20 



B. Rumberg, P. Schmidt-Mende , J. Weikard and H.T. Witt 



centres. One part - hv-j- - is channeled to Chl-a-j- (as far red 

 light), a second part - hVjy - is obviously channeled to a se- 

 cond reaction centre w;here it provides electrons from a natural 

 electron-donor for the reduction of Chl-a-j- • 



hv-r hv-p-r 



Z < — Chl-aj < ...Ai 9 



With hVj-light alone (far red light 7oo - 72o mu) Chl-a-j- should 

 accumulate in its oxidized form. Measuring absorption changes at 

 7o3 mu the measuring beam should already oxidize Chl-a-j-. The mag^ 

 nitude of this oxidation must depend on the intensity of the 

 measuring beam (fig.5)« 



a) In soft measuring light (7o5 mu) practically no Chl-aj - 

 oxidation takes place. A supplementary red flash (-s: 7oo mu) at 

 t^ with hv-p- and hv-p-^-light has the following effect: The hv^- 

 llght oxidizes Chl-ai immediately. The hv-j-j- light provides 

 electrons from a second reaction centre for the reduction of 

 Chl-aj. The reduction takes place in»-^1o"^ sec (fig. 5a). 



b) In medium measuring light (7o5 mu) started at t^ Chl-a-j- 

 accumulates partially in the oxidized form (fig. 5b). A supple- 

 mentary red flash ( ^ 7oo mu) at t, oxidizes the rest of Chl-a-j- 

 immediately. Afterwards all Chl-ai is again reduced in ^ — ^1o~^ 

 sec (fig. 5b and 5b'). Therefore, -fche changes pass the zero line. 



c) In stronger measuring light (7o5 mu) started at t^ nearly 

 all Chl-ay accumulates in the oxidized form (fig. 5c). Therefore 

 in stronger measuring light practically only positive absorption 

 changes take place. These effects are the basic for the follow- 

 ing chapter. 



7. The electron-acceptor Z of Chl-a j 



Changing from soft to stronger measuring light we should aspect 

 - according to the scheme in fig. 5 - a shift from negative ab- 

 sorption changes to positive ones as indicated by fig.5a,b,c. 

 Such a shift has been observed in chlorella (fig. 5a and 5b). 

 However, in chloroplasts without any Hill-oxidants in stronger 

 measuring light no shift to positive absorption changes takes 

 place (fig. 6 top). This indicates that no accumulation of Chl- 

 a+ has occurred. The reason may be that Chl-a+ can be reduced 

 b^ a backflow of elacirons from the electron acceptor Z: 



Z < Chl-aj < ? 



If this is true, trapping of electrons of Z should prevent the 

 backflow and allow an accumulation of Chl-a^ : 



hvj tivjj 

 ox.S-j- < Z < Ghl-a-j- < ■? 



Indeed with the addition of a number of oxidized substances 

 (ox.S-p) positive absorption changes occur which are characteri- 

 stic for accumulation of Chl-a^ (fig. 6 bottom). 



This effect gives two important informations. 



