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Rev Environ Sci Biotechnol (2023) 22:471–504
https://doi.org/10.1007/s11157-023-09656-1
REVIEW PAPER
Allelopathy asasource ofbioherbicides: challenges
andprospects forsustainable agriculture
MariannaKostina‑Bednarz · JoannaPłonka·
HannaBarchanska
Received: 13 February 2023 / Accepted: 27 April 2023 / Published online: 10 May 2023
© The Author(s) 2023
Abstract The sustainable management of the envi-
ronment and crop production in modern agricul-
ture involves dealing with challenges from climate
change, environmental pollution, depletion of natural
resources, as well as pressure to cope with depend-
ence on agricultural inputs. Balancing crop produc-
tivity with environmental sustainability is one of
the main challenges for agriculture worldwide. The
emergence of weeds resistant to synthetic herbicides
generates huge economic losses, so unconventional
weed control strategies, especially those based on
ecological principles, are very much needed in mod-
ern agriculture. Incorporating a natural eco-friendly
approach—allelopathy—as a tool in an integrated
weed control plan by growing specific crops or spray-
ing fields with extracts containing allelopathic com-
pounds can significantly reduce the use of herbicides.
Allelopathy is considered a multi-dimensional phe-
nomenon occurring constantly in natural and anthro-
pogenic ecosystems, by which one organism produces
biochemicals that influence the growth, survival,
development, and reproduction of other organisms.
The objective of this systematic literature review is
to present a comprehensive overview of allelopathy,
define this phenomenon, and classify allelochemi-
cals. This paper also discusses and highlights recent
advances, ongoing research, and prospects on plant
allelopathy management practices applied in agri-
culture, and the underlying allelopathic mechanisms.
The review suggested the holistic view of some alle-
lochemicals as an ecological approach to integrated
weed control and is an important contribution to
future research directions of multidisciplinary pro-
grams, chemoinformatic tools, and novel biotechnol-
ogy methods to plant breeding.
Keywords Allelopathy· Bioherbicides·
Sustainable agriculture· Secondary metabolism·
Weed management· Allelochemicals
1 Introduction
One of the most important elements to sustaining
life and promoting good health is ensuring access to
enough safe and nutritious food. It is estimated that
the agro-tech sector may encounter many difficulties
in securing food production for the rapidly growing
human population in the coming years. The world’s
population is expected to increase by 2 billion persons
in the next 30years, from 7.7 billion currently to 9.8
billion in 2050 (Hernandez-Tenorio etal. 2022). Out
M.Kostina-Bednarz(*)· J.Płonka· H.Barchanska
Department ofInorganic Chemistry, Analytical
Chemistry andElectrochemistry, Faculty ofChemistry,
Silesian University ofTechnology, B. Krzywoustego 6,
44-100Gliwice, Poland
e-mail: marianna.kostina-bednarz@polsl.pl
J. Płonka
e-mail: joanna.plonka@polsl.pl
H. Barchanska
e-mail: hanna.barchanska@polsl.pl
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of the United Nations’ 17 sustainable development
goals, the second goal focuses on agriculture, aim-
ing for zero-hunger levels worldwide while also being
sustainable over time (OECD/FAO 2022). Sustain-
able agriculture can be defined as meeting the food
needs of a growing world population while ensur-
ing minimal impact on the environment and humans
as well as productivity (Lykogianni etal. 2021). To
withstand these harsh challenges induced by abiotic
and biotic factors the world needs to adopt novel and
improved agricultural practices and strategies for
high sustainability and productivity (Khursheed etal.
2022). Among the major biotic constraints, weeds are
considered the most harmful to agricultural produc-
tion (Gharde etal. 2018). Worldwide huge crop losses
have been found to result from heavy weed infesta-
tions. Crop losses due to weeds continue to reduce the
available production of food and cash crops world-
wide. It is very difficult to indicate the yield loss due
to any single weed species; therefore, the loss is esti-
mated as the collective effects by all weed species.
Globally, compared to other biotic factors, weeds
produced the highest potential loss at 34%, with ani-
mal pests and pathogens being less important—with
losses of 18 and 16%, respectively (Głąb etal. 2017).
