FLUORESCENCE-TIME CURVES 1387 



slope of fluorescence wave AB becomes steeper, as anticipated. In Hydran- 

 gea leaves, according to Franck, French and Puck (1941), this increase 

 continued until the light intensity has reached 20 X 10* erg/cm.^ sec, or 

 approximately 40 klux. The height of point B, on the other hand, ceases 

 growing much earlier, e. g., in Hydrangea at about 1.5 X 10" erg/cm.^ sec. 

 (see fig. 33.28; cf. the "light saturation" of the carbon dioxide induction 

 loss, illustrated by fig. 33.8). 



The maximum fluorescence yield, <pmax., reached in point B, often is 

 almost three times the steady yield <p. Thus, the ratio (^max./^ can be con- 

 siderably higher than the ratio </j2/<pi (—1-7), of the yield of fluorescence 

 in strong and weak light (cf. p. 1049). The fluorescence peak during the 

 induction period therefore must be due to a change in the chlorophyll com- 

 plex that enhances fluorescence more strongly than the transformation that 

 occurs in strong, steady light. The maximum is reached so soon (in about 

 1 second) after the beginning of illumination that, at light intensities of the 

 order of 2 X 10* erg/cm.^ sec. {cf. figs. 33.19a and 33.28), only a small frac- 

 tion of the chlorophyll molecules (probably, less than one tenth) could have 

 absorbed a quantum during this period, and thus passed into a chemically 

 difl'erent form. To explain this result, we have the alternatives: either to 

 assume that the form of the photosensitive complex produced during the 

 burst fluoresces so strongly that even the conversion of only 10% of total 

 chlorophyfl into this form increases the average yield of fluorescence by a 

 factor of three or four; or to postulate that the burst is caused by the for- 

 mation of a fluorescence "protector," which prevents both chemical 

 quenching and physical dissipation of energy (i. e., acts like a narcotic), and 

 that the amount of the protector activated by one quantum is sufficient to 

 protect the fluorescence of ten chlorophyll molecules. 



The first fluorescence wave may be accompanied by an almost complete 

 inhibition of both oxygen liberation and carbon dioxide consumption. 

 Here again, the question arises : How can a transformation in which only a 

 few per cent of the total number of chlorophyfl molecules can be actively 

 involved, cause complete inhibition of the photosynthetic apparatus? 

 Franck, French and Puck suggested that the inhibitor, activated in the 

 first moment of illumination, acts in two ways: by settling down and 

 "narcotizing" a part or all of the chlorophyll molecules (and thus promot- 

 ing their fluorescence), and by associating with certain catalyst molecules, 

 and thus inhibiting photosynthesis. Since, according to the above esti- 

 mate, the concentration of this catalyst must be at least one order of mag- 

 nitude lower than that of chlorophyfl, they suggested the "stabilizing" 

 catalyst, Eb (for which a concentration of only 0.1% of that of chlorophyll 

 was deduced from flashing light experiments; cf. chapter 34). 



Fluorescence bursts occur not only upon transition from darkness to 



