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Relationship: 2333

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Decrease, Photosystem II efficiency leads to Decrease, Photosynthesis

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Decrease, Photosystem II efficiency

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Decrease, Photosynthesis

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status
Deposition of ionizing energy leading to population decline via inhibition of photosynthesis	adjacent	High	High	Knut Erik Tollefsen (send email)	Under development: Not open for comment. Do not cite
Oxygen-evolving complex damage leading to population decline via inhibition of photosynthesis	adjacent	High	High	Knut Erik Tollefsen (send email)	Under development: Not open for comment. Do not cite
Binding to plastoquinone B site leading to decreased population growth rate via photosystem II inhibition	adjacent	High	High	Li Xie (send email)	Under development: Not open for comment. Do not cite

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
Lemna minor	Lemna minor	High	NCBI
Chlamydomonas reinhardtii	Chlamydomonas reinhardtii	High	NCBI
Pinus sylvestris	Pinus sylvestris	High	NCBI
Arabidopsis thaliana	Arabidopsis thaliana	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Unspecific	High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
All life stages	High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

The decrease in photosystem II (PSII) efficiency impairs the effectiveness of light-driven electron transport in thylakoid membranes, thereby limiting the reduction of plastoquinone, weakening proton gradient formation, and consequently reducing ATP and NADPH production. Since these energy carriers are crucial for carbon fixation, diminished PSII efficiency directly leads to reduced photosynthetic rates. This relationship exhibits direct mechanistic causality and has been consistently validated across all photosynthetic oxygen-producing organisms.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

A systematic literature search will be conducted in Web of Science, Scopus, and PubMed using combinations of terms related to PSII efficiency (e.g., Fv/Fm, ΦPSII, ETR, chlorophyll fluorescence) and photosynthesis (e.g., oxygen evolution, CO₂ fixation, ¹⁴C uptake). Studies will be included if they report paired measurements of PSII performance and photosynthetic rate in oxygenic phototrophs under controlled exposure conditions. Priority will be given to experiments demonstrating temporal or dose-response concordance. Extracted data will include species, exposure details, endpoints, effect magnitude, and methodological quality to support WoE evaluation and potential quantitative linkage.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Strong empirical evidence supports the relationship between decreased PSII efficiency and reduced photosynthesis. Declines in chlorophyll fluorescence parameters (e.g., Fv/Fm) are consistently associated with reduced oxygen evolution and carbon fixation rates in plants and algae (Maxwell and Johnson, 2000; DELIEU and WALKER, 1981). PSII-inhibiting herbicides such as atrazine and terbutryn directly impair electron transport at the D1 protein QB site, leading to decreased photosynthetic performance (Alfonso et al., 1996; Broser et al., 2011). These studies demonstrate mechanistic causality, dose-response concordance, and cross-taxa consistency, providing strong support for this KER.

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

Biological plausibility was considered as high. Photosystem II (PSII) is the primary site of light-driven charge separation and water oxidation in oxygenic photosynthesis, and its photochemical efficiency governs electron entry into the photosynthetic electron transport chain (Maxwell and Johnson, 2000). Reduced PSII efficiency limits electron transfer from QA to QB, decreasing plastoquinone reduction and proton gradient formation across the thylakoid membrane, thereby constraining ATP and NADPH production (Broser et al., 2011). Because ATP and NADPH drive the Calvin–Benson cycle, impaired PSII function mechanistically results in decreased carbon fixation and overall photosynthetic rate.

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

The empirical support of this KER is considered high.

Rationale: A substantial body of experimental evidence demonstrates strong incidence, dose, and temporal concordance between decreased Photosystem II (PSII) efficiency and reduced photosynthetic performance across cyanobacteria, algae, aquatic macrophytes, and higher plants. Reductions in chlorophyll fluorescence parameters (e.g., Fv/Fm, ΦPSII) are consistently accompanied by declines in oxygen evolution and carbon fixation, with few inconsistencies reported.

