80 MINERAL SALTS ABSORPTION IN PLANTS 



might be achieved in a single step, or, alternatively, anions might 

 be handed on from one cytochrome molecule to another in traversing 

 greater distances (Fig. 28d). 



According to the Lundegardh hypothesis, the transport of 

 anions inwards across a membrane is accompanied by movement 

 of an equivalent number of electrons and hydrogen ions outwards. 

 These are derived from reduced respiratory intermediates, and 

 subsequently combine with molecular oxygen to produce water, 

 the final product of respiration. The maximum number of anions 

 transported by such a mechanism per oxygen molecule consumed in 

 salt respiration is 4, since 4 electrons are utilized in the reduction of 

 a molecule of oxygen to water, and one anion is absorbed per 

 electron transferred. With wheat roots, the ratio observed is 

 commonly less than 1, but Robertson and Wilkins (1948) found 

 with slices of carrot tissue that the ratio approached but did not 

 apparently exceed 4 when the concentration of salts in the medium 

 was increased (Fig. 29). This was taken to indicate that the Lunde- 

 gardh mechanism probably operates in plants. 



Cations are thought to be absorbed passively along "adsorp- 

 tion tracks" under the influence of an electrical gradient 

 created by the absorption of anions (cf. mechanism d, Fig. 13, 

 p. 44). This contention is supported by the lack of a quan- 

 titative relationship between respiration and the absorption of 

 cations. 



Most investigators have found it impossible to accept the 

 Lundegardh hypothesis for various reasons of which the following 

 are perhaps the most important: 



(a) If a single type of carrier molecule is involved in the uptake 

 of all anions, anions should compete with one another for the 

 absorption mechanism when they are supplied simultaneously. In 

 fact, competition occurs between closely related ions such as chloride 

 and bromide, but not between halides, sulphate, nitrate or 

 phosphate (see p. 58). Lundegardh (1955) admitted that cytochrome 

 is unlikely to be the carrier for metabolically important ions, which 

 in effect confines the hypothesis to a mechanism for absorption of 

 chloride. It must be pointed out that nitrate absorption bore a 

 similar relationship to anion respiration as did chloride uptake in 

 the experiments upon which the hypothesis was originally based 

 (Fig. 28a) and if it is now denied that cytochrome acts as a carrier 



