IM CHEMICAL AG K NTS 



meability occur, a compound increasing the loss of cellular nitrogen 

 and phosphorus but not increasing the entrance of substrates (182). 



Finally, we may expect relatively minor effects of sorbed materials, 

 especially on permeability and accumulation. We have referred earlier 

 (Chapter 8) to the possibility that amino acid toxicity is exerted by 

 the exclusion of one acid by another. Among other metabolic effects 

 of yS-propiolactone on Blastomyces dermatitidis is the inhibition of 

 amino acid uptake (119), although this could be secondary. 



Reduction of the rate of solute movement is shown especially well 

 by the studies of Rothstein (356) on the poisoning of yeast respiration 

 by uranium. This appears to be a specific interference with the up- 

 take of hexoses; endogenous respiration and the utilization of pyruvic 

 acid or ethanol are very little affected. Rothstein suggests that uranyl 

 ion complexes with groups at or near the cell surface which are es- 

 sential to sugar uptake, and suggests further that these groups are 

 cellular polyphosphates which are also capable of binding other ions 

 (357). Similar responses to uranium have been found in animal ma- 

 terial (315). 



5. HYDROGEN-ION CONCENTRATION 



In any survey of the literature on fungicides, one is forcibly struck 

 by the common failure to determine or to report the pH of the test 

 solution. For practical purposes this may not be important, at least 

 if the pH is that of the projected use situation, but conclusions on 

 mode of action of ionizable materials are difficult indeed to draw if 

 the acidity is unknown. 



Some effects of pH are obvious: a fungicide may break down more 

 or less rapidly and in so doing yield products of greater or less toxicity; 

 thus, thiosulfate forms a more toxic product (162), and dithiocarba- 

 mates form less toxic materials (432). A second possibility is that the 

 toxicant itself changes the pH to a value unfavorable for growth or 

 spore germination; this can of course be compensated for by specific 

 controls. The chelating agents, as mentioned later, become less effec- 

 tive in binding metals as the pH is lowered; uranyl ion forms complexes 

 with hydroxyl ion at pH values above 4.5 and becomes therefore una- 

 vailable (356). Competition between hydrogen ion and toxic cations 

 is considered in connection with the heavy metals. 



When weak acids or bases are dissolved in water, the degree of ioniza- 

 tion is pH-dependent. This is evident by inspection of the equation 

 for the dissociation of an acid: 



HA ^± H+ + A- (2) 



