SALT ABSORPTION AND METABOLISM 91 



sap was less well buffered than it was following uptake of potassium 

 bromide, and they suggested that the organic acid content of roots 

 might be different after the two treatments. Ulrich (1941) confirmed 

 that when cations are absorbed in excess of anions (e.g. from a 

 solution of potassium sulphate), organic acid anions appear in the 

 sap (cf. Chapter 4, p. 52). Conversely, when an excess of anions are 

 absorbed (e.g. from a solution of calcium bromide), the organic acid 

 content decreases. In Ulrich's experiments there was an approximate 

 agreement between the excess absorption of one ion and the change 

 in organic acid content, except during absorption from solutions of 

 calcium nitrate. Here, the excess of anions over cations absorbed 

 was presumably balanced by loss of other anions, e.g. hydroxyl or 

 bicarbonate. Ulrich suggested that it is the unequal absorption of 

 anions and cations which in some way induces changes in organic 

 acid content to maintain electrical neutrality, whereas Lundegardh 

 (1954) proposed that synthesis or disappearance or organic acid 

 anions (i.e. of negatively charged particles) may be the cause of 

 unequal cation and anion absorption, since it stimulates or retards 

 absorption of cations along an electrical gradient. 



It must be emphasized at this point that the normal operation 

 of the Krebs cycle does not result in accumulation of organic acids; 

 rather this is achieved through carbon dioxide or bicarbonate 

 fixation linked with the cycle. It seems probable that depending on 

 external conditions, e.g. pH and the tissue under investigation, either 

 carbon dioxide or bicarbonate ions are taken up and converted to 

 organic acid anions, which balance part of the cations absorbed. 

 In the case of carbon dioxide fixation, the excess of absorbed cations 

 over anions is replaced in the medium by an equivalent number of 

 hydrogen ions. When bicarbonate is absorbed, equal numbers of 

 positive and negative charges are transported, and no exchange 

 occurs. It is possible that if bicarbonate is taken up as such, it is 

 converted to carbon dioxide at the pH of cell sap before incorpor- 

 ation in organic acid. The various possibilities are represented 

 diagrammatically in Fig. 33. Most of the organic acid anions (0A.~) 

 in cells are located in vacuoles, into which they are transferred, 

 together with mobile cations from cytoplasm where they are 

 synthetized. 



When excess absorption of anions over cations is observed, 

 there is sometimes a reduction in the organic acid content of the 



