1476 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



reaction does not affect the form of the kinetic equations, but only the assumptions 

 concerning the relative values of the rate constants. Franck (and Tamiya) have assumed 

 that the first forward reaction of the photoproduct (its association with the enzyme Eb) 

 must be very fast compared to the back reaction (except when much of the enzyme Eb 

 is occupied by the slow transformation of its complex with the photoproduct, with en- 

 suing light saturation); this assumption was needed to permit effective utilization of 

 quanta in weak light. Burk and Warburg, in their variant of the energy dismutation 

 mechanism, postulated, to the contrary, that the back reaction must be at least twice as 

 fast as the forward reaction (as needed to reduce the net quantum yield from 1 to the 

 highest energetically possible value of about Va)- 



Burk denied the existence of a "bottleneck enzyme" other than chlorophyll (for a dis- 

 cussion of this possibihty, see p. 1030); the maximum expected flash yield thus became 

 Chlo/n, where n (^3) is the "energy dismutation factor", i. e., the number of quanta 

 used for the net production of one molecule of oxygen. Burk thus reverted, in essence, 

 to the early concept that in flash saturation each chlorophyll molecule must produce one 

 molecule of oxygen. With n ^^ 3, this means a seven hundred times higher flash yield than 

 obtained by Emerson and Arnold, or two hundred times greater than obtained by Tam- 

 iya and Chiba. At first, Burk et al. (1951, 1952) implied that such high flash yields ac- 

 tually are observable; a subsequent note (1953) indicated, however, that this was not 

 the case. It was suggested that the failure to obtain the "theoretical" flash yields, P = 

 Chlo/n, is caused by a "solarization"— the same phenomenon that reduces the rate of 

 photosynthesis in excessively strong, steady light (cf. chapter 19, section A3). As 

 mentioned before, Burk et al. thus rejected the concept that, with very brief flashes, the 

 momentary intensity of illumination is irrelevant, and all that matters is the total energy 

 of the flash. Instead, they postulated that, if the momentary intensity of illumination 

 exceeds — even for a few microseconds — the intensity which produces solarization in 

 steady light, the yield of photosynthesis declines in the same proportion as if this in- 

 tensity were applied for an extended period of time. According to Burk et al. (1953), 

 the threshold of solarization for Chlorella lies at about 50 klux; this means that the 

 maximum flash yield obtainable with flashes of 10 ^sec. duration should be not much 

 in excess of that corresponding to a flash energy of 0.5 lux sec. (which seems to be quite 

 incompatible with observations), and flashes of 10 /^sec. should yield, at best, not much 

 more than the amount of oxygen corresponding to a flash energy of 500 lux sec. (close 

 to where Tamiya found flash saturation). 



This theory appears implausible (because "solarization," as described in chapter 19, is 

 a cumulative rather than instantaneous response). It seems incapable of accounting 

 quantitatively for the observations of Emerson and Arnold et al. ; and if it were the cor- 

 rect explanation of Tamiya's observations, the latter should have noticed an "optimum" 

 of flash energy. The theory appears as a reversion from Blackman's recognition of 

 "limiting reactions" to the older concept of "cardinal points" (cf. chapter 26). A de- 

 scription of the actual experiments of Burk and coworkers must be awaited before judg- 

 ing whether they call for such drastic revision of our kinetic concepts. 



Intermittency factors E/j > 1 offer obvious inducements to use intermit- 

 tent light in experiments on mass culturing of algae. Of course, with a 

 light source of a given intensity ^ — c. g., the sun — no advantage could be ex- 

 pected (on the basis of present kinetic knowledge) from the relation E/, > 

 1 if intermittency were achieved simply by blacking out the illumination for 

 certain intervals (e. g., by means of rotating sectors) ; but if the same inter- 



