Coupling Mechanisms Between Terminal Reducing-Power Allocation in Photosynthetic Electron Transport and Metabolic Demand

After photosystem I, the photosynthetic electron transport chain does not simply enter a single “NADPH generation endpoint,” but instead enters a terminal reducing-power allocation layer centered on ferredoxin. This layer simultaneously connects FNR-NADP+ output, Fd-FTR-Trx regulation, nitrogen and sulfur assimilation, cyclic electron flow, alternative electron sinks, and the malate valve. The final distribution pattern is determined by the ATP/NADPH demand ratio, the load of carbon, nitrogen, and sulfur assimilation, the timing of regulatory activation, and the state of environmental stress.

Keywords: photosynthetic electron transport; terminal reducing power; ferredoxin; FNR; cyclic electron flow; nitrogen assimilation; sulfur assimilation; thioredoxin

1. Research Objects and Analytical Boundaries of Terminal Reducing-Power Allocation

1.1 Why the PSI acceptor side constitutes an independent research layer

(1) The hub property of ferredoxin

In chloroplasts, once PSI transfers electrons to ferredoxin(Fd), electrons leave the membrane-embedded main chain and enter a soluble branching stage. At this point, Fd is no longer merely an intermediate carrier of linear electron transport, but becomes a hub node shared by multiple terminal acceptors and regulatory modules. FNR can direct Fd electrons toward NADP+ reduction, the FTR/Trx system can convert them into regulatory signals, nitrite reductase and sulfite reductase can directly consume Fd electrons to complete reductive assimilation, and a portion of electrons can also enter cyclic electron flow and other alternative buffering pathways. Therefore, the core issue on the PSI acceptor side is not whether electrons leave PSI, but which metabolic outlet they preferentially enter after leaving PSI.

(2) The terminal layer is not a subsidiary component of general light-reaction descriptions

If the endpoint of the light reactions is simply summarized as “generation of NADPH and ATP,” many actual phenomena cannot be explained, such as why the PSI acceptor side becomes over-reduced under high light, why cyclic electron flow rises when carbon assimilation is restricted, why changes in nitrogen and sulfur nutritional status rewrite electron partitioning, and why the activation timing of regulatory enzymes affects acceptor-side pressure. Accordingly, the PSI acceptor side should be treated as an independent analytical layer, because it is the interface that truly converts photochemical electron output into organized metabolic outcomes.

1.2 Why terminal allocation must be understood within a metabolic-demand framework

(1) The ATP/NADPH ratio is not intrinsically matched

Linear electron flow simultaneously provides NADPH and ATP supported by the transmembrane proton gradient, but the ATP/NADPH requirements of the Calvin cycle, photorespiration, nitrogen assimilation, sulfur assimilation, and intercompartmental metabolic exchange are not constant. If the system requires ATP rather than additional NADPH, then continuing to force electrons toward the FNR direction will generate relative NADPH excess and elevate reductive pressure on the acceptor side. In other words, terminal allocation is not a process in which reducing power is first generated and then passively consumed by metabolism; rather, the output form must be continuously corrected according to the structure of downstream demand.

(2) Metabolic demand is hierarchical and time-dependent

At the dark-to-light transition, regulatory activation demand often precedes high-flux assimilation demand. During steady-state assimilation, the FNR-NADP+ axis usually dominates. Under high light, low CO2, or low-temperature conditions, however, protection and buffering demands rise rapidly. Therefore, terminal allocation is not only a matter of multiple outlets competing for the same electron pool, but also a matter of multiple metabolic modules sharing PSI acceptor-side reducing power according to temporal order and priority.

2. Major Outlets of Terminal Reducing Power and Their Competitive Relationships

2.1 The FNR-NADP+ main axis

(1) The principal output route for assimilatory reducing power

FNR catalyzes electron transfer from Fd to NADP+, generating NADPH. This pathway is the core outlet for chloroplast production of two-electron reducing equivalents and directly supports the reductive phase of the Calvin cycle, fatty acid synthesis, and many other anabolic processes. Thus, under most steady-state conditions, the FNR-NADP+ axis remains the main pathway for terminal reducing-power output.

