Lecture 6 — 129 — Biochemical Problems 



cell sap. These acids, in association with inorganic 

 bases, potassium, calcium, magnesium, and sodium, 

 together with phosphate, amides, amino acids and 

 other components constitute a buffer system which 

 contributes to the maintenance in living plant cells 

 of a hydrogen ion concentration held within fairly 

 narrow limits, at least for many types of living cells 

 of the plant. But the system is not merely a static 

 one in which only simple chemical buffering deter- 

 mines the hydrogen-ion concentration. The cells are 

 capable of responding to tendencies toward change of 

 hydrogen-ion concentration caused by entering solutes 

 through appropriate metabolic reactions that prevent 

 wide fluctuations in hydrogen ion concentrations. 



Early plant physiologists did not have the ad- 

 vantage of modern concepts of buffer systems and of 

 hydrogen ion activities with appropriate techniques of 

 study, but they were aware that organic acids are 

 associated with the acid-base metabolism of plants. 

 This is illustrated by the following quotation from 

 Pfeffer : 



"By means of this self-regulatory power of in- 

 creasing or decreasing the production of acids, an 

 excessive formation of basic or alkaline substances may 

 be compensated for, while on the other hand a dan- 

 gerous accumulation may be avoided when neutralizing 

 substances are produced in less than normal amount." 



One view of an earlier period was that bases, par- 

 ticularly calcium, absorbed by plants prevent toxicity 

 that would otherwise ensue from the metabolic pro- 

 duction of organic acids, notably oxalic acid. Dunne 

 (1932) examined this supposition in our laboratory 

 by means of experiments on buckwheat plants grown 

 in controlled culture solutions. He came to the op- 

 posite conclusion; namely, that oxalic acid is synthe- 

 sized in part in response to calcium absorbed by the 

 plant. Recently similar conclusions have been drawn 

 from experiments by other investigators on different 

 plants. When nutrient solutions were varied in com- 



