Current IssueEditorial BoardOnline First ArticlesArchive
Current IssueEditorial BoardOnline First ArticlesArchive

Review Article

Allelochemicals as Bioherbicide: Potentials, Applications and Future Perspectives

E
Emine Girişken Güral1
A
Abdul Mumeen Iddrisu1
İ
İnanç Özgen2
1Department of Biotechnology, Institute of Science, Fırat University, Elazığ, Türkiye.
2Department of Bioengineering, Faculty of Engineering, Fırat University, Elazığ, Türkiye.

ABSTRACT

Repeated herbicide use and herbicide resistance have increased interest in allelochemicals for weed management. This narrative review synthesizes published evidence on mechanisms of action, major compound classes, agricultural applications and barriers to bioherbicide development. Allelochemicals can inhibit germination and seedling growth through effects on membrane integrity, cell division, photosynthesis, hormonal signaling and oxidative balance. Phenolics, benzoxazinoids, sorgoleone and terpenoids offer different opportunities for crop breeding, residue management and formulation development. Laboratory activity does not establish field efficacy or crop selectivity. Soil adsorption, microbial transformation, environmental instability, dose variability and non-target effects constrain application. Cover crops and mulches combine chemical interference with physical weed suppression, so their performance cannot be attributed to allelopathy alone. Allelochemicals offer useful components of integrated weed management, but reliable deployment requires standardized formulations and replicated field validation of efficacy, crop safety, environmental fate and economic feasibility.

INTRODUCTION

Weeds compete with crops for light, water, nutrients and growing space, reducing crop yield and quality (Jabran et al., 2015; Mennan et al., 2020). Oerke (2006) estimated substantial potential crop losses from weeds across major crops, emphasizing the importance of effective management. These estimates describe potential losses under specified crop and management conditions and should not be interpreted as a universal loss rate. Sustainable weed management must maintain crop productivity while limiting unnecessary chemical inputs.
       
Synthetic herbicides remain effective weed management tools, but repeated use of the same mode of action selects for resistant populations (Duke and Powles, 2008; Norsworthy et al., 2012). Environmental and non-target risks vary with the active ingredient, formulation, exposure and application conditions (Carvalho, 2017). Resistance and these exposure-related concerns support the development of complementary management strategies rather than assuming that all synthetic herbicides present identical risks.
       
Integrated weed management combines cultural, mechanical, biological and chemical measures (Jabran et al., 2015; Norsworthy et al., 2012). Allelopathy provides one possible contribution through chemical interactions among plants and their environment. Its agricultural value depends on whether biologically active compounds reach susceptible weeds at effective concentrations without unacceptable effects on crops or other organisms (Macias  et al., 2007; Scavo and Mauromicale, 2021).
       
Allelopathy describes beneficial or inhibitory effects mediated by chemicals released by organisms into their surroundings. In plant systems, allelochemicals can be released through root exudation, volatilization, leaching from aerial tissues and decomposition of residues (Cheng and Cheng, 2015; Singh et al., 2003; Weston et al., 2012). They include phenolics, terpenoids, alkaloids, benzoxazinoids and quinones. Direct phytotoxicity and interactions with soil processes contribute to their biological activity.
       
Natural origin does not establish environmental safety. Allelochemicals require compound-specific evaluation of dose, selectivity, persistence and non-target effects (Duke et al., 2000; Macias et al., 2007). Sorgoleone from sorghum, benzoxazinoids from cereals and phenolic compounds illustrate the chemical diversity available for bioherbicide discovery (Li et al., 2010; Niemeyer, 2009; Schulz et al., 2013). Some compounds suppress germination and seedling growth in controlled assays, but this activity must be tested under realistic soil and crop conditions (Khamare et al., 2022).
       
Translation into agricultural practice is limited by soil adsorption, microbial transformation, environmental instability and inconsistent exposure (Inderjit and Duke, 2003; Weston and Duke, 2003). Formulation, dose standardization, crop injury, production cost and regulatory requirements also affect product development. Cover crops, residues and crop varieties may therefore offer practical routes for exploiting allelopathy alongside other weed management measures (Scavo and Mauromicale, 2021).

This review evaluates mechanisms of action, major allelochemical classes, application strategies and barriers to agricultural use. It distinguishes laboratory phytotoxicity from field performance and separates chemical interference from physical crop competition and mulch effects. The discussion identifies research priorities for integrating allelopathy into weed management and developing reproducible, selective bioherbicide formulations (Khamare et al., 2022; Scavo and Mauromicale, 2021).
       
This article is a narrative review of published literature and does not report an original experiment, a systematic review, or a meta-analysis. The synthesis addresses mechanisms of action, allelochemical classes, applications and constraints on bioherbicide development. Laboratory, greenhouse and field findings are considered separately. Reported outcomes are interpreted in relation to exposure conditions, crop selectivity and whether weed suppression can be attributed to chemical interference. No new experimental data or statistical analysis were generated.
       
Institutions: Department of Biotechnology, Institute of Science and Department of Bioengineering, Faculty of Engineering, Fýrat University, Elazýð, Türkiye. Literature coverage period: 1994-2026. This period describes the literature considered and is not an experimental period.
 
Mechanisms of action of allelochemicals
 
Allelochemicals can affect several physiological and biochemical processes, including germination, membrane function, cell growth and energy metabolism (Inderjit and Duke, 2003; Macias et al., 2007). Understanding these responses can guide candidate selection, but a visible growth response does not by itself identify the primary molecular target. Analytical and omics approaches complement conventional bioassays in resolving these mechanisms (Khamare et al., 2022) (Fig 1.)

Fig 1: Release pathways and environmental processes governing exposure to plant allelochemicals.


 
Suppression of germination and early seedling development
 
Allelochemicals commonly inhibit germination or early seedling growth by affecting water uptake, membrane function, reserve mobilization and energy metabolism (Inderjit and Duke, 2003; Macias et al., 2007). Some phenolics and quinones interfere with hydrolytic enzyme activity, reducing the availability of reserves for embryo growth (Li et al., 2010). These responses depend on the compound, concentration and target species; inhibition in an extract assay does not by itself demonstrate allelopathy under field conditions.
       