(2) Whether the main axis remains open depends on the acceptor pool and the consumption side

The FNR direction does not have unlimited inherent capacity to receive electrons. The size of the NADP+ pool, the NADPH/NADP+ ratio, the rate of Calvin-cycle reductive steps, and the downstream capacity for product utilization all feed back to determine how many electrons this pathway can still accept. When NADP+ regeneration is insufficient or NADPH consumption slows, the FNR direction still exists, but its pulling force declines and electrons are redirected to other outlets.

2.2 The Fd-FTR-Trx regulatory outlet

(1) Regulatory use of terminal electrons

Fd can also transfer electrons through FTR to multiple Trx species. The main purpose of this pathway is not the formation of large amounts of stable metabolic products, but rather the rapid conversion of light-reaction signals into metabolic program activation signals through reductive regulation. In this way, Calvin-cycle enzymes, starch-metabolism enzymes, part of the antioxidant system, and several nitrogen- and sulfur-metabolism enzymes are activated.

(2) Although small in flux, the regulatory outlet has an amplifying effect

Compared with the FNR main axis, the Fd-FTR-Trx pathway usually does not represent the largest electron-flux outlet, but it determines whether downstream metabolic systems can receive light-reaction output in a timely manner. If activation of this layer is delayed, the system can enter a state in which the light reactions accelerate first while the metabolic side has not yet fully opened, eventually leading to electron accumulation on the acceptor side and delayed NADPH utilization. Thus, although this outlet may be modest in flux, it occupies a pre-positioned role in overall system control.

2.3 The nitrogen-assimilation outlet

(1) Direct use of Fd electrons in nitrite reduction

Nitrite reductase in the chloroplast directly uses Fd as the electron donor to reduce NO2− to NH4+. This means that inorganic nitrogen assimilation does not necessarily have to pass through the NADPH pool first, but can compete directly for electrons at the Fd layer. Whenever nitrogen-assimilation demand increases, the proportion of Fd electrons diverted toward the NiR direction rises.

(2) Fd-GOGAT extends demand into the reassimilation stage

Ammonium is not the endpoint, because it must subsequently enter the glutamine/glutamate metabolic network. Fd-GOGAT participates in this process, meaning that the pull of nitrogen assimilation on terminal reducing power is not confined to the NiR step alone, but extends further into the organic-nitrogen integration stage. Therefore, enhanced nitrogen assimilation systematically changes the terminal branching pattern, rather than merely increasing the electron consumption of a single reaction.

2.4 The sulfur-assimilation outlet

(1) Sulfite reduction represents a typical reductive-assimilation branch

Sulfite reductase likewise directly uses Fd electrons to convert SO3^2− into more reduced sulfur states. This process shows that sulfur assimilation, like nitrogen assimilation, is a direct reducing-power sink on the PSI acceptor side rather than a secondary consumer after NADPH formation.

(2) Sulfur assimilation is linked to the downstream reductive buffering system

Sulfur assimilation ultimately supports synthesis of cysteine, glutathione, and other sulfur-containing metabolites. Because glutathione in turn participates in antioxidant buffering and maintenance of redox homeostasis, sulfur assimilation is not only a nutrient-assimilation outlet but also an important upstream source for subsequent reductive buffering capacity. Changes in sulfur-assimilation demand therefore feed back on PSI terminal allocation.

2.5 Cyclic electron flow and alternative electron sinks

(1) Cyclic electron flow is an ATP-compensatory outlet

When the system faces relative ATP shortage rather than NADPH shortage, cyclic electron flow can redirect part of the electrons from the Fd side back into the PQ/b6f-related pathway, thereby enhancing the transmembrane proton gradient and supporting additional ATP formation. Its core significance lies not in increasing assimilatory reducing power, but in correcting the ATP/NADPH supply-demand ratio.

(2) Alternative electron sinks function as safety valves on the acceptor side

When carbon assimilation and assimilatory reducing-power demand are insufficient and acceptor-side reductive pressure rises, part of the electrons can be diverted toward alternative sinks such as O2 photoreduction. These pathways usually do not constitute the main steady-state route, but under high light, low temperature, low CO2, or fluctuating light, they can significantly buffer PSI acceptor-side pressure and prevent prolonged electron retention in highly reduced states.

