INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1067 



100%) on the yield of steady fluorescence of leaves and algae. McAlister 

 and Mj^ers (1940) found that an increase in oxygen concentration from 0.5 

 to 20% resulted in a marked increase in fluorescence (c/. fig. 28.51) — an 

 effect opposite to quenching, and probably associated with the inhibiting 

 influence of oxygen on photosynthesis, described in Volume I (chapter 13). 



Shiau and Franck (1947) noted that, at low light intensities, fluorescence 

 of green algae was stronger in nitrogen than in air, but that the increase 

 of (p with increasing light intensity began earlier in air. In some cases the 

 two curves even crossed each other, so that in strong light the aerated sus- 

 pension fluoresced stronger than the nonaerated one. 



We have spoken in this chapter only of changes in chlorophyll fluores- 

 cence caused by internal chemical transformations associated with photo- 

 synthesis — a relation that reveals itself indirectly, by comparison of the 

 influences of light intensity, temperature and poisons on the yields of 

 photosynthesis and fluorescence. In vitro, quenching of chlorophyll fluores- 

 cence can be produced directly, by the addition of certain substances under- 

 going autoxidation, as well as of many oxidants, including free oxygen 

 (chapter 23, section A6). No observations have been made on the 

 quenching of chlorophyll fluorescence m vivo by amines (or other possible 

 substrates of sensitized photoxidation), while the effect of oxygen was 

 described above as complex and probably mostly indirect. 



Shiau and Franck (1947) found that quinone depresses fluorescence in 

 Chlorella, if added in the dark or in light after long anaerobic incubation. 



3. Interpretation of Light Curves of Fluorescence 



In absence of positive information to the contrary, we have assumed, 

 throughout the preceding sections, that all the observed changes of fluores- 

 cence were increases and decreases in the fluorescence yield, tp, without 

 significant shifts in the position of the fluorescence bands. This point 

 could, however, profit by exact investigation. Changes in the structure 

 of the chlorophyll complex (e. g., conversion of X- Chi -HZ to HX-Chl-Z, 

 not to speak of reversible hydrogenation, Chi ^ rChl) could well find ex- 

 pression in the variation of the position and shape of the fluorescence bands ; 

 and the selective spectral sensitivity of the photometric devices used could 

 convert these changes into apparent variations in the intensity of fluores- 

 cence. While it is extremely unlikely that such spectral effects were re- 

 sponsible for all or even a large fraction of the described intensity changes, 

 it might be unwise to ignore their possibility. 



True changes in the intensity of fluorescence can be caused by two fac- 

 tors: alterations in the relative probabilities of fluorescence and energy 

 dissipation in the light-absorbing complex, and changes in the probability 



