Abstract
In higher plants, moderate photoinhibition of photosystem II (PSII) leads to a stimulation of cyclic electron flow (CEF) at low light, which is accompanied by an increase in the P700 oxidation ratio. However, the specific role of CEF stimulation at low light is not well known. Furthermore, the mechanism underlying this increase in P700 oxidation ratio at low light is unclear. To address these questions, intact leaves of the shade-adapted plant Panax notoginseng were treated at 2258 μmol photons m-2 s-1 for 30 min to induce PSII photoinhibition. Before and after this high-light treatment, PSI and PSII activity, the energy quenching in PSII, the redox state of PSI and proton motive force (pmf) at a low light of 54 μmol photons m-2 s-1 were determined at the steady state. After high-light treatment, electron flow through PSII (ETRII) significantly decreased but CEF was remarkably stimulated. The P700 oxidation ratio significantly increased but non-photochemical quenching changed negligibly. Concomitantly, the total pmf decreased significantly and the proton gradient (ΔpH) across the thylakoid membrane remained stable. Furthermore, the P700 oxidation ratio was negatively correlated with the value of ETRII. These results suggest that upon PSII photoinhibition, CEF is stimulated to increase the ATP synthesis, facilitating the rapid repair of photodamaged PSII. The increase in P700 oxidation ratio at low light cannot be explained by the change in pmf, but is primarily controlled by electron transfer from PSII.
Highlights
Light is the driving force of photosynthesis, high light can cause significant photoinhibition of photosystem II (PSII) for leaves of shade-establishing plants (Kitao et al, 2000; Barth et al, 2001; Krause et al, 2004; Huang et al, 2015b, 2016b,c)
Our specific objectives were to (1) investigate whether cyclic electron flow (CEF) stimulation at low light mainly facilitates the synthesis of ATP; and (2) determine whether the increase in P700 oxidation ratio upon moderate PSII photoinhibition is more related to electron flow through PSII (ETRII) rather than the CEF stimulation or the change in pmf
The amplitude of pmf decreased significantly and pH declined slightly. These results indicated that the stimulation of CEF did not trigger the pH-dependent downregulation of photosynthetic electron transport. This stimulation of CEF at low light mainly facilitated the synthesis of ATP
Summary
Light is the driving force of photosynthesis, high light can cause significant photoinhibition of photosystem II (PSII) for leaves of shade-establishing plants (Kitao et al, 2000; Barth et al, 2001; Krause et al, 2004; Huang et al, 2015b, 2016b,c). In addition to contributing ATP synthesis, another function of pH is the down-regulation of photosynthetic electron transport by acidifying the thylakoid lumen (Shikanai, 2014, 2016) This regulation involves two different mechanisms: one is linked to thermal energy dissipation and dissipates excess absorbed light energy as heat from PSII antennae (Takahashi et al, 2009), and the other one is downregulation of Cyt b6/f complex activity and controls the rate of electron transfer to PSI (Suorsa et al, 2012, 2016; Tikkanen and Aro, 2014). Our specific objectives were to (1) investigate whether CEF stimulation at low light mainly facilitates the synthesis of ATP; and (2) determine whether the increase in P700 oxidation ratio upon moderate PSII photoinhibition is more related to ETRII rather than the CEF stimulation or the change in pmf. Before and after high-light treatment, the energy distribution in PSII, the redox state of PSI and proton motive force (pmf ) at a low light of 54 μmol photons m−2 s−1 were determined
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