168 PHYSIOLOGY OF THE FUNGI 



SUMMARY 



All aqueous solutions contain hj^drogen and hydroxyl ions. Hydrogen- 

 ion concentration is most often expressed in terms of S0rensen's scale of 

 pH. The pH scale is logarithmic. Acidity and alkalinity are expressed 

 on the same scale. A pH of 7 indicates equivalent concentrations of 

 hydrogen and hydroxyl ions. Values of less than 7 indicate acidity, and 

 pH values greater than 7 indicate alkalinity. The smaller the pH values, 

 the greater the concentration of hydrogen ions. 



Buffers are substances which tend to maintain the pH of a solution 

 constant when either strong acid or strong alkali is added or when the 

 solution is diluted with water. Mixtures of weak acids or bases and 

 their soluble salts, and amphoteric compounds such as amino acids and 

 proteins act as buffers. The pH range over which a given buffer is effec- 

 tive is a function of the ionization constant of the weak acid (Ka) or base 

 (Kb) from which the buffer is made. The effective pH range of a simple 

 buffer is two pH units. 



The upper and lower pH values between which a fungus is able to grow 

 is called the pH range. The pH ranges of various species are different. 

 Fungi generally tolerate more acid than alkali. The optimum pH range 

 may be different for growth and sporulation. The pH of a medium in 

 which a fungus is growing may change. High buffer concentration and 

 limited growth may keep the changes in pH at a minimum. To follow 

 the changes in pH of a culture medium, frequent determinations should 

 be made. 



Four metabolic processes operate to change the pH of a culture medium : 

 (1) utilization of cations, (2) utilization of anions, (3) formation of acids 

 from neutral metabolites (especially carbohydrates), and (4) formation 

 of bases (especially ammonia) from amino acids and proteins. The net 

 change in pH is the result of the interaction of all of these processes, 



REFERENCES 



Armstrong, J. I.: Hydrogen-ion phenomena in plants. Hydrion concentration and 



buffers in the fungi, Protoplasma 8: 222-260, 1929. 

 Brian, P. W.: Studies on the biological activity of griseofulvin, Ann. Botany (N.S.) 



13:59-77, 1949. 

 BiJNNiNG, E, : Ueber Farbstoff- und Nitrataufnahme bei Aspergillus niger, Flora 



131:87-112, 1930. 

 Canting, E. C: The physiology of the aquatic Phycomycete, Blastocladia Pring- 



sheimii, with emphasis on its nutrition and metabolism, Am. Jour. Botany 36: 



95-112, 1949. 

 *DiMOND, A. E., and G. L. Peltier: ControUing the pH of cultures of PenicilUum 



notatum through its carbon and nitrogen nutrition. Am. Jour. Botany 32 : 46-50, 



1945. 

 Dole, M.: The Glass Electrode. Methods, Applications, and Theory, John Wiley & 



Sons, Inc., New York, 1941. 