2.6 The malate valve and transcompartmental transfer of reducing power

(1) The malate valve relieves local over-reduction in the chloroplast

When NADPH accumulates locally in the chloroplast and internal consumption pathways cannot increase synchronously, part of the reducing equivalents can be converted into transcompartmental metabolic flux through the malate-oxaloacetate shuttle, thereby relieving stromal over-reduction. This pathway does not replace the main assimilation process, but instead provides an additional outlet when local acceptor capacity is temporarily insufficient.

(2) Terminal allocation has a cross-compartmental systems property

Therefore, terminal reducing-power allocation at PSI is not merely a local issue within the chloroplast, but also involves the coordinated participation of the cytosol and mitochondria in handling organic acids, reducing equivalents, and metabolic rebalancing. The terminal layer is in fact positioned at the interface of whole-cell metabolic organization.

Table 1. Major outlets of terminal reducing power in photosynthetic electron transport and their interpretive significance

Outlet direction	Major acceptor/system	Major function	Interpretive significance
NADPH output	FNR, NADP+	Formation of assimilatory reducing power	Indicates whether the main axis of carbon fixation and anabolic metabolism is operating smoothly
Regulatory output	FTR, Trx system	Transmission of reductive regulatory signals	Indicates whether metabolic programs can be activated in time
Nitrogen-assimilation output	NiR, Fd-GOGAT-related processes	Inorganic nitrogen reduction and reassimilation	Indicates whether nitrogen metabolism is actively drawing Fd electrons
Sulfur-assimilation output	SiR and related sulfur-metabolism processes	Formation of reduced sulfur	Indicates whether sulfur assimilation and reductive-buffer demand are increasing
ATP-compensatory output	Cyclic-electron-flow-related modules	Enhancement of ΔpH and promotion of ATP formation	Indicates whether the system is relatively deficient in ATP
Buffering output	O2 photoreduction, malate valve, etc.	Dissipation or transfer of excess reducing power	Indicates whether the PSI acceptor side is under over-reduction pressure

3. How Metabolic Demand Reorganizes Terminal Reducing-Power Allocation

3.1 Stabilization of the main axis when carbon-fixation demand is enhanced

(1) The Calvin cycle determines the sink capacity of the NADPH main route

Under conditions of sufficient CO2 supply, coordinated Calvin-cycle enzyme operation, and smooth utilization of assimilatory products, NADPH consumption capacity remains strong and NADP+ is regenerated in a timely manner. The FNR-NADP+ axis therefore maintains the highest priority. In this case, terminal allocation is characterized by a high-assimilation state dominated by assimilatory output, with regulatory and buffering outputs remaining secondary.

(2) Stabilization of the FNR main axis does not mean that all other outlets are shut down

Even under steady-state conditions where the FNR main axis is dominant, Trx regulation, small amounts of CET, and basal nutrient-assimilation branching still exist, although their relative proportions are lower. Thus, stabilization of the main axis means that electrons preferentially flow toward assimilatory output, not that all other outlets cease entirely.

3.2 Reorganization when carbon fixation is restricted and photorespiration is enhanced

(1) Slower regeneration of the acceptor pool causes reductive pressure

When CO2 is limiting, Rubisco carboxylation declines, Calvin-cycle steps are restricted, or product export is limited, the rate of NADPH consumption falls and NADP+ regeneration slows. The capacity of the FNR direction to receive electrons is thereby reduced, and reductive accumulation readily develops on the PSI acceptor side.

(2) Enhanced photorespiration simultaneously increases ATP demand and reassimilation demand

Under conditions of high O2, low CO2, or high temperature, photorespiration increases, thereby elevating not only ATP expenditure but also the burden of ammonium reassimilation and organic-acid transfer. This shifts the system from a state of “priority to high NADPH output” toward one in which ATP compensation, nitrogen reassimilation, and buffering outputs all become important. In other words, photorespiration does not merely increase energetic cost, but restructures the terminal demand profile.

3.3 Rewriting of acceptor-side competition during enhanced nitrogen assimilation

(1) Inorganic nitrogen supply changes the priority of Fd-electron allocation

When nitrate supply increases, inorganic nitrogen limitation is relieved, or rapidly growing tissues enhance nitrogen-metabolism demand, the competition for Fd electrons by NiR and Fd-GOGAT-related processes increases significantly. Under such conditions, even if the FNR pathway remains operative, its relative share is reduced by the stronger pull of nitrogen assimilation.