Altered membrane permeability and oxidative balance can contribute to reduced seed water uptake and impaired seedling growth. Reactive oxygen species (ROS) may accumulate during allelochemical exposure, promoting lipid peroxidation and membrane injury when antioxidant defenses are insufficient (Wang et al., 2025; Khamare et al., 2022). The timing and magnitude of these responses vary with exposure and target species.
 
Inhibition of cell division and cell elongation
 
Cell division and elongation are important targets of allelochemical exposure. Nishida et al., (2005) demonstrated inhibition of cell proliferation and DNA synthesis in the root apical meristem of Brassica campestris exposed to volatile monoterpenoids from Salvia leucophylla. Changes in mitotic activity and growth regulation can restrict root development, although mechanisms must be established for each compound rather than generalized across all allelochemicals (Inderjit and Duke, 2003; Macias et al., 2007). Root shortening can reduce access to water and nutrients and thereby contribute to reduced competitive ability (Fig 2).

Fig 2: Physiological processes affected by allelochemical exposure in susceptible plants.


 
Disruption of photosynthesis and energy metabolism
 
Photosynthesis is an important target of several allelochemicals. Changes in chlorophyll content, chloroplast structure and electron transport can reduce carbon assimilation and biomass accumulation (Macias et al., 2007). Sorgoleone inhibits photosynthetic electron transport associated with photosystem II, but its effects depend on uptake, tissue exposure and plant developmental stage (Dayan et al., 2009; Weston et al., 2013). Reduced photosynthetic performance can also occur as a secondary consequence of membrane injury or oxidative stress. Distinguishing a primary biochemical target from secondary symptoms is necessary for mechanism-based bioherbicide development.
 
Oxidative stress and reactive oxygen species (ROS) accumulation
 
Excess ROS can damage lipids, proteins and nucleic acids, impairing membrane integrity and cellular metabolism (Macias et al., 2007; Wang et al., 2025). Antioxidant enzymes such as superoxide dismutase, catalase and peroxidases regulate oxidative balance, but their responses vary with the compound and exposure level (Inderjit and Duke, 2003). Oxidative stress can contribute to growth inhibition while also reflecting injury initiated through other pathways. Measurements of ROS and antioxidant activity should therefore be interpreted alongside biochemical and growth responses rather than as proof of a single mechanism.
 
Disruption of hormonal balance
 
Allelochemicals can affect hormone metabolism and signaling, influencing root development, germination and stress responses (Inderjit and Duke, 2003; Macias et al., 2007). Changes involving auxin, ethylene and reactive oxygen species have been demonstrated for benzoic acid-induced primary root inhibition in Arabidopsis (Zhang et al., 2018). Such responses are compound-specific and should not be assumed to occur identically across allelochemical classes.
       
Hormonal responses interact with oxidative stress and growth regulation. Changes in gene expression or hormone abundance can identify pathways involved in phytotoxicity, but they require validation before being interpreted as direct molecular targets (Zhang et al., 2018).
 
Effects at the molecular level and gene expression
 
Transcriptomic and metabolomic approaches reveal changes in stress, growth and metabolic pathways following exposure to allelochemicals (Khamare et al., 2022). For example, Zhang et al., (2018) linked benzoic acid-induced root inhibition to auxin, ethylene and ROS signaling in Arabidopsis. Such findings connect physiological responses with molecular regulation, but expression changes may represent downstream stress responses. Direct target identification requires complementary functional experiments. Molecular evidence also does not establish field efficacy or selectivity without relevant exposure and crop testing.
 
Allelochemical classes and their roles in weed control
 
Allelochemicals span several chemical classes with different structures, release pathways and biological activities. Phenolics, terpenoids, alkaloids, benzoxazinoids and quinones provide representative examples (Li et al., 2010; Macias et al., 2007). Production and release can vary with genotype, developmental stage and environmental conditions (Scavo and Mauromicale, 2021). Classification helps organize candidate discovery, but structure alone does not predict agricultural efficacy or safety.

The phenolic compounds
 
Phenolic allelochemicals include phenolic acids, flavonoids and tannins. Ferulic, p-coumaric and vanillic acids occur in various crop and non-crop plants and can affect membrane integrity, enzyme activity and oxidative balance (Li et al., 2010). Their activity in soil depends on sorption, transformation and continued release from plant material (Weston et al., 2012). In a controlled study, Sun et al., (2021) identified o-coumaric acid in hairy vetch and demonstrated inhibition of alfalfa root growth. This finding supports the importance of chemically characterized bioassays while also showing that crop plants can be susceptible. Phenolic activity should therefore be assessed through dose-response experiments with both target weeds and crops before field recommendations are made.
 
Benzoxazinoids
 
Benzoxazinoids occur in cereals including wheat, maize and rye. DIBOA and DIMBOA, their glycosides and their transformation products contribute to plant defense and allelopathic interactions (Niemeyer, 2009; Schulz et al., 2013; Wouters et al., 2016). Biological activity depends on chemical form, release and subsequent transformation. Their agricultural relevance includes residue management and selection of allelopathic crop genotypes. Because soil processes alter exposure, the presence of benzoxazinoids in plant tissue alone does not establish weed suppression in a field crop (Schulz et al., 2013).
 
Sorgoleone
 
Sorgoleone is a lipophilic benzoquinone produced by sorghum root hairs and released in root exudates (Cook et al., 2010). Its distribution near producing roots and interaction with soil affect exposure to neighboring plants (Weston et al., 2013). Inhibition of photosystem II electron transport is a well-studied component of its activity, but tissue uptake and seedling stage influence the observed response (Dayan et al., 2009). These properties make sorgoleone a useful model for natural-product herbicide research while emphasizing the difference between root-associated production and delivery as a formulated treatment.
       
