1368 



INDUCTION PHENOMENA 



CHAP. 33 



tained a number of carbon dioxide consumption curves in pure commercial 

 nitrogen (0.5% oxygen), but here, too, no attempt was made to realize 

 truly anaerobic conditions (which have to be maintained for some time 

 prior to the beginning of illumination in order to obtain the typical anaero- 

 biotic inhibition effects). Consequently, McAlister and Myers' ''low oxy- 

 gen" curves reflected merely the known favorable effect on photosynthesis 

 of a reduction in the partial pressure of oxygen (attributed in chapters 13 

 and 19 to the avoidance of photoxidation). The only effect of low oxygen 

 concentration noticeable in these curves was a slight increase in rate, 

 spread over most of the induction period. An investigation of the initial 



O 



o 



ZD 



o 

 o 

 a: 



Scenedesmus, 5 x 10"^ g. / 2.5 ml. 



2 3 



TIME, min. 



Fig. 33.18. Absence of induction 

 in Scenedesmus after 3 hours anaero- 

 biosis in Nj (after Shiau and Franck 

 1947). piCOi) = 20 mm. 



Fig. 33.18.^. Induction in Chlorella after 

 3 hours anaerobiosis in N2 (after Shiau and 

 Franck 1947). 



course of photosynthesis under strictly anaerobic conditions, in a stream of 

 oxygen-free nitrogen or hydrogen, was made by Franck, Pringsheim and 

 Lad (1945), using the phosphorescence-quenching method. It was men- 

 tioned before that reversible light saturation curves can be obtained under 

 these conditions, the saturation level being from 50 to 0.1% of that under 

 aerobic conditions, depending on the completeness of the incubation treat- 

 ment. 



In some experiments, no induction at all was observed under anaerobic 

 conditions, as far as the inertia of the apparatus permitted one to judge. 

 This inertia was of the order of 20 seconds. Figure 33.18 gives an example 

 of this behavior in Scenedesmus after 3 hours anaerobiosis. Fluorescence 

 experiments (part B, section 2f) make it likely that 6ne/ induction changes 

 actually occur in the first few seconds. 



After longer anaerobic pretreatment, curves with a clearly delayed as- 

 cent were obtained; their shape indicated a "second wave" of inhibition 

 about 1 minute after the beginning of illumination. Oxygen liberation 



