336 



C. S. French 



THE POST - ILLUMINATION EXCHANGE OF O2 



Brackett, Olsen, and Crickard^ "^ found a stimulation of respir- 

 ation in Chlorella immediately after illumination, and Blinks^'^ 

 saw the effect using two beams of light. 



The difference between green and red light in producing an in- 

 creased uptake of Go after the exposure is shown in figure 1. The 



c 

 o 



(J 

 ^ 40 



V 



-a 



Green 

 Dark 570 



Dark 



_, m, ^ 



Red 

 695 



Dark 



Photost imulotion of 

 respiration 



Photosynthesis rate 



J L 



Time.min 



Fig. 1. The time course of photosynthesis in red and 

 in green light for Porphyridium . Red light makes a pro- 

 duct that increases the rate of O2 uptake during and af- 

 ter exposure. French and Fork ^5;^ 



drop in rate of O2 evolution after a light exposure may take over 

 a minute to become complete even in the presence of a large res- 

 piratory stimulation induced by light. After the rate has reached 

 its minimum value it climbs back to the original dark level as the 

 light-induced material that gives increased O2 uptake is consumed. 

 Its half -life in Porphvridium is about 1.1 min. 



Of course it is possible to explain the observed curves by as- 

 suming two opposite effects of light on O2 exchange. From this 

 point of view we would have an inhibitory effect of light on O2 

 uptake that decayed fairly rapidly, superimposed on a more persis- 

 tent stimulation of O2 uptake. The alternative explanation for 

 the slowness of the return of the rate to its pre-illumination 

 level is that O2 production actually continues at a more or less 

 exponentially decreasing rate after the light is turned off. This 

 continuing O2 evolution has its counterpart in CO2 uptake follow- 

 ing a light exposure--the "pickup" experiments of McAlister^"''. 

 The post-illumination extension of O2 production^^^ is better ex- 

 plained by the idea of parallel rather than consecutive photochem- 

 ical reactions. According to this concept of Brody and Brody^lO-' 