Sorghum residues and cover crops can suppress weed growth, but their performance combines physical competition, residue effects and chemical interference. The contribution of sorgoleone cannot be inferred directly from total weed biomass reduction (Weston et al., 2013). Its hydrophobicity and restricted distribution near producing roots also create challenges for extraction, formulation and consistent field delivery (Duke et al., 2000; Dayan et al., 2009).
 
Terpenoids and volatile organic compounds (VOCs)
 
Terpenoids include volatile compounds from aromatic plants such as Eucalyptus, Artemisia and Salvia. Compounds including 1,8-cineole, camphor and pinene derivatives have been investigated for effects on germination and plant growth (Singh et al., 2003; Verdeguer et al., 2020). Reported mechanisms include changes in membrane function, cell proliferation, respiration and oxidative balance. Terpenoids also participate in broader ecological interactions, so their presence in a plant does not establish an exclusively herbicidal function (Boncan et al., 2020; Kong et al., 2024).
       
Volatility and environmental instability can limit exposure duration and complicate field delivery. Encapsulation and other carrier technologies offer possible approaches for improving stability, but treatment efficacy, crop injury, carrier effects and production costs require direct evaluation (Duke et al., 2024; Verdeguer et al., 2020).
 
Alkaloids and other nitrogenous compounds
 
Alkaloids such as gramine and caffeine provide additional candidates for studying plant growth inhibition. However, their activity and non-target toxicity vary and evidence from other biological systems should not be treated as proof of a weed-specific mechanism (Duke et al., 2000; Shimasaki and Nakano, 2025). Agricultural assessment requires chemical characterization, dose-response analysis and crop-selectivity testing. More broadly, nitrogen-containing metabolites can affect interactions in the rhizosphere, making microbial and environmental responses relevant to safety assessment (Shimasaki and Nakano, 2025).
 
Comparative evaluation of allelochemical classes
 
Compound classes differ in solubility, volatility, transformation and biological activity. These features influence whether a candidate is better investigated through crop traits, residue management, extracts, or formulated products (Scavo and Mauromicale, 2021). Comparisons should use defined compounds and exposure conditions because substantial differences occur within each class. Table 1 summarizes representative chemistry and constraints without assigning universal mechanisms or efficacy rankings.

Table 1: Comparative characteristics of allelochemical classes.



Field applications and case analyses
 
Field performance depends on soil, weather, microbial activity, crop development and management. Controlled assays can therefore overestimate activity where effective exposure is difficult to maintain (Macias et al., 2007; Weston and Duke, 2003). Cover crops, residues and crop varieties integrate chemical release into the production system, but their effects remain sensitive to management and cannot be separated from competition without suitable controls (Kruidhof et al., 2008; Scavo and Mauromicale, 2021).
 
Field-scale use of allelopathic cover crops
 
Rye, wheat and sorghum are widely investigated for weed suppression through biomass production, competition and chemical interference (Weston et al., 2013; Scavo and Mauromicale, 2021). Cover crops can reduce weed establishment, but their effects depend on species, biomass, termination timing and the following crop (Kruidhof et al., 2008). Table 2 summarizes application features qualitatively; pooled efficacy percentages are not presented because the cited studies use different conditions and response measures.

​

Table 2: Weed suppression effects of allelopathic cover crops under field conditions.


       
A cover crop reduces light penetration and changes soil moisture and temperature, alongside any chemical effects. Consequently, field suppression should be interpreted as the combined outcome of physical and biochemical processes (Jabran et al., 2015; Kruidhof et al., 2008). Crop injury or delayed establishment may occur in susceptible crop species. Species choice, residue decomposition and the interval between termination and planting are therefore important management variables.
 
Plant residues and mulch applications
 
Plant residues and organic mulches can release allelochemicals during decomposition while forming a physical barrier to emergence (Weston et al., 2013). Release and transformation depend on residue composition, soil moisture, temperature and microbial activity (Scavo and Mauromicale, 2021). Mulch thickness and physical properties also affect weed emergence independently of allelopathy (Teasdale and Mohler, 2000). These mechanisms must be separated experimentally before attributing suppression to specific chemicals.
 
Field performance of allelopathic crop varieties
 
Crop varieties differ in competitive ability and allelochemical production. Selecting for weed suppression may support management in cereals, but field effects reflect both chemical traits and crop growth characteristics (Worthington and Reberg-Horton, 2013). The amount and composition of released compounds can change with soil and weather, limiting consistency across environments. Genotype screening should therefore include field performance and measurements that distinguish chemical interference from ordinary competition (Scavo and Mauromicale, 2021).
       
Breeding objectives should retain crop yield, quality and establishment while improving weed suppression. Greater allelochemical production cannot be assumed to improve overall agronomic performance. Evaluating weed responses alongside crop outcomes is necessary to identify useful genotypes and potential tradeoffs (Worthington and Reberg-Horton, 2013).
 
Plant extracts and direct field applications
 
Plant extracts provide a direct route for testing allelochemical-based treatments, but laboratory efficacy often exceeds field performance because exposure changes with sunlight, rainfall and soil adsorption (Jabran et al., 2015; Macias et al., 2007). Oraon and Mondal (2022) demonstrated concentration-dependent effects of Streblus asper leaf extracts on chickpea in laboratory assays. This is evidence of phytotoxicity to a crop and reinforces the need to assess crop safety alongside weed suppression. Extract composition and application dose must be standardized before treatment effects can be compared across studies. Formulation approaches may improve delivery, but their field efficacy and production costs require independent validation (Duke et al., 2024). In a field study, Kawuryan et al., (2026) evaluated Leucaena leucocephala and Carica papaya leaf extracts in Japanese cucumber and reported treatment-dependent weed suppression and crop responses. Applications were directed away from crop foliage, so this result should be interpreted within the reported application method and site conditions rather than as proof of universal crop safety.
 
