CHLOROPHYLL FLUORESCENCE AND PHOTOSYNTHESIS 821 



changes in fluorescence intensity in the formation of chlorophyll complexes 

 with surface-active substances ("narcotics") which slow down energy dis- 

 sipation, and at the same time inhibit photochemical sensitization by pre- 

 venting photosensitive substrates from reaching the chlorophyll. This 

 amounts to a weakening of both processes (sensitization and dissipation) 

 which compete with fluorescence ; whereas in theories of the first-mentioned 

 type, onl}' one competing process (sensitization) is affected, while the other 

 two (dissipation and fluorescence), profit equally by the elimination of a 

 common competitor. 



We will now describe more specifically the several suggested mechanisms 

 of interrelation of fluorescence and photosynthesis beginning with the pic- 

 ture used in Volume 1 (chapter 19). 



In scheme 19. Ill (Vol. I, page 547) we attempted to represent the prob- 

 able relationship between sensitization and fluorescence of chlorophyll in 

 vivo. This scheme was formulated primarily for the interpretation of 

 sensitized photoxidation, but essentially similar conditions may be as- 

 sumed to prevail in photosjaithesis as well. The primary process was as- 

 sumed in chapter 19 to be a "tautomerization" of the complex X. Chi. HZ 

 (formed bj- association of chlorophyll with oxidant X and reductant HZ) : 



(24.3) X-Chl-HZ ^X-Chl*-HZ > HXChl-Z 



In photosynthesis, this primary process must be followed by secondary, 

 catalytic reactions, in which HX is oxidized back to X (directh' or indi- 

 rectly) by the carbon dioxide-acceptor compound, {CO2} *, and Z is reduced 

 back to HZ (directly or indirectly) either by water (in ordinary photosyn- 

 thesis of green plants) or by reductants such as H2, H2S or thiosulfate (in 

 the photosynthesis of purple bacteria). 



In this picture, variations in fluorescence can be related to those 

 in photosynthesis in both the above-mentioned ways — by means of 

 primary changes in the probabihty of sensitized chemical reaction, and 

 by means of primary change in the rate of dissipation of energy in the 

 chlorophyll-bearing complex. If the dissipation rate is constant, 

 fluorescence is an indicator of the efficiency with which the excitation en- 

 ergy of chlorophyll is used for the primary photochemical process (equation 

 24.3) : Whenever the latter process is retarded for one reason or another, 

 the sum of the probabilities of the two competing processes — fluorescence 

 and internal dissipation of the excitation energy — increases correspond- 

 ingly. Since fluorescence and internal energy dissipation are two alterna- 

 tive monomolecular processes, the yield of both will be changed in the same 

 proportion. Fluorescence thus becomes an index of the yield of the pri- 

 mary photochemical process, even though the absolute yield of fluorescence 



* ir X = { CO2 , the secondary reaction is the replacement of a reduced by a 

 fresh molecule i CO2 ! • 



