1478 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



and tf = 4: msec. These results confirm that dark intervals an order of 

 magnitude longer than the Emerson-Arnold period of IQ-^ sec. can be ef- 

 fectively utilized for photosynthesis after flashes lasting a few milliseconds 

 (and having, in the above example, an integrated energy of the order of 70 

 X 4 = 280 lux sec). 



7. Hill Reaction in Flashing Light 



Clendenning and Ehrmantraut (1950) studied the Hill reaction in flash- 

 ing Hght, with quinone as oxidant, in whole Chlorella cells (cf. chapter 35, 

 part C). A neon discharge tube (flash duration about 10 /zsec.) was used. 

 The flash yields as function of the duration of the dark period are shown in 



0.6 



I 



o 



X 0.5 



tn 



E 

 E 



- 0.4 h 



I 



< 



UJ 



a. 

 a 



u 



X 



o 



03 - 



0.2 - 



0.1 - 



5 10 15 20 



DARK TIME, hundreths of seconds 



25 



Fig. 34.26. Effect of dark intervals on flash yield of photosynthesis and quinone 

 reduction by Chlorella (after Clendenning and Ehrmantraut 1941). 



fig. 34.26; the curves follow a similar course for the quinone reaction and 

 for photosynthesis (measured in the same Chlorella culture, but in bicarbon- 

 ate solution instead of quinone-containing phosphate buffer). Saturation 

 of both reactions requires dark intervals of 0.03-0.04 sec. at 10° C. The 

 absolute flash yields in fig. 34.26 are 40-50% smaller in quinone than in bi- 

 carbonate at the same flash energy. However, subsequent experiments 

 by Ehrmantraut and Rabinowitch (1952) led to the conclusion that this 

 difference was caused by the higher energy needed (in flashing as well as 

 in steady light) to saturate the Hill reaction. Fig. 34.27 indicates that the 

 saturation level probably is the same for both reactions — namely, about 12 

 mm.* oxygen per gram chlorophyll (i. e., one molecule oxygen per 2000 



