Table 4. Culture Medium for Phacus pyrum 



NB. The low pH medium was buffered with amalic acid; the high pH medium with TEA. 

 * a mixture of Of- and S - glycerophosphates . 



different pH but with histidine and glycylglycine as 

 buffers (Table 4 and 5) . The pH 7 .6 base for Pha - 

 cus gives, with suitable buffers, good growth from 

 pH 5.2 - 8.2; the toxicity of TRIS in the pH 7.8 

 medium is puzzling, since TRIS is not toxic for the 

 pH 5 .5 base . The pH 6 .0 base for Woloszynskia 

 llmnetica is more versatile than the pH 8.0 base. 



In any attempt to understand the reason for the 

 successful use of histidine and glycylglycine as 

 pH buffers, we have to consider: 



a) that histidine is a metal chelator almost as 

 strong as EDTA and that glycylglycine is much 

 weaker; b) that in the zone pH 5 .0-8 .5 , an in- 



crease in pH results in more chelated metal ions 

 and less free metal ion. Therefore, if a chelated 

 metal mixture is kept constant and the pH is raised, 

 this rise will result in a metal deficiency (i.e. 

 more metals should be added or the chelator should 

 be reduced); decreasing the pH results in an excess 

 of free metals (i.e. one should add more chelator 

 or reduce the metals); c) how much the chelator is 

 in excess of the quantity needed for the 1:1 chela- 

 tion of the trace metals (and therefore how much Ca 

 and Mg will be chelated); d) whether the chelator 

 employed is a bulky molecule unable, to or slowly 

 penetrating, the cell, or whether it is small enough 



91 



