NUTRITION 401 



on one or more nutrients in others — and various degrees of partial 

 dependence on the environment. 



Experimentally, nutritional independence of the spore is difficult to 

 prove; it is essential that washed spores, free of materials carried over 

 from the growth medium, be used, or that spores be collected in such 

 a way as to prevent contamination with nutrient materials (163). 

 Washing, of course, must be as brief as possible in order to minimize 

 any loss of materials from the interior of the spore. Washed ascospores 

 of Neurospora tetrasperma germinate in distilled water (274) and are 

 therefore to be classed as nutritionally independent. The uniformly 

 good germination of rust uredospores and aeciospores which have been 

 collected by shaking from the host plant suggests that these, too, are 

 independent of the environment; it has been noted earlier (Table 1) 

 that uredospores are unusually high in reserve lipids. Although it is 

 perhaps justifiable to assume that the larger resting spores of fungi are 

 nutritionally independent, critical evidence is lacking. Conidia of the 

 powdery mildew fungi, collected, like the rust spores, by shaking, germi- 

 nate well on water, although sugars and several natural materials im- 

 prove germination (167, 307, 309, 317). 



Nutritional dependence is more easily demonstrated and is probably 

 more common than independence. Many fungus spores germinate very 

 poorly or not at all without added nutrients; examples include basidio- 

 spores of Psalliota bispora and other Hymenomycetes (94, 95, 105), 

 ascospores of Sordaria fimicola (29, 43), and conidia of several imper- 

 fect fungi (75, 194). Other fungi, too numerous to mention individ- 

 ually, germinate fairly well in water but do so more completely or more 

 rapidly in nutrient media (52, 53, 75, 85, 124, 202) or in contact with 

 complex biological products (35, 36, 38, 60, 101, 113, 147, 151, 158). 

 Since unwashed spores have been used in many of these studies, nutri- 

 tional deficiencies are probably more acute than they appear to be. 



The simplest view is that the three reactions just described — com- 

 plete independence, partial dependence, and complete dependence — 

 reflect the adequacy or availability of the reserve substances of the spore. 

 Critical studies on washed spores show that requirements may be simple 

 or complex. Washed spores of Sclerotinia jructicola require a carbon 

 source (163), those of Fusarium roseum need both carbon and nitrogen 

 (255), and spores of Glomerella cingulata must be supplied carbon, 

 nitrogen, phosphorus, and sulfur (161). Responses to minerals, e.g., 

 magnesium (163), are not necessarily nutritional. Spores are so sensi- 

 tive to ions that minerals such as magnesium may act primarily as 

 antidoting agents. 



Mutants of Neurospora crassa which are deficient for amino acids 



