Proton motive force partitioning links energy and redox balance to photoprotection and carbon gain

Under high light conditions, rapid water oxidation at PSII and a full Q-cycle at cytochrome b6f deposit protons into the lumen so that pmf is carried predominantly as ΔpH around the pH 5.5 set point. Anion influx through the thylakoid bestrophin-like channel VCCN1 relieves Δψ and thereby biases the partition toward ΔpH; plants lacking VCCN1 accumulate a larger electrical term and are more vulnerable during rapid light increases, consistent with the logic of the operating-regime map (Sacksteder et al., 2000; Li et al., 2002; Cramer et al., 2011). Under these conditions, ΔpH protonates PsbS within seconds and activates VDE near its acidic optimum, while ΔpH-gated “photosynthetic control” at b6f restricts plastoquinol oxidation until carbon sinks are fully engaged, protecting PSI during the ramp (Degen and Johnson, 2024).

At low and moderate light, lumen pH stabilizes nearer pH 6.0, the chemical term is smaller, and Δψ contributes a larger fraction of the pmf unless it is actively dissipated. KEA3 becomes important for shaping this regime: by exchanging stromal K+ for luminal H+, KEA3 hastens ΔpH relaxation after down-steps, limits needless carry-over of qE, and improves PSII quantum efficiency in fluctuating light; genetic and overexpression analyses show that tuning KEA3 levels or regulation speeds recovery from high to low light without sacrificing the capacity to protect during the next up-step (Armbruster et al., 2016; Wang and Shikanai, 2019). Because stromal pH remains near pH 8 in this regime, Calvin-cycle enzymes stay in a light-activated state, but the weaker acidification means that effective Δψ relief is essential to prevent PSI over-reduction when light suddenly increases (Michelet et al., 2013).

During light-to-dark transitions, both compartments relax toward pH 7.4, the ΔpH term collapses, and Δψ decays, extinguishing the driving force for ATP synthesis and allowing NPQ to dissipate. Inner-envelope K+/H+ antiporters KEA1 and KEA2 are now fully engaged to neutralize the stroma, complementing thylakoid-level H+ efflux and explaining the rapid loss of the electrochromic signal at darkening; functional analyses show that KEA1/KEA2 activity is dispensable for maintaining alkaline stroma in the light but is critical for fast down-regulation of stromal pH in the dark, in line with the behavior depicted in the schematic (Correa Galvis et al., 2020; Dukic et al., 2022; Juhaszova et al., 2022).

Partitioning matters because the two components of pmf have distinct physiological consequences. A larger ΔpH fraction promptly engages qE and b6f control and is therefore inherently protective during up-steps, whereas an oversized or persistent Δψ-dominant pmf is problematic because it fails to generate the ΔpH signal needed for protective feedbacks (Tikhonov, 2018; Wilson et al., 2021). Experimental manipulations that elevate the electrical term at the expense of ΔpH increase PSII damage and depress productivity under fluctuating light, underscoring why leaves are wired to counter-ion flux limits Δψ build-up, thereby allowing proton pumping to manifest primarily as ΔpH during illumination and to release ΔpH quickly when light falls (Cruz et al., 2001).

Electrochromic-shift spectroscopy remains the standard for partitioning pmf and estimating ATP-synthase proton conductance in vivo, but quantitative use requires spectral controls and reporting conventions so that Δψ estimates are not confounded by overlapping pigment signals; coupling ECS with fluorescence-based NPQ kinetics and P700 redox traces allows the ΔpH-dependent switches and PSI safety to be operated alongside the partition itself (Johnson, 2011; Brestic et al., 2015; Tikkanen et al., 2015; Kono et al., 2017). Finally, structural work on the chloroplast ATP synthase provides a mechanistic basis for how proton efflux is gated during transients and explains why adjustments in gH+ markedly reshape ΔpH formation and decay across the three regimes (Johnson et al., 2014; Davis et al., 2017; Wilson et al., 2021).

