EFFECTS OF ASCORBIC ACID 



>59 



Figure 1 shows the results of six experiments on the quinone- 

 stimulated Mehler reaction. Oxygen evolution rates are plotted above 

 and oxygen consumption rates below the base line. In each experi- 

 ment the first period is a dark control. The second period shows the 

 oxygen production rate of a quinone-Hill reaction. Thereafter the 

 preparation (now quinone-stimulated, of course) exhibits a Mehler 

 reaction in which oxygen uptake proceeds at twice the rate of oxygen 

 evolution. Then, upon addition of ascorbic acid, the influence of this 

 substance on the individual production and consumption components 

 of the stimulated Mehler reaction is observed. The final period is 



b z+3 

 z o 



S •- +2 



w 5 + 1 



V> -I 



t <-2 



O. 



< 



12 3 45 



12 3 4 5 



12 3 4 5 



12 3 4 5 



12 3 4 5 



12 3 45 



Exp I 



Exp 2 



Exp 3 



Exp 4 



Exp 5 



Exp 6 



1.5- Dark Rates 3 - Mehler Reaction 



2 - OuiNONE Reaction 4- Memler, after 



Ascorbic Acid Tip 



Fig. 1. Effect of ascorbic acid on quinone-stimulated Mehler reaction. 



again dark. It is apparent that the stimulation of net oxygen con- 

 sumption brought about by the ascorbic acid is attributable to both 

 increased consumption and decreased production. 



At present our interpretation of these results lies along the lines 

 of Mehler's original suggestion that ascorbic acid oxidation in the 

 light utilizes an oxygen precursor. In terms of the detailed reaction 

 steps below which have been postulated to account for the oxygen 

 metabolism of the chloroplasts, we note that at all but the lowest 

 light intensities (at which the photolysis reaction (F) becomes 

 rate-limiting) the back reaction (G) may be important. When quinone 

 is being reduced, its reaction (H) must effectively compete with the 

 back reaction (G) so that oxj^gen evolution (J) is enhanced. When 



