ULTRAVIOLET RADIATION 



431 



Figure 5. The survival of 

 macroconidia (average number 

 of nuclei 2.67 per conidium) 

 of Neurospora crassa as a 

 function of ultraviolet dosage. 

 Redrawn from Atwood and 

 Norman (5), by permission of 

 the National Academy of Sci- 

 ences. 



60 120 180 



Irradiation time, seconds 



240 



species with dark spores tend to be more resistant (36, 46), and a pro- 

 tective action of wall pigments in Puccinia graminis uredospores has 

 been claimed (29). Pigmentation alone, however, cannot explain spe- 

 cific differences in sensitivity. 



The curve generated by a plot of survival against dose is usually one 

 of two types: 



1. The exponential curve, in which the logarithm of the fraction 

 surviving is a linear or approximately linear function of dosage. This 

 curve is characteristic of microconidia of Neurospora crassa (Figure 4) 

 and spores of Penicillium spp. (2, 46), Aspergillus melleus (33), and 

 Streptomyces flaveolus (80). 



2. The sigmoidal curve (Figure 5), which is the more common type, 

 reported in studies of Aspergillus spp. (73, 158), Mucor dispersus and 

 Rhizopus suinus (33), Ustilago zeae (85), Glomerella cingulata (95), 

 Fusarium eumartii (126), Trichophyton mentagrophytes (64), and 

 Streptomyces griseus (121). 



The significance of these curve shapes has been debated vigorously, 

 and it may be too early for a categorical explanation. By itself, the 

 sigmoidal curve could be interpreted as expressive of variability in 

 the spore population, and Markert (95) has shown such variability in 

 sensitivity in Glomerella cingulata. However, this interpretation ap- 

 plied to the exponential curve requires a highly improbable distribu- 

 tion of sensitivity within the population. Studies on Neurospora crassa 



