PH DEPENDENT CHANGES IN REACTIVITY OF PRIMARY ELECTRON-ACCEPTOR OF SYSTEM-II IN SPINACH-CHLOROPLASTS TO EXTERNAL OXIDANT AND REDUCTANT

Changes in the rates of dark oxidation and reduction of the primary electron acceptor of System II by added oxidant and reductant were investigated by measuring the induction of chlorophyll fluorescence under moderate actinic light in 3-(3'',4''-dichlorophenyl)-1,1-dimethylurea-inhibited chloroplasts at pH values between 3.6 and 9.5. The rate of dark oxidation of photoreduced primary acceptor was very slow at all the pH values tested without added electron acceptor. The rate was accelerated by the addition of ferricyanide in the whole pH range. It was dependent approximately on the 0.8th power of the ferricyanide concentration. The rate constant for the oxidation of the primary acceptor by ferricyanide was pH-dependent and became high at low pH. The value at pH 3.6 was more than 100 times that at pH 7.8. The pH-dependent change in the rate constant was almost reversible when the chloroplasts were suspended at the original pH after a large pH change (acid treatment). An addition of carbonylcyanide m-chlorophenylhydrazone or heavy metal chelators had little effect on the rate of dark oxidation of the primary acceptor by ferricyanide. The dark reduction of the primary acceptor by sodium dithionite also became faster at low pH. Apparently at low pH the primary acceptor of System II becomes accessible to the added hydrophilic reagents even in the presence of 3-(3'',4''-dichlorophenyl)-1,1-dimethylurea.