(2) Carbon-nitrogen coordination determines whether this branching becomes sustained

If enhanced nitrogen assimilation is not matched by corresponding carbon skeletons and ATP support, it can only create short-term competition. If carbon and nitrogen supply are coordinated, however, this branching can be converted into a sustained metabolic reorganization. Therefore, the influence of nitrogen assimilation on terminal allocation should not be understood solely as the presence or absence of NiR substrate, but rather judged within the overall nutritional state.

3.4 Reorganization when sulfur assimilation and antioxidant demand increase

(1) Sulfur assimilation both consumes electrons and changes downstream buffering capacity

When sulfur-metabolism demand rises, SiR branching is enhanced, and more Fd electrons are directed toward formation of reduced sulfur. This process not only immediately consumes acceptor-side electrons, but also determines subsequent capacity for synthesis of cysteine and glutathione.

(2) Antioxidant demand can in turn pull sulfur assimilation upward

Under elevated oxidative stress, demand for glutathione-related buffering increases and can in turn enhance the pull on sulfur assimilation. This means that changes in terminal allocation are regulated not only by assimilation itself, but also by feedback from downstream buffering-system demand.

3.5 Protective reorganization under fluctuating light, low temperature, and high light

(1) The priority of protection rises under fluctuating environments

Under fluctuating light, the ability of the light reactions to deliver electrons changes more rapidly than the ability of the metabolic side to adjust flux. In such conditions, if allocation remains absolutely dominated by the FNR direction, the acceptor side readily enters repeated states of reductive accumulation. Accordingly, the importance of rapid Trx regulation, CET, and alternative electron sinks increases.

(2) Low temperature and high light together amplify mismatch risk

Low temperature slows enzymatic assimilation rates, whereas high light increases electron input to the acceptor side. When these occur together, the system readily shifts from a high-assimilation state into a high-protection state. Under such conditions, the primary goal of terminal allocation is no longer maximization of assimilatory output, but prevention of irreversible damage on the PSI acceptor side.

4. Buffering and Protection Mechanisms Under Mismatch States

4.1 ATP-relatively-insufficient mismatch

(1) Core contradiction

This type of mismatch is characterized not by lack of reducing power, but by ATP generation lagging behind metabolic expenditure. If electrons continue to be forced toward the FNR direction, the relative excess of NADPH is further amplified.

(2) Main buffering pathway

In this case, the most important buffering pathway is cyclic electron flow. It can increase proton motive force and promote ATP formation without further increasing the NADPH burden, thereby correcting the supply-demand ratio. Such scenarios typically arise when photorespiration is enhanced, ion-transport load increases, or certain energy-intensive assimilation conditions prevail.

4.2 NADPH-relatively-excessive mismatch

(1) Core contradiction

The essence of this mismatch is that NADPH generation exceeds the capacities for consumption and regeneration, resulting in NADP+ insufficiency and sustained over-reduction on the acceptor side. If not diverted in time, electrons remain accumulated in Fd and downstream carriers, increasing the risk of reactive oxygen species formation and photoinhibition.

(2) Main buffering pathways

The major buffering pathways in this case include alternative electron sinks, the malate valve, and to some extent CET cooperation. Their shared function is not to improve assimilation efficiency, but to lower acceptor-side pressure and return the system from a state of “electron accumulation” to one of “controlled output.”

4.3 Regulatory-activation-lag mismatch

(1) Core contradiction

During the dark-to-light transition or a rapid increase in light intensity, the light reactions can elevate electron output within a short period, whereas activation of the Calvin cycle and related metabolic enzymes requires time. If activation of the regulatory layer lags behind the increase in electron input, a transient state arises in which reducing power has nowhere to go.

(2) Main buffering pathway

The Fd-FTR-Trx system is an important correction layer for this type of mismatch. By preferentially converting part of the electrons into regulatory signals, it allows the metabolic side to enter a receptive state as quickly as possible. If this layer responds inadequately, the system may still experience acceptor-side pressure even when the NADP+ pool and other outlets are not yet fully exhausted, simply because activation is not synchronized.

4.4 Nutrient-assimilation-driven mismatch

(1) Core contradiction

When nitrogen or sulfur assimilation demand suddenly rises, competition for Fd electrons is rewritten. The issue is not that no electrons are available on the acceptor side, but that the original allocation structure biased toward the FNR main axis is pulled again by newly strengthened assimilatory outlets, thereby creating new proportional tension among carbon fixation, reductive buffering, and nutrient assimilation.