The proton-circuit framework used throughout this review is intentionally modular: proton deposition by LEF/CEF builds pmf, ATP synthase spends pmf as ATP with an effective proton conductivity g(H+), and counter-ion pathways reshape how much of the instantaneous pmf is expressed as ΔpH versus Δψ. While this conceptual circuit is useful for interpreting light-transient phenotypes, many of its key internal states (Δψ, lumen pH, and individual ion fluxes) are only partially observable in vivo and often must be inferred from composite signals such as the electrochromic shift (ECS), chlorophyll fluorescence, and P700 redox kinetics. Mechanistic ordinary differential equation (ODE) models provide a compact way to formalize these couplings, test whether a given mechanistic interpretation is self-consistent, and quantify how changes in specific conductances (e.g., ATP synthase or ion pathways) are expected to reshape the timing of ΔpH formation and relaxation under defined light regimes. Recent ODE-based studies—particularly the series by Lyu et al. explicitly simulate Δψ, ΔpH, and pmf during light transitions while incorporating key physical features such as thylakoid ion currents and membrane-surface ion binding, offering an important systems-level complement to experimental approaches (Lyu and Lazár, 2017, 2022; Lyu and Lazár, 2023; Lyu and Lazár, 2024a, b).

Most ODE formulations relevant to this review share a common “state-flux” structure. The internal state can be represented by a small set of time-dependent variables, including lumen proton activity (often expressed as lumen pH), trans-thylakoid electric potential (Δψ), and total pmf (Δp = Δψ + (2.303RT/F)ΔpH), together with optional redox states (e.g., PQ pool, PSI acceptor side) depending on model scope. These states evolve as the net result of fluxes that correspond directly to the circuit elements discussed in Sections 2–7:

Proton deposition (source term): a light-dependent proton influx to the lumen driven by LEF and augmented by CEF (with pathway-specific contributions depending on whether PGR5/PGRL1 and/or NDH are represented).

Proton efflux (valve term): proton return through ATP synthase, typically parameterized as a conductance or kinetic term that is closely related to experimentally estimated g(H+).

Charge-compensation and partitioning (shaper terms): ionic currents (anion influx and/or cation efflux) and exchange processes (e.g., K+/H+ antiport) that dissipate Δψ and thereby control how efficiently continued proton pumping is converted into ΔpH.

In a minimal dynamical representation, lumen acidification can be expressed conceptually as:

and Δψ can be treated as an electrical state set by a membrane capacitance and net ionic currents:

where JH+pump aggregates LEF/CEF-linked proton deposition, JH+ATPase reflects ATP synthase proton efflux (linked to g(H+) and substrate availability), Inet captures the balance of proton-generated charge separation and counter-ion currents (e.g., Cl- influx through VCCN1/CLCe-like terms and cation fluxes), βlumen is an effective lumen buffering capacity, and Cm is an effective thylakoid membrane capacitance. Although different models implement these terms at different levels of detail, the central advantage is that the framework forces explicit accounting of how fluxes and constraints jointly determine ΔpH:Δψ partitioning over time (Lyu and Lazár, 2017, 2022).

A primary contribution of mechanistic ODE models is that they separate three concepts that are easily conflated in narrative descriptions: (i) total pmf magnitude, (ii) pmf parsing into ΔpH versus Δψ, and (iii) the kinetics with which parsing changes during transients. By explicitly representing the competing timescales of proton pumping, ATP synthase discharge, and counter-ion flux, models can reproduce situations where pmf rises quickly but lumen acidification is delayed (Δψ-heavy transient) or where ΔpH persists because ATP synthase throughput is limited by kinetic regulation and/or substrate supply (Lyu and Lazár, 2017, 2023).

A second contribution is that several recent ODE formulations incorporate electrostatic features that are difficult to isolate experimentally, including membrane-surface ion binding and Donnan/diffusion components of electric potential. These features provide a mechanistic route for Δψ behavior that is not simply a “short-lived spike,” and they offer quantitative hypotheses for when electrical contributions could persist or become functionally consequential despite substantial ΔpH expression (Lyu and Lazár, 2022).

