THEORY OF ALTERNATING LIGHT EFFECTS 1445 



the rate of carboxylation; and the average rate of carboxylation in inter- 

 mittent hght can at best approach (but never exceed) the rate of carboxyhi- 

 tion in continuous hght. (Ea cannot work more efficiently in the dark 

 than it does in hght, when practically all acceptor is maintained in the de- 

 carboxylated state by intense photosynthesis.) 



Briggs (1941) also has discussed the efficient utilization for photosynthesis of dark 

 intervals of the order of several seconds (in addition to those of the order of 10"^ second), 

 and suggested two catalytieal components: one with a concentration approximately 

 equal to that of chlorophyll, and a relatively long working period (of the order of several 

 seconds) corresponding to the concentration of the carbon dioxide acceptor, A, and the 

 worldng time of the carboxylating catalyst, Ea, in our hypothesis; and one with a con- 

 centration about 500 times smaller and a working period of the order of 0.01 second 

 (corresponding to concentration and working time of catalyst Eb in Franck's picture). 

 Briggs suggested that the second catalyst is an intermediate between chlorophyll and the 

 carbon dioxide acceptor complex {cf. the position of the system HX/X in some of our 

 schemes, e. g., scheme 7.1 in Vol. I). However, the identification of the hmiting catalyst, 

 Eb, with an intermediate oxidation-reduction system in this position is improbable, 

 because of fluorescence phenomena {cf. chapter 28, part B). 



In part B of this chapter, when discussing more recent experiments with 

 flashing light, we will again find evidence of the favorable effect of dark in- 

 tervals of the order of 0.1-1 second on light energy utilization, and discuss 

 several new attempts to interpret these results. It is particularly notable 

 that Gilmour et at. (cf. section B7) found these effects also in the Hill reac- 

 tion of chloroplast f rag-men ts, in M^hich carbon dioxide takes no part at all ; 

 this seems to indicate that if two (or more) enzymatic reactions (of the 

 type discussed by Franck and Weller, and by Briggs) are responsible for 

 these complexities of induction and intermittency effects, both of them 

 belong (or can belong, if more than two reactions are involved) to the part 

 of the photosynthetic reaction sequence concerned with the photochemical 

 oxidation of water and liberation of oxygen. Tamiya has attempted to 

 show that all intermittent light results can be explained by a single enzy- 

 matic reaction with kinetic characteristics different from those postulated by 

 Franck and Herzfeld (on the basis of the data of Emerson and Arnold) ; for 

 a discussion of his suggestion, see section B6 below. 



We see that, in all cases of catalytic yield limitation, the question asked 

 at the beginning of ttiis chapter : whether an increase in photosynthesis can 

 be achieved by regular interruption of illumination, e. g., by means of a 

 rotating sector, must be answered in the negative. The factor ijt never ex- 

 ceeds unity, and the factor irs never exceeds 2. (We have shown this foi 

 alternaling light; we will see in part B that the same is true for the factor 

 ill in flashing light, but that the factor ij^ may acquire, in such light, 

 values much higher than 2.) Whether the same is true when the poorly 

 understood phenomena of "injury" and "fatigue" come into play, is uncer- 