Integrated Weed Management is a long-term com-
prehensive approach to controlling and mitigating
infestation in fields incorporating physical, genetic,
biological, cultural, and chemical weed management
techniques (Jabran etal. 2015). Farmers usually rely
on quick and effective synthetic herbicides which
represent the backbone of the agri-food sector in its
endeavor to secure food production and suppress yield
losses, but their application is perceived as an obsta-
cle to the achievement of sustainability (Lykogianni
etal. 2021). The excessive use of chemical herbicides
has contributed significantly to soil degradation, envi-
ronmental pollution, and adverse effects on non-target
organisms and has been proven to have deleterious
effects on human health. The consequences of the
inappropriate adjustment of herbicides to the weed
species occupying the fields, the use of herbicides
at the wrong plant developmental stage and under
unsuitable weather conditions are the accumulation
of active compounds in the soil, the accumulation of
weed species, and the acceleration of the evolution of
resistant biotypes (Motmainna etal. 2021). Long-last-
ing exploitation of herbicides with one target site in
plants has resulted in the evolution of weeds resistant
to herbicides (Soltys etal. 2013). The complete exclu-
sion of chemical control of weeds is impossible with
current agrochemical practices, so it is necessary to
develop novel classes of herbicides with new mecha-
nisms of action and target sites. Reducing the large-
scale use of herbicides and introducing organic pro-
duction systems requires the combined efforts of all
actors in the food value chain (Möhring and Finger
2022). In the face of climate change, increasing con-
sumer awareness of crop protection products and fer-
tilization, coupled with unconventional methods to
ensure safe and superior agricultural products are in
high demand (Głąb etal. 2017).
The need for safe food production and eco-friendly
trends in weed management forces scientists to
develop innovative solutions. There is a growing
need for new herbicides with safer toxicological and
environmental profiles. Natural compounds provide
a wide selection of potential new environmentally
safe herbicides, so-called “bioherbicides”, which are
based on compounds produced by living organisms
(Soltys etal. 2013). Bioherbicides are broadly defined
as products derived either from living organisms or
their secondary metabolites to suppress target weed
populations without harming the environment (Scavo
and Mauromicale 2021).
Recently, among the proposed approaches,
research on allelopathy has become increasingly
prevalent in weed management for agroecosystems
(Hoang Anh et al. 2021). Allelopathy, through its
wide range of benefits, may become a promising solu-
tion to the problems of environmental pollution and
the evolution of herbicide resistance (Jabran et al.
2015). Allelopathy, known since ancient times, is a
natural phenomenon in which different organisms
affect the functioning of other organisms in their
vicinity, negatively or positively by releasing sec-
ondary metabolites (Bajwa 2014). The phytotoxic
properties of allelochemicals, or biologically active
metabolites exuded by higher plants, fungi, or micro-
organisms provide a source of practical solutions for
weed control.
Allelopathic compounds are a suitable substitute
for synthetic herbicides because they do not have
residual or toxic effects, however, so far only 3% of
the approximately 400,000 known compounds in
plants that show allelopathic activity have been rec-
ognized as acting as bioherbicides, although more
than 2000 plant species (39 families) have strong
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allelopathic effects (Li etal. 2019). The deployment
of allelopathic cover crops, intercropping, the inclu-
sion of allelopathic plants in crop rotation, and the use
of their residues as mulch are important for ecologi-
cal, sustainable, and integrated weed control systems
(Jabran et al. 2015). The most significant challenge
to sustainable modern crop protection is the limited
availability of bioherbicides. For current researchers,
allelopathic plants can be a source for identifying and
isolating new allelopathic substances. After examin-
ing their bioactivity under laboratory and field condi-
tions, promising compounds can be recommended for
novel natural herbicide development for sustainable
agriculture (Motmainna et al. 2021). Despite their
many advantages, allelochemicals have some limita-
tions for direct use as bioherbicides. It is complicated
to explain the different modes of action of each class
of allelochemical and to determine how environmen-
tal conditions affect their success. Problems in their
commercial deployment arise from the complexity of
application in crop fields due to easy degradation, and
the complicated registration processes required by
authorities.
This article presents a comprehensive and updated
review of the herbicidal potential of allelopathy. The
discussion begins with a description of the origin of
allelopathy and an explanation of the controversy
over the proper definition of the allelopathy phe-
nomenon and its characterization in a mathematical
model. Furthermore, the physiological and ecologi-
cal mechanisms underlying plant allelopathy, factors
influencing the release of allelochemicals, examples
of selected secondary metabolites, and classification
of allelochemicals are highlighted in this overview.
