Oxidation of P700 Induces Alternative Electron Flow in Photosystem I in Wheat Leaves

Oxygen (O₂)-evolving photosynthetic organisms oxidize the reaction center chlorophyll, P700, in photosystem I (PSI) to suppress the production of reactive oxygen species. The oxidation of P700 is accompanied by alternative electron flow in PSI (AEF-I), which is not required for photosynthetic linear electron flow (LEF). To characterize AEF-I, we compared the redox reactions of P700 and ferredoxin (Fd) during the induction of carbon dioxide (CO₂) assimilation in wheat leaves, using dark-interval relaxation kinetics analysis. Switching on an actinic light (1000 μmol photons m^-2 s^-1) at ambient CO₂ partial pressure of 40 Pa and ambient O₂ partial pressure of 21 kPa gradually oxidized P700 (P700⁺) and enhanced the reduction rate of P700⁺ (vP700) and oxidation rate of reduced Fd (vFd). The vFd showed a positive linear relationship with an apparent photosynthetic quantum yield of PSII (Y[II]) originating at point zero; the redox turnover of Fd is regulated by LEF via CO₂ assimilation and photorespiration. The vP700 also showed a positive linear relationship with Y(II), but the intercept was positive, not zero. That is, the electron flux in PSI included the electron flux in AEF-I in addition to that in LEF. This indicates that the oxidation of P700 induces AEF-I. We propose a possible mechanism underlying AEF-I and its physiological role in the mitigation of oxidative damage.