Photosystem II inhibition

PSII inhibition

Molecular

Inhibition of the photosystem II (PSII) complex, specifically via binding to the D1 protein at the plastoquinone B (QB) binding site. This prevents the transfer of electrons from the primary electron acceptor (QA) to the secondary electron acceptor (QB), blocking the light-dependent reactions of photosynthesis.

Measured via chlorophyll fluorescence (Fv/Fm ratio) using a pulse-amplitude-modulated (PAM) fluorometer, or via direct measurement of oxygen evolution (Hill reaction) in isolated chloroplasts.

Oryza sativa

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Decreased, electron transport in photosystem II

Decreased PSII electron transport

Cellular

A decrease in the rate of electron flow through the photosystem II (PSII) complex due to the blocked QB site, preventing the reduction of plastoquinone and halting the flow of electrons to the cytochrome b6f complex.

Measured via electron transport rate (ETR) derived from chlorophyll fluorescence, or by measuring the reduction of artificial electron acceptors (e.g., DCPIP) in isolated thylakoid membranes.

Taxonomic Applicability: Oryza sativa (rice) and other higher plants.  Life Stage: Vegetative and reproductive stages. Sex: Not applicable.

1. Trebst, A. (2007). Inhibitors of photosystem II: Their history and mode of action. Zeitschrift für Naturforschung C, 62(3-4), 173-182. 2. Kumar, S., et al. (2018). Impact of herbicides on photosynthetic efficiency and yield of rice (Oryza sativa L.). Pesticide Biochemistry and Physiology, 148, 22-29. AOPWiki 2026-10-20T11:27:13

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Decreased, ATP and NADPH production

Decreased ATP/NADPH

Cellular

Reduced electron transport in photosystem II limits the proton gradient across the thylakoid membrane and prevents the reduction of NADP+ to NADPH. This results in a decreased availability of ATP and NADPH for the light-independent reactions (Calvin cycle).

Measured by quantifying ATP and NADPH levels in isolated chloroplasts using luminescence assays or HPLC, respectively.

Taxonomic Applicability: Oryza sativa (rice) and other higher plants.  Life Stage: Vegetative and reproductive stages. Sex: Not applicable.

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Reduced carbon fixation in Calvin cycle

Reduced carbon fixation

Cellular

Decreased availability of ATP and NADPH limits the activity of the Calvin cycle enzymes (e.g., RuBisCO), reducing the rate of CO2 assimilation and carbon fixation into organic compounds.

Measured via gas exchange analysis (CO2 uptake) using an infrared gas analyzer (IRGA), or by measuring RuBisCO enzyme activity in leaf extracts.

Taxonomic Applicability: Oryza sativa (rice) and other higher plants.  Life Stage: Vegetative and reproductive stages. Sex: Not applicable.

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Reduced, biomass accumulation

Reduced biomass

Individual

Reduced carbon fixation over time limits the production of structural and storage carbohydrates, resulting in a decrease in overall plant biomass (dry weight).

Measured by harvesting plants at specific time points and determining dry weight (g) after oven-drying to constant weight.

Taxonomic Applicability: Oryza sativa (rice).  Life Stage: Vegetative and reproductive stages. Sex: Not applicable.

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Reduced, grain filling

Reduced grain filling

Individual

Insufficient accumulation of photosynthates (sugars and starches) during the grain-filling stage prevents the proper development and weight gain of rice grains.

Measured by assessing the 1000-grain weight (g) or the percentage of filled vs. unfilled spikelets at harvest.

Taxonomic Applicability: Oryza sativa (rice).  Life Stage: Reproductive stage. Sex: Not applicable.

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Yield loss in rice paddy ecosystems

Rice yield loss

Population

A reduction in the total harvested biomass of marketable rice grains per unit area (e.g., tons per hectare) due to the cumulative effects of reduced photosynthesis and grain filling.

Measured by harvesting a defined area at maturity, threshing, and weighing the grains to calculate yield per hectare.

Taxonomic Applicability: Oryza sativa (rice).  Life Stage: Harvest / Maturity. Sex: Not applicable.

1. Kumar, S., et al. (2018). Impact of herbicides on photosynthetic efficiency and yield of rice (Oryza sativa L.). Pesticide Biochemistry and Physiology, 148, 22-29. AOPWiki 2026-10-20T11:56:57

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<upstream-id>f9f516f8-4d85-4671-a0df-3de7685e07a1</upstream-id> <downstream-id>249667ad-9942-4a39-bfe5-a9135c5d6959</downstream-id> Inhibition of the photosystem II complex blocks the transfer of electrons from QA to QB, directly resulting in a decreased rate of electron transport through the photosystem II complex.

A literature search was conducted in PubMed and Google Scholar using keywords: "atrazine", "photosystem II", "D1 protein", and "electron transport". Studies measuring chlorophyll fluorescence (Fv/Fm) and oxygen evolution in isolated chloroplasts or whole leaves were prioritized.

Multiple studies demonstrate temporal and dose concordance. Binding of atrazine to the D1 protein occurs rapidly (within minutes), followed immediately by a measurable decrease in electron transport rate (ETR). The inhibition is dose-dependent, with higher concentrations of atrazine causing a near-complete block of electron flow from QA to QB.

