"long" and "short" induction 1315 



in the first 10-20 minutes of illumination as a function of the duration of 

 incubation (fig. 33.6) indicated that two different dark processes, one much 

 slower than the other, were involved in bringing about the "short" induc- 

 tion. The same appeared to be the case in McAhster's observations of 

 carbon dioxide uptake by wheat (fig. 33.9), and in Franck and Wood's 

 measurement of fluorescence during the induction period. 



Since it is generally difficult to maintain completely steady conditions 

 of photosynthesis for extended periods, particularly in higher plants, it is 

 uncertain whether "induction" phenomena lasting for as long as several 

 hours can be treated as significant demonstrations of the working of the 

 photosynthetic mechanism ; many of them may be due to variations in the 

 changes in colloidal properties of the protoplasm, and other transforma- 

 tions only indirectly related to the intrinsic mechanism of photosynthesis. 

 In the steady state, the photosynthetic quotient, Qp = — AO2/ ACO2, 

 is unity. This makes it possible to speak of the "rate of photosynthesis" 

 without specifying whether we have in mind absorption of carbon dioxide 

 or Hberation of oxygen. The observers who first studied induction took it 

 for granted that the same must be true also during the induction period. 

 This is not necessarily the case. Recent advances in the understanding of 

 photosynthesis lead to the conclusion that the reduction of carbon dioxide 

 and the oxidation of water are two more or less separable catalytic processes. 

 In the steady state of photosynthesis, the two catalytic systems must work 

 at the same rate. In the dark, however, these systems could be— and prob- 

 ably are— deactivated to different degrees. If the catalytic system engaged 

 in the reduction of carbon dioxide has become oxidized in the dark, it may, 

 upon illumination, utilize the hydrogen supplied by the photochemical 

 process for its own reduction, before it starts transferring hydrogen to 

 carbon dioxide; while the catalytic system that takes hydrogen from water 

 and liberates oxygen will, from the beginning, work in the normal way. 

 Inversely, reduction of the water-oxidizing catalytic system in the dark 

 may delay oxygen liberation in the light, without affecting the reduction of 

 carbon dioxide. The extent of such "asymmetric" induction losses is 

 limited by the ciuantity of the catalysts available in the cells. It appears 

 that the most abundant of them are present in concentrations similar to the 

 concentration of chlorophyll. The "induction asymmetry" may therefore 

 reach the order of magnitude of one carbon dioxide (or oxygen) molecule 

 missing per chlorophyll molecule, which is ecjuivalent to the maximum 

 photosynthetic production in 20 or 30 seconds {cf. Table 28. V). This Hmi- 

 tation imposed on induction losses affecting only oxygen liberation or car- 

 bon dioxide consumption does not apply to inhibition losses affecting photo- 

 synthesis as a whole. 



It must be clear from the above remarks that no investigation of indue- 



