BESSEL KOK AND GEORGE HOCH 



411 



We finally sliouki draw attention to the remarkable phenomenon 



that in chloroplasts and broken Andcystis cells PMS j)roved to be an 



inhibitor ol the positive shilt (as much as it stimulated the negative 



change) and that conversely lerricyanide and some other Hill reagents 



(inhibitors of the negative change) act as stimulators. 



Sensitization of Photophosphorvlation in the Presence of PMS 



The striking effect that PMS exerts on the absorption shifts and 

 the peculiar type of DCMU - insensitive "closed" photophosphoryla- 

 tion which is catalyzed by PMS (11, 10), made us wonder what type 

 of sensitization is involved in the latter process. 



Fig. 10 shows two rate (inorganic phosphate esterified per unit 

 time) vs. intensity curves measured with totally absorbing chloro- 

 plast suspensions. Two monochromatic lightbeams of different wave- 

 lengths were used: one at 670 m/x, where chlorophyll b and chloro- 

 phyll a both absorb, one at 710 m^, where almost all incident light is 

 absorbed by chlorophyll a. 



These data show a better rate at the medium and high intensities. 



lOO 



% Absorption ^ 

 Change 



-100 



10 



10 



10 " 10 " 10 



Concentration of DCMU 



Fig. 9. Effect of dichlorophenyl dimetliylurea (DCiMU) upon the 700 mfi absorp- 

 tion changes in Anorystis. Dashed arrow in lower corner indicates that in 10' M 

 DCMU the (now negative) background change induced with 600 m/i light equals 

 the one induced by 700 m/x light. 



