IxniAL STEPS I.\ PHOTOSYXTHESIS 371 



The straishtncss of the curve in Fig. 3 at longer light periods indi- 

 cates that the second reservoir is tilled by a zeio-oidcr reaction, which 

 is quite reasonable for an enzymatic reaction. If the light period is fur- 

 ther increased, the curve in Fig. 3 should level off when the second 

 reservoir is saturated. 



Discussion 



Tamiya: How higli was your maximum yield per flash? 



Bassham: The values are given in Table I, but it should be mentioned that 

 some algae give five times higher yields per flash than others depending on growth 

 conditions. 



Tamiya : What is the factor determining the variation? 



Bassham : The yield depends principally upon the chlorophyll content of the 

 algae. When one prolongs the flash time, one obtains two or three times higher 

 oxygen yields than with a short flash. This eff'ect increases as the temperature 

 increases. 



Tamiya : Are you sure that your shortest flashes were saturating? 



Bassham: Yes. 



Tamiya : The dark period for saturation was much longer than that in Emerson's 

 experiment? 



Bassham : Yes, much longer. 



Whittingham: We did some work on flashing light; it was dropped because 

 of the War. One difficultj^ that bothered us then was that, when one shakes a 

 Chlorella suspension, one superimposes upon the flashes given from the outside 

 flashes owing to the cells' being moved from a region of relative high light intensity 

 to one of relatively low intensity and back. 



I wonder whether Dr. Bassham or Dr. Kok could tell us what was the percent- 

 age absorption of incident light in their suspensions. 



Shibata: Less than 50%. Even if variations in intensity caused by shaking 

 were as high as 20%, this did not matter because the light was far above satura- 

 tion. 



Rabinowitch: The present difficulty with the theories based on the original 

 experiments of Emerson and Arnold is that Tamiya has since discovered, and Kok 

 confirmed, a yield-enhancing effect of dark periods much longer than the original 

 "Emerson-Arnold period" of about 10"^ second at room temperature, which be- 

 comes apparent when the flashes last longer than 10 ~* second, and that in this 

 case the flash yield becomes temperature-dependent. This indicates that there 

 is some kind of a "reservoir" of intermediates, which longer flashes can fill and 

 from which material for further transformation can be extracted in prolonged dark 

 periods. 



Gilmour, who has obtained similar results in the study of the Hill reaction, 

 devised a rather complicated mechanism. In his theory, there is a side reservoir, 

 in which the primary photochemical products are stored, later to be taken back 

 into the main reaction sequence. Kok finds that a simpler mechanism is sufficient. 

 All one needs, according to him, is to substitute a two-step transformation of the 

 photoproduct for a one-step transformation; one of the two enzymatic components 

 (Kok's E) acts as a "reservoir," filled up by longer lasting flashes. 



