Induced Radiation Resistance during Low-Dose-Rate γ Irradiation in Plateau-Phase Chlorella Cells

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Abstract
The unicellular green alga Chlorella is well suited to studies of radiosensitivity to acute and protracted irradiation, the end point being cell survival. The survival curve of cultures in plateau phase after low-dose-rate irradiation reflected the gradual appearance of radioresistance. Resistance to acute irradiation was also seen following protracted irradiation. The induced radioresistance was a function of the total radiation dose delivered at low dose rate. It did not result from the selection of radioresistant cells; it depended very little on the age of the culture, regressed spontaneously when low-dose-rate irradiation was discontinued, and completely disappeared in progeny of low-dose-rate irradiated cells. The resistance did not appear to be linked to the repair of potentially lethal damage. We suggest that this phenomenon of low-dose-rate irradiation-induced radioresistance could be linked to repair stimulation.