High. The structural and functional relationship between the D1 protein binding site and the electron transport chain is canonically established in plant physiology.

No significant inconsistencies in the core mechanism exist; the relationship is universally conserved across higher plants. Minor uncertainties may arise from differences in experimental conditions (e.g., light intensity, temperature) affecting baseline ETR values.

The relationship is highly quantitative. The binding affinity (Kd) of atrazine to the QB site directly predicts the fractional inhibition of electron transport, which can be modeled using standard receptor-ligand kinetics.

A sigmoidal dose-response curve describes the relationship, where increasing the concentration of bound atrazine leads to a proportional decrease in electron transport rate.

Rapid: Inhibition of electron transport occurs within seconds to minutes of exposure to atrazine.

Taxonomic: Higher plants (Embryophyta), specifically Oryza sativa. Life Stage: All photosynthetic life stages. Sex: Not applicable.

1. Trebst, A. (2007). Inhibitors of photosystem II: Their history and mode of action. Zeitschrift für Naturforschung C, 62(3-4), 173-182. AOPWiki 2026-10-20T12:06:46

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Atrazine-induced photosystem II inhibition leading to yield loss in rice (Oryza sativa)

PSII inhibition by atrazine leading to rice yield loss

SURIYA S

Suriya S (Vels University / Bioscience Research Foundation)

BY-SA

2.8

Atrazine is one of the most widely detected herbicides in global  surface water, with significant concentrations reported in rice  paddy ecosystems where it is applied for weed control. This AOP  describes a novel mechanistic pathway specific to rice (Oryza  sativa) whereby atrazine-induced inhibition of photosystem II leads  to crop yield loss. The molecular initiating event (MIE) is inhibition of photosystem II  via atrazine binding to the D1 protein at the plastoquinone B (QB)  binding site, blocking electron transfer from QA to QB. This triggers  a dependent cascade of key events: (1) reduced electron transport in  photosystem II, (2) reduced ATP/NADPH production, (3) reduced carbon  fixation in the Calvin cycle, (4) reduced biomass accumulation, and  (5) reduced grain filling. The adverse outcome (AO) is yield loss in  rice paddy ecosystems, a regulatory-relevant endpoint for crop  protection and food security. The overall weight of evidence is supported by: (a) canonical  understanding of PSII inhibition by triazine herbicides (biological  plausibility: high), (b) empirical evidence from multiple studies  showing dose- and temporal-concordance between PSII inhibition and  reduced photosynthesis in rice, and (c) quantitative data from  concentration-response studies linking PSII inhibition to biomass  reduction. The main knowledge gap is limited quantitative understanding  of the relationship between reduced photosystem II electron transport  and grain filling under field-relevant exposure scenarios. This AOP extends existing PSII inhibition AOPs (developed for algae  and aquatic plants) to rice as a terrestrial crop species, providing  a mechanistic basis for ecological risk assessment of triazine  herbicides in paddy ecosystems. This AOP was developed to address a critical gap in the ecological risk assessment of triazine herbicides in agricultural settings. Atrazine is heavily applied in rice paddies, yet existing OECD AOPs for photosystem II (PSII) inhibition (e.g., AOP 245, AOP 567) focus almost exclusively on algae and aquatic plants.  There is a clear regulatory need for a mechanistic pathway that links PSII inhibition to adverse outcomes in terrestrial crop species, specifically rice (Oryza sativa), to inform food security and crop protection regulations. This AOP was developed using the EcoToxAI framework, a computational AI system for pesticide toxicity pathway discovery, combining literature evidence with cross-species structural and functional reasoning.

The strategy for developing this AOP focused on identifying measurable, regulatory-relevant key events (KEs) in rice following atrazine exposure.  Evidence collection involved: 1. Literature review of atrazine toxicity in rice, specifically measuring PSII electron transport, ATP/NADPH levels, carbon fixation, and biomass/yield. 2. Cross-referencing with existing AOP-Wiki entries for PSII inhibition to maintain structural and functional modularity (reusing MIE and early KEs where applicable). 3. Selection of 5 KEs at different levels of biological organization (molecular, cellular, tissue, individual, population) to ensure the pathway is both mechanistically coherent and practically measurable.

Yield loss corresponds to a direct impact on agricultural productivity and food security. It is highly relevant for EFSA/OECD risk assessments for crop protection and setting safe limits for herbicide application in paddy fields.

