OXYGEN EXCHANGE DURING THE SHORT INDUCTION PERIOD 1323 



photosynthetic apparatus in light proceeds only to the level sufficient for 

 the maintenance of the maximum steady rate possible at the prevailing 

 Hght intensity (this maximum being determined by factors not involved 

 in induction). A similar conclusion was reached earlier by Gaffron and 

 Franck (c/. part C). 



Steemann-Nielsen's observations of the time required for the prepara- 

 tion of induction in the dark are iUustrated by figure 33.6. The curve 

 showing the initial rate in light as a function of the length of the preceding 



10 ^0 (0 Ao 100 uo mo 140 no 250 



Fig. 33.6. Average rate of O2 liberation by Fucus serratus in first 5 min. of light 

 of 23 klux at 4.8° C, as function of preceding dark incubation (after Steemann- 

 Nielsen 1942). Abscissa, minutes dark after activation to Pmax. at 23 klux. Ordi- 

 nate, per cent of Pmax. (net). 



dark interval appears to consist of two parts : About one half of the total 

 induction loss is developed after 10 minutes in darkness; 50 minutes of 

 additional incubation are required to reduce the rate by another factor of 

 1/2. A similar relation was found for the carbon dioxide exchange (Mc- 

 Alister, fig. 33.9). This indicates the superposition of two different induc- 

 tion-producing dark reactions: one being completed in about 10 minutes 

 (at 18° C), the other requiring several hours. Fluorescence measurements 

 (part B) also revealed two dark deactivation processes — one appeared 

 to reach saturation in about 1 minute ; the second one was much slower. 



The duration of the dark period required to bring about a certain per- 

 centage inhibition was found by Steemann-Nielsen to be independent of 

 temperature, if incubation and illumination were carried out at the same 

 temperature. Experiments in which only the incubation temperature was 

 varied (while illumination temperature was kept constant) showed, on the 

 other hand, that the absolute rate of the deactivation process was— as 



