ABSOLUTE QUANTUM YIELDS OF FLUORESCENCE 1 1 1 



correction was evaluated by using various sharp cut-off red filters 

 to separate the fluorescence from the incident light, thus obtaining 

 v^alues for fluorescence differently influenced by self-absorption. 

 Three filters of various shades of red were used. Total fluorescence 

 yields were computed by dividing the value obtained with each 

 filter, by the fraction of the total fluorescence transmitted by it (de- 

 termined from the fluorescence spectrum and the transmission curve 

 of the filter) . The three values of the total yield at zero concentration 

 obtained in this way can be in turn extrapolated to zero reabsorption 

 (by plotting them as a linear function of the slopes of the straight lines 

 fitting the three sets of experimental points). We arrive in this way at 

 a \'alue of ^ unaff'ected by all forms of self -absorption, "internal" 

 as well as "external." 



The quantum yields of fluorescence of chlorophyll in Chlorella, 

 excited with an intensity of 50 ergs/(cm.^ sec.) at X = 436 m^u are 

 plotted in Fig. 2 as a function of the concentration of the algal sus- 

 pension. Using the above-described procedure to correct for self- 

 absorption, we obtained for the limiting quantum yield of fluores- 

 cence, at this intensity of excitation, a value of 2.7% — about ten 

 times that reported by Wassink and co-workers — confirming the pre- 

 diction of Duysens. The time available for the migration of excitation 

 energy between chlorophyll molecules is, therefore, ten times as long 

 as it had been assumed to be on the basis of earlier data. 



The quantum yield of fluorescence of chlorophyll in plant cells has 

 been generall}'' assumed to be independent of the intensity of the ex- 

 citing light in w^eak light, but to increase at high, photosynthesis- 

 saturating intensities (12 j. We have found, however, that the yield 

 is also a function of the intensity of the exciting light at intensities as 

 low as 0.01 of that required for compensation of respiration by photo- 

 synthesis. Figure 3 indicates that fluorescence yield, and thus also the 

 average lifetime of excited chloroph^'ll in cells, is about 50% longer 

 at compensation than it is at very low mtensities. (Similar observa- 

 tions were reported by Brugger at this Symposiinu.) 



References 



1. Forster, L. 8., and Livingston, R., J. Chem. Phys., 20, 1315 (1952). 



2. Prins, J. A., Nature, 134, -157 (1934). 



3. Vermeulen, D., Wassink, E. C, and Reman, G. H., Enzymohgia, 4, 254 (1937). 



4. Wassink, E. C, and Kersten, J. A. H., Enzymologia, 11, 282 (1944). 



