Photoinactivation and long-term photosensitivity of diploid and disomic strains of the smut fungusUstilago violacea

Abstract

Diploid and disomic strains ofUstilago violacea, containing high cellular concentrations of cytochrome c and various cellular concentrations of carotenes, were subjected to high levels of radiation from incandescent, fluorescent, and UV sources to characterize inactivation kinetics. Additionally, these strains were grown under high and low levels of radiation from the fluorescent and incandescent sources, to characterize their long-term resistance to photoinactivation by visible light with different spectral qualities. Survival kinetics in response to incandescent radiation were characterized by either an exponential decay for the low carotene strain, or an initial loss of viability, with recovery and a subsequent gradual loss of viability for the strains containing high carotene concentrations. Long-term colony survival was suppressed at the high incandescent level for the low carotene strain only. Long-term incandescent exposure did not affect the other strains. Survival kinetics in response to fluorescent and UV radiation were characterized as exponential decays for all strains, however, the exposure time leading to 90% loss in viability LD90 was quantitatively related to carotene content in the form of a power function, where LD90 increased as the total carotene content per cell. We also determined that the ratio of colored carotenes to cytochrome c in the cells was quantitatively related to the rate of viability loss during UV exposure. In all cases, including the long-term fluorescent experiment, the strains containing higher concentrations of carotenes were more resistant to destruction by the different radiation sources.