for the increase in the number of aberrations at late
fixation times. Pretreatment with 1 mu g/ml of mitomycin C, an alkylat
ing agent, before the exposure to 10 mg/l BLM, resulted in cross-adapt
ation, which drastically reduced the number of BLM-induced aberrations
. This was especially evident for the number of aberrations and the nu
mber of cells affected at late fixations.