300 B. KOK 



appeared to differ from that of photosynthesis; blue Hght is relatively 

 more active in the former. 



Though it is still a subject of study, we are inclined to accept the 

 view that photoinhibition is only indirectly correlated with photo- 

 oxidation. In the latter process the oxygen consumption might 

 possibly be sensitized via pigment molecules which are disorganized 

 in the former. Figure 5 summarizes our kinetic model of photosyn- 

 thesis and photoinhibition. 



Discussion 



Rabinowitch : When speaking of the primary photoproduct, what do you moan 

 by "stable?" Compared to what? To the Emerson-Arnold period of 0.02 second? 



Amon : What is the time between flashes? 



Strehler : How do you explain the slower than linear increase of flash yield with 

 the longer dark time? 



Kok: In each flash, all the primary acceptor, U, is converted into U* (r/. se- 

 quence 1-3) and this component, in turn, rather quickly converts E into E*. 

 The slowest step is the formation of oxygen with regeneration of E. So far, it has 

 been impossible accurately to measure the time course of the oxygen evolution 

 after a single flash. Instead, we apply a regular sequence of flashes; under such 

 conditions, a steady state is approached in which, during the flash, as much U* 

 is formed as is removed in the subsequent dark period. 



During a relatively short dark time, only a fraction of U* is drained off. After 

 the first few light-dark cycles, all the available E will be converted into E* while 

 E molecules liberated by reaction (3) are needed to react with U*. Due to this 

 "buffer" action, the rate of oxygen evolution will be constant during not-too-long 

 dark periods; as long as this is true, the flash jaeld will be proportional to the 

 dark time, and the slope of the yield vs. dark time curve will be equal to the maxi- 

 mum rate in continuous light. 



The range of dark times over which this linear relation holds depends on the 

 type of algae; at 30°C. it may extend up to 5 or 10 m sec. When the dark time is 

 extended so much that a substantial fraction of U* is used up during it, the re- 

 generated E molecules will not find a reaction partner. Reaction (3) will then slow 

 down and the rate of oxygen evolution (which is equal to A;3(E*)) will drop. In 

 this picture the component U* is supposed to be "stable," i.e., to have a lifetime 

 long enough to permit its "reloading" with E molecules for some time after the 

 flash. A minimum demand for this lifetime would be of the order of 30 m sec. at 

 30°C., and longer at lower temperatures. 



Rabinowitch : The difference between what you are now suggesting and what 

 was more or less generally assumed in the past, is that the dependence of the 

 flash yield on the length of the dark period is (within certain limits) linear and 

 not exponentially declining, because the first photoproduct can be stored for a 

 while, and the enzyme can operate on it several times — not just once, as in 

 Franck's theory. 



