Influence of radiation dose rate on somatic mutation induction in Tradescantia stamen hairs

Both X and gamma rays have been used to investigate the effect of dose rate on the induction of pink mutant cells in stamen hairs of Tradescantia clone 02. For inflorescences exposed to doses of 60-80 rads of X rays at 0.3 to 503 rads/min there was a progressive increase in the number of $\text{mutations}/\text{rad}/10^{4}$ hairs as the dose rate was increased from 0.3 to 105 rads/min. However, 300 rads/min did not further increase the mutation rate, and at 503 rads/min the effect was slightly less, but approximately equal to that at 30 rads/min. With 58-79 rads of 137 Cs gamma rays the mutation rate increased progressively with dose rate over the entire range studied (0.0034-30 rads/min). Some inflorescences were held at 8 C before and during irradiation to slow development so that low dose rates (3.4-12 mrads/min) could be delivered within a given development period. The mutation rate during cold treatment was higher than when the same dose rates were given during a period of more rapid development under standard growth conditions. Dose-response relationships were established at three radiation intensities, 0.05, 0.5, and 5.0 rads/min, and the resulting mutation rates were related to those from a previously established curve derived at 30 rads/min. Mutation response was independent of dose rate at 10 rads and below, but a progressively lower incidence of mutation was associated with the 5.0 and 0.5 rad/min intensities over at least a 30- to 160-rad range of doses. Response to 0.05 rads/min was not different from that at 0.5 rads/min over the entire 5- to 30-rad range of doses investigated.