Allelopathy in integrated weed management systems
 
Allelopathic practices can complement cover cropping, mechanical control, crop rotation and appropriately selected herbicide treatments (Jabran et al., 2015; Mennan et al., 2020). Sathishkumar et al., (2020) reviewed the use of crop and tree allelopathy in weed management, including residues and extracts. Such strategies should be evaluated against explicit weed control and crop yield objectives rather than assuming a fixed percentage reduction in herbicide use. Table 3 identifies the contributions and constraints of alternative management combinations (Norsworthy et al., 2012; Scavo and Mauromicale, 2021).

​

Table 3: Contribution of allelopathic strategies in integrated systems.


 
Challenges and limitations encountered in bioherbicide development
 
Laboratory-field discrepancy
 
Laboratory bioassays establish phytotoxic potential under defined exposure, but soil and field environments alter the amount of active compound reaching a target plant. Sorption to organic matter or minerals, microbial transformation and weather-dependent transport can reduce or modify activity (Inderjit and Duke, 2003; Li et al., 2013). Extract assays also confound chemical interference with effects of pH, osmotic conditions and nutrients unless suitable controls are included. Field validation therefore requires realistic exposure measurements and comparisons that distinguish allelopathy from competition or physical mulch effects (Khamare et al., 2022; Sahrir et al., 2023).

Stability and persistence issues
 
Persistence differs among compounds and soils. Photodegradation, hydrolysis, volatilization, microbial metabolism and adsorption can change both activity and bioavailability (Weston et al., 2012; Li et al., 2013). Rainfall and irrigation may redistribute compounds away from the treated area. Short persistence can limit residual weed control, whereas prolonged exposure may increase crop injury or effects on non-target organisms. Assessing the parent compound and its transformation products is therefore more informative than treating rapid degradation as an automatic safety advantage (Duke et al., 2000; Sahrir et al., 2023).
 
Dose standardization and application challenges
 
Dose standardization is complicated by variation in genotype, plant part, developmental stage, extraction procedure and storage. An extract concentration expressed only as a percentage cannot be transferred across studies without defining the mass, solvent volume and composition. Both target weeds and crops require dose-response testing under relevant soil conditions (Jabran et al., 2015; Scavo and Mauromicale, 2021). Application timing, weed stage and delivery method should be reported because they determine effective exposure. Evidence of non-target activity must be assessed for each candidate rather than inferred from natural origin (Duke et al., 2024).
 
Formulation, legal regulations and technological limitations
 
Limited solubility, chemical instability and variable composition complicate formulation. Encapsulation and controlled-release carriers may improve delivery, but efficacy must be evaluated together with carrier toxicity, shelf life, manufacturing cost and scalability (Duke et al., 2024). A successful laboratory formulation is not sufficient evidence of an economically viable agricultural product. Candidate formulations should be compared with current weed management practices using crop yield, operational costs and treatment frequency as practical endpoints.
       
Registration requirements depend on the jurisdiction and product category. Natural origin does not exempt a candidate from evaluation of efficacy, exposure and toxicological or environmental risks. Product adoption also depends on availability, reliable performance, ease of use and cost relative to existing practices (Scavo and Mauromicale, 2021). These considerations should enter development early, alongside biological screening.

FUTURE PERSPECTIVES AND RESEARCH GAPS

Research should prioritize compounds and management practices with reproducible activity under realistic exposure. Strong inhibition in a screening assay is an initial finding, while commercial or agronomic usefulness requires crop selectivity, predictable delivery and consistent performance (Duke et al., 2024; Khamare et al., 2022). The priorities below address the transition from chemical discovery to agricultural application.
       
Mechanistic studies should distinguish direct biochemical targets from general stress responses. Transcriptomic and metabolomic analyses can identify candidate pathways, but complementary biochemical assays and dose-response experiments are needed to test causality. Multiple affected pathways do not establish that a treatment will prevent resistance; resistance selection and cross-resistance require direct evaluation (Duke et al., 2000, 2024).
       
Omics approaches should be linked to measurements of compounds and transformation products in soil and plant tissues. Such measurements help determine whether concentrations associated with laboratory activity are achieved in the rhizosphere or after application. Studies should report sampling time, analytical recovery and exposure conditions to support comparisons across sites (Khamare et al., 2022; Weston et al., 2012).
       
Microbial degradation and transformation influence persistence and biological activity. Future work should connect changes in microbial communities with functional measurements rather than assuming that a community shift improves weed suppression. Evaluating soil-dependent transformations can identify conditions that support or limit candidate performance (Li et al., 2013; Khamare et al., 2022).
       
Breeding for weed suppression should evaluate allelopathic traits alongside competitive ability, yield, crop quality and stress tolerance. Genetic or biochemical evidence of greater allelochemical production requires confirmation of agronomic benefit across environments. Weed suppression caused by canopy structure or growth rate must be separated from chemical interference (Worthington and Reberg-Horton, 2013; Scavo and Mauromicale, 2021).
       
Formulation research should compare conventional and controlled-release delivery under the same field conditions. Stability, crop injury, non-target effects and the fate of carrier materials should accompany efficacy measurements. Precision application may reduce unnecessary exposure, but its value depends on demonstrated treatment performance and practical cost (Duke et al., 2024).
       
Replicated trials across locations and seasons are essential for defining where allelopathic practices are dependable. Trials should include untreated controls, current management comparators, crop yield, weed community responses and cost measures. Combining these data with compound-specific environmental assessment will support evidence-based integration into weed management (Norsworthy et al., 2012; Scavo and Mauromicale, 2021).
 

CONCLUSION

Allelochemicals offer useful components of integrated weed management through crop traits, residues, extracts and candidate bioherbicide formulations. Their agricultural value depends on reproducible weed suppression at crop-safe doses under realistic field conditions. Standardized composition, reliable delivery and replicated field trials are the key requirements for translating laboratory phytotoxicity into practical use. Environmental fate, non-target effects and economic feasibility must be assessed alongside efficacy before routine agricultural deployment.

CONFLICT OF INTEREST

 The authors declare that they have no conflicts of interest related to this manuscript.

