84 MINERAL SALTS ABSORPTION IN PLANTS 



(/) As Russell (1954) pointed out, the theory of Lundegardh 

 denies the possibiUty that cells can store an ability to absorb salts 

 as a result of prior respiration. He obtained evidence that previous 

 respiration facilitates the subsequent movement of phosphate into 

 barley roots under conditions of reduced metabolism, Ketchum 

 (1939) suggested that dark-induced uptake of phosphate in 

 phosphate-deficient cells of Nitzschia closterium exposed previously 

 to light, might be due to combination between the ion and 

 phosphate-binding substances synthesized in light. Lowenhaupt 

 (1956) found similarly that Potomogeton leaves v^ill take up extra 

 calcium ions in the dark, following exposure of the leaf to light in 

 the absence of calcium. 



Laties (1959) has shown that when potato slices are transferred 

 from a high temperature in the absence of salt to salt at °C, uptake 

 of chloride proceeds quite rapidly at first, and only gradually falls. 

 He suggested that this may be attributed to the presence of a "carrier 

 precursor", which is synthesized at high temperature, and gradually 

 consumed at °C. 



ig) The high selectivity of cation uptake in many plants seems 

 unlikely if a passive mechanism of absorption such as that visualized 

 by Lundegardh is operative. Although it is not impossible that 

 preference exhibited by the binding substances {Y' Fig. 28c) could 

 be responsible for some discrimination between cations, it is unlikely 

 that this could account for the extreme specificity of cation absorp- 

 tion which is observed. 



{h) Cation absorption seems to occur to a large extent indepen- 

 dently of anion absorption in some plants, notably in yeast, and in 

 some marine algae. 



b. The redox pump. Conway (1953, 1955) proposed an electro- 

 chemical mechanism for the transport of cations in muscle cells and 

 yeast which resembles in certain respects that proposed by Lunde- 

 gardh for anions. It is suggested that ions (M+) become bound to a 

 reduced respiratory intermediate {X~) in accordance with the 

 equation : 



The uncharged complex so formed diffuses across the cell 

 membrane and becomes oxidized by transferring an electron (e) 



