1458 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



dark rest long enough for the carboxylation equihbrium to be estabhshed 

 (despite the inactivation of a large part of the carboxylating enzyme by 

 cyanide). The first flashes should then give the usual maximum flash 

 yield; only when the initial reserve of the A-C02 complexes, formed in the 

 long dark period, is exhausted, will the yield be limited to the number of 

 A-C02 complexes restored during the short dark interval immediately 

 preceding the flash. By inserting after several flashes, a dark interval of 

 sufficient length, it should be possible to produce enough reduction sub- 

 strate, A-C02, to assure the maximum yield in a new series of flashes, and so 

 on. In other words, if the decrease in flash yield caused by cyanide is due 

 to the retardation of a supply reaction that can effectively utilize dark 

 intervals much longer than 0.01 second, such extended intervals need to be 

 inserted only once after a series of flashes to remove completely the cyanide 

 inhibition. This consideration led Rieke and Gaffron (1943) to carry out 

 experiments in flashing light in which a rapid series of flashes was followed 

 by a single long dark interval. (This was achieved by an appropriate ar- 

 rangement of sectors in a rotating disc.) Table 34. Ill shows a typical 

 result. When the 16 flashes/sec. were evenlj^ spaced, the intervals (0.06 

 second) were insufficient for the cyanide-poisoned reaction to renew all 

 the substrate used up during the flash, thus explaining the observed con- 

 siderable inhibition (by 22%). It will be noted that the residual rate in 

 flashing light (0.43 mm.Vmin.) was practically the same as in continuous 



Table 34.III 



Influence of Cyanide on Flash Yield in Chlorella pyrenoidosa 

 (after Rieke and Gaffron 1943)" 



Rate of O2 production (corr. for respiration), in mm.Vmin. 



16 flashes/sec, 16 flashes/sec, 



Continuous light evenly spaced in groups of 4 



Nopoison 2.08 0.55 0.34 



SXIO-^MKCN 0.42 0.43 0.33 



Inhibition (%) 80 22 <3 



" Comparison of the rates in continuous and flashing illumination; thin suspension 

 of algae in carbonate buffer (85 parts M/10 NaHCOj -f- 15 parts M/10 K2CO3); gas 

 phase, air; 20% absorption of light of 0.578 rafi; intensity about twice that needed for 

 saturation; temperature, 20° C. 



light, being determined in both cases by the rate of formation of A-C02 by 

 the nonpoisoned fraction [EaI^ of the catalyst Ea- When, however, the 16 

 flashes were gathered in 4 groups of 4 flashes each, the cyanide had no ef- 

 fect on the rate— since, now, the long intervals between the groups were 

 sufficient to supply all the required substrate even in the poisoned state. 

 (The required quantity of A-C02 was smaller than with regularly spaced 



