Abstract
The water-water cycle in chloroplasts is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electron derived from water in PSII, and its prototype was acquired already by cyanobactertia. The primary function of W-W cycle is immediate scavenging of reduced species of oxygen prior to their interaction with the stromal enzymes, inhibiting CO2 fixation. The scavenging complex of ROS on PSI complex is designed well to satisfy above requirements. As far as W-W cycle operates without leak of ROS, it functions as alternative route of the linear electron transport, allowing fine adjustment of the photoproduction ratio of ATP/NADPH and dissipation of excess electrons or photons. The requirement of oxygen to the start of CO2 fixation in maize, and to the suppression of photoinhibition in Symbiodinium in coral, suggests the participation of W-W cycle in photosynthetic processes other than rapid scavenging of ROS.
