RESPONSE AND ITS MEASUREMENT 



441 



1. THE RESPONSE TO TOXICANTS AND ITS MEASUREMENT 



For practical reasons most of our information on the response of 

 fungi to toxic agents relates to the inhibition of spore germination or 

 of mycelial growth. However, other effects are occasionally observed 

 and may be briefly described. The most important aspect of response 

 is, of course, its measurement, since any investigation of mode of action 

 presupposes an accurate, reproducible assay of the biological effect of a 

 fungicidal or fungistatic agent. 



Morphological Responses. Morphological abnormalities of growing 

 hyphae are caused by many toxicants, including such diverse agents as 

 diphenyl (340), dyes (363), benzothiazole (142), solanin (2), and 

 sodium tungstate (256). Other instances, as well as data on abnormal 

 development of germ tubes, are tabulated by Horsfall (174). Thymol 

 and carvacrol cause swelling of conidia of Aspergillus niger (450), as 

 does phenol (420). 



Griseofulvin, an antibiotic produced by Penicillium spp. (Chapter 

 6), causes stunting, branching, distortion, and other abnormalities of 

 growth in virtually all chitin-walled fungi tested, not in the yeasts or 

 Oomycetes, which have non-chitinous walls (Chapter 2). Brian (42) 

 suggests that griseofulvin either acts as a growth regulator or interferes 

 with a growth regulator controlling wall extension; spore germination 

 of Botrytis allii is not prevented by a concentration 500 times that which 

 affects germ tube growth (43). Griseofulvin is taken up by green 

 plants (74, 403) and protects to a considerable degree against infection 

 by Alternaria solani (44). A few fungi which probably have chitin 

 walls are nonetheless insensitive (304). 



Camphor causes the formation of large, possibly diploid, conidia 

 (361, 362). Cholic acid and sodium taurocholate induce the formation 

 of uninucleate cells in the dikaryon of several basidiomycetes, prob- 

 ably converting a clamp initial to a separate branch (286). 



Mitotic Poisons. Many fungitoxic substances are known to inhibit 

 mitosis in other organisms (137, 174). Exposure of basidia of Gymno- 

 sporangium clavipes to penicillin or streptomycin results in abnormal 

 development and interference with meiosis (27). In yeast, penicillin 

 inhibits cell division without inhibiting growth, so that filaments are 

 formed; reversal experiments suggest that sulfhydryl groups essential 

 to cell division are blocked by the antibiotic (310). 



The Response of Developmental Stages. Specific inhibition of spor- 

 ulation has both theoretical and practical importance. Simultaneous 



