430 PHYSIOLOGY OF THE FUNGI 



EXERCISE 7 

 The Influence of Hydrogen-ion Concentration on Growth and Sporulation 



This experiment is outlined to demonstrate (1) that the pH require- 

 ments for optimum growth and reproduction vary with the different 

 species of fungi, (2) that the pH of the culture medium changes during the 

 growth of the fungus, (3) the techniques by which pH changes may be 

 followed during growth, (4) that the pH at the time of sporulation may be 

 considerably different from that during most rapid growth, (5) that the 

 pH changes are also influenced by the composition of the medium. For a 

 discussion of pH, see Chap. 8. The pH of liquid media is more easily 

 tested than that of solid media by colorimetric methods; hence liquid 

 media should be used in this exercise. Twenty-five milliliters of medium 

 in a 250-ml. Erlenmeyer flask gives rapid, even growth of many fungi and 

 has been found to be quite satisfactory. Two different media are given 

 below, each set at four different pH values. For convenience each is 

 given a separate number. 



Media 



38. Glucose-asparagine (otherwise, basal medium as given in Ex. 4), pH 3.0 



39. As above, pH 4.0 



40. As above, pH 6.0 



41. As above, pH 8.0 



42. Sucrose-nitrate (otherwise, basal medium as given in Ex. 4), pH 3.0 



43. As above, pH 4.0 



44. As above, pH 6.0 



45. As above, pH 8.0 



Prepare these media, adjust the pH of each, and distribute to flasks 

 before autoclaving. Then use one flask of each medium to determine the 

 pH after autoclaving. This value should be considered the "initial pH." 

 Each student should select one fungus and inoculate eight flasks of each 

 medium listed above. Incubate the flasks at 25°C. or at room tempera- 

 ture. The pH of the culture filtrate should be determined at three differ- 

 ent times during the period of active vegetative growth (about the fourth 

 and eighth days) and at about the time or shortly after maximum growth 

 is reached (sporulation of some fungi will occur at this time). Duplicate 

 cultures should be used for each determination. See your instructor 

 regarding the method of determining pH of culture medium. The 

 sucrose-nitrate media may not be favorable for the growth of some of the 

 fvmgi selected for use in this exercise. 



Records. The relative amounts of growth should be recorded at the 

 time of each pH determination. If more accurate growth measurements 

 are desired, the mycelium can be dried and weighed. Also record the 

 time of the earliest sporulation and the amount at subsequent intervals. 