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    Review Article

    Allelochemicals as Bioherbicide: Potentials, Applications and Future Perspectives

    E
    Emine Girişken Güral1
    A
    Abdul Mumeen Iddrisu1
    İ
    İnanç Özgen2
    1Department of Biotechnology, Institute of Science, Fırat University, Elazığ, Türkiye.
    2Department of Bioengineering, Faculty of Engineering, Fırat University, Elazığ, Türkiye.

    ABSTRACT

    Repeated herbicide use and herbicide resistance have increased interest in allelochemicals for weed management. This narrative review synthesizes published evidence on mechanisms of action, major compound classes, agricultural applications and barriers to bioherbicide development. Allelochemicals can inhibit germination and seedling growth through effects on membrane integrity, cell division, photosynthesis, hormonal signaling and oxidative balance. Phenolics, benzoxazinoids, sorgoleone and terpenoids offer different opportunities for crop breeding, residue management and formulation development. Laboratory activity does not establish field efficacy or crop selectivity. Soil adsorption, microbial transformation, environmental instability, dose variability and non-target effects constrain application. Cover crops and mulches combine chemical interference with physical weed suppression, so their performance cannot be attributed to allelopathy alone. Allelochemicals offer useful components of integrated weed management, but reliable deployment requires standardized formulations and replicated field validation of efficacy, crop safety, environmental fate and economic feasibility.

    INTRODUCTION

    Weeds compete with crops for light, water, nutrients and growing space, reducing crop yield and quality (Jabran et al., 2015; Mennan et al., 2020). Oerke (2006) estimated substantial potential crop losses from weeds across major crops, emphasizing the importance of effective management. These estimates describe potential losses under specified crop and management conditions and should not be interpreted as a universal loss rate. Sustainable weed management must maintain crop productivity while limiting unnecessary chemical inputs.
           
    Synthetic herbicides remain effective weed management tools, but repeated use of the same mode of action selects for resistant populations (Duke and Powles, 2008; Norsworthy et al., 2012). Environmental and non-target risks vary with the active ingredient, formulation, exposure and application conditions (Carvalho, 2017). Resistance and these exposure-related concerns support the development of complementary management strategies rather than assuming that all synthetic herbicides present identical risks.
           
    Integrated weed management combines cultural, mechanical, biological and chemical measures (Jabran et al., 2015; Norsworthy et al., 2012). Allelopathy provides one possible contribution through chemical interactions among plants and their environment. Its agricultural value depends on whether biologically active compounds reach susceptible weeds at effective concentrations without unacceptable effects on crops or other organisms (Macias  et al., 2007; Scavo and Mauromicale, 2021).
           
    Allelopathy describes beneficial or inhibitory effects mediated by chemicals released by organisms into their surroundings. In plant systems, allelochemicals can be released through root exudation, volatilization, leaching from aerial tissues and decomposition of residues (Cheng and Cheng, 2015; Singh et al., 2003; Weston et al., 2012). They include phenolics, terpenoids, alkaloids, benzoxazinoids and quinones. Direct phytotoxicity and interactions with soil processes contribute to their biological activity.
           
    Natural origin does not establish environmental safety. Allelochemicals require compound-specific evaluation of dose, selectivity, persistence and non-target effects (Duke et al., 2000; Macias et al., 2007). Sorgoleone from sorghum, benzoxazinoids from cereals and phenolic compounds illustrate the chemical diversity available for bioherbicide discovery (Li et al., 2010; Niemeyer, 2009; Schulz et al., 2013). Some compounds suppress germination and seedling growth in controlled assays, but this activity must be tested under realistic soil and crop conditions (Khamare et al., 2022).
           
    Translation into agricultural practice is limited by soil adsorption, microbial transformation, environmental instability and inconsistent exposure (Inderjit and Duke, 2003; Weston and Duke, 2003). Formulation, dose standardization, crop injury, production cost and regulatory requirements also affect product development. Cover crops, residues and crop varieties may therefore offer practical routes for exploiting allelopathy alongside other weed management measures (Scavo and Mauromicale, 2021).

    This review evaluates mechanisms of action, major allelochemical classes, application strategies and barriers to agricultural use. It distinguishes laboratory phytotoxicity from field performance and separates chemical interference from physical crop competition and mulch effects. The discussion identifies research priorities for integrating allelopathy into weed management and developing reproducible, selective bioherbicide formulations (Khamare et al., 2022; Scavo and Mauromicale, 2021).
           
    This article is a narrative review of published literature and does not report an original experiment, a systematic review, or a meta-analysis. The synthesis addresses mechanisms of action, allelochemical classes, applications and constraints on bioherbicide development. Laboratory, greenhouse and field findings are considered separately. Reported outcomes are interpreted in relation to exposure conditions, crop selectivity and whether weed suppression can be attributed to chemical interference. No new experimental data or statistical analysis were generated.
           
    Institutions: Department of Biotechnology, Institute of Science and Department of Bioengineering, Faculty of Engineering, Fýrat University, Elazýð, Türkiye. Literature coverage period: 1994-2026. This period describes the literature considered and is not an experimental period.
     
    Mechanisms of action of allelochemicals
     
    Allelochemicals can affect several physiological and biochemical processes, including germination, membrane function, cell growth and energy metabolism (Inderjit and Duke, 2003; Macias et al., 2007). Understanding these responses can guide candidate selection, but a visible growth response does not by itself identify the primary molecular target. Analytical and omics approaches complement conventional bioassays in resolving these mechanisms (Khamare et al., 2022) (Fig 1.)

    Fig 1: Release pathways and environmental processes governing exposure to plant allelochemicals.


     
    Suppression of germination and early seedling development
     
    Allelochemicals commonly inhibit germination or early seedling growth by affecting water uptake, membrane function, reserve mobilization and energy metabolism (Inderjit and Duke, 2003; Macias et al., 2007). Some phenolics and quinones interfere with hydrolytic enzyme activity, reducing the availability of reserves for embryo growth (Li et al., 2010). These responses depend on the compound, concentration and target species; inhibition in an extract assay does not by itself demonstrate allelopathy under field conditions.
           
