Abstract
In higher plants, the generation of proton gradient across the thylakoid membrane (ΔpH) through cyclic electron flow (CEF) has mainly two functions: (1) to generate ATP and balance the ATP/NADPH energy budget, and (2) to protect photosystems I and II against photoinhibition. The intensity of light under which plants are grown alters both CEF activity and the ATP/NADPH demand for primary metabolic processes. However, it is unclear how the role of CEF is affected by the level of irradiance that is applied during the growth and measurement periods. We studied the role of CEF at different light intensities in leaves from sun- and shade-grown plants. At 849 μmol photons m-2 s-1, both types of leaves had nearly the same degree of CEF activation. Modeling of the ATP/NADPH demand revealed that, at this light intensity, the contribution of CEF toward supplying ATP was much higher in the sun leaves. Meanwhile, the shade leaves showed higher levels of non-photochemical quenching and the P700 oxidation ratio. Therefore, at 849 μmol photons m-2 s-1, CEF mainly helped in the synthesis of ATP in the sun leaves, but functioned in photoprotection for the shade leaves. When the light intensity increased to 1976 μmol photons m-2 s-1, CEF activation was greatly enhanced in the sun leaves, but its contribution to supplying ATP changed slightly. These results indicate that the main role of CEF is altered flexibly in response to light intensity. In particular, CEF mainly contributes to balancing the ATP/NADPH energy budget under sub-saturating light intensities. When exposed to saturating light intensities, CEF mainly protects photosynthetic apparatus against photoinhibition.
Highlights
Light energy is harvested by photosynthetic antenna complexes
In the shade leaves, values of JO under 800 and 2000 μmol photons m−2 s−1 changed slightly (Table 2). These results suggested that, under 2000 μmol photons m−2 s−1, the sun leaves needs more ATP supply rather than linear electron flow (LEF) to maintain the high rate of photorespiration
cyclic electron flow (CEF) primarily assists in maintaining a balance in the ATP/NADPH ratio under sub-saturating light conditions but tends to mainly participate in photoprotection for photosystem I (PSI) and PSII under saturating light conditions
Summary
Light energy is harvested by photosynthetic antenna complexes It is used for the synthesis of ATP and NADPH, which are consumed by primary metabolic processes such as photosynthetic CO2 assimilation and photorespiration. If ATP were consumed faster than NADPH, the lack of NADP+ would rapidly induce the limitation of linear electron flow (LEF), decreasing proton translocation and ATP synthesis. If NADPH were to be consumed at a greater rate than ATP, Flexibility of cyclic electron flow proton translocation through ATP synthase would be reduced due to limiting ADP, This would cause an increase in the proton gradient between the thylakoid lumen and stroma ( pH), restricting plastoquinol oxidation at cytochrome b6/f and, down-regulating LEF (Kanazawa and Kramer, 2002; Tikkanen and Aro, 2014). Because pool sizes of ATP and NADPH are relatively small and fluxes through primary metabolism are large (Noctor and Foyer, 2000; Avenson et al, 2005; Cruz et al, 2005; Amthor, 2010), the stoichiometric balancing of ATP and NADPH must be regulated rapidly quickly when responding to a particular level of irradiance
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