Consideration is also given to the world’s major plant
crops that exhibit allelopathic potential and strategies
such as intercropping with allelopathic weed plants,
the use of allelopathic cover crops and residues, and
rotational sowing of allelopathic plants for practi-
cal weed control in field crops are discussed. The
paper discusses examples of new tools of molecular
genetics, proteomics, and metabolomics, as well as
modern and sophisticated methods of chemistry and
biochemistry that could lead to the development of
substances, perhaps based on the structure of particu-
lar compounds found in nature, that could be used
without risk as selective and eco-friendly herbicides.
The publication compares and critically evaluates the
effectiveness of synthetic plant protection products
with those of natural sources. Finally, a reflection on
existing problems and suggestions for future research
directions in this field is presented to provide a useful
reference for forthcoming studies on plant allelopathy
and its use in the creation of new bioherbicides with
novel, unexploited target sites.
2 Genesis anddefinition oftheallelopathy
phenomenon
A detailed literature review determining the origins
of the term allelopathy leads to the work of Hans
Molisch in 1937. The more historically oriented stu-
dents recognized A.P. de Candolle’s theory of the
early nineteenth century as a starting point. Review-
ing the literature, it is apparent that the history of
allelopathy has only been superficially investigated,
even though it has been very popular in plant ecol-
ogy over the last decades. In the book “The History
of Allelopathy” Willis tried to bring to light most of
the writings that have touched on allelopathy span-
ning the period from antiquity until about 1957. In
the mid-1950s there were almost synchronous pub-
lications of three books about allelopathy: a 1955
monograph by Grümmer in German, a book on the
effects of allelopathic substances in agriculture by
Chernobrivenko (1956) in Russian, and a little-known
but valuable monograph in English by Hubert Martin
(1957) entitled Chemical Aspects ofEcology in Rela-
tion to Agriculture (Willis 2007).
This should be emphasized that allelopathy is not
a completely new phenomenon, it has been described
in the literature already for more than 2000years. The
first references to the general ‘sickening of the soil’
by plant toxins that negatively affected the growth of
other plants were published in ancient manuscripts.
After reading Willis’s book, which contains an exten-
sive collection of examples of theses by classical
authors from Ancient Greece and Rome, can con-
clude that the concept of allelopathy and the contro-
versy surrounding it has been with us for a very long
time. Although it is discussed and quantified in a rela-
tively large number of papers, the existence of allel-
opathy as a natural ecological process is still consid-
ered doubtful by many.
The idea of allelopathy and its genesis has been
very superficially investigated, but with the growing
trend in plant ecology, it is once again in the spotlight.
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Molisch derived allelopathy from the two Greek
words: “allelos”, mutual, and “pathos”, suffering
(Molisch 1938). Although the Greek term was trans-
lated as mutual harm, Molisch in his discussion indi-
cated that the meaning of allelopathy should include
any direct or indirect effect of one plant or microor-
ganism on another. He emphasized that for him the
phenomenon includes both harmful and stimulatory
interactions on the target organism and surrounding
organisms (Willis 2007). Molisch’s definition was
expanded and used in the practice to describe the
chemical communication and chemical interactions
between different organisms in general. The chemi-
cals involved in this process, called allelochemicals,
can be primary metabolites (directly involved in fun-
damental physiological processes of an organism) or
secondary metabolites, which are not contributing
to the survival of plants but produce some products
that aid them in their normal growth and development
(Chaïb etal. 2021).
Based on previous definitions of allelopathy and
attempts to explain its mechanism of action, the Inter-
national Allelopathy Society (IAS), defined allelopa-
thy as the science that “studies any process involving
secondary metabolites produced by plants, algae, bac-
teria, and fungi that influence the growth and devel-
opment of agricultural and biological systems” (Duke
2010). One of the most contentious points in deter-
mining the definition of allelopathy is to distinguish
it from the phenomenon of competition. In the envi-
ronment it is impossible to separate these two mecha-
nisms, therefore scientists recognize allelopathy as
part of the competition for resources. This arisen can
be solved by Muller’s proposed term “interference” as
a general influence between plants, thus encompass-
ing both allelopathy and competition (Rice 1984).
Allelopathy is different from the competition the fact
that it involves the removal or reduction of some fac-
tor from the environment that is required by some
other plant sharing the habitat (Chaïb et al. 2021).
Competition is related to the acquisition of various
resources: light, water, food, minerals, pollinators,
and root space. In an environment of insufficient sup-
ply, a simple way to survive is to inhibit the growth of
competitive plants and thereby decrease the consump-
tion of limited resources by those competitors.
