442 CHEMICAL AGENTS 



Figure 1. The response of 

 germination (curve 1) and oxy- 

 gen uptake (curve 2) of Neuro- 

 spora sitophila conidia as a 

 function of the concentration of 

 cycloheximide. Respiration was 

 measured in the presence of 

 sucrose, germination after re- 

 moval of the toxicant. Redrawn 

 from McCallan, Miller, and 

 10 ^00 1000 Weed (247), by permission of 



Cycloheximide, Mg per mg spores the B °y ce Thompson Institute 



for Plant Research, Inc. 



and fully quantitative determinations of both growth and sporulation 

 are difficult; preliminary data suggest that the high metal requirements 

 of sporulation (Chapter 11) make the process especially sensitive to 

 chelating agents, e.g., dimethylglyoxime, but many other types of toxi- 

 cant also reduce reproductive activity (179, 332, 333, 340, 342, 399, 428, 

 448, 457). 



It is generally believed that mycelial cells are less sensitive to toxicants 

 than are spores; this is borne out by direct comparisons for sulfanila- 

 mide (125, 147) and cycloheximide (430a, 431). The difference may, 

 however, be related to uptake (246a). 



Inhibition of Respiration. Inhibition of respiratory activity as 

 measured by oxygen uptake is often parallel to growth inhibition (106, 

 269, 309, 334), and it has even been tentatively suggested that the prin- 

 cipal activity of Captan 1 is on pyruvic carboxylase (160). However, 

 most studies, on very different compounds, show that in general endo- 

 genous and substrate respiration is much less sensitive to a wide range 

 of fungicides than is spore germination or growth (83, 125, 127, 132, 

 203, 224). 



Figure 1 is drawn from an extensive study of fungicidal activity, 

 judged by killing of spores, and spore respiration; the figure shows 

 that a compound, in this case cycloheximide, kills at concentrations 

 which do not measurably reduce oxygen uptake. The data of Klopping 

 (203) show a similar relation between fungistatic action and inhibition 

 of respiration; 8-hydroxquinoline has no effect on respiration at a con- 

 centration 100 times the fungistatic concentration. 



Low concentrations of toxicants often stimulate respiration (247). 

 It seems likely that membrane permeability to substrate is increased 



1 N-(trichloromethylthio)-4-cyclohexene-l,2-dicarboximide. 



