1444 PHOTOSNYTHESIS IN INTERMITTENT LIGHT CHAP. 34 



and therefore can be used repeatedly during a single dark period (while Eb 

 is supposed to act on unstable intermediates and is therefore available only 

 once in each dark interval). Because of this difference, the frequency of 

 alternations that gives the most favorable intermittency factor under the 

 conditions of Ea limitation is not equal to one ''action period" of this 

 catalyst, but is determined in a more complex way by the time that the 

 available quantity of Ea requires to saturate the acceptor with carbon 

 dioxide to such an extent as to give the maximum possible yield of photo- 

 synthesis during the subsequent hght period. 



Franck has concluded, from the "pick-up" experiments described in chapter 8 

 (page 206, c/. also figs. 10 and 11 in chap. 33), that the time required to carboxylate all 

 acceptor (once it has been totally decarboxylated) is about 20 seconds. This then must 

 be the upper limit for the optimum frequency of alternations. (There certainly can be 

 no advantage in extending the dark periods beyond the time required for complete re- 

 carboxylation; while the simultaneous extension of light periods is disadvantageous 

 since in longer flashes the A-C02 complex is more completely decarboxylated, thus caus-, 

 ing an approach to the conditions of steady illumination and decreasing the favorable 

 intermittency effect.) To obtain a lower limit of the most favorable frequency of 

 alternations, one has to know the rate of processes by which the complex A-C02 is de- 

 carboxylated in light. We presented in chapter 8 (page 167) and chapter 29 (p. 1086) 

 evidence that led Franck to assume that each absorption act, whether it contributes to 

 photosynthesis or not, may cause the decarboxylation of an acceptor molecule (because 

 back reactions can result in a decomposition of A-C02 into free acceptor and carbon 

 dioxide). If this is so (some difficulties of this hypothesis were mentioned in chap. 33), 

 the rate of decarboxylation in light is proportional to light intensity, even in the satura- 

 tion range (at least, until we come into the intensity region where the yield of fluorescence 

 increases, as described in chap. 28B, thus revealing a decrease in the rate of the primary 

 photochemical process). From the frequency of absorption acts (estimated on page 

 838) and the concentration of the acceptor molecules (estimated on page 204), we can 

 deduce that, in light of approximately 10,000 lux, the velocity constant of the photo- 

 chemical decarboxylation of A-COz complexes (partly by reduction and partly by dis- 

 sociation caused by back reactions) is of the order of l/sec, and in light of 100,000 lux, 

 of the order of 10/sec. 0.1 to 1 second must then represent the lower limit for favorable 

 intermittency effects (i/E > 1 ) in the case of Ea limitation. Combined with the above- 

 mentioned upper limit (20 seconds), these estimates define a region that in fact roughly 

 corresponds to the range in which intermittency factors larger than 1 have been found 

 by Warburg, McAlister, Briggs and Weller and Franck. This lends support to Franck's 

 attribution of this anomaly to an inadequate quantity of the carboxylating catalyst, Ea. 

 This interpretation implies that, in plants that show the anomaly, the rate-limiting proc- 

 ess in strong continuous light also is the "preliminary" carboxylation reaction, rather 

 than the "stabilizing" reaction catalyzed by Eb. This should be recognizable, e. g., by a 

 greater sensitivity of the maximum rate to cyanide (c/. chapter 12, sect. Al). According 

 to Weller and Franck, Hydrangea leaves generally behave as if they were deficient in 

 enzyme Ea. 



It may be useful to point out that in the case of Ea limitation, the 

 maximum obtainable value of Ije still is 2 (ijt < 1). Under no conditions 

 can the average yield of photosynthesis in alternating light be higher than 



