86 CARBON NUTRITION 



mon in the fungi (160, 166, 227). Although most of such cases have 

 not been analyzed, the factor involved is almost certainly the forma- 

 tion of toxic metabolic products. In Entomophthora spp. the effect 

 of high concentrations of glucose is clearly to increase the acidity of 

 the medium (242), and the lowered pH resulting from organic acid 

 accumulation is probably the most general cause of poor growth at 

 high sugar concentrations. 



The carbohydrate concentration at which growth-depressant effects 

 become evident is of course affected by other constituents of the 

 medium, especially buffers and the nitrogen source, but there are also 

 specific effects. Thus, growth of Ustilago violacea falls off above 0.2 

 per cent glucose (180), that of Penicillium javanicum only above 21.3 

 per cent (227). It appears probable, although there are very few 

 data, that in those fungi which grow well at high sugar concentrations 

 (above 10 per cent) the factor which ultimately limits growth is osmotic 

 pressure (Chapter 1). 



Probably because so much of the early work on fungi concerned the 

 acid-tolerant species of Aspergillus and Penicillium, the media in com- 

 mon use for fungi are too high in available carbohydrate, giving a 

 distorted picture of growth and metabolism. Ideally, the level of 

 carbohydrate to be used should be determined by experiment for each 

 organism; perhaps as a rough rule of thumb, 2 per cent of available 

 carbohydrate in a liquid medium may be suggested as an upper limit, 

 considering the amounts of nitrogen usually recommended in media. 

 Westerdijk (232) suggests that agar stock culture media for fungi con- 

 tain not more than 3 per cent carbohydrate. 



9. enzyme induction: adaptive growth 



and the utilization of mixed carbon sources 



In this chapter we have had occasion to note that "adaptive" utiliza- 

 tion of substrates occurs in the fungi. Operationally, adaptive growth 

 is usually detected in an experiment in which a relatively large popu- 

 lation of spores in a given medium grows only after prolonged incuba- 

 tion. The organism is said to adapt itself to the carbon source. It 

 must be understood that in this type of experiment it is possible — and 

 in some instances very probable — that the results can be explained by 

 selection of a mutant pre-existing in the inoculum or arising during 

 the long incubation period. 



In a related type of experiment, it is found that a fungus produces 

 much more of a given enzyme if cultivated on the substrate of the 

 enzyme. The literature on this phenomenon is very large, and has 