The allelopathic effect is largely stress-depend-
ent. In practice, a plant for example, if it is defi-
cient in mineral elements would suffer both the
phytotoxic effect of allelopathic compounds and the
stress of nutrient deficiency, so its growth would
be inhibited. This can be evidenced by the fact
that competition is often associated with 43–57%
of interference (An etal. 2008). This supports the
idea that one effective means of plant action is the
release of phytotoxic substances into a shared habi-
tat to inhibit the growth of competing plants, which
would confirm Bais’s “novel weapons hypothesis”
thesis about allelopathy for invasion success (Bais
et al. 2003). When considering a more complex
environment than a simple laboratory-scale experi-
ment, it is important to include all factors that may
influence the allelopathic effect, which can be addi-
tive, synergistic, or antagonistic.
Allelopathy has been viewed in a multifaceted
approach, as shown by a growing number of works
by scientists from diverse fields worldwide. Academ-
ics also began to develop mathematical modeling to
separate the concept of allelopathy from the com-
petition, establishing the fundamentals of allelopa-
thy and its ecological role. Only a few authors have
been challenged to develop an allelopathic interaction
model based on experimental data or field studies
(An 2005). They proposed a model for considering
allelochemical production that is based on the prin-
ciples: (1) release and degradation are not reversible,
(2) the amount of compounds released is propor-
tional to the rate of degradation of the allelochemi-
cals, (3) the amount of allelopathic compounds in
plants is proportional to the rate of their release, (4)
considered is the total allelochemical production, (5)
the production, transformation, and decomposition
of allelochemicals are considered a system, which
consists of two processes, i.e. allelochemical release
and dissipation, and three compartments (An et al.
2003). Such generalized model processes, while not
always satisfactory, will be useful in explaining pat-
terns of species interactions and point the way to fur-
ther research. An (2005) develop the assumption that
the allelochemical content in living plants declines
with age and plant stress and is reflected by the corre-
sponding dynamics in the environment. Under normal
conditions, the concentration of allelopathic com-
pounds can be relatively stable in the plant and these
biomolecules can be in an inactive state. This mathe-
matic study may enable us also to understand why the
results of allelopathic research are inconsistent when
conducted under different stages of plant growth, and
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why results change as experimentation proceeds (An
2005).
3 Mechanism ofaction ofallelochemicals
Over the years, the phenomenon of allelopathy has
been well documented, while the precise understand-
ing of the mechanisms of action of allelopathic com-
pounds remains incompletely understood. One of the
crucial challenges in allelopathy is to determine the
specific mechanism of action of allelochemicals in
association with their diverse chemical nature and
multiple target sites in plants. The in-depth investi-
gation of the biochemical and physiological changes
caused by the allelochemicals on selected weed
and crop species is essential to develop an effective
weed management strategy based on the allelopathy
phenomenon. Plant allelocompounds have differ-
ent mechanisms of action based on activities such as
repellency, growth inhibition, protein denaturation,
respiratory impairment, and other effects, depend-
ing on the type of botanical compound and weeds
(Gawronska and Golisz 2006). To use an appropriate
strategy in plant management, it is very important to
understand all the existing chemical, biological, and
physical interactions between the plant and the weed
(Lengai etal. 2020).
The release of allelochemicals is possible under
dry and semi-arid conditions as a result of volatiliza-
tion, whereby these substances are absorbed as vapors
by angiosperm plants, or can be absorbed from the
condensate of these vapors in the form of dew, or
the condensate can reach the soil and be absorbed by
the roots (Kassam etal. 2019). Figure1 presents the
overall processes of allelopathy and the factors influ-
encing allelopathy.
Another mode of action is through leaching by
irrigation, dew or precipitation, or plant residues
which transport allelopathic substances from the
above-ground plant parts to the soil or other plants.
The direct source of allelochemical entry into the
rhizosphere of the soil is root excretion. The most
complex and least understood method is when toxic
substances are produced during the decomposition
of plant residues or are formed by chemical trans-
formation of starter materials by microorganisms
due to the presence of microbial enzymes (Mehdiza-
deh and Mushtaq 2019). In most cases, allelopathic
interference is the result of the concerted action of
different compounds. The reactions of plants to dif-
ferent allelochemicals are strongly influenced by the
Fig. 1 Scheme of crop-weed interaction including factors of constituting and inhibiting allelopathy (Belz 2007) (created with https://
www. biore nder. com/)
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concentration value; at a certain concentration, they
inhibit the development of a given species, and at
another concentration, they can enhance the develop-
ment of the same species or another. Additionally, the
fate of allelopathic compounds depends on the inter-
actions and kinetics of individual processes occur-
ring, under specific natural conditions, and at a spe-
cific location (Mehdizadeh and Mushtaq 2019).
