314 RADIATION BIOLOGY 



At high hicident Ught intensities the capacity of system (1) of Sect. 2 will 

 become a limiting factor for the energy transfer, especially if a sufficient amount 

 of carbon dioxide is available. The annihilation of activated acceptor molecules 

 appears unhampered, and with increasing light intensity the energy transfer 

 gradually becomes limited by the supply of molecules ready to accept energy. 

 This leads to an increase in the concentration of excited bacteriochlorophyll, 

 which gives rise to an increased fluorescence yield. It seems, furthermore, that 

 activated energy acceptor, which does not react in system (3) of Sect. 2, is recon- 

 verted after some time into nonactivated acceptor, ready to accept energy again. 

 This can be considered the reversal of a reaction of the type 



REH + F -^ RE + FH, 



which may occur if FH cannot be reconverted to F in the usual way owing to lack 

 of carbon dioxide. It thus seems that the amount of energy acceptor at the sys- 

 tem of energy transfer is less easily lowered at high light intensities in the absence 

 of carbon dioxide than in its presence. This leads to a lower stationary concen- 

 tration of excited bacteriochlorophyll and thus to a lower fluorescence (cf. also 

 Wassink et al, 1942, p. 323). 



Another set of observations on fluorescence which are important 

 in understanding the mechanism of photosynthesis will now be con- 

 sidered. Many observers, e.g., Kautsky and coworkers and Wassink 

 and coworkers, have found that upon illumination of a photosynthetic 

 system, such as a leaf or a suspension of algae, fluorescence undergoes a 

 number of distinct and reproducible changes. Some observations made 

 by Wassink and Katz (1939) on the fluorescence of algal suspensions of 

 known density under controlled conditions, which recently received 

 renewed attention because of their bearing on the mechanism of photo- 

 synthesis, are of interest (Calvin and Benson, 1948). Wassink and Katz 

 (1939) found that a suspension of Chlorella when illuminated under nitro- 

 gen at first shows a much higher fluorescence than when illuminated 

 under air. After some 20 min the fluorescence of the suspension in 

 nitrogen reaches practically the level of that in air. These observations 

 indicate that in Chlorella a reduced state is connected with a higher 

 fluorescence than a more oxidized one. It is in accordance with this 

 explanation that the decay of fluorescence observed under prolonged 

 illumination — also in air but much stronger in nitrogen — is prevented by 

 cyanide (Fig. 5-12). Obviously this is due to inhibition of oxygen pro- 

 duction by cyanide. In general, the fluorescence-time curve consists of 

 a rise, followed by a decay. The rise is insensitive to cyanide. A sus- 

 pension of Chlorella in the presence of a concentration of cyanide which 

 is just sufficient to stop photosynthesis shows upon illumination a rise of 

 fluorescence above its starting point, after which the fluorescence intensity 

 remains unchanged for at least 3 min. The rise is steeper with higher 

 light intensities or with lower oxygen tensions. These facts have led to 

 the conclusion that one of the most direct effects of Ught upon the photo- 



