Mechanism of the Initial Steps in Photosynthesis* 



J. A. BASSHAM and K. SHIBATA, Radiation Laboratory, Universitij of 



California, Berkeley, California 



Many experiments have been carried out with intermittent hght 

 in the hope of elucidating the mechanism of the initial steps of reac- 

 tions in photosynthesis. But most previous studies have been con- 

 cerned with the change of the photosynthetic yield per flash of a 

 fixed duration with different dark periods. From the results, various 

 theories were proposed, including the theory of a "photosynthetic 

 unit" (1,2). Tamiya and Chiba (3) found that the maximum yield 

 becomes temperature-dependent when the light intensity is very 

 high. In place of the "photosynthetic unit" theory, they proposed 

 a different mechanism in which the sensitizer S (probably chloro- 

 phyll) is converted to the excited sensitizer (S*), after which S* 

 reacts with other chemicals to bring about photosynthesis or is de- 

 activated by a second-order temperature-dependent reaction. Despite 

 differences in these theories, there is similarity in one respect in that 

 they assume only one compound to be excited or one reservoir to be 

 filled during the course of a flash, with other steps in photosynthesis 

 occurring during a subsequent dark period. 



The preliminary experiment reported here was carried out to study 

 the effect of the flashing-light period on the maximum yield per flash 

 when a long dark period and a sufficiently high intensity are provided 

 to saturate photosynthesis even with the flashing hght. As will be 

 shown later, it appears that at least two reservoirs are filled, or par- 

 tially filled, in the course of light periods used in these experiments. 



METHODS 



The system used in this experiment to measure photosynthetic 

 rate was the same as the system for our quantum requirement ex- 

 periments (4), although the total volume was changed to increase the 

 sensitivity, and the surface area of the cell containing the algal sus- 



* The work described in this paper was sponsored bj^ the U. S. Atomic Energy 

 Commission. 



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