Radiation Responses of Fern Spores during Their First Cell-cycle

Abstract

SummarySpores of the Royal fern, Osmunda regalis, were irradiated at different times after water uptake and before their first mitosis. 3H-thymidine labelling showed that light-grown cells synthesized DNA between 10 and 22 hours after water uptake, and half of these reached mitosis between 24 and 30 hours. Dark-grown cells did not synthesize DNA nor divide until given light. Sensitivity to killing by x-rays determined by D0 values of survival curves increased by a factor of 4·5 throughout interphase in light-grown cells but levelled off after 6 hours in dark-grown cells. The suggested correlation between increasing sensitivity and DNA synthesis was thus in the direction opposite to that seen in many other cell types. We conclude that a basic connection between radiation sensitivity and DNA synthesis is contra-indicated. The sparing effect of splitting the radiation dose into two equal fractions was approximately equal throughout interphase and was similar in light- and dark-grown cells. We conclude that this effect was independent of DNA synthesis. Survival was increased when mitosis was delayed by maintaining the cells in the dark for a period after irradiation. We interpret this as indicating rescue from potentially lethal damage.