    Altered membrane permeability and oxidative balance can contribute to reduced seed water uptake and impaired seedling growth. Reactive oxygen species (ROS) may accumulate during allelochemical exposure, promoting lipid peroxidation and membrane injury when antioxidant defenses are insufficient (Wang et al., 2025; Khamare et al., 2022). The timing and magnitude of these responses vary with exposure and target species.
     
    Inhibition of cell division and cell elongation
     
    Cell division and elongation are important targets of allelochemical exposure. Nishida et al., (2005) demonstrated inhibition of cell proliferation and DNA synthesis in the root apical meristem of Brassica campestris exposed to volatile monoterpenoids from Salvia leucophylla. Changes in mitotic activity and growth regulation can restrict root development, although mechanisms must be established for each compound rather than generalized across all allelochemicals (Inderjit and Duke, 2003; Macias et al., 2007). Root shortening can reduce access to water and nutrients and thereby contribute to reduced competitive ability (Fig 2).

    Fig 2: Physiological processes affected by allelochemical exposure in susceptible plants.


     
    Disruption of photosynthesis and energy metabolism
     
    Photosynthesis is an important target of several allelochemicals. Changes in chlorophyll content, chloroplast structure and electron transport can reduce carbon assimilation and biomass accumulation (Macias et al., 2007). Sorgoleone inhibits photosynthetic electron transport associated with photosystem II, but its effects depend on uptake, tissue exposure and plant developmental stage (Dayan et al., 2009; Weston et al., 2013). Reduced photosynthetic performance can also occur as a secondary consequence of membrane injury or oxidative stress. Distinguishing a primary biochemical target from secondary symptoms is necessary for mechanism-based bioherbicide development.
     
    Oxidative stress and reactive oxygen species (ROS) accumulation
     
    Excess ROS can damage lipids, proteins and nucleic acids, impairing membrane integrity and cellular metabolism (Macias et al., 2007; Wang et al., 2025). Antioxidant enzymes such as superoxide dismutase, catalase and peroxidases regulate oxidative balance, but their responses vary with the compound and exposure level (Inderjit and Duke, 2003). Oxidative stress can contribute to growth inhibition while also reflecting injury initiated through other pathways. Measurements of ROS and antioxidant activity should therefore be interpreted alongside biochemical and growth responses rather than as proof of a single mechanism.
     
    Disruption of hormonal balance
     
    Allelochemicals can affect hormone metabolism and signaling, influencing root development, germination and stress responses (Inderjit and Duke, 2003; Macias et al., 2007). Changes involving auxin, ethylene and reactive oxygen species have been demonstrated for benzoic acid-induced primary root inhibition in Arabidopsis (Zhang et al., 2018). Such responses are compound-specific and should not be assumed to occur identically across allelochemical classes.
           
    Hormonal responses interact with oxidative stress and growth regulation. Changes in gene expression or hormone abundance can identify pathways involved in phytotoxicity, but they require validation before being interpreted as direct molecular targets (Zhang et al., 2018).
     
    Effects at the molecular level and gene expression
     
    Transcriptomic and metabolomic approaches reveal changes in stress, growth and metabolic pathways following exposure to allelochemicals (Khamare et al., 2022). For example, Zhang et al., (2018) linked benzoic acid-induced root inhibition to auxin, ethylene and ROS signaling in Arabidopsis. Such findings connect physiological responses with molecular regulation, but expression changes may represent downstream stress responses. Direct target identification requires complementary functional experiments. Molecular evidence also does not establish field efficacy or selectivity without relevant exposure and crop testing.
     
    Allelochemical classes and their roles in weed control
     
    Allelochemicals span several chemical classes with different structures, release pathways and biological activities. Phenolics, terpenoids, alkaloids, benzoxazinoids and quinones provide representative examples (Li et al., 2010; Macias et al., 2007). Production and release can vary with genotype, developmental stage and environmental conditions (Scavo and Mauromicale, 2021). Classification helps organize candidate discovery, but structure alone does not predict agricultural efficacy or safety.

    The phenolic compounds
     
    Phenolic allelochemicals include phenolic acids, flavonoids and tannins. Ferulic, p-coumaric and vanillic acids occur in various crop and non-crop plants and can affect membrane integrity, enzyme activity and oxidative balance (Li et al., 2010). Their activity in soil depends on sorption, transformation and continued release from plant material (Weston et al., 2012). In a controlled study, Sun et al., (2021) identified o-coumaric acid in hairy vetch and demonstrated inhibition of alfalfa root growth. This finding supports the importance of chemically characterized bioassays while also showing that crop plants can be susceptible. Phenolic activity should therefore be assessed through dose-response experiments with both target weeds and crops before field recommendations are made.
     
    Benzoxazinoids
     
    Benzoxazinoids occur in cereals including wheat, maize and rye. DIBOA and DIMBOA, their glycosides and their transformation products contribute to plant defense and allelopathic interactions (Niemeyer, 2009; Schulz et al., 2013; Wouters et al., 2016). Biological activity depends on chemical form, release and subsequent transformation. Their agricultural relevance includes residue management and selection of allelopathic crop genotypes. Because soil processes alter exposure, the presence of benzoxazinoids in plant tissue alone does not establish weed suppression in a field crop (Schulz et al., 2013).
     
    Sorgoleone
     
    Sorgoleone is a lipophilic benzoquinone produced by sorghum root hairs and released in root exudates (Cook et al., 2010). Its distribution near producing roots and interaction with soil affect exposure to neighboring plants (Weston et al., 2013). Inhibition of photosystem II electron transport is a well-studied component of its activity, but tissue uptake and seedling stage influence the observed response (Dayan et al., 2009). These properties make sorgoleone a useful model for natural-product herbicide research while emphasizing the difference between root-associated production and delivery as a formulated treatment.
           
