The Slow Phase of the Electrochromic Shift in Relation to the Q-Cycle in Thylakoids

The electrochromic signal in oxidising conditions with whole-chain electron transport from water to ferricyanide or methyl viologen was compared with that under reducing conditions with electron transport from duroquinol to methyl viologen. Under oxidising conditions, in a train of flashes at 10 Hz, the slow phase in the growth of the electrochromic signal was not apparent after a few flashes whilst under reducing conditions it persisted. The slow phase of the electrochromic signal was also examined particularly under conditions favouring the operation of an apparent Q-cycle, namely oxidising conditions plus the addition of low concentrations of valinomycin and flashes given at 5-10 Hz. The slow phase was retained in the presence of valinomycin at flash rates up to 10 Hz and its decay was accelerated. The half-time for the slow phase was 18-20 ms (whole-chain), or 3-5 ms (reducing conditions, duroquinol/methyl viologen). There was a lag of 3 ms before the rise of the slow phase with ferricyanide. The onset was accelerated by valinomycin under oxidising conditions. The slow phase was sharply inhibited by 2-n-heptyl- and 2-n-nonyl-4-hydroxyquinoline N-oxide (HQNO and NQNO) to at least half its maximum extent by 0.1 �M (HQNO) or 0.05 �M (NQNO); further inhibition took place in the micromolar range. The slow phase is discussed in terms of probable electrogenic events in the cytochrome b/f complexes and their kinetics. We support the hypothesis that its basic cause is the transverse, intramembrane passage of electrons and show that this passage is controlled by the prevailing intramembrane potential difference, an estimated 85-140 mV sufficing for half inhibition of the slow phase.