374 PHOTOMECHANICAL CONSIDERATIONS 



tion of illumination and thus a much more complicated equation than 

 Equation 15 is required to evaluate his experimental data. 



DISCUSSION 



Based on the assumption that all electron flow observed in the 

 photosynthetic bacteria is of the cyclic type, a simple mathematical 

 derivation leads to the conclusion that the rate of cytochrome reduc- 

 tion on interruption of illumination may be equal to or greater than 

 the rate of cytochrome oxidation on illumination. Preliminary obser- 

 vations on R. rubrum indicate that under a particular set of con- 

 ditions the kinetics of cytochrome of type c meet this criterion, al- 

 though it does not apply at low temperatures or in the presence of 

 glycerol. In a pale green mutant of Chlamydo monas , the light-off rate 

 for cytochrome/ exceeds the light-on rate by three times. 



This method may have possibilities for investigation of the nature 

 of electron flow in a variety of photosynthetic systems. 



SUMMARY 



Under the assumptions made in our mathematical derivation, the 

 rate of cytochrome reduction on interruption of illumination should be 

 at least as great as the rate of cytochrome oxidation on illumination 

 in systems involving cyclic electron flow. The rate of cytochrome re- 

 duction may be considerably less than that of the rate of cytochrome 

 oxidation in noncyclic systems. For a particular set of conditions the 

 light-induced kinetics of cytochrome of type c in/?, nibnim are con- 

 sistent with existence of cyclic electron flow, but those of cytochrome 

 in a pale green mutant of Chlatnydomoua's are not. 



REFERENCES 



1. Diiysens, L. N. M., Reversible photooxidation of a cytochrome pigment in 

 photosynthesizing Rhodo spirillum rub rniu. Nature, 173, 692 (1954). 



2. Chance, B., and Smith, L., Respiratory pigments of Rhodospirillu))/ rubrum. 

 Nature, 175, 803 (1955). 



3. Olson, J. M., and Chance, B., Oxidation-reduction reactions in the photo- 

 synthetic bacterium Chromatium. Arch. Biochem. Biophys., 88, 26 (1960). 



4. Horio, T., and Yamashita, J., Electron transport system in facultative 

 photoheterotroph: Rhodospirillum rubrioii. This volume, p. 275. 



5. Baltscheffsky, M., and Baltscheffsky, H., Photophosphorylation in /?/?oc/o- 

 spirillum rubrum. About the electron transport chain and the phosphoryla- 

 tion reactions. This volume, p. 195. 