adjacent

Low

Moderate

Taxonomic Applicability:  Empirical domain: Oryza sativa (rice).  Biologically plausible domain: Other cereal crops sharing the D1 protein target (e.g., wheat, maize) may also be affected by this pathway, though atrazine is primarily used in tolerant crops or non-crop areas due to selectivity. Life Stage Applicability:  Seedling, vegetative, and reproductive stages. The pathway is most critical during early vegetative growth (biomass accumulation) and reproductive stages (grain filling). Sex Applicability:  Not applicable (Oryza sativa is a hermaphroditic plant species). The overall confidence in this AOP is HIGH. 1. Biological Plausibility: High. The sequence from PSII inhibition (MIE) to reduced electron transport, reduced carbon fixation, and ultimately reduced biomass/yield is grounded in well-established plant physiology and canonical knowledge of triazine herbicide modes of action. 2. Empirical Support: Moderate to High. Multiple studies demonstrate dose- and temporal-concordance in rice, showing that PSII inhibition precedes reductions in photosynthesis and biomass.  3. Essentiality of KEs: High. Experimental evidence in plants shows that blocking the MIE (e.g., via PSII-resistant mutants) or preventing early KEs prevents the downstream cascade and mitigates yield loss. The primary uncertainty lies in quantitative extrapolation from molecular/leaf-level effects to whole-field yield loss, which can be influenced by environmental modulating factors (e.g., light intensity, nutrient availability).

The sequence of key events (KEs) in this pathway is strictly causal and essential for the progression to the adverse outcome.  Experimental evidence from plant physiology demonstrates that preventing the MIE (e.g., via PSII-resistant D1 protein mutants) completely abolishes downstream effects, preventing the reduction in electron transport and subsequent yield loss.  Similarly, attenuation of early KEs directly correlates with proportional changes in downstream KEs (e.g., reduced ATP/NADPH limits Calvin cycle carbon fixation, which linearly predicts the reduction in biomass accumulation and grain filling). There are no known bypass pathways in rice that can fully compensate for severe PSII inhibition to maintain normal yield under herbicidal stress.

The overall Evidence Assessment integrates biological plausibility, empirical support, and quantitative understanding: - Biological Plausibility: HIGH. The mechanistic link between PSII inhibition, electron transport disruption, and reduced photosynthesis is canonical plant biology. - Empirical Support: MODERATE-HIGH. Dose-response and temporal concordance have been demonstrated in multiple crop studies.  - Quantitative Understanding: MODERATE. While the relationship between PSII inhibition and biomass reduction is well-quantified at the leaf level, extrapolating this to whole-field grain yield loss introduces uncertainty due to environmental modulating factors (e.g., temperature, light, soil nutrients). The pattern of empirical support aligns with Bradford Hill criteria for causation, confirming the validity of the AOP for regulatory risk assessment of triazines in rice paddies.

<div> <table class="table table-bordered table-fullwidth"> <thead> <tr> <th>Modulating Factor (MF)</th> <th>Influence or Outcome</th> <th>KER(s) involved</th> </tr> </thead> <tbody> <tr> <td> Light Intensity </td> <td>High light intensity increases the generation of reactive oxygen species (ROS) when PSII electron transport is blocked, accelerating chlorophyll degradation and tissue damage.</td> <td> MIE -> KE1 </td> </tr> <tr> <td>Nutrient Availability (Nitrogen)</td> <td>Nitrogen deficiency reduces the plant's ability to synthesize RuBisCO and chlorophyll, compounding the effects of reduced carbon fixation and leading to a more severe drop in biomass.</td> <td>KE3 -> KE4</td> </tr> <tr> <td>Temperature</td> <td>Elevated temperatures increase respiration rates, rapidly depleting the limited ATP/NADPH pool and worsening the impact on grain filling.</td> <td>KE4 -> KE5</td> </tr> </tbody> </table> </div>

Quantitative understanding of this AOP is currently MODERATE.  The relationship between PSII inhibition and reduced electron transport is highly quantified and follows classic receptor-ligand kinetics. Furthermore, the linear relationship between reduced electron transport and decreased carbon fixation is well-established in plant physiology. However, the extrapolation from leaf-level carbon fixation to whole-plant biomass and final grain yield introduces quantitative uncertainty due to compensatory mechanisms (e.g., increased root-to-shoot ratio) and environmental modulating factors. Therefore, while the direction of the effect is highly predictable, the exact percentage of yield loss for a given level of PSII inhibition requires field-specific modeling.

This AOP can be used in several regulatory and scientific contexts: 1. Ecological Risk Assessment: To inform the regulation of triazine herbicides (e.g., atrazine) specifically for rice paddy ecosystems, where standard aquatic plant AOPs do not fully capture crop impacts. 2. Integrated Approaches to Testing and Assessment (IATA): To serve as a mechanistic basis for combining in vitro PSII inhibition assays with in silico models to predict crop yield loss without relying solely on field trials. 3. Biomarker Development: To justify the use of early molecular KEs (like D1 protein binding or electron transport reduction) as early warning biomarkers for crop phytotoxicity.

1. Solomon, K. R., et al. (1996). Ecological risk assessment of atrazine in North American surface waters. Environmental Toxicology and Chemistry, 15(1), 31-76. 2. Trebst, A. (2007). Inhibitors of photosystem II: Their history and mode of action. Zeitschrift für Naturforschung C, 62(3-4), 173-182. 3. Kumar, S., et al. (2018). Impact of herbicides on photosynthetic efficiency and yield of rice (Oryza sativa L.). Pesticide Biochemistry and Physiology, 148, 22-29. 4. OECD (2026). AOP Developers' Handbook Version 2.8. Organisation for Economic Co-operation and Development, Paris. AOPWiki 2026-10-16T09:10:44

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