822 FLUORESCENCE OF PIGMENTS IN VIVO CHAP. 24 



( < 1%) is much too small to make it a significant competitor of this process. 

 For example, if the yield of the primary photochemical process drops from 

 80 to 40%, and the sum of the yields of dissipation and fluorescence there- 

 fore increases from 20 to 60%, the yield of each of these two processes will 

 increase by a factor of 3. If the yield of fluorescence was <p = 0.2% be- 

 fore it will become 0.6% afterward. 



The occurrence of antiparallel changes of the yields of fluorescence and 

 photosynthesis was discovered by Kautsky in his investigations of the in- 

 duction period (for an example, cf. fig. 33.19A), and we now know that this 

 type of correlation is quite common, both in induction phenomena and in 

 the steady state. However, the relation between the yields of photosyn- 

 thesis and fluorescence is not always that of antiparallelism. Sometimes, 

 yield of photosynthesis changes strongly without an appreciable change 

 in 3deld of fluorescence (cf., for example, fig. 28.24 in which "light satura- 

 tion" of photosynthesis has no counterpart in the — steadily increasing- 

 intensity of fluorescence). In other cases, e. g., in some types of induction 

 phenomena, photosynthesis and fluorescence both change in the same di- 

 rection (cf. fig. 33.22C). The picture of the mechanism of photosynthesis 

 used above to explain the usual antiparallehsm of photosynthesis and 

 fluorescence can, however, be used also to explain how exceptions from this 

 antiparallelism can arise. 



In the first place, fluorescence competes only with the primary photo- 

 chemical reaction — not \vith the over-all process of photosynthesis. The 

 rate of photosynthesis, as measured by the liberation of oxygen or consump- 

 tion of carbon dioxide, often is determined, not (or not only) by the ef- 

 ficiency of the primary photoprocess, but also by the rate of one or several 

 of the associated dark, catalytic reactions. Among these are reactions 

 that convert the primary photoproducts into the stable end products of 

 photosynthesis. AVhen these "finishing" reactions are too slow to keep 

 pace with the primary photochemical process (a situation that may arise, 

 for example, in excessively strong hght, or at low temperature, or in the 

 presence of certain poisons), the primary photoproducts will accumulate to 

 a certain extent, but will then disappear by back reactions. The quantum 

 yield of photos^mthesis will thus be reduced, but that of fluorescence need 

 not be affected at all, since the primary photochemical process — which 

 alone competes with fluorescence — continues at full speed. This can 

 explain the occurrence of light saturation of photosynthesis without simul- 

 taneous increase in the yield of fluorescence (a phenomenon to which we 

 have referred above). 



In Volume I (cf., for example, chapter 7) we have considered, in addition 

 to"finishing" dark reactions (which, as just stated, are likely to have no effect 

 on fluorescence at all), also catalytic reactions of "preparatory" character. 



