1442 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



at ^-values below 1 minute should bring the factor Iib back to unity, but 

 could not make it higher than 1. The experimental intermittency factor 

 increases, however, far above this value, and approaches 2 at alterna- 

 tion frequencies of the order of 100/sec. (c/. Table 34.1, and figs. 34.3 

 and 34.4c). This behavior becomes understandable if one assumes that 

 short dark intervals can be efficiently utiUzed for the completion of the 

 dark reaction that limits the rate of photosynthesis in strong continuous 

 light. In the limiting case of very short intervals, the rate-determining 

 catalyst will be as fully occupied during the dark interval as it is in light, 

 leading to an intermittency factor of ^/B ^^ 2 {ijt ^^ 1). We may thus 

 conclude, from the alternating light experiments, that the catalytic reac- 

 tion that limits the rate of photosynthesis in strong light can continue for 

 about 0.01 second after the cessation of illumination. 



According to the theory of Franck and Herzfeld, this reaction is the 

 transformation of the intermediates produced by the photochemical proc- 

 ess proper, which prevents them from reacting back. The catalyst that 

 brings about this "stabilization" was designated by Eb in several reaction 

 schemes presented in chapters 7, 9 (Vol. I), 24 and 28 (Vol. II, 1). As 

 stated before, this hypothesis of Franck and Herzfeld is not bound to the 

 specific reaction mechanism suggested by these authors (scheme 7VA), but 

 can be used also in conjunction mth other reaction schemes. 



The fact that the catalyst Eb can work in the dark only for a limited 

 length of time (about 0.01 second at room temperature), irrespective of the 

 intensity of the preceding flash, can be understood if it is assumed that this 

 catalyst acts on an unstable substrate. If the flash had produced more 

 light products than the catalyst can handle at one time, only the batch that 

 has become associated with the catalyst immediately after the light reaction 

 is saved from back reactions and contributes to the final yield. We have 

 already used this picture in chapter 32 (sect. 4) in explaining the maximum 

 number of oxygen molecules that can be produced by a flash. (We have 

 postulated that this number is determined by the number of available mole- 

 cules of Eb; it may be either equal to Eb, or smaller by a factor of n, de- 

 pending on whether Eb has to operate once, or n times — perhaps, four or 

 eight times — to bring about the liberation of one molecule of oxygen.) 



Experiments in flashing light (to be discussed in section B) have per- 

 mitted a more precise determination of the "working period" of Eb — 

 about 0.02 second at 20° C. (c/. Table 34.11). If the intermittency ef- 

 fect in alternating light were determined, in the region t < 1 minute, only 

 by this catalytic action period (which we will call the Emerson-Arnold 

 period) the factor ijn could exceed unity only for dark intervals of this 

 order of magnitude. Instead, we find in Table 34.1 that ijE is higher than 

 unity even for intervals as long as 15 seconds. 



