Rothamsted Symposium on Trace Elements 62 



Now it has been found experimentally in sand culture ex- 

 periments at Long Ashton that symptoms of iron deficiency can be 

 induced in sugar beet by the addition of Cu+ + , Mn++ or Co+ + 

 ions and by other ions also. The severity of induced chlorosis 

 varies widely, however, and is in the order of Co+ + , > Cu + +, 

 >> Mn++ which bears no relation to the Eq values shown. 

 Furthermore, it is unlikely that free Co+ + + ions could exist in 

 living plants, and in order for any ion to cause oxidation by loss of 

 an electron that ion must first be oxidised to the higher valency 

 state. Thus oxidation reduction potentials of reactions involving 

 simple ions cannot account for experimental observations. 



It is probable that many metals exist in plants as complex 

 compounds and the following examples show that the Eq values for 

 reactions with complex ions may differ greatly from those with the 

 simple ions. 



Thus for CoCCnOe^- = CoCCn)63- +e-, Eo = +0.83V, 

 and in the Fe++^Fe+ + + change where the iron is present as a 

 complex with o. phenanthrolene Eq = — 1.14.V. It is thus evi- 

 dent that it is not possible to formulate the action of metals such as 

 copper or manganese in oxidation-reduction reactions particularly 

 in relation to the ferrous-ferric iron equilibrium until detailed 

 knowledge is available as to the nature of the particular metallic 

 compounds involved, and of their respective oxidation-reduction 

 potentials. 



Addendum:— li{ecent experiments (Erkama, 1949: Acta Chem. Scand. 

 3:850-857) have shown that the effect of copper on the iron uptake of peas, 

 in sterile solution cultures, is always correlated with the effect of copper on 

 the oxidation-reduction potential of the culture solution. There is reason 

 to believe that the conditions in the plant sap, as regards redox-relationships, 

 are somewhat similar to those in the culture solution. On the other hand, 

 the interpretation of the oxidation-reduction systems, by the measured redox- 

 potential, has no significance. With regards to Eo values, it should be 

 emphasized that oxidation-reduction equations are based on the assumption 

 that the systems are in a state of equilibrium. However, a living cell is 

 never in equilibrium in respect to all its redox-systems. 



