28 



PROBLEMS IN PHOTOSYNTHESIS 



delayed light of very low intensity. According to Arnold and Davidson (2), 

 the emission spectrum of this chemiluminescence — observed with Chlorella — 

 is identical with the fluorescence spectrum of chlorophyll a. The discoverers 

 of this phenomenon attribute it to an enzymatically catalyzed recombination 

 of substances produced early in photosynthesis. The production of this 

 delayed light would thus be partly a reversal of photosynthesis, in which at 

 least one — still unknown — reaction would be reversible. If, however, the 

 spectrum of the delayed light is identical with the fluorescence spectrum, it 

 must be concluded that the chemiluminescence is due to the excited chloro- 

 phyll molecule. Though Brugger and Franck (9) built up a series of hy- 

 potheses, based partly upon the delayed light phenomenon, to try to explain 

 the photochemical part of photosynthesis, the question still arises whether 

 the phenomenon really has anything to do with true photosynthesis. 



Chlorophyll is ir} vivo a stable substance with respect to air and to light. 

 However, after strong illumination or when photosynthesis is inhibited by 

 poisons, the leaves become yellow or colourless due to photochemical decom- 

 position of the pigment. This bleaching process is accelerated by excess of 

 O2. According to Gaffron (26), the excited chlorophyll gives its excitation 

 energy to an acceptor A 



Chi* + A -> Chi + .4* 



and the excited acceptor produced reacts with Oo 



A* + Go -^ AOi 



Fig. 12. Kautsky's photooxidation cycle. 



Kautsky (33) proposes a cyclic process. Substance A undergoes a cyclic 

 transformation represented in Figure 12. The excited chlorophyll activates 

 A to A"^. A metabolite X reacts with A* to produce a reduced form A' 

 and an oxidized form X'. The change of A via A* to A' means nothing 

 but the photooxidation of X. The substance A is regenerated in the dark by 

 oxidation of A' with Oo. This back oxidation has some similarity with the 

 induced respiration discovered by Burk and Warburg (see § 35). 



Another hypothesis based upon the formation of the HOo radical, has been 

 put forward by Weiss (64). 



