92 



W. L. Butler and N. I. Bishop 



transients following the onset of illumination. In the sympli- 

 fied electron transport chain proposed by Duysens and Sweers^ ^> 



H20 -> (system 2) -^ Q -> cyt -^ P -> (system l) -^ PN 

 Q will quench the fluorescence of the chlorophyll a in system 2 

 but the reduced state, QH, will not quench. Light absorbed by 

 system 2 will reduce Q to QH, thus increasing fluorescence while 

 light absorbed by system 1 will oxidize QH to Q, thereby quench- 

 ing the fluorescence. 



P is a small amount of a chlorophyll absorbing near 700 nm. 

 Kok (^^ showed that this component, which he called P 700, is 

 oxidized and thereby bleached by light absorbed by system ^/ai?d 

 that it is reformed by light absorbed by system 2. Butler ^^ ^ 

 studied this component (denoted G-705 in his work) by low-temper- 

 ature absorption and fluorescence excitation spectroscopy and 

 showed that energy absorbed by a large bulk of chlorophyll a is 

 transferred by inductive resonance to P. Presumably, P is the 

 energy trap for system 1. Duysens and Sweers ^ ^ have also pro- 

 posed that Q is the energy trap for system 2. The chlorophyll 

 transfers excitation energy to Q but not to QH. So far, there 

 is no direct spectroscopic evidence for this energy trap. In the 

 scheme proposed by Duysens, system 1 and system 2 should be re- 

 moved from the path of electron flow. They drive the electron 

 transport chain by energy transfer to P and Q. 



Butler (3) attempted to account for the fluorescence yield 

 changes of chlorophyll a on the basis of energy transfer from 

 chlorophyll a to P but not to Pqx- This explanation, however, 

 did not account for the action of far-red light in decreasing 

 the fluorescence yield. Duysens' scheme, in which Q is the 

 quencher of system-2 chlorophyll, accounts satisfactorily for 

 the experimental observations on fluorescence yield changes and 

 relates these to changes in the electron flow along the electron 

 transport chain. The fluorescence yield measurements thus be- 

 come a convenient assay to determine if the electron flow is 

 functioning. Such measurements will be reported in the present 

 paper on mutants of Scendesmus which have specific blocks in the 

 photosynthetic electron transport chain. 



METHODS AND MATERIALS 



The instrument, previously (3) used to measure the relative 

 fluorescence yield immediately following a brief actinic irradi- 

 ation, was modified so that the fluorescence excited at low in- 

 tensity could be measured during the actinic irradiation. The 



