92 



MINERAL SALTS ABSORPTION IN PLANTS 



cells, and this is accompanied by an increased respiratory quotient. 

 The mechanisms involved here are presumably the reverse of those 

 just discussed, namely carbon dioxide or bicarbonate release from 

 organic acid, accompanied by a decrease in the hydrogen ion 

 concentration of the medium. 



Further evidence that organic acid metabolism and salt absorp- 

 tion are linked has been obtained by experiments involving Krebs 

 cycle inhibitors. Machlis (1944) observed that iodoacetate and 



MEDIUM 



M"- 



(a) 



H*-t- 



CO; 



(b) 



M^- 



Hco;- 



M"- 



(c) Hco; 



PROTOPLASM 



-^M*- 



H +0A" 



->- M - 



-*- Organic acid 



-^iVl'" 



-^M" 



-^ OA" 



VACUOLE 



-^-M^ 



-^-OA 



-M" 



-*-0A' 



-^M""- 



->-M*- 



-^-COo— *^Organic acid 

 -H'^+OA • 



-^M" 



-OA' 



Fig. 33. Organic acid metabolism and cation absorption. For explanation, 



see text. 



malonate inhibit both salt absorption and respiration in barley roots. 

 The effects of iodoacetate were reversed completely by adding malate, 

 succinate or fumarate, and those of malonate were partially reversed 

 by these acids. Ordin and Jacobson (1955) confirmed that pre- 

 treatment of barley roots with various inhibitors of the Krebs cycle 

 enzymes prevents ion absorption without entirely suppressing 

 respiration. They concluded that the Krebs cycle and phosphoryla- 

 tion control the synthesis of ion-carriers, while cytochrome oxidase 

 functions in salt uptake by facilitating energy release through 

 metabolism. 



