1482 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



is present in these bacteria in the ratio of one molecule for 400 BChl 

 molecules — or 400n, where n may be V4 or Vs- The duration of dark inter- 

 vals required to achieve the full flash yield in bacteria — i. e., the constant 

 /cea is as yet unknoAvn. However, it is likely that the rate-limiting reac- 

 tion is the same in bacterial as in ordinary photosynthesis; and, if this is 

 so, one sees no reason why the required dark intervals should be different. 

 Confirmation would be of some interest. 



Rieke and Gaffron (1943) conducted some experiments on the photo- 

 reduction of hydrogen-adapted Scenedesmus in flashing light. As discussed 

 before, in chapter 6, the rate-limiting reaction is in this case the sup- 

 ply of hydrogen by the hydrogenase system. Whenever the rate of the 



15 10 15 



TIME BETWEEN FLASHES x '/i2o sec. 

 Fig. 34.28. Length of "stabilizing period" in photoreduction 

 (after Rieke and Gaffron 1943). Rate of photoreduction as a func- 

 tion of time interval between flashes in a group of two. 



primary photochemical process exceeds the maximum rate of the 

 hydrogen supply, an accumulation of the intermediate oxidation products 

 takes place, and causes a rapid "de-adaptation," i. e., return to normal 

 photosynthesis. Limited hydrogen supply should have the same influence 

 on flash yield as the Hmited carbon dioxide supply had in EA-deficient (e. g., 

 cyanide-poisoned) algae. This j^rediction was verified by Rieke and Gaf- 

 fron by means of experiments Avith flashes grouped in pairs. When uni- 

 formly spaced flashes were used, the maximum yield of carbon dioxide 

 reduction per flash was found to be approximately equal to that in non- 

 adapted cells of the same species; but the dark intervals required for the 

 full flash yield were much longer, since the supply of hydrogen during the 

 intervals had to sufl5ce for the reduction of all intermediates produced by a 

 flash. Otherwise, not only would the yield per flash be smaller (as in the 

 case of cyanide-poisoned plants), but the accumulation of oxidation inter- 

 mediates would have brought about immediate "deadaptation." 



When flashes were grouped in pairs — with the total number of flashes 

 per minute unchanged — it was found that the interval between two flashes 