The soil solution plays an important role in allel-
opathy, there are many compounds considered to be
allepathic, mainly secondary metabolites that are
secreted into the rhizosphere and affect the develop-
ment of plants growing in the vicinity of allelopathic
plants (Duke 2010). Many studies describe the multi-
functionality of root exudates, e.g. protection against
herbivores, change of soil chemical properties, and
stimulation of plant growth. It has also been shown
that plant secondary metabolites produced inhibit
microbially mediated denitrification in the rhizos-
phere of invasive plants, thereby impacting growth
and available nitrogen for the invasive community
(Latif etal. 2017). Many potentially allelopathic sub-
stances exhibit weak biological activity on plants in
the soil due to rapid leaching from the root zone of
highly water-soluble compounds and their instability
or rapid degradation by microorganism (Mehdizadeh
and Mushtaq 2019). Recent research has proven that
the synergism of allelopathic substances can increase
bioavailability, because of preferential distribution,
which leads to increased persistence of allelochemi-
cal mixtures in the soil matrix. This relationship has
complicated the study of allelopathy, but by the same
token, it has also emphasized that it is worthwhile to
consider compounds previously excluded as allelo-
chemical on an individual basis because they may, in
combination with other constituents, exhibit allelo-
pathic effects (Mehdizadeh and Mushtaq 2019).
For the widespread use of crops and their rhizo-
spheres to control weeds, it is necessary to have a
comprehensive knowledge of plant-soil interactions,
identify metabolites, and determine their persistence
(Mwendwa et al. 2021). Metabolic profiling is a
modern tool for assessing the presence and quantity
of allelochemicals in the rhizosphere and provides
additional insight into the complex interplay between
plants and their associated rhizospheric microorgan-
isms. Developing research to investigate the transfor-
mation of allelochemicals in soil should pay attention
to interactions between metabolites and biotic and
abiotic matrix components that affect transport, deg-
radation, and phytotoxicity.
Phytotoxic allelochemicals include aminophenox-
azinones, which are converted from benzoxazinoids
produced by wheat as well as in the rhizosphere by
soil microbiota (Macías etal. 2019). Benzoxazinoids
are unique bioactive metabolites produced by certain
members of the Poaceae including maize, wheat, rye,
and some dicots (Mwendwa etal. 2021). For exam-
ple, Mwendwa etal. (2021) describe for the first time
the production of aminophenoxazinones in Australian
soil, demonstrating that weed suppression through
allelochemical production can be enhanced under
field conditions for some wheat varieties (Latif etal.
2017).
3.1 Impact of allelochemicals on the plant growth
regulator system
Allelopathic compounds can cause severe growth
disturbances and even lead to complete plant stunt-
ing. The expression of specific activity of volatile
monoterpenes in blocking mitotic division and inhib-
iting cell elongation was observed (Kumar et al.
2020). The action of compounds such as coumarin,
cineole, and scopoletin results in the formation of
cells with altered shapes, abnormally formed nuclei,
and highly vacuolated cellular structures (Inder-
jit and Keating 1999). It was shown that exogenous
coumarins can also delay mitosis in onion growth tip
cells, and coumarins secreted from Anthoxanthum
odoratum can inhibit the growth of Zoysia japonica
seedlings (Chou 1999).
Many studies performed on extracts obtained from
different plant species have demonstrated that the
toxic effects of allelochemicals are based on delay-
ing seed germination and inhibiting seedling growth.
The inhibition of growth can also generate anatomi-
cal-morphological deformations at the root tip. Phe-
nolic acids, like coumarins, exhibit similar inhibitory
effects on plant growth and induce root morphologi-
cal deformities (Kumar et al. 2020). Treatment of
soybean seedlings with benzoic and cinnamic acids
resulted in deformities such as no trichomes, no lat-
eral roots, and a tendency of roots to grow horizon-
tally (Sathishkumar et al. 2020). Other researchers
also described the effect of ferulic acid and p-cou-
maric acid as limiting cucumber leaf expansion,
p-hydroxybenzoic acid and vanillic acid contained in
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red bell pepper root exudates as auto toxic—inhibit-
ing the growth of embryonic roots of peppers grow-
ing in monoculture crops (Rice 1984).
