It has been indicated that cyclic electron flow (CEF) around photosystem I (PSI) plays an important role in photosynthesis and photoprotection in plants, especially under environmental stresses. Generally, environmental stresses inhibit the Calvin cycle and subsequently increase the risk of photoinhibition of PSI and photosystem II (PSII). Mutation of genes encoding proteins involved in CEF increased PSII (or/and PSI) photinhibition on condition of excess light energy. In plants, CEF generates proton gradient across thylakoid membrane (pH) controls electron transfer from PSII to PSI and thus protect PSI through increasing the P700 oxidation ratio and preventing over-reduction of P700 reaction centers. However, PSI is sensitive to some specific environmental stresses, such as chilling-light stress, prolonged drought. Interestingly, PSI is susceptible to high light in some shade-established plants although they had significantly CEF activity. Thus, the role of CEF in photoprotection for PSI under environmental stresses needs urgently further studies. CEF alleviates PSII photoinhibition through two different mechanisms: one is linked to activation of non-photochemical quenching (NPQ) and prevents the inhibition of repair of photodamaged PSII, the other one is independent of NPQ and suppresses photodamage to PSII. The former mechanism has been documented clearly more or less, but the latter mechanism is far from complete. Because the two-step scheme have documented that photodamage is attributable to light absorbed directly by manganese in the oxygen-evolving complex. However, it is unclear whether CEF-dependent generation of pH protect oxygen-evolving complex against photodamage. At present, whether the CEF-dependent generation of pH provides supplemental ATP synthesis remains controversy. Studies with mutants indicated that the CEF-dependent generation of pH provided supplemental ATP synthesis under low light in rice and Arabidopsis thaliana. Other studies indicated that the role of CEF in ATP synthesis under high light is not significant in rice and Arabidopsis thaliana, indicated by the similar rate of photosynthesis in WT and CEF mutants. However, according to C3 photosynthesis model, CEF responded to ATP/NADPH demand under high, but not low light. Therefore, an important project is to illuminate how CEF balances ATP/NADPH that responds to environmental conditions. The aim of this Research Topic is to present and compare recent results on our understanding of regulation of CEF under environmental stresses, and determine the specific role of CEF in plant growth, photoprotection and acclimation to environment conditions.

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