1382 INDUCTION PHENOMENA CHAP. 33 



Shiau and Franck (1947) noted that the second wave is much more pro- 

 nounced, and its crest occurs later, in nitrogen than in air (c/. fig. 33.47). 



Fluorescence-time curves of the general type of figure 33.19 were obtained by 

 Franck and Levi ( 1934) also with leaf extracts in acetone or alcohol; but all changes were 

 much slower than in vivo. The activation took about 1 minute and the decay more than 

 10 minutes. It would be interesting to find out whether these changes bear more than 

 accidental similarity to the fluorescence effects in living cells. Franck and Levi (1934) 

 and Franck and Wood (1936) suggested that such a relation may be brought about by 

 the presence, in leaf extracts, of lipophilic plant constituents with which chlorophyll 

 can form photosensitive complexes, analogous to those postulated in living cells. How- 

 ever, the fluorescence-time curves obtained by Knorr and Albers (1935) with solutions 

 of pure chlorophylls a and b also showed a succession of maxima and minima (accom- 

 panied by changes in the position of the fluorescence bands); these fluctuations con- 

 tinued for several hours and ended with the bleaching of the solution. 



Kautsky, Hirsch and Davidshofer (1932) and Kautsky and Zedlitz 

 (1941) reported that grana pi'ecipitates, obtained from mashed Saponaria 

 leaves by the method of Noack (c/. chapt. 14, part B), also showed an initial 

 increase of fluorescence, but only a very slow, if any, subsequent decay, 

 (a feature that they associated with the incapacity of the grana for photo- 

 synthesis). Under nitrogen, or in the presence of urethan, even the initial 

 increase was absent, and the fluorescence-time curves of the grana were 

 perfectly horizontal. 



Shiau and Franck (1947) made analogous observations with whole 

 chloroplasls isolated from spinach or tobacco. The initial rise was present 

 both in air and in nitrogen, but was completely suppressed by urethan. 

 However, this rise was comparatively slow: At 3.0 X 10^ erg/cm. ^ sec, 

 and 4° C, the growth of cp continued for alwut 30 seconds, after which the 

 curve became horizontal. Dark periods of as much as 5 minutes in air, 

 or 15 minutes in nitrogen were required to repeat the curve (c/. the much 

 lower figures for living cells on page 1383). 



The rise of (p was "light-saturated" considerably earlier in isolated chloro- 

 plasts than in whole cells; as in the latter, it was insensitive to cyanide, 

 and to carbon dioxide starvation. Ruptured chloroplasts "aged" faster 

 than whole chloroplasts (the symptom of aging being a decrease in the initial 

 slope of the fluorescence-time curve). 



Fluorescence— time curves, similar to those of free chloroplasts or grana, 

 were obtained by Kautsky and U. Franck (1943) and Shiau and Franck 

 (1947) also with mechanically injwed leaves (fig. 33.23). 



2. Influence of Different Factors on Fluorescence-Time Curves 



(a) Duration of Incubation 



The rise of the first fluorescence wave can be explained, as suggested 

 above, by the combination of a thermal and a photochemical process, 



