Somatic mutation frequencies in Tradescantia stamen hairs treated with relatively low thermal neutron fluxes.

Young inflorescence-bearing cuttings of Tradescantia clone BNL 02, a blue/pink heterozygote, were treated with thermal neutrons for 12-120 s in the Irradiation Tube set in the Heavy Water Facility of the KUR Reactor of the Research Reactor Institute, Kyoto University. In this facility, practically all components of the neutron flux were thermal neutrons. The thermal neutron fluxes measured simultaneously with gold foils were 2.90-33.3 x 10(10) nth cm-2, and the gamma-ray contamination per 10(10) nth cm-2 was 2.58 x 10(-4) C/kg (1.0 R). The induced somatic pink mutation frequencies in the stamen hairs increased linearly with thermal neutron flux at a rate of 1.69 pink mutant events per 10(4) hair-cell divisions per 10(10) nth cm-2 in Experiments 1 and 2, but at a higher rate of 3.87 pink mutant events per 10(4) hair-cell divisions per 10(10) nth cm-2 in Experiment 3. The relatively small thermal neutron fluxes applied were calculated to have resulted in absorbed doses of 16.8-193 mGy, and the absorbed doses of contaminating gamma rays were 28.0-321 mGy. Therefore, these mutation frequencies could be converted into 109 and 251 pink mutant events per 10(4) hair-cell divisions per gray, including the contaminating gamma rays, and were 2.14-4.92 times higher than the 51.0 pink mutant events per 10(4) hair-cell divisions per gray for relatively small doses of acute X rays which was obtained in earlier studies. The relative biological effectiveness of heavy particles (p, alpha and 7Li) produced by thermal neutrons was calculated to be between 11.0 and 35.4 compared with acute X rays rather than with 18-20-h exposure to gamma rays, the value of 11.0 being considered to be more reliable.