(2) Main buffering pathway

Under such conditions, the true buffer is not a single dissipative pathway, but the reestablishment of coordination among carbon, nitrogen, and sulfur metabolism. If ATP supply, carbon skeletons, and downstream metabolic integration are insufficient, this pull evolves into a new systemic mismatch.

Table 2. Major mismatch types in terminal reducing-power allocation and their buffering pathways

Mismatch type	Typical manifestation	Main buffering pathway	Functional outcome
Relative ATP insufficiency	Relative excess on the NADPH side, insufficient ATP support	Enhanced cyclic electron flow	Increases ATP formation and relieves ratio imbalance
Relative NADPH excess	Insufficient NADP+ regeneration, acceptor-side over-reduction	Alternative electron sinks, malate valve, partial CET	Lowers acceptor-side pressure and reduces photoinhibition
Regulatory activation lag	Light reactions accelerate faster than metabolic activation	Fd-FTR-Trx regulatory layer	Accelerates enzyme activation and shortens the mismatch window
Enhanced nutrient-assimilation pull	Increased NiR/SiR-related branching	Rebalancing of carbon, nitrogen, and sulfur metabolism with energetic compensation	Maintains nutrient assimilation and overall homeostasis

5. Scientific Questions and Experimental Interpretation Framework

5.1 Scientific questions

(1) Which outlet dominates terminal allocation under current conditions

In studying this topic, the first issue is to determine where electrons are mainly flowing. Does the FNR-NADP+ axis predominate, are NiR/SiR-related branches enhanced, or have CET and buffering outputs become the main corrective layers? If this question is not addressed first, subsequent interpretation of changes in photosynthetic efficiency or metabolic output can easily become unfocused.

(2) What determines the over-reduction threshold on the PSI acceptor side

The transition from reversible accumulation to functional damage on the acceptor side is not a simple effect of light intensity, but a system-level threshold jointly determined by the rate of NADP+ regeneration, assimilation flux, CET capacity, alternative electron-sink capacity, Trx activation efficiency, and the load of nutrient assimilation. Threshold determination is one of the most explanatory issues in studies of terminal allocation.

(3) Whether carrier selectivity changes the branching pattern

Different Fd isoforms, the localization state of FNR, and the preferential pairing relationships of downstream acceptors such as NiR and SiR may determine whether terminal allocation exhibits tissue specificity, condition specificity, and developmental-stage specificity. Therefore, carrier selectivity should not be simplified as a background variable, but instead regarded as a potential mechanistic layer determining branching plasticity.

5.2 Experimental interpretation framework

① At the main-pathway judgment level, P700 redox state, PSI acceptor-side limitation, NADPH dynamics, FNR-related output, and flux through downstream assimilatory enzymes should be measured jointly; chlorophyll-fluorescence parameters alone cannot substitute for interpretation of the acceptor side.

② At the ratio-imbalance judgment level, ATP/ADP status, NADPH/NADP+ status, ΔpH-related information, and changes in CET should be analyzed simultaneously to distinguish between “the system is relatively ATP-deficient” and “the system is relatively deficient in electron-consuming capacity on the acceptor side,” which are mechanistically different conditions.

③ At the nutrient-pull judgment level, nitrite reduction, glutamate synthesis, sulfite reduction, and cysteine/glutathione-related indicators should be measured to confirm whether Fd electrons are being actively drawn away by nitrogen and sulfur metabolism.

④ At the protection-and-buffering judgment level, O2-related photoreduction, malate-valve-related metabolites, reactive oxygen species status, and PSI photoinhibition indicators should be combined to determine whether the system has shifted from a high-assimilation state into a high-protection state.

⑤ At the functional-output judgment level, interpretation should ultimately return to integrated readouts including CO2 assimilation efficiency, nitrogen and sulfur assimilation rates, fluctuating-light adaptability, PSI stability, and activation state of downstream metabolic enzymes. Only when structural-layer, branching-layer, and functional-layer results are consistent does a terminal-allocation model gain strong explanatory power.

