An elusive factor affecting mutation frequency in Tradescantia stamen hairs: its influence on r.b.e

A comparison of data from several experiments designed to determine either relative biological effectiveness (r.b.e.) or oxygen-enhancement ratios (o.e.r.) as a function of neutron energy revealed a puzzling variability in mutational response for pink mutations in Tradescantia stamen hairs. For some reason that we cannot determine, when the exposure fixtures and accompanying experimental procedures were changed as necessitated by the two studies, the expected mutation frequencies changed for X-rays and for four neutron energies. The other four neutron energies gave the expected results. As a consequence of these changes in mutation frequencies, the magnitude of r.b.e. values changed as well as the relationship of r.b.e. to neutron energy. Consequently, when r.b.e. values were determined using a closed but aerated exposure fixture instead of exposures made in free air, the resulting r.b.e. versus neutron-energy curve more closely paralleled that found by others for the inhibiton of root growth in Vicia faba.