97 



W. L. Butler and N. I. Bishop 



wild-type cells. The fluorescence yield is increased further in 

 the presence of actinic light, but all wavelengths are equally 

 effective. The same results were obtained by adding DCMU to the 

 chloroplasts in Fig. 2 and the same results would have been ob- 

 tained by adding DCMU to the wild-type Scenedesmus cells. The 

 fluorescence measurements indicate that system 2 reduces Q in 

 the light but that the electron transport chain is blocked some- 

 place between P and Q because system 1 and P cannot oxidize QH. 

 The block in the electron transport chain prevents the reduction 

 of TPN which is needed for CO2 fixation but does not prevent 

 system-2 activated transport of electrons from H2O through Q to 

 benzoquinone. 



RELATIVE "^ 

 FLUORESCENCE 

 INTENSITY 



3 







_ o o o 2 <«- 



O U) U3 «5 <D O 



o o o 2 "^ 

 in o If) oj *r 



10 <0 tD yJ ° 



O O O Os- 



2: lo o to o)^ 



O (O tf> tf> ^ o 



wild type 

 (3Xsens.) 



CO2 mutont 



Oj mutant 



min- 



TIME 



Fig. 3. Relative fluorescence intensity (same measurement 

 as Fig. 2) from Scenedesmus wild-type, "CO2" mu- 

 tant No. 8 and "O2" mutant No. 11 cells. Sensi- 

 tivity of fluorescence measurement was increased 

 3-fold for wild-type cells so that the fluores- 

 cence yield of the wild-type cells is approximate- 

 ly 1/3 that of the mutant. 



