Evidence for Recovery from X-Ray Damage in Chlamydomonas

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Abstract
The effect, on subsequent colony formation, of irradiation of cells of Chlamydomonas reinhardi with X-rays at a high dose rate has been studied. A survival curve was obtained, the shape of which cannot readily be accounted for by a mechanism involving killing by a specific number of quanta. As in organisms whose survival curves are simpler, death rarely occurs until 1 to 5 cell generations have elapsed after irradiation. Cells are unusually large at the time of death. The LD50 is approximately 4.5 kr. When exposure to the X-ray beam is interrupted for about 15 minutes, survival after a given dose is increased, provided that the cells are at a suitable temperature during the treatment. Below 10[degree]C, little or no increase occurs even after an hour. Under favorable conditions, the second dose kills fewer cells than it would if given immediately after the first dose. When the number of cells killed by the first dose is accounted for, however, the fraction killed by the second dose is at least as great as would be expected if the first dose had not been given. Two or three interruptions of the dose result in higher survival than a single interruption, but not high enough to suggest that an irradiated cell population can tolerate more irradiation than a previously unirradiated population. There is evidence that illumination between irradiations enhances the increase in survival. These observations can be reconciled with target theory by making assumption which deprive the theory of much of its usefulness. It is suggested that the effect of interruption of the exposure is due to recovery, possibly associated with a metabolic process, rather than to a change caused by irradiation itself. It is further suggested that considerations of "hit number" may not be applicable to this organism.

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