CARBON NUTRITION 367 



6. CARBON NUTRITION 



The classical work on the influence of nutrition, and for that matter 

 of all environmental factors, on reproduction in fungi is that of Klebs 

 (172, 173, 174). In relation to nutrition, his major points may be 

 formulated briefly: 



1. Sexual and asexual reproduction are most likely to occur when 

 a vigorous mycelium exhausts its nutrients or is transferred to a medium 

 low in nutrients. 



2. Formation of fruiting structures therefore occurs over a narrower 

 range of nutrient concentrations than does vegetative growth. 



3. Fruiting structures of different fungi, or different spore forms of 

 the same fungus, have specific requirements for nutrient concentration. 

 In Saprolegnia mixta and Sporodinia grandis the sexual spore stage 

 requires a higher nutrient level than the asexual stages. 



With minor modifications — most importantly that the concentration 

 of the carbon source is usually more decisive than that of other nu- 

 trients — these generalizations still stand. It would be tedious indeed 

 to list all the fungi that conform to Klebs' predictions; some of the 

 more significant modern results with pure cultures deserve, however, 

 some attention. 



Suppression, relative or absolute, of reproductive stages at higher 

 concentrations of carbohydrate — concentrations that permit heavy veg- 

 etative growth — has been found in studies of ascomycetes (59, 137, 

 141), phycomycetes (267), and imperfect fungi (44, 45, 82). More ex- 

 tensive reviews of the literature on this topic are available (140, 185). 



The same phenomenon can be demonstrated more neatly by transfer 

 experiments, in which a mycelium growing in a concentrated medium 

 is transferred to or allowed to grow into a more dilute medium. Such 

 experiments almost always confirm the first principle of Klebs (6, 81, 

 82, 142, 185, 219, 296). In general, transfer to water is less successful 

 than transfer to a dilute medium. 



Finally, it follows from the first principle that spore formation 

 should occur at the point in the culture cycle at which the carbon 

 source is substantially exhausted; studies on myxomycetes (62, 238) 

 demonstrate this principle clearly. The same response has been shown 

 in Melanconium fuJigineum in a different manner: heavy inoculation 

 of an agar surface accelerates sporulation because of the more rapid 

 utilization of nutrients (285). 



