820 FLUORESCENCE OF PIGMENTS IN VIVO CHAP. 24 



sensitizing activity (revealed through parallel measurements of the yields 

 of photosynthesis and fluorescence) has made fluorescence measurements 

 an important tool in the kinetic analysis of photosynthesis. We will 

 therefore restrict ourselves in the present chapter to some general considera- 

 tions of this relationship, postponing more detailed description of experi- 

 mental results and their interpretation to the several chapters in part IV 

 dealing with the effects of light intensity, temperature, carl)on dioxide and 

 other external factors, on the kinetics of photos^mthesis. 



Fluorescence is one of the several ways in which excited chlorophjdl 

 molecules can dispose of their energy. Others include energy dissipation 

 (conversion into vibrational energy and ultimately into heat), and photo- 

 chemical reactions (either involving the chlorophyll molecule itself, or sen- 

 sitized by it). The intrinsic capacity of the excited chlorophyll molecule 

 to fluoresce (the monomolecular fluorescence constant kf, or its reciprocal, 

 the ''natural life time" of the excited state, t/) can be considered as con- 

 stant as long as the absorption spectrum of the chlorophyll molecule re- 

 mains essentially unchanged. The intrinsic capacity for energy dissipa- 

 tion (the monomolecular dissipation constant kt) and the rate of energy loss 

 through chemical reactions (rate constants ki, k^. ■ ■, which can be mono- 

 molecular or bimolecular) are, on the other hand, subject to changes de- 

 pending on the association of the chlorophyll molecule with other molecules 

 before excitation, and on its encounters with other molecules during excita- 

 tion (as discussed in the sections of chapter 23 dealing with the quenching 

 and self-quenching of chlorophyll fluorescence in vitro). The variations of 

 chlorophyll fluorescence in vivo associated with variations in the rate of 

 photosynthesis must therefore be attributed to changes in the composition 

 or structure of the chlorophyll-bearing molecular complex (and consequent 

 alterations in the values of monomolecular constants of dissipation and 

 chemical quenching), and to changes in the probability of the chlorophyll 

 complex encountering, during the excitation time, molecules capable of 

 serving as effective "physical" or "chemical" quenchers (and consequent 

 alterations in the values of bimolecular constants of quenching) . 



The several more or less detailed interpretations of fluorescence changes 

 in photosynthesizing plants, which have been suggested, all are based on 

 these general ideas but differ in emphasis laid on one or the other specific 

 mechanism of quenching. Some (Kautsky; Wassink and Katz) attribute 

 the main function to "chemical quenching" by the reactants taking part 

 in photosynthesis, and consider each increase in fluorescence as evidence of 

 a decrease in the efficiency of the sensitized photochemical process (and 

 consequent decline of chemical quenching), and each decrease in fluores- 

 cence as evidence of increased efficiency of utilization of excitation energy 

 for the sensitized photochemical reactions (and consequent increase of 

 chemical quenching). Others (Franck) see the most important cause of 



