INITIAL STEPS IN PHOTOSYNTHESIS 369 



spectively. As can be seen in this figure, Pf increases linearly at short 

 dark times and levels off to a maximum value. The beha\'ior of this 

 change of Pf as a function of the dark period agrees with the results 

 which ha\-e been obtained by other in\-estigators. 



For the purpose of discussion the effect of the variation in light 

 period, ^P'e was defined as the difference l)etween the Pf value for 

 zero dark period and the maximum Pf value for each curve. The 

 values of AP'e and the corresponding absolute values of AP^ are listed 

 in Table I. 



TABLE I. Extra Yield of Oxygen per Flash in Photosynthesis as a Function of 



Flash Time 

 Ch (total chlorophyll-a content) = 117 X lO"* mole/25 ml. of suspension 



AP'« is the measure of oxygen generation per flash in the dark in 

 excess of the oxygen generation in the light. We postulate that, in the 

 course of light period, a first reservoir is filled by the energy of light 

 and must be full by the end of the light period because the photosyn- 

 thetic rate is saturated against light intensity. Presumably, this initial 

 step of photosynthesis would be a photochemical reaction, and some 

 purely chemical reaction may follow it with the additional possibility 

 of some deactivation process. But, if only one reservoir is filled during 

 the light flash and is responsible for all further reactions of photosyn- 

 thesis, ^P'e should be constant and independent of the light period, 

 because the light intensity is strong enough to saturate the reservoir 

 in any light period used. This would be true, no matter what the 

 mechanism of reactions relating to this particular reservoir might be. 



In Fig. 3, AP', is plotted against relative light period. As can be seen, 

 AP', increases with increasing light period instead of remaining con- 

 stant—the anticipated "saturation" is indicated by the dotted hori- 

 zontal line. The rate of increase becomes almost linear at longer light 

 periods. This result indicates that there is a second reservoir which is 



