PREPARATION, PRESERVATION AND ACTIVATION OF CHLOROPLASTS 1551 



The plants grew well (and, implicitly, photosynthesized normally), but 

 the chloroplasts obtained by the disintegration of their leaves (which, as 

 expected, had no measureable Cl~ content) evolved only very little oxygen 

 upon illumination in the presence of quinone. Addition of protoplasmic 

 fluid from the same plants (which, too, was chloride-free) did not activate 

 them for the Hill reaction, but addition of potassium chloride brought it 

 under way. Bromide had the same effect, while effects of KNO3 and KI 

 were much weaker, and NaF had no effect at all (fig. 35.11). Sulfate, 

 phosphate, thiocyanate and acetate also were without influence. The con- 

 centration of chloride required to fully activate chloride-free chloroplasts 



Fig. 35.12. Effect of chloride on photochemical activity of chloroplast dispersions 

 (after Milner, Koenig and Lawrence 1950). 



was about 7 X 10 ~^ mole/liter — an amount which could not have been 

 left undiscovered in CI --starved plants, or lost in chloroplasts extraction. 

 Arnon concluded that chloride is not a natural component of the photosyn- 

 thetic apparatus, but is required to prevent photochemical deactivation 

 of separated chloroplasts. The (almost) irreversible inactivation of chloro- 

 plasts caused by preillumination of chloride-free chloroplast suspension 

 in the absence of an oxidant could in fact be prevented by the addition of 

 chloride. The chloride exercised a certain protective effect also on the 

 deterioration of chloroplasts in the dark (see Warburg and Liittgens 

 (1946); French and Milner (1951) were unable to stabilize disintegrated 

 chloroplasts by the addition of phosphate buffer and KCl, as recommended 

 by Warburg). Holt, Smith and French (1950) found that the deactivation 

 of chloroplast dispersions in light (which, in their experiments, occurred 

 in presence as well as in the absence of an oxidant) could not be reversed by 

 chloride. 



