FIGURE 1.

Acceptor-side limitation and excess electron flow promotes CEF or in its absence leads to the irreversible damage of PSI centers. The linear electron flow coming from PSII (gray dashed arrow) is the source of electrons for the PSI reaction center (P700⁺/P700) that transfers electrons from the chlorophyll excited state (P700^*) and subsequently delivers to downstream acceptors within PSI (F_X, F_A, F_B iron-sulfur centers) then to stromal electron carriers (ferredoxin, FNR, NADP⁺) (light gray arrow). When CO₂ fixation decreases, acceptor-side limitation gradually leads to accumulation of NADPH and overreduction of stromal and PSI electron carriers (light red arrow). In this case electrons are redirected to O₂ either at the level of NADPH without the production of reactive oxygen species (ROS; gray dashed arrow) or produce the very reactive superoxide anion radical (${\text{O}}_{\text{2}}^{•-}$) at the level of F_X, F_A, F_B, Fd, and FNR with a rate exceeding the detoxification process. Thus ${\text{O}}_{\text{2}}^{•-}$ will irreversibly destroys the centers (red arrows) resulting in an inability to oxidize *P700 and on a longer time scale the degradation of the entire PSI complex. Preventing this scenario, cyclic electron flow triggers downregulation of linear electron flow at the site of PSII and cytb₆f by enhancing proton accumulation in the lumen.