Evidence:

Dose concordance: Increasing concentrations of PSII inhibitors (e.g., diuron, atrazine, terbutryn) cause progressive declines in Fv/Fm and ΦPSII, which are quantitatively paralleled by reductions in oxygen evolution and CO₂ assimilation rates. In controlled experiments, decreases in fluorescence-based PSII efficiency strongly correlate with inhibition of electron transport and reduced photosynthetic activity (Maxwell and Johnson, 2000; Delieu and Walker, 1981).

Incidence concordance: Impaired PSII photochemistry, reflected by altered QA reoxidation kinetics and reduced quantum yield, is consistently associated with diminished photosynthetic oxygen evolution in functional assays across taxa (Sundby et al., 1993; Broser et al., 2011).

Temporal concordance: Reductions in PSII efficiency occur rapidly (minutes to hours) after herbicide exposure or photoinhibitory stress, followed by measurable decreases in carbon fixation and net photosynthesis, supporting upstream–downstream sequence consistency (Macinnis-Ng and Ralph, 2003; Wilkinson et al., 2015).

Genetic/functional concordance: Mutations or damage affecting PSII reaction center components reduce photochemical efficiency and are directly accompanied by decreased photosynthetic capacity, supporting causal linkage between PSII impairment and reduced photosynthesis (Ohad and Hirschberg, 1992; Alfonso et al., 1996).

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

Some variability exists across species, light regimes, and exposure conditions. PSII efficiency reductions do not always translate proportionally to decreased whole-organism photosynthesis due to compensatory mechanisms, alternative electron pathways, or short-term acclimation. Differences in measurement methods and recovery dynamics may also introduce variability in observed effect magnitude.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Modulating Factor (MF)	MF Specification	Effect(s) on the KER	Reference(s)
Light intensity	Low vs. high irradiance; fluctuating light	High light amplifies PSII photoinhibition and accelerates decline in photosynthesis; low light may partially mask PSII impairment	Maxwell and Johnson, 2000
Temperature	Suboptimal vs. optimal thermal range	Alters membrane fluidity and enzyme kinetics, modifying electron transport and recovery capacity	Sundby et al., 1993
Nutrient status	Nitrogen or iron limitation	Reduces chlorophyll content and D1 repair capacity, strengthening coupling between PSII decline and photosynthesis reduction	Maxwell and Johnson, 2000
Species-specific traits	NPQ capacity, cyclic electron flow, antenna size	Enhanced photoprotective mechanisms buffer translation of PSII impairment into reduced photosynthesis	Maxwell and Johnson, 2000
Exposure duration	Acute vs. chronic exposure	Short-term inhibition may be reversible; prolonged exposure leads to sustained reduction in photosynthesis	Macinnis-Ng and Ralph, 2003

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

The quantitative understanding is high. PSII efficiency endpoints consistently show equal or greater sensitivity than whole-photosynthesis measurements, reflecting upstream positioning in the pathway. In Rhodomonas salina, the EC₅₀ for PSII photoinhibition (ΔF/Fm′) was 1.71 µg/L diuron, compared to 6.27 µg/L for growth (Thomas et al., 2020). Oxygen evolution in Synechococcus elongatus was inhibited by 50% at 20 µg/L diuron and 65 µg/L atrazine (Jones et al., 2003). Regulatory assessments report similar EC₅₀ values for atrazine based on fluorescence (232 µg/L) and oxygen evolution (222 µg/L), demonstrating quantitative concordance (USEPA, n.d.).

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

A strong response–response relationship exists between PSII efficiency and photosynthetic. Declines in Fv/Fm or ΦPSII correlate proportionally with reductions in electron transport, oxygen evolution, and CO₂ fixation. This quantitative alignment supports a mechanistically consistent, predictive linkage between upstream photochemical impairment and downstream photosynthetic performance.

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

The time-scale of this KER is rapid and sequential. Decreases in PSII efficiency typically occur within minutes to hours following exposure to PSII inhibitors or photoinhibitory stress. Reductions in oxygen evolution and carbon fixation follow shortly thereafter, while longer-term decreases in growth emerge under sustained exposure conditions.

