INDUCTION AFTER PHOTOXIDATION 



1373 



depended on the duration and intensity of photoxidation. If the latter 

 had gone too far, no complete recovery was possible. The photosynthetic 

 capacity could be restored more or less completely also by a dark rest. 



In chapter 19, we described a second method for replacing photosynthe- 

 sis by photoxidation: very intense illumination. A period of photoxida- 

 tion induced in this way also is followed by an induction period upon return 

 to photos3mthesis in more moderate light. Figure 33.18D, taken from the 

 work by Mj^ers and Burr (1940), shows that the longer the cell suspension 

 has been exposed to extremely strong light (28,000 foot-candles, or 300 

 klux), the slower and less complete is its recovery in moderate light (1000 



10 20 30 



LENGTH OF EXPOSURE TO 23,000 f.-c, min. 



Fig. 33.18E. Residual photosynthetic activity after varying ex- 

 posures to 23,000 foot-candles (after Myers and Burr 1940). Rate 20 

 equals 100% for this batch of cells under 7500 foot-candles. 



foot-candles). Curve D shows that in this case, too, recovery occurs in 

 darkness as well as in light. After a long exposure (3-5 hours) to intense 

 light (> 10,000 foot-candles) no recovery is possible at all. Figure 33.18E 

 shows the residual photosynthetic efficiency (at 7500 foot-candles) as a 

 function of the duration of exposure to 23,000 foot-candles. 



Franck and French suggested that the aftereffects of a period of photoxidation have 

 a double origin — the oxidation of some constituents of the enzymatic apparatus of 

 photosynthesis, and the burning up of the reserves of intermediate products. The oc- 

 currence of the first effect is clearly demonstrated by the fact — mentioned in chapter 19 

 — that photoxidation inhibits photosynthesis and not merely counterbalances it. The 

 natural explanation of such a "catalytic" effect is the destruction (oxidation) of one or 

 several members of the enzymatic system involved in photosynthesis. (Franck and 

 French beheve that the most probable substrate of oxidation is Ea, the catalyst responsi- 

 ble for the formation of the carbon dioxide-acceptor complex.) 



