336 Mineral Nutrition of Plants 



chloroplast suspension which was exposed to light in the absence of 

 chloride, this anion was added simultaneously with the oxidant. The 

 results are shown in Figure 3. The pre-exposure to light in the absence 

 of chloride inactivated the oxygen evolution system of the chloroplasts. 

 This inactivation was irreversible. The subsequent addition of chloride 

 had only a slight reactivating effect. On the other hand, a vigorous 

 oxygen evolution giving stoichiometric yields resulted from the chloro- 

 plasts which had received added chloride during their exposure to 

 light. Thus, chloride appeared to exert protective action on some essen- 

 tial photosynthetic factor that in the absence of this anion was irreversi- 

 bly destroyed by light. Chloride also seemed to exert some protective 

 action on the chloroplasts in the dark. There was evidence of inactiva- 

 tion from shaking the chloroplasts in the manometer vessels at 15 ° C. 

 for a period equal to the light exposure. The inactivation in light, how- 

 ever, was much more pronounced. It goes without saying that the 

 identification of this substance would be of great physiological interest. 

 Experiments along this line have been under way in our laboratory, 

 but no statement is possible at this time. 



In addition to chloride, Warburg and Liittgens (52) reached a con- 

 clusion of great significance with regard to zinc. They considered that 

 the photochemical evolution of oxygen by chloroplasts is catalyzed by 

 a metal and that in all probability the metal concerned is zinc. The 

 evidence for this conclusion was as follows. The oxygen evolution re- 

 action was strongly inhibited by o-phenanthroline, a well-known metal 

 complex former; this reagent forms complexes with bivalent iron, 

 nickel, cobalt, and zinc. The o-phenanthroline inhibition was reversed 

 and the chloroplast fully reactivated by adding an excess of zinc. (In 

 the example cited by Warburg and Liittgens, ten times as much as 

 would be required to bind the o-phenanthroline.) The addition of 

 iron brought only partial reactivation. If, however, the metal was added 

 to o-phenanthroline prior to placing it in contact with the chloroplasts, 

 then zinc was not the only element which prevented inhibition: iron 

 was equally effective, as were divalent cobalt and nickel ions. Warburg 

 and Liittgens (52) reasoned that since o-phenanthroline inhibition was 

 observed only when this reagent was still free to combine with the 

 metals in the chloroplasts, it is probable that some metal is involved as 