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

PSII impairment activates feedback mechanisms such as increased non-photochemical quenching (NPQ) and cyclic electron flow, which transiently compensate for reduced photochemical efficiency (Maxwell and Johnson, 2000). However, sustained PSII inhibition enhances reactive oxygen species (ROS) formation, leading to oxidative damage of the D1 protein and amplifying photosynthetic decline through feedforward stress pathways (Sundby, Chow and Anderson, 1993; Alfonso et al., 1996).

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Taxonomic applicability: This KER applies to all oxygenic photosynthetic taxa possessing Photosystem II (PSII), including cyanobacteria, chlorophytes (green algae), diatoms, macrophytes (e.g., Lemna spp.), seagrasses, and higher terrestrial plants. The mechanistic linkage is conserved because PSII structure and electron transport function are highly evolutionarily conserved across these groups.

Sex applicability: Not sex-specific. Photosynthesis is not sexually dimorphic; therefore, the KER is applicable to both male and female individuals in dioecious plant species, as well as hermaphroditic or clonal organisms.

Life-stage applicability: Applicable across all photosynthetically active life stages, including vegetative growth stages, juvenile and adult plants, and algal exponential growth phases. Sensitivity may vary with developmental stage due to differences in chloroplast density, metabolic demand, and repair capacity, but the mechanistic relationship remains consistent.

Chemical applicability: Most relevant for PSII-targeting chemicals (e.g., triazines, phenylureas, triazinones) and stressors causing photoinhibition, oxidative damage to D1 protein, or disruption of thylakoid electron transport.

Environmental applicability: Relevant under light-exposed conditions in freshwater, marine, and terrestrial ecosystems. The linkage may be modulated by irradiance, temperature, and nutrient status.

References

List of the literature that was cited for this KER description. More help

Alfonso, M., Pueyo, J.J., Gaddour, K., Etienne, A.-L., Kirilovsky, D. and Picorel, R., 1996. Induced new mutation of D1 serine-268 in soybean photosynthetic cell cultures produced atrazine resistance, increased stability of S2QB⁻ and S3QB⁻ states, and increased sensitivity to light stress. Plant Physiology, 112(4), pp.1499–1508.

Broser, M., Glöckner, C., Gabdulkhakov, A., Guskov, A., Buchta, J., Kern, J., Müh, F., Dau, H., Saenger, W. and Zouni, A., 2011. Structural basis of cyanobacterial Photosystem II inhibition by the herbicide terbutryn. Journal of Biological Chemistry, 286(18), pp.15964–15972.

Delieu, T. and Walker, D.A., 1981. Polarographic measurement of photosynthetic oxygen evolution by leaf discs. New Phytologist, 89(2), pp.165–178.

Macinnis-Ng, C.M.O. and Ralph, P.J., 2003. Short-term response and recovery of the seagrass Zostera capricorni to the herbicide diuron. Marine Environmental Research, 55(2), pp.153–166.

Maxwell, K. and Johnson, G.N., 2000. Chlorophyll fluorescence—A practical guide. Journal of Experimental Botany, 51(345), pp.659–668.

Ohad, N. and Hirschberg, J., 1992. Mutations in the D1 protein of photosystem II affect herbicide binding and electron transport properties. Plant Cell, 4(3), pp.273–282.

Sundby, C., Chow, W.S. and Anderson, J.M., 1993. Effects on Photosystem II function, photoinhibition, and herbicide binding caused by mutation of the D1 protein. Photosynthesis Research, 36(2), pp.123–135.

Wilkinson, A.D., Collier, C.J., Flores, F. and Ralph, P.J., 2015. Assessing the toxicity of herbicides to tropical seagrasses using chlorophyll fluorescence. Marine Pollution Bulletin, 95(2), pp.449–455.

Jones, R.J., Muller, J., Haynes, D. and Schreiber, U., 2003. Effects of herbicides diuron and atrazine on corals of the Great Barrier Reef, Australia. Marine Ecology Progress Series, 251, pp.153–167.

Thomas, M.C. et al., 2020. Toxicity of ten herbicides to the tropical marine microalgae Rhodomonas salina. Scientific Reports, 10, Article 7521.

U.S. Environmental Protection Agency (EPA), n.d. Ambient Aquatic Life Water Quality Criteria for Atrazine