Studies on chlorophyll fluorescence in chloroplasts II. Effect of ferricyanide on the induction of fluorescence in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea

The effect of preincubating spinach chloroplasts with ferricyanide on the time courses of chlorophyll-α fluorescence in the presence of 3-(3,4-dichlorophyl)-1,1-dimethylurea (DCMU) was studied. When DCMU was absent from the preincubation mixture, but was added just before the onset of excitation light, preincubation of chloroplasts with ferricyanide markedly affected the fluorescence kinetics. The rise-rate was lowered and consequently the area above the induction curve (S/Fv), which is proportional to the pool size of the electron acceptor(s) for photosystem 2, increased. The maximum increase in the S/Fv was attained after 3 min and 10 min, respectively, of preincubation with 5×10−4M and 3×10−5M ferricyanide. When DCMU was present during preincubation with ferricyanide, the effect of ferricyanide in increasing the S/Fv, was completely eliminated. The effect of ferricyanide was also suppressed by addition of ferrocyanide to the preincubation mixture. The redox potential of the ferri-ferrocyanide mixture which produced 50% suppression of the ferricyanide effect was about 360 mV. A similar dependency of the ferricyanide effect on the redox potential was observed in Tris-treated chloroplasts. However, the redox potential of cytochrome b-559 was markedly lowered by Tris-treatment. These results were explained by assuming the occurrence of a secondary electron acceptor, R, between the reaction center of photosystem 2 and the DCMU-sensitive site.