334 



Mineral Nutrition of Plants 



1500- 



1000 - 



.chl 



500 



0.002 



0.006 



0010 

 KCI (M) 



~y s s^- 



0.067 



Figure 2. Effect of KCI concentration on rate of oxygen 

 evolution by illuminated chloroplast fragments. Q ^ 1 = cubic 

 millimeters of oxygen, per hour, per milligram of chlorophyll, 

 computed from data obtained for the six-minute period from 

 i min. to 7 min. after turning on the light, t = 20 C. Con- 

 ditions not specified were similar to those given in the legend 

 for Figure i. 



chloride either in the nutrient medium or in the plant. The other anion 

 capable of giving full activation of photochemical oxygen evolution, 

 bromide, although it is readily absorbed and tolerated by plants in 

 appreciable amounts, is not a common constituent of plants or soils, 

 and there is even less reason for suspecting it as being essential for 

 plant growth. 



If the view that chloride or bromide is a coenzyme of photosynthesis 

 in vivo is to be abandoned, how can the effect of these anions in vitro 

 be explained? We have formulated the hypothesis that, while in the 

 intact green cell photosynthesis goes on without the participation of 

 either chloride or bromide, once the cell is broken, there is a rapid 

 light-induced deterioration of some cellular substance essential for 

 the photochemical evolution of oxygen by chloroplasts. Chloride or 

 bromide is able to protect this substance against inactivation, but 

 the intact cell accomplishes this in some other manner. This would 

 explain the superfluousness of the halide in the in vivo system as con- 

 trasted with its requirement in the in vitro system. 