    Sorghum residues and cover crops can suppress weed growth, but their performance combines physical competition, residue effects and chemical interference. The contribution of sorgoleone cannot be inferred directly from total weed biomass reduction (Weston et al., 2013). Its hydrophobicity and restricted distribution near producing roots also create challenges for extraction, formulation and consistent field delivery (Duke et al., 2000; Dayan et al., 2009).
     
    Terpenoids and volatile organic compounds (VOCs)
     
    Terpenoids include volatile compounds from aromatic plants such as Eucalyptus, Artemisia and Salvia. Compounds including 1,8-cineole, camphor and pinene derivatives have been investigated for effects on germination and plant growth (Singh et al., 2003; Verdeguer et al., 2020). Reported mechanisms include changes in membrane function, cell proliferation, respiration and oxidative balance. Terpenoids also participate in broader ecological interactions, so their presence in a plant does not establish an exclusively herbicidal function (Boncan et al., 2020; Kong et al., 2024).
           
    Volatility and environmental instability can limit exposure duration and complicate field delivery. Encapsulation and other carrier technologies offer possible approaches for improving stability, but treatment efficacy, crop injury, carrier effects and production costs require direct evaluation (Duke et al., 2024; Verdeguer et al., 2020).
     
    Alkaloids and other nitrogenous compounds
     
    Alkaloids such as gramine and caffeine provide additional candidates for studying plant growth inhibition. However, their activity and non-target toxicity vary and evidence from other biological systems should not be treated as proof of a weed-specific mechanism (Duke et al., 2000; Shimasaki and Nakano, 2025). Agricultural assessment requires chemical characterization, dose-response analysis and crop-selectivity testing. More broadly, nitrogen-containing metabolites can affect interactions in the rhizosphere, making microbial and environmental responses relevant to safety assessment (Shimasaki and Nakano, 2025).
     
    Comparative evaluation of allelochemical classes
     
    Compound classes differ in solubility, volatility, transformation and biological activity. These features influence whether a candidate is better investigated through crop traits, residue management, extracts, or formulated products (Scavo and Mauromicale, 2021). Comparisons should use defined compounds and exposure conditions because substantial differences occur within each class. Table 1 summarizes representative chemistry and constraints without assigning universal mechanisms or efficacy rankings.

    Table 1: Comparative characteristics of allelochemical classes.



    Field applications and case analyses
     
    Field performance depends on soil, weather, microbial activity, crop development and management. Controlled assays can therefore overestimate activity where effective exposure is difficult to maintain (Macias et al., 2007; Weston and Duke, 2003). Cover crops, residues and crop varieties integrate chemical release into the production system, but their effects remain sensitive to management and cannot be separated from competition without suitable controls (Kruidhof et al., 2008; Scavo and Mauromicale, 2021).
     
    Field-scale use of allelopathic cover crops
     
    Rye, wheat and sorghum are widely investigated for weed suppression through biomass production, competition and chemical interference (Weston et al., 2013; Scavo and Mauromicale, 2021). Cover crops can reduce weed establishment, but their effects depend on species, biomass, termination timing and the following crop (Kruidhof et al., 2008). Table 2 summarizes application features qualitatively; pooled efficacy percentages are not presented because the cited studies use different conditions and response measures.

    ​

    Table 2: Weed suppression effects of allelopathic cover crops under field conditions.


           
    A cover crop reduces light penetration and changes soil moisture and temperature, alongside any chemical effects. Consequently, field suppression should be interpreted as the combined outcome of physical and biochemical processes (Jabran et al., 2015; Kruidhof et al., 2008). Crop injury or delayed establishment may occur in susceptible crop species. Species choice, residue decomposition and the interval between termination and planting are therefore important management variables.
     
    Plant residues and mulch applications
     
    Plant residues and organic mulches can release allelochemicals during decomposition while forming a physical barrier to emergence (Weston et al., 2013). Release and transformation depend on residue composition, soil moisture, temperature and microbial activity (Scavo and Mauromicale, 2021). Mulch thickness and physical properties also affect weed emergence independently of allelopathy (Teasdale and Mohler, 2000). These mechanisms must be separated experimentally before attributing suppression to specific chemicals.
     
    Field performance of allelopathic crop varieties
     
    Crop varieties differ in competitive ability and allelochemical production. Selecting for weed suppression may support management in cereals, but field effects reflect both chemical traits and crop growth characteristics (Worthington and Reberg-Horton, 2013). The amount and composition of released compounds can change with soil and weather, limiting consistency across environments. Genotype screening should therefore include field performance and measurements that distinguish chemical interference from ordinary competition (Scavo and Mauromicale, 2021).
           
    Breeding objectives should retain crop yield, quality and establishment while improving weed suppression. Greater allelochemical production cannot be assumed to improve overall agronomic performance. Evaluating weed responses alongside crop outcomes is necessary to identify useful genotypes and potential tradeoffs (Worthington and Reberg-Horton, 2013).
     
    Plant extracts and direct field applications
     
    Plant extracts provide a direct route for testing allelochemical-based treatments, but laboratory efficacy often exceeds field performance because exposure changes with sunlight, rainfall and soil adsorption (Jabran et al., 2015; Macias et al., 2007). Oraon and Mondal (2022) demonstrated concentration-dependent effects of Streblus asper leaf extracts on chickpea in laboratory assays. This is evidence of phytotoxicity to a crop and reinforces the need to assess crop safety alongside weed suppression. Extract composition and application dose must be standardized before treatment effects can be compared across studies. Formulation approaches may improve delivery, but their field efficacy and production costs require independent validation (Duke et al., 2024). In a field study, Kawuryan et al., (2026) evaluated Leucaena leucocephala and Carica papaya leaf extracts in Japanese cucumber and reported treatment-dependent weed suppression and crop responses. Applications were directed away from crop foliage, so this result should be interpreted within the reported application method and site conditions rather than as proof of universal crop safety.
     
