GAS EXCHANGE DURING THE LONG INDUCTION PERIOD 1363 



curves were interpreted by Harder as the result of superposition of four processes: 

 activation ( = induction), deactivation ( = light injury), re-adaptation to strong light 

 (renewed rise) and "fatigue" (expressed in the final decline of the rate). 



Other authors who have studied the course of photosynthesis over long 

 periods obtained less complex curves. Bukatsch (1935), for example, noted 

 with green algae {Spirogyra, Zygnema, Mougeotia and Cladophora), after 

 an extended dark rest, only a smooth increase in rate that lasted for 

 about 1 hour, and was followed by constant oxygen production. Gessner 

 (1937) often found, in experiments with higher aquatic plants {Elodea, 

 Potomageton and Ceratophyllum), no long induction periods at all; the 

 rate was constant from the first hour of illumination. (Two such curves 

 were reproduced in figure 26.9.) Other curves of the same observer 

 showed, however, an increase in rate during the first 1 or 2 hours of illu- 

 mination, especially after a prolonged period of darkness. However, the 

 induction never exceeded 2.5 hours, even after dark periods of 40 days (c/. 

 fig. 33.16). Finally, it was mentioned in section 2 that Steemann-Nielsen 

 (1942) observed, in Fucus, only induction phenomena of 10-30 minutes 

 duration, even after incubation periods of 16 hours. 



Apparently, a long induction period after prolonged dark incubation is a 

 fairly common, but not general, phenomenon; whether its occurrence is 

 closely related to the kinetic mechanism of photosynthesis, or is more or less 

 incidental, is as yet uncertain. Perhaps, vigorous circulation of the me- 

 dium in the experiments of Gessner and Steemann-Nielsen had something 

 to do with the absence of a prolonged induction period in their experiments. 

 It was mentioned in section 1 that, even if no prolonged induction oc- 

 curs after a dark incubation of several hours, the extent of the "short" in- 

 duction is affected in a manner showing the superposition of a slow de- 

 activating process upon the fast deactivating reaction responsible for induc- 

 tion after a few minutes of dark rest. This twofold origin of short induc- 

 tion losses is clearly shown both by Steemann-Nielsen's (1942) curve of 

 losses of oxygen liberation by Fucus (fig. 33.6), and by McAhster's (1939) 

 curve of losses of carbon dioxide uptake by Triticum (fig. 33.9). 



A special kind of induction phenomena has been recently described as 

 following the transfer of certain algae from acid into alkaline media. 



In chapter 37D, we will describe the experiments of OsterUnd (1951, 

 1952) on the photosynthesis of Scenedesmus quadricauda in carbon dioxide 

 and in bicarbonate solutions. He noted the occurrence of a long induction 

 period (order: 30-50 minutes) in algae grown in acid, carbon dioxide-rich 

 solutions, and transferred into carbon dioxide-poor bicarbonate solution 

 before exposure to light; the same algae showed none, or only the usual 

 short, induction if exposed to Ught in the original carbon dioxide solution. 

 OsterUnd suggested as explanation a slow photoactivation of a factor in- 



