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C.P. Whittingham and P.M. Bishop 



together with a 697 mp flash, and a 700 mp background in conjunc- 

 tion with a 644 mp flash. The yield from a single flash, after a 

 long dark period, fired at the beginning of the experiment and at 

 regular intervals during the course of it, was again used as a 

 measure of the decline in chloroplast activity. 



Effect of a single short flash 



No oxygen yield was obtained from a single short flash from 

 chloroplasts reducing TPIP. Witt et al . (13) using a rapid spec- 

 tropho tome trie assay, showed that TPIP was reduced by chloroplasts 

 instantly within a flash of 10 psec. ; reduction was followed by 

 rapid partial re-oxidation of the dye in the dark. In order to 

 estimate the amount of TPIP reduced in a single flash, several 

 flashes had to be given, and Witt calculated that, in a single 

 flash, two molecules of dye were reduced; this corresponded to 

 the evolution of one molecule of oxygen per 12,000 chlorophyll 

 molecules. It seems possible, therefore, that the amount of oxy- 

 gen liberated by a single flash of 100 psec, originally very 

 small, is reduced by competing back reactions to an amount which 

 is below the limits of detection of the measuring apparatus. 



A very small oxygen yield was obtained from a single short 

 flash when catalytic amounts of TPIP were present together with 

 ferricyanide. Presumably the TPIP reduced photochemically by the 

 chloroplasts in the light, is reoxidixed chemically more rapidly 

 by the substrate amounts of ferricyanide present, than by molec- 

 ular oxygen, so that a measurable amount of oxygen is released. 



No oxygen yield was obtained from a single short flash with 

 chloroplasts and ferricyanide, either on its own, or with the 

 addition of phosphorylating or uncoupling agents. The absence of 

 a fast re-oxidation of the ferricyanide reduced by chloroplasts 

 in the light, has been reported on several occasions. It seems 

 possible, that in the case of ferricyanide no oxygen is produced 

 as a result of a single short flash. 



The production of oxygen from a single short flash thus re- 

 sults when chloroplasts are reducing TPIP, or ferricyanide with 

 catalytic amounts of TPIP; this is in contrast to the situation 

 occurring with ferricyanide alone since then no oxygen yield from 

 a single short flash is apparent. 



Pre-excitation by a short flash on the yield of a following long 

 flash 



Oxygen was produced as a result of a single long flash and a 

 brief flash was found to increase the yield from a long flash in 



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