Some Observations on Irradiation Effects in Yeast

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Abstract
Two strains of yeast, a haploid and a diploid, were irradiated with X-rays, and survival curves were obtained by colony counts. In haploid yeast, the viability decreased exponentially with increasing X-ray dose, whereas the diploid strain exhibited a plateau in the dose-response curve similar to that previously described by other workers for haploid yeast. The number of survivals and the shape of the survival curves varied with the physiological condition of the cells at the moment of irradiation. Thus, for example, in baker''s yeast, deprivation of a source of N prior to irradiation resulted in a certain degree of protection, when compared with the survival of fresh nonstarved cells. Considerable protection was further afforded in both strains of yeast if cells were grown and irradiated in high concentration of substrate (55% fructose). Lastly, N deprivation interposed between irradiation and plating also favored recovery. Reasons are suggested for these changes in support of the view that indirect effects of irradiation contribute to a large extent to loss of viability in yeast.

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