The state 2 transition in the cyanobacterium Synechococcus 6301 can be driven by respiratory electron flow into the plastoquinone pool

State 1‐state 2 transitions in the cyanobacterium Synechococcus 6301 were observed using a modulated fluorescence measurement system. State 1 transitions could be reversed even when PS II turnover was completely inhibited by DCMU. In starved cells, in which the rate of respiratory electron flow was greatly decreased, the extent of this reversion to state 2 was reduced. This suggests that state 2 transitions can be driven by respiratory electron flow as well as by PS II turnover and provides further evidence for the intersection of photosynthetic and respiratory electron transport in cyanobacteria. We propose that excitation energy distribution in this organism is controlled by the redox level of plastoquinone or a closely associated component of the electron transport chain.