428 PHYSICAL AGENTS 



Chapter 11. It must be recalled that exposure to "white light" may, 

 depending on the source, involve exposure to some ultraviolet; heating 

 effects, too, cannot de disregarded unless specifically excluded. 



Several early observations, reviewed by Smith (127), and later data 

 (20, 58, 75, 113, 129) agree that light retards mycelial growth; dry 

 weight measurements confirm this for Sclerotinia fructigena (55) and 

 Karlingia rosea (60). The growth of Botrytis squamosus on agar is 

 completely inhibited by light (112). 



Light stimulation of growth, infrequently reported (60, 127), has 

 been most carefully analyzed in Blastocladiella emersonii (18). In 

 this organism dry weight increases of up to 141 per cent, at constant 

 temperature, are induced by illumination. It is observed further that 

 fixation of carbon dioxide is increased by light, and it is proposed that 

 light acts in some way to accelerate the reductive carboxylation of 

 succinic acid and arketoglutaric acid. 



Vegetative mycelium, like reproductive structures, may be photo- 

 tropic (122) and may show zonation in response to light and dark 

 periods (25). Pigmentation is frequently dependent upon or enhanced 

 by light (127); Centrospora acerina forms its red pigment after a very 

 short exposure (107). Melanogenesis in Neurospora crassa is some- 

 what reduced by illumination (123). The effect of light on carotene 

 formation has been mentioned elsewhere (Chapter 6). 



It has been recognized at least since 1860 (61) that strong light in- 

 hibits spore germination. Whether spores are more or less sensitive 

 than mycelium is in doubt (127). Obviously, any ultraviolet con- 

 tamination is especially serious, and there is no doubt that many of 

 the lethal effects of light can be credited to a short wavelength com- 

 ponent. However, in both Erysiphe graminis (117) and Puccinia gra- 

 minis (30, 31) experiments with filters implicate the longer wave- 

 lengths, in the yellow-red region, and exclude ultraviolet. Moist spores 

 are more susceptible than dry (23). 



A number of reports — too many to disregard — suggest that spore 

 germination is increased by light or even requires light (47, 48, 71, 

 106, 130, 155, 160). The mechanism of such an effect is difficult to 

 imagine, and it can only be said at this time that further quantitative 

 work under accurately controlled conditions is necessary. 



Photodynamic action has been demonstrated by Kaplan (78) in 

 Penicillium notatum: conidia vitally stained with erythrosin are sensi- 

 tized so that visible light, normally innocuous, becomes mutagenic. 

 Eosin-stained cells of Neurospora crassa appear from limited data to 

 respond similarly (34). Methylene blue intensifies the zonation re- 

 sponse of Penicillium notatum to light (120a). 



