228 Mineral 'Nutrition of Plants 



increase in concentration within the cell, balancing the excess cation. 

 An internal decrease in organic acid anions accompanies an excess in- 

 ward migration of anions. The balancing effect of organic acids is often 

 reflected in the process of respiration of the cells; for example, the 

 ratio of carbon dioxide evolved to oxygen involved is increased in 

 accord with a metabolic respiration of organic acids. These relation- 

 ships are shown in Table II from Ulrich (84). 



Respiration and accumulation 



Several mechanisms for active or metabolic inorganic solute ac- 

 cumulation have been suggested. Some seem untenable, while others do 

 not, at least with present knowledge, seem to explain all the various ob- 

 servations. The suggestion of chemical union between cations and 

 some protoplasmic constituent, associated with a pronounced hydrogen 

 ion gradient, does not appear to be in accord with observations from 

 studies with various pH values of the external medium (compare 

 {31,78) and (53,54))- The proposal that all accumulation of ions is an 

 exchange phenomenon seems to be incomplete. Ionic exchange, through 

 or between solutions or by surface migration, is a real part of the over- 

 all process of inorganic solute movement, but does not appear to ex- 

 plain the major metabolic accumulation. Various anions and cations 

 are produced metabolically which might exchange for solute ions 

 from the external medium, yet their production to even high concen- 

 trations within cells does not necessarily lead to an inward migration 

 and accumulation of inorganic solute. Briggs (5) and Brooks (7, 8) 

 have been strong proponents of this hypothesis (compare 13). All in- 

 vestigators recognized at least a partial role in the movement of in- 

 organic solute by diffusion, Donnan, and exchange phenomena. Most 

 of them propose, in addition, some mechanism whereby energy can be 

 expended by the cell through oxidative metabolism and applied to- 

 ward accumulation and retention of solute. The suggested cause and 

 effect relationship between the enhanced respiration and metabolic 

 accumulation of inorganic solute, has been pointed out earlier. 



Observations of net solute influx, from measurements on internal 

 and external media of cell or integrated systems, indicate an over-all 

 migration of inorganic solute from a phase of lower to a phase of higher 