Finally, recent work has emphasized sensitivity analysis as a practical bridge between modeling and experiment: by ranking which parameters most strongly influence predicted Δψ/ΔpH dynamics or observable signatures (e.g., ECS or fluorescence kinetics), models can guide which measurements are most informative and which engineering interventions are most likely to shift leaf performance in targeted regimes (Lyu and Lazár, 2024a, b).

To keep this section focused, Table 4 highlights representative ODE-based studies that explicitly simulate Δψ, ΔpH, and pmf dynamics while exploring the influences of ion fluxes, ATP synthase activity, and membrane electrostatics. These studies are not intended as an exhaustive modeling review; rather, they exemplify how formal dynamical models can be used to (i) test the internal consistency of mechanistic interpretations of ECS/fluorescence/P700 data, (ii) quantify how changing conductances shifts ΔpH “timing,” and (iii) identify parameter regimes where ΔpH-gated control might decouple from expected photoprotective outcomes.

This review describes a proton-centric view of leaf photosynthesis in which the partitioning of the thylakoid proton motive force (pmf)—the balance between ΔpH and Δψ—links light harvesting, ATP synthesis, photoprotection, and downstream carbon metabolism. Proton pumps in LEF and CEF build pmf; ATP synthase and thylakoid ion pathways (KEA3, VCCN1, CLCe) shape how much pmf is expressed as lumen acidification and how rapidly that acidification forms and relaxes; and pH-sensitive effectors translate lumen acidification into rapid qE and ΔpH-gated control at cytochrome b6f, safeguarding PSI during transients. Stromal alkalinization, Mg²+ mobilization, and thioredoxin activation contribute to “switching on” Calvin–Benson–Bassham (CBB) enzymes, while CEF and metabolite shuttles help balance ATP/NADPH. Crucially, carbon metabolism is not merely a downstream “consumer”: stromal metabolite pools—especially ADP and inorganic phosphate (Pi), and their recycling via export/TPU constraints—feed back on ATP synthase throughput and therefore on how quickly pmf is spent and how long ΔpH-dependent regulation persists.

In fluctuating light, photosynthetic performance can often be understood less as a problem of maximizing pmf or NPQ capacity and more as a problem of timing: leaves need to generate sufficient ΔpH quickly during light up-steps to protect PSI, and then relax ΔpH in a timely manner during down-steps so that quenching does not unnecessarily suppress CO2 assimilation. Within this framework, the operating-regime map—together with a common diagnostic set (multi-wavelength ECS for ΔpH/Δψ and g(H+), NPQ and P700 kinetics, and gas exchange)—provides concrete, testable hypotheses and a practical basis for making future datasets more directly comparable across studies and species.

A key implication of this synthesis is that a proton−circuit framework is most powerful when treated as a quantitative organizing principle with explicit boundary conditions, rather than a universal, single−variable explanation of leaf performance. pmf and its ΔpH:Δψ partitioning provide a mechanistic bridge between light reactions, rapid photoprotective switching, and ATP production, but the realized ΔpH kinetics and downstream outcomes emerge from coupled physical constraints and metabolic feedbacks in intact leaves. Stromal metabolite pools—especially ADP and inorganic phosphate (Pi)—can directly modulate ATP synthase throughput (g(H+)) and thus the rate at which pmf is converted into ATP or retained as lumen acidification, feeding back on both ΔpH−dependent regulation and CO2 assimilation. Accordingly, strengthening the integration of proton−centric diagnostics with Pi/adenylate status and sink activity (and, where feasible, incorporating these variables into quantitative models) will be essential for identifying when ΔpH timing is the dominant determinant of protection–productivity trade−offs and when additional metabolic or regulatory layers set the limiting behavior (Table 4).