EFFECTS OF ASCORBIf ACII) 201 



to oxygen evolution to influence the back reaction. It is of course 

 still speculative whether or not naturally occurring ascorbic acid 

 functions in this way. 



Discussion 



Lumry: I have two questions. What is known about oxygen reduction un- 

 stimulated by quinone? Has anyone been able to reverse the stimulation pro- 

 duced by quinone? 



Brown : When studied manometrically the rate of net oxygen consumption in 

 the quinone-stimulated Mehler reaction usually is more than twice that of the 

 unstimulated reaction. The enhancement factor varies with different chloroplast 

 preparations and may even be zero. With a given batch of "cooperative" chloro- 

 plasts results are consistent, however. The effect of quinone concentration on the 

 rate of the subsequent (stimulated) Mehler reaction after the quinone reduction 

 is over has been investigated, and this relation is a roughly hj^ierbolic curve 

 saturating at about 2 X 10 "^ molar. We originally thought that the quinone- 

 stimulated Mehler reaction proceeded exactly twice as fast as the unstimulated 

 when enough quinone was present to saturate the stimulation effect; it now seems 

 that the factor usually is above two. Also the effect of ascorbic acid on the un- 

 stimulated Mehler reaction has been studied, and it was found that a slight but 

 consistent stimulation of oxygen production occurred in that case. 



In answer to your second question, we have not observed a reversal of the 

 quinone stimulation on the Mehler reaction. Depending on experimental condi- 

 tions the rates of either stimulated or unstimulated Mehler reactions decline 

 with time— say 10% or no more than 20% in a couple of hours. Within limits 

 of manometric measurement no appreciable return to the unstimulated reaction 

 rate has been observed. 



Lumry: Quinone and perhaps other Hill oxidants play a very strange role in 

 several chloroplast reactions. We have noticed that the change-over from one 

 type of Hill reaction to another, produced by increasing the oxidant concentra- 

 tion, takes place in just the same range as the stimulating effect. It is also in this 

 region that fluorescence quenching by Hill oxidants begins to appear. The paral- 

 leUsm between effects of quinone on the Mehler reaction and on the Hill reaction 

 may ultimately prove to be less close than it now appears, but certainly the entire 

 matter is worthy of much more detailed investigation. It would, for example, be 

 interesting to see if other Hill oxidants can stimulate the Mehler reaction, since 

 all we have investigated have similar effects on the Hill reaction. Perhaps one 

 might find a new way to get manganese ion into photosynthesis in such studies. 

 The latter substance, like quinone, presents some highly intriguing possibilities 

 but neither can be said to be well studied. 



Brown: Stimulation of the Mehler reaction by Mn + + has been studied also. 



Here the effect is more striking than with quinone. Somewhat higher stimulation 



factors are observed and the effect saturates at about 0.5 X 10"' molar. 



Tamiya : Does oxygen compete with quinone in the Hill reaction? 



Brown : Everyone assumes that they compete. Initially you start out with a 



concentration of quinone high relative to that of oxygen and you end up with the 



