FLUORESCENCE-TIME CURVES 



1389 



When the Hght intensity is suddenly decreased, e. g., from 16 X lO'* to 

 0.8 X 10* erg/em.- sec, the yield of fluorescence drops first to a level even 

 lower than (pi (the value that ordinarily corresponds to weak illumination) ; 

 and several minutes are required for the return to the steady value (c/. 

 fig. 33.30). At low temperatures, this "undershooting" of the steady fluo- 

 rescence intensity may be replaced by a "hysteresis," or slow adjustment of 

 <p to its final value (cf. fig. 33.31), indicating that the conditions conducive 

 to strong fluorescence — and thus presumably to low yield of photosynthesis 

 — survive, at 0° C, for about a second after the transition from strong to 

 weak light. This reminds us of the observations of Steemann-Nielsen, in 



Fig. 33.30. Fluorescence changes in Hy- 

 drangea leaves after a sudden decrease 

 of light intensity (after Franck, French 

 and Puck 1941). At normal temperature, 

 fluorescence first drops below the steadj^ 

 level, then recovers slowly. 



Fig. 33.31. Fluorescence changes in 

 Hydrangea leaves after sudden decrease of 

 light intensity (after Franck, French and 

 Puck 1941). At 0°, fluorescence requires 

 about 1 sec. to decline to its steady level, 

 showing the survival of the highly fluores- 

 cent material present in intense light. 



which a ciualitatively similar picture was found for oxygen liberation; 

 however, the "induction after transition to weaker light" lasted in Stee- 

 mann-Nielsen's experiments for several minutes (at 4° C), as against only 

 a second in figure 33.31. 



We have dealt so far only with the effect of light intensity on the first 

 wave of fluorescence. Figure 33.27 shows that the second maximum also is 

 shifted with increasing intensity, in a way which indicates that it, too, is 

 due to a photochemical reaction that promotes fluorescence, and to a coun- 

 teracting thermal reaction. However, the relations seem to be more com- 

 plex than in the first maximum. For example, Wassink and Katz (1939) 

 noted that, in the presence of cyanide (which prevents the final decay, DE 

 in fig. 33.19), the ascending slope, CD, is affected not only by light inten- 

 sity, but also by temperature {cf. figs. 33.39 and 33.40). Figure 33.41 indi- 



