INDUCTION AFTER PHOTOXIDATION 1371 



from the reduction of carbon dioxide, accumulated in the plant (by some fermentation 

 reaction) during the dark period; the subsequent steady liberation of oxygen could be 

 based on continued slow carbon dioxide production by the same fermentation process. 



Hill and Whittingham (1953) studied the induction phase in Chlorella 

 and Scenedesmus by their spectroscopic method (c/. section 2) also under 

 anaerobic conditions. They noted (in agreement with the earlier observa- 

 tions by Franck and co-workers) that the induction period was more 

 strongly affected by dark anaerobiosis in ChloreUa than in Scenedesmus. 

 After anaerobic dark periods up to 80 minutes (at 15° C.) the half-time of 

 reactivation was increased from 0.5 to 2-3 minutes, and after 17 hours, to 

 about 7 minutes. In this case, as contrasted to that of aerobic incubation, 

 the induction period is longer the lower the temperature; also, the reacti- 

 vation is faster the stronger the illumination (while the duration of aerobic 

 induction is independent of light intensity over a wide range, and even 

 decreases in weak light). The anaerobic inhibition could be completely 

 removed by addition of a small amount of oxygen in the dark, provided 

 the anaerobic incubation had not lasted too long. 



Because they noted an effect on induction even after an anaerobic incubation of 

 only a few minutes, Hill and Whittingham suggested a return to the Willstatter-Kautsky- 

 Warburg hypothesis of direct oxygen participation in photosynthesis, in preference to 

 the concept of Gaffron, Franck and others, according to which anaerobic inhibition is 

 due to the accumulation of fermentation products and inactivation of enzymes (see in 

 this connection, chapter 13, p. 327, this chapter, p. 1365, and chapter 37D, section 3). 



Olson and Brackett (1952) noted that anaerobic dark incubation of Chlorella leads 

 to an initial total inhibition of oxygen evolution, which may last up to 2 minutes (after 

 an incubation of 1-3 hours); after a shorter, or much longer, incubation period (-CI 

 hour or 18-24 hours), inhibition does not occur. Once the period of total inhibition is 

 over, the "light adaptation" curve follows the same time course as after aerobic incuba- 

 tion. 



Franck, Pringsheim and Lad made use of the high sensitivity of the 

 phosphorescence method to study oxygen liberation by single light flashes, 

 and found evidence of both primary and secondary induction losses in the 

 flash yield as well. The first flash produced less oxygen than the sec- 

 ond one, and so forth. In Scenedesmus, the yield per flash soon became 

 steady, while in Chlorella, a second depression was noticeable. 



7. Induction after Photoxidation 



Since induction probably is a consequence of oxidation-reductions and 

 autoxidations in the dark, we can expect characteristic induction effects to 

 occur also after a period of photoxidation in light (brought about in one of 

 the several ways discussed in chapter 19). 



Observations on these aftereffects of photoxidation were made by 