6 Related Research Products

Table 3. Product table related to research on terminal reducing-power allocation in photosynthetic electron transport

Name	CAS No.	Experimental stage	Key use	Use notes
Paraquat dichloride	1910-42-5	PSI acceptor-side stress layer	Serves as an artificial electron acceptor to draw electrons from the PSI terminal side and construct an acceptor-side oxidative-stress model	Suitable for short-term evaluation of PSI acceptor-side capacity and protection threshold; excessive dosage can introduce a strong ROS background
DCMU	330-54-1	PSII inhibition layer	Blocks electron donation from PSII to the PQ pool and is used to separate PSI acceptor-side processes	Suitable for distinguishing donor-side and acceptor-side limitation; should be interpreted together with P700 parameters
DBMIB	6969-49-9	Cyt b6f inhibition layer	Blocks electron transfer from PQH2 to Cyt b6f and is used to analyze the relationship between linear and cyclic electron flow	Suitable for use alongside DCMU and Antimycin A to identify electron-flow branch position
Nigericin	28380-24-7	ΔpH regulation layer	Modulates the transmembrane pH gradient and is used to analyze the effect of proton motive force on CET and terminal allocation	Suitable for distinguishing ΔpH-dependent processes; short-term treatment conditions should be carefully controlled
Valinomycin	2001-95-8	Transmembrane ion-gradient layer	Alters the membrane-potential component and is used to analyze coupling between Δψ/ΔpH partitioning and terminal electron branching	Commonly combined with Nigericin for decomposition analysis of proton motive force
Potassium ferricyanide	13746-66-2	Artificial electron acceptor layer	Used in in vitro electron-transport systems to accept electrons and analyze photochemical electron-output capacity	More suitable for isolated systems and thylakoid electron-transport activity measurements
2,6-Dichlorophenolindophenol	956-48-9	Artificial electron acceptor layer	Used for measurement of photochemical electron-transport activity and reduction rate	Suitable for in vitro electron transport and acceptor-side activity analysis
Sodium bicarbonate	144-55-8	Carbon-assimilation pull layer	Increases inorganic carbon supply and is used to analyze the pull of enhanced CO2 fixation on the FNR main axis	Suitable for combined measurement with NADPH/NADP+, assimilation rate, and P700 parameters
Glyceraldehyde-3-phosphate	591-59-3	Calvin-cycle output layer	Used to analyze feedback from carbon-fixation product accumulation on terminal reducing-power redistribution	Suitable for integrated analysis of carbon assimilation and acceptor-side pressure
Potassium nitrate	7757-79-1	Nitrogen-source supply layer	Establishes a nitrate-assimilation background and is used to compare terminal branching changes under different nitrogen nutritional conditions	More suitable for medium- to long-term nutritional treatments
Ammonium chloride	12125-02-9	Reduced-nitrogen control layer	Provides an ammonium background and is used to compare electron-partition changes when the requirement for inorganic nitrogen reduction declines	Suitable as a control nitrogen source for nitrate-based systems
L-Glutamine	56-85-9	Nitrogen-metabolism coupling layer	Used to establish an organic-nitrogen supplementation background and evaluate changes in inorganic-nitrogen reduction demand	Suitable for carbon-nitrogen coupling and rescue experiments
Sodium sulfite	7757-83-7	Sulfur-assimilation outlet layer	Serves as a sulfite-reduction substrate and is used to analyze branching of Fd electrons toward sulfur assimilation	Suitable for use with cysteine- and glutathione-related indices
Magnesium sulfate	10034-99-8	Sulfur-source supply layer	Establishes a sulfur nutritional background and is used to compare the effect of sulfur supply on terminal branching	Suitable for nutritionally stratified cultivation experiments
L-Cysteine	52-90-4	Sulfur-metabolism rescue layer	Used to evaluate feedback of sulfur-end-product rescue on sulfur-assimilation pull and terminal allocation	Suitable for modeling decline in sulfur-assimilation demand
Reduced glutathione	70-18-8	Reductive buffering layer	Establishes a high reductive-buffering background and is used to analyze the effect of antioxidant demand on terminal reducing-power occupation	Suitable for paired analysis with GSSG/ROS indices
Oxidized glutathione	27025-41-8	Redox-balance layer	Used to analyze chloroplast reductive buffering and oxidative-pressure status	Suitable for models of reducing-power allocation mismatch
N-acetyl-L-cysteine	616-91-1	Antioxidant buffering layer	Used to buffer oxidative pressure and observe relief of acceptor-side over-reduction	Suitable for evaluation of PSI protection and alternative electron-sink function
Ascorbic acid	50-81-7	Antioxidant and recovery layer	Participates in ROS scavenging and helps maintain reductive-system stability	Suitable for photoinhibition recovery experiments
L-Malic acid	97-67-6	Reducing-equivalent shuttle layer	Used to analyze exchange of reducing power between chloroplasts and the cytosol/mitochondria	Suitable for research on the malate valve and intercompartmental coordination
Oxaloacetic acid	328-42-7	Malate-valve-related layer	Used to detect potential changes in the demand for export of chloroplast reducing equivalents	Suitable for joint interpretation with malate and NAD(P)H status
Disodium EDTA	6381-92-6	In vitro system control layer	Controls metal-ion background in reaction systems and reduces nonspecific oxidation reactions	Suitable for isolated enzymatic systems involving FNR, Fd, NiR, and related components
Phenylmethylsulfonyl fluoride	329-98-6	Protein-extraction protection layer	Used during chloroplast-protein extraction and enzyme-activity measurement to inhibit protein degradation	Suitable for preparation of FNR-, Fd-, and Trx-related proteins

