824 FLUORESCENCE OF PIGMENTS IN VIVO CHAP. 24 



of the primary photochemical process, and the net result will be a simul- 

 taneous decline in the yields of both fluorescence and photosynthesis. 

 (In other words, fluorescence, freed of one of its two competitors — the 

 primary photoprocess — will face a stronger second competitor — internal 

 dissipation — and will suffer a net loss.) The energy-dissipating properties 

 of the chlorophyll-containing complexes may differ somewhat in different 

 species and even strains (otherwise, the yield of fluorescence would be ex- 

 actly the same in all plants) ; this may explain why the rationing of carbon 

 dioxide (or outright starvation) apparently has a different effect on fluores- 

 cence in leaves (investigated by Franck, French and Puck 1941 and Mc- 

 Alister and Myers 1940), purple bacteria (investigated by Wassink, Katz 

 and Dorrestein 1942) and diatoms (investigated by Wassink and Kersten 

 1944). In the first case, denial oi carbon dioxide caused a considerable 

 increase of (p at high light intensities {cf. figs. 28.25, page 1048); in the 

 second case, it caused a slight increase of (p at moderate intensities and a 

 decrease at high intensities (fig. 28.30) ; in the third case, (p declined in 

 strong light in the presence of carbon dioxide, but remained constant in the 

 absence of carbon dioxide (fig. 28.28). 



The primary photochemical process can be retarded not only by a defi- 

 ciency of reactants, e. g., {CO2} or thiosulfate, which are needed to restore 

 the photosensitive complex, but also by narcotization, that blankets the 

 complex and prevents its contact with the reactants. In this case, too, 

 one can expect a simultaneous effect on the probability of internal dissipa- 

 tion — this time in the direction of making the dissipation slower. Narcotics 

 are therefore likely to enhance fluorescence in a twofold way : by prevent- 

 ing "chemical quenching" by the primary photochemical process, and by 

 exercising a "protective" action of the type discussed in chapter 23, (page 

 776), i. e., by "wrapping in" the excited molecules, and weakening in this 

 way all energy-dissipating interactions with neighboring molecules. 



According to Franck et al. (1941,1947,1949), formation of protective 

 "narcotic" layers can be brought about by supplying plants externally 

 with substances such as chloroform or urethan, and also by metabolic 

 reactions — particularly those occurring under anaerobic conditions. Such 

 "internal" narcotization effects are supposed by Franck and co-Avorkers to 

 be responsible for the induction effects after an extended period of darkness 

 (see later in chapter 33) , for the inhibition of photosynthesis by anaerobic 

 incubation (cf. chapter 13, Vol. I, and chapter 33 in Part 2) and for the 

 "midday depression" (cf. chapter 26, page 873). In all these cases, cessa- 

 tion or retardation of photosynthesis is accompanied by enhancement of 

 fluorescence, and the effect on fluorescence often is considerably stronger 

 than could be attributed to the limitation of the primary photochemical 

 process. 



