1412 INDUCTION PHENOMENA CHAP. 33 



Franck's interpretation of the "second induction wave" is only possible 

 because 1 or 2 minutes is all the time required for the whole chlorophyll 

 complex to undergo photochemical transformation in moderate light, if the 

 quantum yield is of the order of 1.) 



The apparent absence of an induction period attributable to the re- 

 plenishment of photochemical intermediates permits two interpretations: 

 either such intermediates do not exist, or they are stable. The first alter- 

 native leads to reaction schemes with a single photochemical primary proc- 

 ess. As stated before, Franck and co-workers (who based their considera- 

 tions on scheme 7.VA) chose the second alternative, and postulated that 

 the photochemical intermediates survive, without decomposition, dark 

 periods of the order of several minutes. Furthermore, they suggested that, 

 even if the dark periods last for several hours or days, the intermediates do 

 not disappear completely — perhaps because they are regenerated by oxida- 

 tion processes. More recent concepts of the chemical mechanism of 

 photosynthesis (chapter 36) favor the alternative of a single photochemi- 

 cal reaction. 



(b) Thermal Intermediates 



The building up of "thermal" intermediates, i. e., of photochemical 

 products that can be converted in the dark to the final products of photo- 

 synthesis, could give rise to an induction period of any duration — depend- 

 ing on the quantity of these products required for the maintenance of a 

 stationary state. This quantity is likely to increase in proportion with the 

 rate of photosynthesis; therefore, the duration of the induction period of 

 this origin could be more or less independent of light intensity (at least, 

 in the region where the rate is approximately proportional to light inten- 

 sity). In other words, the assumption of thermal intermediates as source 

 of induction is not open to the two objections raised above against the at- 

 tribution of this phenomenon to photochemical intermediates. 



However, other arguments can be adduced against this hypothesis as 

 well. In discussing slow diffusion as a possible source of induction, we 

 noted that irreversibility of induction losses makes it impossible to accept 

 diffusion as an adequate explanation. The same can be said about the 

 attribution of induction to thermochemical intermediates. If an initial 

 delay of the gas exchange is caused by the accumulation of an intermediate 

 from which oxygen can be evolved (or which can absorb carbon dioxide) by 

 a dark reaction, the accumulated intermediates should continue to evolve 

 oxygen (or absorb carbon dioxide), after the cessation of illumination, for 

 a period eciual to the induction period. 



