THEORY OF ALTERNATING LIGHT EFFECTS 1443 



The results of McAlister (fig. 34.3) are somewhat less extreme: the 

 iiE values found by him are < 1 at 15 seconds, and practically equal to 1 at 5 

 seconds; but they reach 1.2 at 0.5 second and 1.6 at 0.1 second — periods 

 which are still too long to be effectively occupied by the Emerson-Arnold 

 reaction. Briggs (1941), too, found Ije = 1.6 for t — 0.6 second, a value 

 even somewhat higher than Warburg's values in Table 34.1. 



Weller and Franck (1941) noted the need to explain the favorable 

 effect of dark periods of the order of 1-10 seconds, and proceeded to 

 repeat Warburg's experiments under a variety of conditions. In some 

 cases, the Iie values remained below unity until t reached the order of 

 magnitude of the Emerson- Arnold period; but in others, they exceeded 

 1 even at much longer intervals. Weller and Franck suggested that this 

 "premature" rise of ijE occurs when a second catalytic reaction of photo- 

 synthesis becomes rate-limiting, and thus influences the phenomena of 

 intermittency. According to Franck and Herzfeld, the stabilizing cata- 

 lyst, Eb, is usually limiting in strong continuous light, and in the presence 

 of abundant carbon dioxide. However, under certain conditions, the 

 limiting influence may pass partially or completely to other factors, par- 

 ticularly those associated with the carbon dioxide supply. 



When the carbon dioxide concentration is low, or the diffusion path 

 offers high resistance (as in the case of closed stomata), the limiting process 

 may be the diffusion of carbon dioxide to the chloroplasts; in this case, dark 

 intervals can be utilized for the re-establishment of the carboxylation 

 equilibrium by diffusion (as this was first suggested by Willstatter and Stoll 

 in the discussion of the results of Brown and Escombe). The length of the 

 dark period required for this purpose must depend on specific conditions. 



Intermittency effects caused by slow diffusion are, however, unlikely to 

 occur in unicellular algae suspended in buffer solutions (which were used in 

 the Warburg experiments) and should be absent in the carbon dioxide- 

 saturated state (since the diffusion supply can always be improved by an 

 increase in the external concentration of carbon dioxide). However, a car- 

 bon dioxide supply limitation of a different nature may occur even under 

 these conditions if the quantity of the available carboxylating enzyme, Ea, is 

 insufficient. In this case, the maximum rate of supply of carbon dioxide to 

 the acceptor — and thus also the maximum over-all rate of photosynthesis — 

 cannot be improved by an increase in the external carbon dioxide concentra- 

 tion (as discussed in chapter 27, page 917) . If, because of E a deficiency, car- 

 boxylation becomes rate-limiting in strong continuous light, dark intervals 

 can be utilized for recarboxylation of the "denuded" acceptor. The time re- 

 quired depends not merely on the action period of Ea, but also on its concen- 

 tration. This difference from the case of limitation by a deficiency of Eb is 

 caused by the fact that Ea acts on a stable substrate (carbon dioxide). 



