326 VITAMIN REQUIREMENTS 



least over a certain range, growth will be proportional to the supply of 

 the required factor or factors. Many natural and induced deficiencies 

 are or appear to be of this type, which is the most easily detectable. 



A somewhat more complicated situation arises if a fungus is able to 

 synthesize a vitamin, but so slowly that under the usual conditions of 

 culture the rate of all other processes is potentially faster than vitamin 

 synthesis. Here we find the organism growing slowly in the absence of 

 exogenous vitamin but responding to an external supply by a faster rate 

 of growth. Such partial deficiencies are also common, perhaps more 

 common than complete deficiencies. Examples include the require- 

 ments of particular fungi for thiamine (16, 65, 183, 186), biotin (144, 

 216), pyridoxine (66, 67, 216, 217), nicotinic acid (291), and inositol 

 (238). 



ft also follows that any growth-regulating process — including vita- 

 min synthesis — may be limiting under some conditions and not under 

 others. A vitamin requirement that is manifest only under certain 

 circumstances is termed a conditioned requirement. The known types 

 of conditioned deficiency in fungi may be classified as follows: 



1. The deficiency is apparent or more severe at particular tempera- 

 tures, pH levels, or salt concentrations. 



2. The requirement for a vitamin is reduced or, more rarely, elimi- 

 nated by provision in the medium of a precursor or of a metabolite for 

 the synthesis of which the vitamin is essential. 



3. The deficiency is limited to or more acute at a particular stage of 

 development. 



Environmental factors conditioning vitamin deficiencies include tem- 

 perature, acidity, and salt concentration. A mutant of Nenrospora 

 crassa, for example, requires riboflavin only at relatively high tempera- 

 tures (172). Sclerotinia camelliae requires more inositol at 26° C than 

 at 18° but is inhibited by inositol at 27° (8); a temperature-dependent 

 inositolless mutant of N. crassa is also known (113). Sordaria fimicola 

 requires thiamine only if the initial pH of the culture is less than 4.0, 

 possibly because thiamine synthesis is inhibited by hydrogen ion (139). 

 A similar phenomenon in mutants of Neurospora spp. which require 

 pyridoxine appears, however, to reflect a requirement for free ammonia 

 rather than a direct effect of acid (268). The influence of pH on the 

 p-aminobenzoic acid requirement seems, finally, to be exerted through 

 permeability effects (325). Pythium butleri requires exogenous thia- 

 mine only in a high-salt medium (211). 



The second type of conditioned requirement is more easily ex- 

 plained: a precursor of the vitamin or a metabolite in the synthesis of 



