143G 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



of day and night probably belong to this class. Very short intervals (of the 

 order of 1 second or less) also may cause an improvement of the energy con- 

 version yield, because they allow the dark catalytic reactions of photo- 

 synthesis to run to completion, restoring the photosynthetic apparatus 

 to its full efficiency at the beginning of each new light period. 



In the intermediate range of frequencies — of the order of 1/min. or 

 1/hr.— alternating illumination can be expected to cause a depression of 

 the yield (i/e < 1, in < 0.5), because dark intervals of this length permit 

 the development of induction phenomena, which occupy most of the 

 subsequent light periods. Thus, plotting Ije and in against log t, we can 

 expect to obtain a curve of the shape shown in figiu'e 34.1. 



Short 



induction 



period 



-2-10123456 

 LOG / , sec 



Fig. 34.1. Expected trend of intermittency factors for 

 equal dark and light periods. 



(In this curve, the "bursts" and "gulps" which complicate induction 

 phenomena have not been taken into consideration. This is legitimate 

 when the volume of these extra components of the gas exchange is con- 

 siderably smaller than that of the induction losses.) 



The first observations of the actual effect of intermittent light on photo- 

 synthesis were made by Brown and Escombe (1905). They reported that, 

 under certain conditions, as much as one half, or even three quarters, of the 

 total incident light could be taken away by a rotating sector without sig- 

 nificantly decreasing the yield of photosynthesis (this means in ^^^ 1 and 

 iiE ^ 2-4). 



Willstatter and StoU (1918) suggested that, since Brown and Escombe 

 worked with strong light and a limited supply of carbon dioxide, their re- 

 sults could have been due to the exhaustion of carbon dioxide in the im- 

 mediate neighborhood of the chloroplasts during each flash and thus bear no 

 relation to the intrinsic kinetic mechanism of photosynthesis. This inter- 



