Inhibition site of the electron transport system in lettuce chloroplasts by fumigation of leaves with SO_2

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In chloroplasts isolated from SO_2-fumigated leaves at 2.0 ppm, electron flow from water to 2,6-dichloroindophenol (DCIP) was inhibited, but the electron flow from reduced DCIP to methyl viologen was not affected. Neither diphenylcarbazide nor MnCl_2 could restore the activity of the DCIP-Hill reaction of SO_2-injured chloroplasts. Electron flows, from water to ferricyanide or to silicomolybdic acid, were inhibited in a degree similar to that of the DCIP-Hill reaction. The rate of carotenoid photobleaching in the presence of carbonyl cyanide-m-chlorophenylhydrazone was suppressed and paralleled the inhibition of the DCIP-Hill reaction. In SO_2-injured chloroplasts, the variable part of the fluorescence transient was diminished, and the fluorescence yield lowered by SO_2 was increased with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or more pronouncedly by incubating the sample with sodium dithionite. However, the yield could not recover to the level found in non-fumigated chloroplasts. With SO_2 fumigation, the time required to reach steady-state level of fluorescence became longer in the absence of DCMU, but was not altered in the presence of DCMU. The pool size of the primary electron acceptors decreased with SO_2 fumigation. We concluded that SO_2 inactivated the primary electron donor or the reaction center itself. The mode of SO_2 action in the electron transport chain is discussed.
日本植物生理学会の論文