Table 4. Product table related to research on terminal reducing-power allocation in photosynthetic electron transport

Catalog No.	Name	Grade and purity	Corresponding research stage	Suitable research direction/application
F1445242	Ferredoxin-NADP reductase	—	FNR-NADP+ main axis	Suitable for in vitro reconstruction of the Fd→FNR→NADP(H) reduction system and analysis of the allocation capacity of terminal electrons toward the NADPH-output main axis
N1515945	Nitrite Reductase (NiR) Activity Assay Kit (NO₂⁻, Micro Method)	BioReagent	Nitrogen-assimilation outlet layer	Suitable for detecting changes in NiR activity and determining whether more Fd electrons are directed toward inorganic nitrogen assimilation
N1515946	Nitrite Reductase (NiR) Activity Assay Kit (NO₂⁻, Colorimetric Method)	BioReagent	Nitrogen-assimilation outlet layer	Suitable for routine colorimetric detection of NiR activity and comparison of the strength of nitrogen-assimilation pull under different nutritional and light environments
G1505937	Glutamate Synthase (GOGAT) Activity Assay Kit (UV Micro Method)	BioReagent	Nitrogen reassimilation layer	Suitable for analyzing downstream reassimilation capacity following NiR and evaluating sustained occupation of terminal reducing power by nitrogen assimilation
N1515808	NAD-Malate Dehydrogenase（NAD-MDH）Activity Assay Kit (UV Micro Method)	BioReagent	Organic-acid metabolism coupling layer	Suitable for detecting NAD-MDH activity and analyzing coupling between the malate-oxaloacetate cycle and intercompartmental reducing power
N1515938	NAD-Malate Dehydrogenase (NAD-MDH) Activity Assay Kit (UV Colorimetric Method)	BioReagent	Organic-acid metabolism coupling layer	Suitable for comparing NAD-MDH activity changes under different treatments and assisting analysis of intercompartmental metabolic buffering status
N1515807	NADP-Malate Dehydrogenase（NADP-MDH）Activity Assay Kit (UV Micro Method)	BioReagent	Malate-valve layer	Suitable for detecting chloroplast NADP-MDH activity and determining whether reducing power is being allocated to the malate valve and intercompartmental transfer pathways
N1515937	NADP-Malate Dehydrogenase (NADP-MDH) Activity Assay Kit (UV Colorimetric Method)	BioReagent	Malate-valve layer	Suitable for routine colorimetric analysis of NADP-MDH activity and auxiliary evaluation of chloroplast reducing-equivalent export capacity
EJ1513175	Mouse Malate Dehydrogenase, Cytoplasmic (MDH1) ELISA Kit	BioReagent	Malate-metabolism detection layer	Suitable for detecting MDH1 level changes and supplementing expression-layer readouts of the malate-metabolism pathway
M1515806	Mitochondrial Malate Dehydrogenase(mMDH) Activity Assay Kit (UV Micro Method)	BioReagent	Mitochondrial coordination layer	Suitable for evaluating mitochondrial-side malate-metabolism responses and analyzing coordination between chloroplast terminal reducing-power allocation and mitochondria
M1515936	Mitochondrial Malate Dehydrogenase (mMDH) Activity Assay Kit (UV Colorimetric Method)	BioReagent	Mitochondrial coordination layer	Suitable for routine detection of mMDH activity and supplementation of functional readouts of intercompartmental metabolic coordination
M139683	Malate dehydrogenase (MDH) from Thermus sp.	Biologically active, ActiBioPure™, native, high performance, EnzymoPure™, ≥100 U/mg enzyme powder	Malate valve/enzymatic reconstruction layer	Suitable for constructing MDH-related in vitro enzymatic systems and analyzing organic-acid shuttling and reducing-equivalent export demand
