Lethal and mutagenic effects of ultraviolet radiation on Glomerella conidia.

IN comparative studies of the mutagenic action of ionizing and ultraviolet radiation on conidia of the fungus Glomerella (5) it was evident that the shape of the mutation curve was different for the two types of radiation. The dose-mutation curve for the surviving spores was essentially linear for the ionizing radiations, but with the ultraviolet radiation the mutation curve rose with increasing dose to a peak and then declined with further increases in dose. Studies on the mutagenic effects of radiation on other fungi have generally yielded the same types of curves (1, 2, 3, 7). Although various supplementary treatments of the radiated spores have been shown to alter the frequency of mutants found at any dose, the basic shape of the curve is not changed by these treatments (10, 11). The types of survival curves obtained by plotting the logarithm of the survivors against dose of radiation also vary with the type of radiation. Heavily ionizing radiation results in an exponential survival curve (8, 13) while the less densely ionizing x-radiation results in a sigmoid curve; many investigators have, however, obtained exponential survival curves with x-rays. The non-ionizing ultraviolet radiation has generally resulted in sigmoid survival curves (l), although Norman (6) obtained exponential survival curves with Neurospora microconidia after ultraviolet radiation. Ultraviolet radiation of bacteria has yielded either sigmoid or exponential survival curves depending upon the genetic makeup of the bacteria (12). Of these various radiobiological effects perhaps the most perplexing has been the shape of the mutation curve with ultraviolet. No satisfactory explanation has yet been advanced for the decline in the mutation rate at high doses of ultraviolet (3). The present investigation was undertaken primarily to elucidate the factors involved in this peculiarity of the dose-mutation curve with ultraviolet and to relate this effect if possible to some characteristic of the survival curve.