1450 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



amounts of limiting catalyst actively engaged in photosynthesis (A) in 

 saturating continuous light, and {B) in flashing light of the same integrated 

 intensity, for two different flash frequencies (5i, B2). The average yield of 

 photosynthesis, which is proportional to the sum of the shaded areas, is 

 highest in A and lowest in B2. 



Instead of plotting the average yield of photosynthesis in flashing light 

 against the average light intensity (as in fig. 34.6), one can plot the yield 

 per flash, P against the energy of a single flash, I. Such "flash saturation 

 curves" (fig. 34.8) can be constructed from the same data used in figure 



TIME 



Fig. 34.7. Photosynthesis in continuous Ught and flashing hght of the same inte- 

 grated intensity, /, for two different frequencies. Flashes are "saturating," i. e., each 

 produces enough intermediates to occupy all molecules of Eb. Photosynthesis begins 

 at the saturation level (which is proportional to Eb ), and declines exponentially during 

 each dark interval. Black rectangles, Ught. Shaded areas, photosynthesis (i. e., prog- 

 ress of the limiting dark reaction). 



34.6. At first, P is proportional to I and independent of r ( = t* + ta); this 

 increase continues as long as practically all intermediates, formed during the 

 flash, can be utilized by catalyst Eb during the subsequent dark period. 

 With increasing flash energy, however, Eb, even if it is fully occupied dur- 

 ing the entire dark interval, ceases to be able to transform all intermediates 

 formed in the flash; therefore the flash saturation curves bend, one after 

 another, and approach the horizontal. The saturation yield is propor- 

 tional to the length of the dark intervals as long as the latter are very short 

 compared with the working period, t^, of Eb, and thus can be fully utilized 

 for the dark reaction catalyzed by this agent. When ta becomes commen- 

 surate with ^B, full utilization of the dark intervals becomes impossible 