    Allelopathy in integrated weed management systems
     
    Allelopathic practices can complement cover cropping, mechanical control, crop rotation and appropriately selected herbicide treatments (Jabran et al., 2015; Mennan et al., 2020). Sathishkumar et al., (2020) reviewed the use of crop and tree allelopathy in weed management, including residues and extracts. Such strategies should be evaluated against explicit weed control and crop yield objectives rather than assuming a fixed percentage reduction in herbicide use. Table 3 identifies the contributions and constraints of alternative management combinations (Norsworthy et al., 2012; Scavo and Mauromicale, 2021).

    ​

    Table 3: Contribution of allelopathic strategies in integrated systems.


     
    Challenges and limitations encountered in bioherbicide development
     
    Laboratory-field discrepancy
     
    Laboratory bioassays establish phytotoxic potential under defined exposure, but soil and field environments alter the amount of active compound reaching a target plant. Sorption to organic matter or minerals, microbial transformation and weather-dependent transport can reduce or modify activity (Inderjit and Duke, 2003; Li et al., 2013). Extract assays also confound chemical interference with effects of pH, osmotic conditions and nutrients unless suitable controls are included. Field validation therefore requires realistic exposure measurements and comparisons that distinguish allelopathy from competition or physical mulch effects (Khamare et al., 2022; Sahrir et al., 2023).

    Stability and persistence issues
     
    Persistence differs among compounds and soils. Photodegradation, hydrolysis, volatilization, microbial metabolism and adsorption can change both activity and bioavailability (Weston et al., 2012; Li et al., 2013). Rainfall and irrigation may redistribute compounds away from the treated area. Short persistence can limit residual weed control, whereas prolonged exposure may increase crop injury or effects on non-target organisms. Assessing the parent compound and its transformation products is therefore more informative than treating rapid degradation as an automatic safety advantage (Duke et al., 2000; Sahrir et al., 2023).
     
    Dose standardization and application challenges
     
    Dose standardization is complicated by variation in genotype, plant part, developmental stage, extraction procedure and storage. An extract concentration expressed only as a percentage cannot be transferred across studies without defining the mass, solvent volume and composition. Both target weeds and crops require dose-response testing under relevant soil conditions (Jabran et al., 2015; Scavo and Mauromicale, 2021). Application timing, weed stage and delivery method should be reported because they determine effective exposure. Evidence of non-target activity must be assessed for each candidate rather than inferred from natural origin (Duke et al., 2024).
     
    Formulation, legal regulations and technological limitations
     
    Limited solubility, chemical instability and variable composition complicate formulation. Encapsulation and controlled-release carriers may improve delivery, but efficacy must be evaluated together with carrier toxicity, shelf life, manufacturing cost and scalability (Duke et al., 2024). A successful laboratory formulation is not sufficient evidence of an economically viable agricultural product. Candidate formulations should be compared with current weed management practices using crop yield, operational costs and treatment frequency as practical endpoints.
           
    Registration requirements depend on the jurisdiction and product category. Natural origin does not exempt a candidate from evaluation of efficacy, exposure and toxicological or environmental risks. Product adoption also depends on availability, reliable performance, ease of use and cost relative to existing practices (Scavo and Mauromicale, 2021). These considerations should enter development early, alongside biological screening.

    FUTURE PERSPECTIVES AND RESEARCH GAPS

    Research should prioritize compounds and management practices with reproducible activity under realistic exposure. Strong inhibition in a screening assay is an initial finding, while commercial or agronomic usefulness requires crop selectivity, predictable delivery and consistent performance (Duke et al., 2024; Khamare et al., 2022). The priorities below address the transition from chemical discovery to agricultural application.
           
    Mechanistic studies should distinguish direct biochemical targets from general stress responses. Transcriptomic and metabolomic analyses can identify candidate pathways, but complementary biochemical assays and dose-response experiments are needed to test causality. Multiple affected pathways do not establish that a treatment will prevent resistance; resistance selection and cross-resistance require direct evaluation (Duke et al., 2000, 2024).
           
    Omics approaches should be linked to measurements of compounds and transformation products in soil and plant tissues. Such measurements help determine whether concentrations associated with laboratory activity are achieved in the rhizosphere or after application. Studies should report sampling time, analytical recovery and exposure conditions to support comparisons across sites (Khamare et al., 2022; Weston et al., 2012).
           
    Microbial degradation and transformation influence persistence and biological activity. Future work should connect changes in microbial communities with functional measurements rather than assuming that a community shift improves weed suppression. Evaluating soil-dependent transformations can identify conditions that support or limit candidate performance (Li et al., 2013; Khamare et al., 2022).
           
    Breeding for weed suppression should evaluate allelopathic traits alongside competitive ability, yield, crop quality and stress tolerance. Genetic or biochemical evidence of greater allelochemical production requires confirmation of agronomic benefit across environments. Weed suppression caused by canopy structure or growth rate must be separated from chemical interference (Worthington and Reberg-Horton, 2013; Scavo and Mauromicale, 2021).
           
    Formulation research should compare conventional and controlled-release delivery under the same field conditions. Stability, crop injury, non-target effects and the fate of carrier materials should accompany efficacy measurements. Precision application may reduce unnecessary exposure, but its value depends on demonstrated treatment performance and practical cost (Duke et al., 2024).
           
    Replicated trials across locations and seasons are essential for defining where allelopathic practices are dependable. Trials should include untreated controls, current management comparators, crop yield, weed community responses and cost measures. Combining these data with compound-specific environmental assessment will support evidence-based integration into weed management (Norsworthy et al., 2012; Scavo and Mauromicale, 2021).
     

    CONCLUSION

    Allelochemicals offer useful components of integrated weed management through crop traits, residues, extracts and candidate bioherbicide formulations. Their agricultural value depends on reproducible weed suppression at crop-safe doses under realistic field conditions. Standardized composition, reliable delivery and replicated field trials are the key requirements for translating laboratory phytotoxicity into practical use. Environmental fate, non-target effects and economic feasibility must be assessed alongside efficacy before routine agricultural deployment.

    CONFLICT OF INTEREST

     The authors declare that they have no conflicts of interest related to this manuscript.

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