312 PHYSIOLOGY OF THE FUNGI 



nature stored up the "necessary potentialities," which then permitted 

 conidium production in darkness. MyceHum growing from the pieces 

 of leaf into the agar did not produce spores in darkness. This is an 

 interesting theory regarding a possible delayed action of light upon sporu- 

 lation. It also seems possible that the leaf tissue of the host may furnish 

 some nutrient necessary for sporulation which is not contained in potato- 

 glucose agar. Perhaps light is essential to the synthesis of this material 

 by the fungus. 



It was demonstrated recently (Barnett and Lilly, 1950) that an isolate 

 of Choanephora cucurhitarum requires both light and darkness for the forma- 

 tion of conidia, but these factors have little or no apparent influence upon 

 the formation of sporangia. This fungus was grown under a number of 

 conditions, but none was found which overcame the need for either light 

 or darkness. Cultures incubated in the laboratory under natural alter- 

 nating light and darkness produced abundant conidial heads during the 

 second and third nights after inoculation. Exposure to artificial light 

 for 2 days after inoculation followed by darkness gave similar results, but 

 an exposure in the reverse order resulted in no conidia. Cultures under 

 continuous artificial light (65 foot-candles) and those in total continuous 

 darkness failed to form conidial heads. Continuous light of low intensity 

 (less than 1 foot-candle) , however, did permit the formation of numerous 

 conidial heads in the usual period. A summary of the important results 

 is presented in Fig. 59, together with an outline of a proposed hypothesis 

 to explain the results. We may assume that light, or its absence, affects 

 two metabolic reactions, or groups of reactions, which are essential to 

 conidium formation by C. cucurhitarum. Light, which is essential to 

 reaction A, apparently inhibits reaction B, which must occur in darkness 

 or weak light. The reaction in light must be'^followed by the reaction in 

 darkness, if conidia are to be formed. Continuous bright light favors 

 only reaction A, while continuous darkness permits only reaction B. 

 Both reactions occur simultaneously in continuous light of low intensity. 



A different isolate of C. cucurhitarum was studied by Christenberry 

 (1938), who found that alternating periods of light and dark, 12 hr. each, 

 gave the best sporulation. Red-yellow light was more favorable to 

 conidium formation than the shorter rays. This isolate formed conidia 

 in total darkness. 



The beneficial effect of alternating light, or a period of light followed 

 by darkness, was demonstrated (Timnick et al., 1951) for the formation of 

 ascospores by Diaporthe phaseolorum var. hatatafis. Cultures grown in 

 continuous darkness formed only a few perithecia, which contained 

 abundant ascospores. In continuous bright light numerous perithecia 

 were formed, but relatively fewer ascospores were produced. A long 

 period of light followed by darkness gave many perithecia with abundant 

 ascospores. 