It was also observed that the inhibitory effect on
seedling growth and germination of the phenolic
acids (vanillic, ferulic, p-coumaric, and p-hydroxy-
benzoic acids) depended more on the type of corn
hybrid than the type of acid. The mixture of the inves-
tigated acids produces a stronger inhibitory effect
than these acids alone. This is consistent with the the-
ory of the synergistic action of these compounds (Par-
thasarathy etal. 2021). The main indicator revealing
the presence of alleloinhibitors in the environment is
the inhibition of elongational growth of roots and aer-
ial parts. This negative effect may already be visible
during seed swelling—changes leading to anatomical
distortions in the seed coat and the seed stocks occur
at this point. The result is delayed germination, and
the accompanying inhibition of embryonic root elon-
gation which can cause plant death (Liu etal. 2021).
Decreased levels of auxins in tissues can result
in a slowing of the growth rate of plants. Mono-
hydroxy phenols, which are cofactors of indoleac-
tic acid (IAA)-oxidase accelerate the degradation
of auxins, while polyphenols and dihydroxy acids
have the opposite effect, inhibiting the decarboxyla-
tion of IAA, resulting in accelerated growth by the
lack of amelioration of this growth enhancer as the
plant matures (Bogatek etal. 2005). Adverse effects
of auxins and hydroxamic acids are also possible—
because of these acids reduce the ability of auxins
to bind to receptor sites on cytoplasmic membranes.
Some allelochemicals similar to auxins stimulate
ethylene biosynthesis, so this may be a kind of plant
indicator for the presence of allelochemicals in the
environment (Bogatek etal. 2005).
3.2 Changes in cell membrane permeability
An important change caused by allelochemicals is
their effect on the structure and functioning of cyto-
plasmic membranes (Soltys et al. 2013). Allelo-
chemicals determine the course of plant growth and
development by changing the state of cytoplasmic
membranes, thereby affecting the course of biochemi-
cal and physiological processes in different parts of
cells (Fig.2). Damage to cell membranes affects the
entire metabolism and all physiological processes.
The consequence of structural changes is the limita-
tion of the functioning of enzymatic proteins that
affect intermembrane ion transport, accumulation,
and water balance. The hydration of plant tissues
affects the state of the stomatal apparatus, thus the
process of photosynthesis (Soltys etal. 2013).
Compounds with allelopathic activity may also
form molecular assemblies in mitochondrial and
chloroplast membranes and modify electron transport
therein or decrease cellular adenosine triphosphate
Fig. 2 The effects of allelochemicals on morphological properties and growth rates in plants (created with https:// www. biore nder.
com/)
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(ATP) formation. Alleloinhibitors can also negatively
impact cytoplasmic membrane processes by alter-
ing the function of phytohormones by reacting with
receptor sites (Cheng and Cheng 2015). Other inhibi-
tors that disintegrate lipid-soluble membrane struc-
tures, such as the terpenoids and quinones sorgoleone
and juglone, can also be adsorbed on the membrane
surface (Gniazdowska etal. 2004).
Scientists researching saponins substantiated
their high inhibitory potential against higher plants
and microorganisms by observing strong damage
mainly in the roots of wheat seedlings (Macías etal.
2019). Saponins interact with the cytoplasmic mem-
branes’ constituent elements, proteins, and lipids. As
a result of this action, browning of the apical meris-
tem occurs, leading in the next stage to the death of
the root system (Rice 1984). The effects of saponins
are focused on lowering the activity of membrane
enzymes such as nicotinamide adenine dinucleotide
(NAD) oxidase and malate dehydrogenase. Interac-
tions of saponins with membrane components are a
major cause of saponin-induced damage (de Bertoldi
etal. 2009).
Allelocompounds have a significant impact on the
water balance in the plant. An example of an allelo-
compound that affects the water equilibrium of plants
is ferulic acid, which has a negative effect on water
uptake by cucumbers, beans, and tomato seedlings
(Mamolos 2008). Compared to p-coumaric acid, it
has a stronger inhibitory effect on water uptake by
the roots (Schandry and Becker 2020). According to
these researchers, plant growth regulators are also
involved in the mechanism of action of allelochemi-
cals. P-coumaric and ferulic acids, as well as extracts
from many weeds with previously known allelo-
pathic properties, caused effects dependent on the
concentration of compounds. In the case of higher
concentrations, they caused an increase in leaf diffu-
sion resistance and closure of stomata, while at lower
concentrations—they decreased the water potential,
which was the result of a decrease in the osmotic
potential and the pressure potential (Chou 1999).
