1408 INDUCTION PHENOMENA CHAP. 33 



2. Building Up of Intermediates 



Osterhout and Haas (1918), in the very first discussion of induction in 

 photosynthesis, pointed out two possible mechanisms : the building up of 

 intermediates, and the activation of catalysts. All subsequent explana- 

 tions of the induction losses belong to one of these two general types. For 

 positive induction, one can postulate utilization of intermediates accumu- 

 lated in the preceding period (preceding dark period for positive induction 

 in fight, preceding fight period for positive induction in dark). Transient 

 inactivation of catalysts by fight can also produce a kind of positive induc- 

 tion in the form of a temporary decfine of gas exchange below the initial, 

 normal level {i. e., not in the form of a burst of gas exchange in excess of the 

 steady level, which can be due only to the utilization of accumulated inter- 

 mediates). 



Closer analysis shows that the distinction between the two mechanisms 

 is not as sharp as one may think. The kinetic role of catalysts that are 

 reversibly changed during a chemical reaction (e. g., by alternating oxida- 

 tion and reduction, or carboxylation and decarboxylation) is in many re- 

 spects similar to that of intermediates. For example, if carbon dioxide in 

 the complex A -002 is hydrogenated, in the course of photosynthesis, in 

 several steps, the compounds k-YiCOi, A -112002, . . .are reduction inter- 

 mediates, while the free acceptor A is a catalyst; nevertheless, at the be- 

 ginning of illumination, the concentration [A] may have to be built up in 

 exactly the same way as the concentrations [A-H002], etc., since at the 

 end of a dark period all acceptor molecules may be in the form of the 

 complex A 002, while a certain quantitj^ of free A is required for the car- 

 boxylation reaction, OO2 + A -^ A • OO2, to keep pace with the photosyn- 

 thetic process as a whole. 



Similarly, if the hydrogen atoms move, during photosynthesis, from 

 the reductant, A'H20, to the oxidant, A -002, through the intermediary of 

 an oxidation-reduction catalyst, X/HX (c/. chapter 7), and if, after a dark 

 period, all molecules of this catalyst are present in the same form (e. g., the 

 oxidized form, X) the concentration of the other form has to be built up, 

 in light, like that of an intermediate, until the reaction in which this form 

 takes part {e. g., HX + A -002 -^ X -1- A -11002) can keep pace with the 

 over-all progress of photosynthesis. 



The specific character of induction effects due to catalj^sts appears when 

 their inactivation is brought about by inhibition (and not by accumulation 

 in one form) and this inhibition is removed "autocatalyticafiy" by the 

 products of photosynthesis. In this case, induction may assume the form 

 of "waves," so characteristic of many experimental induction curves of 

 photosynthesis. The building up of intermediates, on the other hand, can 



