ROLE OF THE CARBON DIOXIDE ACCEPTOR IN INDUCTION 1413 



As illustrated by figure 33.2, no such recovery of induction losses of 

 photosynthesis in the dark has been observed in the study of oxygen libera- 

 tion. In measurements of carbon dioxide consumption, a "dark uptake" 

 of carbon dioxide after the cessation of illumination was noted by McAlis- 

 ter, Blinks and Skow and Emerson and Lewis (c/. next section), but only 

 under very special conditions; usually induction losses of carbon dioxide 

 are as irreparable as those of oxygen {cj. fig. 33.8). 



To sum up : Two views are possible of the role of intermediates in induc- 

 tion. The first one — which fits best into the picture of photosynthesis as a 

 single photochemical oxidation-reduction followed by dark, catalytic reac- 

 tions — assumes that the only intermediates in photosynthesis are "ther- 

 mal," and that these do not occur, in the steady state, in concentrations 

 approaching that of chlorophyll. The second hypothesis, which follows if 

 one postulates two or more successive light reactions, and thus admits 

 the existence of one or several photochemical intermediates (with sta- 

 tionary concentrations of the same order of magnitude as that of chloro- 

 phyll), postulates that these intermediates are still present in the cells 

 even after prolonged dark intervals. The two concepts are not mutually 

 exclusive, since photosynthesis may involve both thermal and photochemi- 

 cal intermediates. In this case, we can assert that the first ones do not 

 occur in ciuantities commensurate with those of chlorophyll, while the 

 second ones do not disappear from the cells in the dark. 



The possible function of respiration intermediates in the induction (par- 

 ticularly "positive" induction) will be discussed in section 5. 



3. Role of the Carbon Dioxide Acceptor in Induction 



It was stated before that, formally, the carbon dioxide acceptor. A, may 

 play the role of an intermediate. After a dark rest, this acceptor is in 

 thermodynamic equilibrium with external carbon dioxide; depending on 

 the concentration of the latter and the dissociation constant, it may be 

 either completely or partially carboxylated. As discussed in chapter 27, 

 the carboxylation equilibrium may be disturbed in light, in consequence 

 of rapid consumption of carbon dioxide by the photochemical system, and 

 insufficient replacement (because of slow difTusion or slow carboxylation). 

 In any case, the disturbed carboxylation equilibrium should be restored by 

 a "pick-up" of carbon dioxide after the cessation of illumination, and this 

 phenomenon has actually been observed under conditions favoring the 

 denudation of the acceptor, such as strong light, low carbon dioxide supply, 

 or cyanide poisoning of the carboxylase, Ea {cJ. chapter 8, page 200, and 

 figs. 21, 22 and 33.11). 



Whenever a "pick-up" occurs at the end of illumination, this can be 



