23 



B. fiumberg, F. Schmidt-Mende , J. Weikard and H.T. Witt 



redox potential of plastoquinone ( — Volt) is very similar to 

 that of X (--^0 Volt). This encouraged us to look for the rela- 

 tion of X and plastoquinone. 



b) Petroleum ether extraction of chloroplasts (fig. 12) re- 

 sults in a decrease of the absorption changes of X. Following 

 condensation of synthetic plastoquinone results in a complete 

 reappearance of the difference spectrum or even more (15% 



Both results indicate that X is closely related or identical 

 with plastoquinone (15) • But plastoquinone in vitro is not ac- 

 companied by absorption changes at 475 S-nd 515 niu. If X is iden- 

 tical with plastoquinone, we have to assume that in vivo the re- 

 action of plastoquinone influences the absorption of its sur- 

 rounding pigments, resulting in changes at 475 and 515 niu. 



4. The difference spectrum of plastoquinone 



Absorption changes which are directly related to the reduction 

 of pure plastoquinone in vitro show changes with a maximum at 

 254 mu (15). Corresponding changes should occur in vivo. V/e have 

 observed light induced changes in active chloroplasts (17) which 

 correspond to those in vitro. 



5. Separation of the difference spectrum of plastoquinone 



We reported that with addition of DPIP (ferricyanide in excess), 

 the difference spectrum of Chl-a-j- vanishes while that of X is 

 furthermore in action. Because or the demonstrated relation of X 

 and plastoquinone we supposed that under these conditions also 

 the difference spectrum of plastoquinone should be still in ac- 

 tion. Measurements at 254 mu show indeed that with addition of 

 DPIP and ferricyanide the magnitude of the changes at 254 mu are 

 influenced not at all while those of Chl-aj at 7o5 mu are com- 

 pletely quenched. 



6. Relation between absorption changes of X and plastoquinone 



The absorption changes of plastoquinone (254 mu) and X (475 and 

 515 mu) were measured in chlorella and chloroplasts (with DPIP + 

 Ferricyanide) in relation to light intensities, duration of the 

 flash, temperature and pH. We added also different concentratiom 

 of CMU and changed the content of plastoquinone by different ex- 

 tractions. Under all these conditions the magnitude as well as 

 the time course at 254 mu and 515 niu correspond to each other. 

 In fig. 15 the changes at 515 and 254 mu are compared as function 

 of extraction and recondensation of plastoquinone. Fig. 14 shows 

 the decrease of both changes with increasing extractions of pla- 

 stoquinone. The changes of Chl-aj (upper curve) at 7o5 mu are 

 influenced not at all. This indicates in a different way (com- 

 pare 5) that plastoquinone seems not to be involved in the re- 

 action cycle of Chl-aj. 



7 . X and plastoquinone respectively as electron acceptor 

 of water oxidation 



That X is the electron acceptor of lastly water in reaction cyc- 

 le II, we gave in the last years several evidences. A further 



