EMANUEL EPSTEIN IO3 



and ion absori)lion is furnished by experimenls wilh respiratory inhibitors or 

 anaerobiosis. From aerated solutions, excised barley roots will rapidly absorb 

 KCl or RbCl. Increasing the O2 content of the atmosphere used to 100% causes 

 no significant increase in the rate of absorption. However, upon diminishing 

 the O2 concentration, uptake rapidly falls off at O2 concentrations below ap- 

 proximately 5% and approaches zero at complete anaerobiosis (22). 



Indications are that in many plant tissues the Krebs cycle is operative (11) 

 and that the cytochrome system is the principal terminal oxidase system (19). 

 lodoacetate and malonate inhibited respiration and the absorption of Br by 

 excised barley roots in approximately parallel manner in the experiments of 

 ^lachlis (31). Addition of malate, succinate, fumarate and citrate reversed the 

 inhibition of both processes by iodoacetate. Malonate inhibition of absorption 

 was also reversed by these salts, but the inhibition of respiration by malonate 

 was but slightly affected. 



Respiration and ion absorption are inhibited by cyanide (30, 31, 39, 44) and 

 carbon monoxide, and the latter inhibition of both processes is reversed by 

 light (39, 42, 44). The evidence that the cytochrome system is involved and 

 other observations are the basis of Lundegardh's theory of 'anion respiration' 

 (30). When root tissue is respiring in distilled water, addition of neutral salts 

 frequently causes an increase in the rate of respiration. According to the theory 

 the 'anion respiration' is quantitatively and causally related to the active 

 transport of anions. The electron transference toward molecular oxygen oc- 

 curring in the valance change Fe"'""'^ :^ Fe"*^ of the cytochromes is considered 

 to be accompanied by an equivalent transport of anions in the opposite direc- 

 tion. The absorption of cations is conceived as a passive consequence of the 

 active transport of anions. 



The work of Hoagland and his associates (20, 22, 23) and our own work (see 

 below) did not support the conclusion that a dichotomy exists between the 

 mechanisms of cation and anion transport, as visualized by Lundegardh. In 

 the e.xperiments of Steward and Preston (40), the effects of neutral salts on 

 respiration were related to the cations rather than the anions of the salts. More 

 recently Robertson el al. (39) have shown by the use of dinitrophenol in experi- 

 ments with carrot discs that the relation between respiration and salt absorption 

 may be less direct than would be predicted on the basis of Lundegardh's theory. 

 In recent experiments in which barley roots were exposed to K"*" adsorbed on 

 synthetic cation exchangers (i.e., in the absence of absorbable anions) an in- 

 crease in the rate of O2 uptake by the tissue was observed which was com- 

 parable to that resulting from exposure to KCl (15). 



THE CARRIER HYPOTHESIS 



Indirect evidence obtained largely in the last ten or fifteen years has strength- 

 ened earlier suggestions that the absorption of ions by plant cells involves the 