One of the most significant changes caused by
active allelochemicals is their influence on the struc-
ture and functioning of cytoplasmic membranes.
Allelochemicals determine the progress of plant
growth and development by changing the state of
cytoplasmic membranes and, consequently, affect the
course of biochemical and physiological processes in
different parts of cells (Lengai and Muthomi 2018).
When the cell membranes are damaged, there are
changes in the entire metabolism and every physi-
ological process. These potent destructive effects are
attributed to phenols, which are among the most plen-
tiful and common compounds synthesized in plants
that manifest toxic effects even at low concentrations
(Mushtaq etal. 2020).
The mechanism of modification of membrane per-
meability by phenolic compounds is mainly based
on the decrease of membrane potential induced by
them; the collapse of transmembrane potential in
mitochondria induced by salicylic acid is similar.
Studies have shown that the permeability of cyto-
plasmic membranes to electrolytes is mediated by
phenolic acids, and that p-coumaric and ferulic acids
are more effective in membrane disintegration than
p-hydroxybenzoic and vanillic acids, and that their
destructive action is based on peroxidation of mem-
brane lipids (Doblinski etal. 2003). As another exam-
ple, the toxic activity of benzoic acid and cinnamic
acid occurs under the generation of free radicals that
damage cytoplasmic membranes. These radicals can
be formed, for example, during the oxidative transfor-
mation of phenols that occur during cell wall lignifi-
cation. Allelopathic inhibitors also affect the activity
of certain phytohormones that are initiated by recep-
tor sites on membranes by altering their function
by reacting with receptor sites (Gniazdowska etal.
2015).
3.3 Influence on water and nutrient uptake
Muscolo etal. (2001), investigated the effects of the
concentration and type of phenolic compounds in
forest soils with different vegetative cover on Nipus
laricio seed germination and found that lack of seed
germination was strongly correlated with inhibi-
tion of glycolysis enzymes and the oxidative pentose
phosphate pathway involved in the first stages of seed
germination. These kinds of analyses clarify biologi-
cal interactions in forest soils for natural regeneration
and reforestation processes (Muscolo et al. 2001).
Allelochemicals have a significant effect on the sup-
ply of mineral substances to plants, and their optimal
concentration is necessary for the proper growth and
development of the plant. Substances contained in
decomposing plant residues, extracts, and leachates
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may be responsible for altering the mineral content of
the plants under study (Muscolo etal. 2001).
Many works have discussed the problem of under-
standing the mechanism of the effect of phenolic
inhibitors on ion uptake by roots of higher plants
(Akemo etal. 2000). It was found that as a result of
the destruction of the transmembrane electrochemi-
cal potential of cytoplasmic membranes, there is an
increase in permeability to phenolic acids. The abil-
ity of phenolic compounds to inhibit respiration and
metabolism in mitochondria, resulting in changes
in ATP synthesis, which is responsible for efficient
membrane transport should also be considered. The
main effects induced by phenolic acids that can
alter mineral concentrations in the plant body have
been recognized, and their effect depends mainly on
the type of ligands in the benzoic ring, for example
hydroxyl groups can mitigate the inhibitory effect.
Phenolic acids can produce effects characterized by
direct action—modifying the rate of uptake, as well
as long-term, for instance changing the concentration
of ions in plant tissues (Głąb etal. 2017). Based on
many literature items, it can be assumed that most
phenolic acids decrease the level of essential ele-
ments in plant materials except chlorogenic acid,
which in the case of tissues of the plant Amaranthus
retroflexus caused a decrease in the level of phospho-
rus, increased nitrogen, and had no effect on the level
of potassium. Salicylic acid also negatively affects ion
uptake by inhibiting barley roots’ absorption of potas-
sium and phosphorus ions. Benzoic and cinnamic
acids cause growth inhibition of soybean (Glycine
max) seedlings because of disturbances in ion uptake
and transport in the plant; the interferences are addi-
tionally accompanied by morphological distortion of
the roots. The rate of ion uptake differed depending
on the cinnamic acid concentration in the rhizosphere
soil. For phosphate ions, inhibition occurred at higher
concentrations, and acid toxicity increased with
decreasing soil pH (Hoang Anh etal. 2021).
3.4 Impairment in the antioxidant system and effect
on plant photosynthesis
There is a strong correlation between respira-
tion and patterning processes, so compounds that
cause disruptions in respiration processes also
have a significant impact on the growth process.
