95 



W. L. Butler and N. I. Bishop 



effectively. The action spectra of Fig. 1 are strickingly simi- 

 lar to the enhancement spectrum for chlorella shown by Myers and 

 Graham CU). 



The action spectra show that the wavelengths absorbed prefer- 

 entially by system 2 are the most effective in increasing the 

 fluorescence yield. Green plants do not show a clear-cut re- 

 sponse to system-1 absorption because of the spectral overlap be- 

 tween systems 1 and 2. Beyond 680 nm, however, the absorption 

 spectrum of system 2 falls markedly and the direct absorption by 

 P becomes more important so that Q can be driven largely to the 

 oxidized state by far-red light. It was previously shown that 

 the most effective wavelengths for decreasing the fluorescence 

 yield are those absorbed directly by P \^J. Far-red light does 

 not depress the fluorescence yield of the leaf in air much below 

 the value for a dark leaf. Thus, Q is largely oxidized in the 

 dark in air. 



The fluorescence yield which obtains when the Q is largely re- 

 duced can be observed by adding DCMU. This herbicide apparently 

 blocks the electron transport chain between Q and cytochrome so 

 that P cannot oxidize QH to). The addition of DCMU causes the_ 

 fluorescence yield to increase approximately 3-fold. This maxi- 

 mal fluorescence yield also obtains momentarily during the tran- 

 sient fluorescence burst which occurs when a darkened leaf is 

 first placed in bright light. The effect of DCMU on spinach 

 chloroplasts is shown in Fig. 2. The chloroplast preparation 

 without DCMU shows a typical two-pigment response. Light at 

 650 nm increases the fluorescence yield somewhat more than 600-nm 

 light and much more than 690-nm light. After the addition of 

 DCMU (10-^M) the fluorescence yield increased slowly under the 

 influence of the weak, meas-uring light and abruptly with the 

 650-nm actinic beam. The fluorescence yield drops somewhat when 

 the actinic light is turned off, indicating that QH can be oxi- 

 dized to some extent either by a back reaction or by dark metab- 

 olism. The light-induced fluorescence yield in the DCMU-treated 

 chloroplasts is independent of wavelength, showing that system 1 

 or P have no influence on Q. The small drop at 690 nm is an 

 artifact due to the small amount of the 690-nm actinic light 

 which leaks through the 710-nm cut-off filter. In the case of 

 the chloroplasts without DCMU, the marked depression of the fluo- 

 rescence yield at 690 nm is due largely to the oxidation of QH 

 by system 1 and P. 



The light-induced fluorescence yield changes have been used 

 to determine if both pigment systems were operating in mutants 



