INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1075 



28.27) is quite different from the "midpoint" i// — the latter is situated at 

 much higher hght intensities. The ^-curves too, often are hyperbolae; 

 this is, for example true for the kinetic models leading to equations (28.51E) 

 (slow reaction with CO2), and (28.22) (slow primary back reaction). On 

 the other hand, equations (28.51G) (Ee-limitation) and (28.51K) ("narco- 

 tization") mdicate cvbic equations for <p = /(/). This is of interest in con- 

 nection with Franck's idea of "self-regulation" of photosynthesis: Franck 

 envisages a narcotization mechanism which would not affect photosynthesis 

 (and fluorescence) at low light intensities, but would become operative 

 rather suddenly when a finishing dark reaction ceases to be able to cope 

 with the photoperoxides produced by the primary photochemical process, 

 and would shut off a part of the chlorophyll apparatus sufficient to reduce 

 the formation of photoperoxides to the amount which the limiting reaction 

 can handle. The assumption of a self-regulating mechanism of this kind is 

 a tempting hypothesis, because of the general importance which "feed- 

 backs" and "servomechanisms" have acquired in mechanical interpreta- 

 tions and imitations of life processes. If such a mechanism were operative, 

 the curves [{Chi} ] = /(/) (and with this, also the curves, <p = /(/)) would 

 have to have sigmoid shapes as indicated by dotted line in figure 28.27. 



It was shown above that half-saturation of fluorescence w^ill occur simul- 

 taneously with the half-saturation of photosynthesis whenever the latter 

 corresponds to one-half of all chlorophyll complexes being in the inactive 

 state. This will be the case, e. g., when saturation is caused by insufficient 

 supply of carbon dioxide (or other reactants) . On the other hand, if satura- 

 tion is caused by slow removal of photoproducts from chlorophyll (e. g., 

 by EB-limitation in mechanism 28.41), we can expect half-saturation of 

 fluorescence to require stronger light than half-saturation of photosynthesis. 

 This seems to be the case in figure 28.26 (Hydrangea leaf) where half -satura- 

 tion of <p occurs at about 60 kerg, and that of P at about 30 kerg. 



We will now review briefly the experimental results described in sections 

 1 and 2 {cf. figures 28.24-28.51) in the light of these theoretical concepts. 

 The occasionally observed Hght saturation of photosjmthesis unaccom- 

 panied by changes in the yield of fluorescence (illustrated most strikingly 

 by fig. 28.24) must indicate that saturation was caused by a finishing dark 

 reaction that did not affect the composition of the chlorophyll complex, 

 either directly or indirectly (through the formation of a "narcotic"). This 

 may be an example of pure "catalyst B" limitation, and saturation by 

 secondary back reactions. It can be asked whether, under these conditions, 

 a change in <p will be observed at some higher intensity in the saturation 

 region, when the primary process becomes too fast for some preparatory 

 reaction to keep pace with it. A\Tiether this is the case might depend on 

 whether the back reactions give products suitable for direct use in the pri- 

 mary photochemical process, or products that have to undergo again the 

 slow preparatory catalytic reactions. For example, if the back reaction is: 



