1474 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



give show this confirmation to extend only up to 109 lux sec, while the 

 measurements at which the Emerson-Arnold maximum flash yield was ex- 

 ceeded required flash energies of 200-1000 lux sec. It is therefore con- 

 ceivable that the main reason for the difference between the results of 

 Tamiya and Chiba, and of Emerson and Arnold, is to be sought in the use 

 of flashes of very different duration — microseconds in one case and milli- 

 seconds in the other. Qualitatively, such a difference is to be expected, 

 according to Franck's "single batch" theory, when the duration of the 

 flash ceases to be negligible compared to the postulated working time of 

 the limiting enzyme (about 3 X 10~^ sec. at 20° C). In this case, some 

 molecules of the stabilizing enzyme operate once during and once again 

 after the flash. (In Tamiya's picture, an enzyme molecule could operate 

 more than once after the flash !) As in Tamiya's theory, this repeated oper- 

 ation of the enzyme Eb must make the yield a function of temperature. 

 However, it does not seem that the results of Tamiya and Chiba can be 

 explained quantitatively in this way. In order that the yield during a 

 saturating flash may become equal to the yield after the flash, its duration 

 must equal the reciprocal (monomolecular) rate constant of the transfor- 

 mation process (for ktf to equal Jl"^ ke~'^'' dt, if/ must be equal to 1/A;). In 

 Tamiya and Chiba's experiments, flash yields twice the Emerson- Arnold 

 "maximum yield" were obtained at 15° C, with flashes lasting as little 

 as 3 msec, while the reciprocal rate constant of the Emerson-Arnold reac- 

 tion is about 40 msec, at 13°C. (c/. table 34.11). 



It is obvious from this example that the correction for the finite duration of the flash 

 is not negligible in experiments with rotating discs, and that an exact determination of 

 flash duration is therefore important. (It can be argued that Tamiya's measurement of 

 the "half -width" of the oscillograph record of the flash led to an underestimation of its 

 effective length.) 



That flash duration is not a sufficient reason for the difference between 

 the results of Tamiya and of Emerson is further indicated by Weller and 

 Franck's results, which, despite the use of "long" flashes, agreed Avith those 

 of Emerson et al. 



It seems that the assumption of (at least) two different reactions af- 

 fecting the flash yield cannot be avoided. The first possibility^ndicated 

 above — is that two hack reactions in the photosensitive complex may com- 

 pete with a single rate-limiting reaction (such as the regeneration of the 

 "finishing" catalyst, Eb). Weller and Franck (1941) suggested a different 

 postulate — that of a second limiting reaction of the preparatory type (spe- 

 cifically, a stage in the carbon dioxide fixation). They used this hypothesis 

 to explain the behavior of certain plants in flashing light, and, in particular, 

 to interpret the effect of cyanide on the flash yield (cf. section B3 above). 

 This postulate could provide a quahtative interpretation also of Tamiya's 



