THE MECHANISM OF PHOTOSYNTHESIS 



311 



that they should be mirror images (Mliller, 1874; Kautsky and Hirsch, 

 1934), but this turned out to be too simple. It can easily be seen that 

 those changes in photosynthesis which influence directly or indirectly 

 the energy transfer from chlorophyll will have an influence on the inten- 

 sity of fluorescence. It is, however, not strictly necessary for all changes 

 in the rate of photosynthesis to be accompanied by changes in fluo- 

 rescence, since every type of inhibition of the photosynthetic process does 

 not necessarily influence the energy transfer from chlorophyll. Numer- 

 ous studies have been made in recent years from the point of view of the 

 connection of chlorophyll fluorescence and photosynthesis. They were 

 initiated by the extensive investigations of Kautsky and Hirsch (1934), 

 who studied especially fluorescence induction phenomena. Comparative 

 studies of fluorescence and photosynthesis were undertaken by McAlister 

 and Myers (1940), by Franck et al. (see, for example, Franck and Gaffron, 

 1941; Franck, 1949, 1951; Shiau and Franck, 1947), by Ornstein et al. 

 (1938), and by Wassink et al. (see, for example, Wassink et al., 1938; 

 Wassink and Katz, 1939; Wassink et al., 1942; Wassink and Kersten, 

 1943-1945). Wassink (1951b) gave 

 a detailed discussion of the work 

 from these various groups. 



The facts which bear on the 

 mechanism of energy transfer and 

 which permit distinction between 

 partial processes of photosynthesis 

 closely related to the energy trans- 

 fer and those which are unrelated 

 will now be considered. Some re- 

 sults obtained with purple sulfur 

 bacteria {Chromatium, strain D) are 

 of special interest. Under normal 

 conditions of photosynthesis, with 

 excess hydrogen donor and excess 

 carbon dioxide, the fluorescence is 

 increased at light intensities for 

 which photosynthesis shows light 

 saturation (Wassink et al., 1942). 

 Under conditions of limited supply 

 of hydrogen donor, the change in 

 amount of fluorescence occurs at 

 lower light intensities (Fig. 5-9). 

 It has been concluded from these observations that the presence of a 

 hydrogen donor is essential for the transfer of energy from bacterio- 

 chlorophyll excited by light. When excess hydrogen donor is present, 

 lowering of temperature affects fluorescence in much the same way as 



o 



3 



2 3X10" 



INCIDENT INTENSITY, ergs /cm2-sec 



Fig. 5-9. Fluorescence of Chromatium, 

 strain D, as a function of hydrogen-donor 

 concentration (No -f- 5 per cent CO 2, 

 phosphate bufTer pH 6.3, 29°C). (Fro7ti 

 Wassink et al., 1942.) 



