232 Mineral Nutrition of Plants 



against the over-all negative potential of the root surface. The iron, 

 with its variable dual charge in the cytochrome complex, is considered 

 to play a role as a carrier of anions from a higher oxidative level ex- 

 ternally to a lower level internally. The cations are visualized as mov- 

 ing through isolated mosaic areas of protoplasm by exchange for other 

 cations, primarily internally produced (e.g., H+), without the direct 

 expenditure of energy. A different temperature coefficient for the ab- 

 sorption of anions and cations was found by Wanner (89, go) and is 



K + 



cr 



\ 



\ 



VACUOLE 



4- 



H 4 



SUBSTRATE 

 I 



DEHYDROGENASE 



\ 



VACUOLAR 

 SURFACE 



CELL 

 SURFACE. 



EXTERNAL 

 SOLUTION 



Figure 23. Schematic representation of the electron and anion transport 

 system. Solid lines represent chemical reactions; broken lines represent 

 movements of substances; Fe+++, oxidized cytochrome; Fe++, reduced 

 cytochrome. — Robertson and Wilkins (68) 



used in support of the anion respiration theory. This idea has received 

 some support in a recent discussion by Robertson and Wilkins (68). 

 Their diagram of the proposed mechanism is reproduced as Figure 23. 

 The Lundegardh hypothesis is quite interesting and appears to be 

 in accord with many observations of respiration and accumulation. The 

 limitation which is made between anion respiration and accumulation, 

 rather than inclusion of the necessity for direct expenditure of energy 

 for cation accumulation as well, has been questioned by some investi- 



