Fluorescence Yield of Chlorophyll in Chlorella as a 

 Function of Light Intensity* 



JOHN E. BRUGGER, University of Chicago (Pels Fund), Chicago, Illinois 



It is known that Franck's explanation of the so-called Kok effect 

 has been based on the assumption that photochemical reduction of 

 intermediates of respiration is involved. The question arises whether 

 there might be a corresponding effect on the fluorescence. In examin- 

 ing the published curves of McAlister, we found that it is un- 

 certain whether the data for fluorescence intensity versus irradia- 

 tion extrapolate linearly to the origin. It was decided to investigate 

 this point further. We have found that the fluorescence yield is in- 

 deed lower in the intensity range below compensation of respiration 

 than in the region between compensation and saturation, but no 

 attempt has been made to find out whether the transition of the 

 yield occurs exactly at compensation. This effect varies in its strength 

 with external conditions, as does the Kok effect. However, unlike the 

 somewhat erratic appearance of the latter, the break of the fluores- 

 cence yield in the neighborhood of compensation was visible in all 

 our curves so long as photosynthetic activity was not suppressed. 



An incandescent lamp with blue filters was the source of irradia- 

 tion. Light intensity was varied by inserting neutral density filters in 

 the beam. Fluorescence was measured with a 1P22 multiplier photo- 

 tube equipped with an appropriate red filter. The photosignal was 

 amplified and recorded. The light-detecting system responded 

 linearly with signal input. To locate compensation and saturation 

 intensities on the illumination scale, a modified Warburg manometer 

 was used with which chlorophyll fluorescence as well as oxygen evolu- 

 tion could be followed simultaneously. In most measurements, the 

 manometer was replaced by a sample holder having the same 

 geometry but fitted with a sintered glass bottom so that various gas 

 mixtures could be passed through the suspension. One-milliliter 

 samples of -^0.02% Chlorella were used. These had a transmission 

 (1-cm. cuvette) of 55% at 4400 A and 85% at 7800 A. The algae 



* This work was partially sponsored by the Office of Naval Research. 



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