R1505812	Recombinant Malate Dehydrogenase (MDH)	Biologically active, recombinant, ActiBioPure™, high performance, EnzymoPure™, ≥50 U/mg enzyme powder; ≥200 U/mg protein	Malate valve/recombinant enzyme system	Suitable for in vitro construction of MDH-related reaction systems and study of reducing-equivalent export and metabolic coupling
M196993	Malate Dehydrogenase，recombinant from bacteria	EnzymoPure™, >550 units/mg	Malate valve/high-activity enzymatic layer	Suitable for high-activity MDH enzymatic systems and enhanced validation of malate-valve-related mechanisms
N276326	NADPH	≥93%	Cofactor and reducing-power donor layer	Suitable for FNR-related enzymatic systems, NADPH consumption/supplementation experiments, and analysis of terminal reducing-power supply-demand relationships
N1499724	NADPH tetracyclohexanamine	Moligand™, 10 mM in DMSO	Cofactor derivative layer	Suitable for constructing NADPH-related treatment systems for donor-condition intervention and comparison experiments
N302057	NADPH (tetracyclohexanamine)	≥96%	Cofactor derivative layer	Suitable for in vitro enzymatic and electron-transfer studies requiring higher-purity NADPH derivatives
N464348	β-Nicotinamide adenine dinucleotide 2′-phosphate reduced tetrasodium salt hydrate（β-NADPHtetrasodium salt hydrate）	≥97%(HPLC)	FNR-NADP(H) axis	Suitable for NADPH quantitative supplementation, reducing-power balance studies, and enzyme-kinetics experiments
N1510346	β-Nicotinamide adenine dinucleotide 2′-phosphate reduced tetrasodium salt hydrate（β-NADPHtetrasodium salt hydrate）	≥99%	FNR-NADP(H) axis	Suitable for terminal electron-allocation and coupled enzymatic experiments requiring higher cofactor purity
C103029	Coenzyme II reduced tetrasodium salt	≥95%	Reducing-power output layer	Suitable for NADPH donor supplementation, downstream reductive reaction driving, and reducing-power buffering studies
C1505573	Coenzyme Ⅱ NADP(H) Content Assay Kit (WST-8, Micro Method)	BioReagent	NADP(H) acceptor-pool status layer	Suitable for quantitative analysis of NADP(H) pool changes and evaluation of FNR-main-axis accessibility and acceptor-side reductive pressure
A1515819	ATP Content Assay Kit (AHM, Micro Method)	BioReagent	Energy-ratio layer	Suitable for determination of ATP status and assessment of whether terminal allocation has shifted from high NADPH output to ATP-compensatory demand
R353922	ROS fluorescent probe DHE	—	Oxidative-pressure and protection layer	Suitable for detecting ROS accumulation caused by mismatch in terminal reducing-power allocation and assisting evaluation of PSI acceptor-side over-reduction and transition to protective state

The key to terminal reducing-power allocation in photosynthetic electron transport is not whether electrons reach the PSI acceptor side, but how these electrons continuously reorder their priorities according to the demands of carbon fixation, nitrogen and sulfur assimilation, regulatory activation, and photoprotection. Only by placing the FNR-NADP+ main axis, the Fd-FTR-Trx regulatory outlet, nitrogen and sulfur assimilation branches, cyclic electron flow, alternative electron sinks, and the malate valve into one unified framework can the “endpoint of the light reactions” be elevated from a simple description of NADPH generation to a true analytical object for understanding chloroplast energy-ratio control and metabolic homeostasis.

For more related articles, please see below:

[1] Cellular Electron Transfer Networks and Mechanisms of Metabolism-Signaling Coupling