Lethal and mutagenic effects of ultraviolet radiation on Glomerella conidia

Abstract

1. 1) The survival curves of ultraviolet irradiated Glomerella conidia have a sigmoid shape. The sigmoid survival curve may be described as a 7 hit curve (approximately) or as indicating radiation-produced toxic substances with a threshold for lethal action.2. 2) At high doses of radiation the survival curve levels off to form a long plateau. This plateau in the curve indicates the presence of a small proportion of resistant spores among the sensitive population. The resistant spores may be observed under the microscope and are characterized by the presence of melanin pigment in the spore in contrast to the great majority of Glomerella conidia which appear colorless. The proportion of resistant spores increases with increasing age of the spore culture.3. 3) With increasing dose of ultraviolet radiation the mutation curve rises to a peak and then declines at higher doses of radiation. This decline in the frequency of mutants among the surviving spores is due to an increasing proportion of resistant pigmented spores among the surviving population at high doses. The mutation curve for sensitive spores alone does not decline at the higher doses of radiation.