1376 INDUCTION PHENOMENA CHAP. 33 



permits one to follow the very rapid changes in the photosynthetic appara- 

 tus, such as must occur during the induction period. The recording of 

 fluorescence has first led to the realization of the great complexity of the 

 induction phenomena, particularly to the discovery of the developments 

 that take place in the first one or two seconds of illumination. 



That the fluorescence of chlorophyll changes in a characteristic way 

 during the induction period of photosynthesis was first noted by Kautsky 

 and Hirsch (1931).* Kautsk}^ has since devoted a series of investigations 

 to this subject (1931-1943). The reliability of his earlier observations 

 (1931-1937) was limited by inadequate technique (which involved excita- 

 tion by ultraviolet light, and visual estimation of fluorescence intensity). 

 More recently, he has gone over to excitation by visible light and automatic 

 registration of fluorescence, and more reliable data have been collected in 

 this way, particularly by Kautsky and U. Franck (1943). Systematic 

 determinations of the intensity of the chlorophyll fluorescence during the 

 induction period have also been made by Wassink and co-workers (1939, 

 1942, 1944), McAlister and Myers (1940), Franck, French and Puck (1941) 

 and Shiau and Franck (1947). Reviews of the subject have been written 

 by Kautsky and U. Franck (1948), J. Franck (1949) and Wassink (1951). 



Nothing is known so far of changes in the fluorescence spectrum of 

 chlorophyll, which are not impossible, if the chemical structure of chloro- 

 phyll itself, or of its associates in the "photosensitive complex," under- 

 goes changes in the transition from darkness to light and back. 



Correlation of measurements of fluorescence intensity with gas ex- 

 change measurements would be much safer if experiments of both kinds 

 were carried out with the same plant objects. Otherwise, there is a danger 

 that, in attempting to present a comprehensive picture of induction phe- 

 nomena, one may try to fit together pieces from several different jigsaw 

 puzzles. A step in the right direction was taken by McAlister and Myers, 

 who measured simultaneously the carbon dioxide absorption and the fluo- 

 rescence intensity of wheat plants and ChloreUa suspensions. 



The analysis of the gas exchange leads to the conclusion that the most 

 conspicuous induction phenomena observed under aerobic conditions can 

 be formally explained by three processes: a preparatory dark reaction, 

 which produces a "precursor," or "potential inhibitor" (or removes or in- 

 activates a catalyst that, when active, prevents the accumulation of an 

 inhibitor); a photochemical reaction, which "activates" the inhibitor; and 

 a second, nonphotochemical process by which the inhibitor is removed. 

 Special explanations are required for the occurrence of a second inhibition 

 period, and for the carbon dioxide "burst" observed by Blinks and Skow 



* It is worth recalling here that French and Young found no induction of the fluo- 

 rescence of phycoerythrin in vivo, indicating that the composition and environment of its 

 molecules remain unchanged by illumination. 



