Studies on Electron-Transport Reactions of Photosynthesis in Plastome Mutants of Oenothera

Fluorescence characteristics and light-induced absorbance changes of 5 plastome mutants of Oenothera, all having a defect in photosynthesis, were investigated to localize the site of the block in their photosynthetic mechanism and to relate mutational changes in the plastome to specific biochemical events in photosynthesis. In 4 of the mutants examined photosystem 2 was largely, or completely, nonfunctional. Excitation of system 2 did not cause reduction of oxidized cytochrome f in these mutants. The system-2 dependent absorbance change at 518 mmu seen in normal leaves was absent in the mutants. Moreover, the mutants had a high initial fluorescence in the presence and in the absence of 3- (3,4-dichlorophenyl)-1,1-dimethylurea, which did not change during illumination, indicating that the reaction centers of system 2 were affected by the mutations. Photosystem 1 functioned normally.A fifth mutant had an impaired photosystem 1. Even high intensity far-red light did not lead to an accumulation of oxidized cytochrome f as was seen in normal plants. Photosystem 2 was functioning, as evidenced by the fast reduction of the primary system-2 oxidant, and by the characteristics of the 518-mmu absorbance change.Because 1 of the 2 photosystems is functional in all mutants, and because they all have the enzymes of the photosynthetic carbon cycle, it appears that the effect of the mutation is specific. The results suggest that the plastome controls reactions within the electron-transport chain of photosynthesis.