Daniel I. Arnon 331 



these authors to conclude that chloride is a coenzyme of photosynthesis 

 was as follows. The isolated chloroplasts lose their capacity for oxygen 

 evolution after several washings in water. They can be reactivated, 

 however, by adding the cytoplasmic fluid. The factor in the cytoplasmic 

 fluid responsible for the reactivation of the chloroplast was found to 

 be heat-stable. An analysis disclosed that the cytoplasmic fluid contained 

 chloride in 0.08 molar concentration. The addition of chloride alone 

 as M/150 potassium chloride brought about complete reactivation. Of 

 the other anions tried, bromide was almost as effective, iodide and 

 nitrate much less so, and fluoride, sulfate, thiocyanate, phosphate, and 

 all the cations tried were without effect. Since chloride was the effec- 

 tive anion found in sufficient concentration in the cytoplasmic fluid, 

 Warburg and Liittgens concluded that it was the natural coenzyme of 

 photosynthesis. 



Impressive as this chain of biochemical evidence was in support of 

 chloride as a coenzyme of photosynthesis, it posed at once a rather per- 

 plexing physiological problem from the standpoint of plant nutrition. 

 Chloride is not generally regarded as an essential element for the growth 

 of higher plants. Is it then possible that plants can get along in nutrient 

 solutions without a coenzyme required for photosynthesis, a process 

 indispensable for growth ? The fact that Warburg and Liittgens found 

 appreciable amounts of chloride in their plants was not surprising. 

 Chloride is widely distributed in soils and readily absorbed by most 

 plants. Its presence in the plant, however, was hitherto regarded as 

 incidental. 



We undertook to investigate the problem by growing sugar beet and 

 chard, in nutrient solutions without chloride.* As was expected the 

 plants made excellent growth in the nutrient solution to which no 

 chloride was added. The chloroplasts from these plants were isolated 

 (4) and their oxygen evolution under the influence of light was 

 measured manometrically, by a technique (//) similar to that used by 

 Warburg and Liittgens. 



Our results disclosed important areas of agreement with those of 

 Warburg and Liittgens as well as several differences. An analysis of 



*These data have been published separately: D. I. Arnon, and F. R. Whafley, 

 Science, 110:554 (1949). 



