Proton motive force in plant photosynthesis dominated by ΔpH in both low and high light

The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking similar to 515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (Delta pH) and electric potential (Delta) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (Delta A505) and energy-dependent nonphotochemical quenching (qE; Delta A535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among Delta A505, Delta A535, and ECS is diminished, such as npq4 (lacking Delta A535) and npq1npq4 (also lacking Delta A505), the parsing method implies the Delta psi contribution is virtually absent and pmf is solely composed of Delta pH. Conversely, in plants where Delta A535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where Delta A535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Delta psi to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Delta psi is caused by Delta A505 and Delta A535 overlap, confirming pmf is dominated by Delta pH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

Automated summary

The proton motive force (pmf) across the thylakoid membrane plays a crucial role in coupling photosynthetic electron transport and ATP synthesis. The use of electrochromic carotenoid and chlorophyll absorption band shift (ECS) to measure pmf in leaves has become widespread, but the interpretation of the results remains controversial. This study demonstrated that the majority of the pmf attributed to Delta psi by the ECS parsing method is actually caused by the overlap of Delta A505 and Delta A535, confirming that pmf is mainly composed of Delta pH following continuous illumination.