These effects are mainly observed in mitochondrial
metabolism—specifically in the Krebs cycle and res-
piratory chain transformations (Kumar et al. 2020).
Studies leading to a comparison of the activity of
individual allelochemicals belonging to different
classes on the respiration rate of soybean cotyledons
described that due to the small size of α-pinene and
cinnamic acid particles, they more efficiently pen-
etrated the tissues and thus accelerated the damage
to mitochondria, compared to quercetin and juglone
(Peñuelas et al. 1996). Glycosides also negatively
affect the respiration process. Resin glycosides were
found to inhibit membrane ATPase in Ipomea tricolor
(Calera etal. 1995).
Disturbances in the photosynthesis process caused
by allelocompounds may be one of the reasons for
the inhibitory effects on plant growth. Among phe-
nolic acids, the greatest toxicity is revealed by sali-
cylic acid, but in general phenolic compounds have
a weak effect on the photosynthesis process (Latif
et al. 2017). Phenolic compounds can block the
release of oxygen by chloroplasts, but high concen-
trations are required to induce significant changes.
Based on studies conducted on a cell suspension of
Abutilon theophrasti, it was shown that at the highest
tested concentrations, vanillic and ferulic acid inhib-
ited photosynthesis and protein synthesis, respec-
tively, while chlorogenic and p-coumaric acids did
not inhibit any of the physiological processes (Latif
etal. 2017). Disturbances in photosynthesis can also
be caused by short-chain organic acids, sesquiterpene
lactones, indole alkaloids, or flavonoids. For example,
artemisinin, one of the sesquiterpene lactones, is an
extremely active inhibitor of photosynthesis because
it reduces this process even at low concentrations.
The effect of allelopathic substances on photosynthe-
sis is also reflected as a change in chloroplast activity,
which is particularly strong for the quinone inhibitors
sorgoleone and juglone. Sorgoleone detected in sor-
ghum (Sorghum bicolor) already at low concentra-
tions induced a 50% inhibition of oxygen secretion by
Glycine max leaves (Głąb etal. 2017).
3.5 Effect on the functions and activities of enzymes
Many allelochemicals are known that can be both
inhibitory and activating on enzymatic systems. A
reduction in protein synthesis by phenols is very com-
mon, as well as changes in the metabolism of porphy-
rin compounds caused by phenolic acids. This results
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in the inhibition of chlorophyll and hemoglobin syn-
thesis, which is necessary for root papilla formation
by legumes (Bogatek etal. 2005). Based on an exper-
iment conducted on a research material consisting of
cucumber stems and leaves, cultivated on a medium
containing high amounts of phenolic compounds,
it was found that the activity of IAA oxidase was
higher than in the plants cultivated on media with a
low content of these compounds. On the other hand,
the results of other studies show that phenolic com-
pounds, mostly phenol acids, stimulate the increase of
guaiacol peroxidase enzyme activity in white clover
roots (Bogatek etal. 2005).
The fundamental effect of allelopathic compounds
on the cell membrane is based on the modification of
the transmembrane barrier, resulting in morphologi-
cal and physiological changes that inhibit growth and
development (Mushtaq etal. 2020). Allelochemicals
generally affect cellular processes but do not damage
cellular organelles. The direct visible effects of their
phytotoxic properties are for example, swollen seeds,
rotting or swollen root tips, an absence of root hairs,
the reduced extension of roots and twisting of their
axis, diminished dry biomass accumulation, repressed
germination rate and reduced reproductive potential
(Mushtaq etal. 2020).
4 Factors affecting therelease ofallelochemicals
The most significant challenge associated with allelo-
pathic weed control is the low concentration of alle-
lochemicals in the source plants and the difficulty in
synthesizing these compounds for large-scale com-
mercial use. A proposal for solving this problem is
to use stress factors, which generally potentiate the
synthesis of allelocompounds in plants. In addition,
biotic and abiotic factors influence the expression of
allelopathy and are, therefore, a tool to manipulate
the persistence, concentration, and fate of allelopathic
compounds in the environment (Inderjit and Keating
1999). Stress-induced stimuli received by the plant at
the cellular level follow different perceptual or signal
transduction pathways, resulting in direct metabolic
response and activation of gene expression. Reac-
tions usually activated under the influence of environ-
mental signals include the formation of enzymes and
the synthesis of stress proteins, hormones, and stress
metabolites, subject to feedback control (Scavo and
Mauromicale 2021). In addition, it has been observed
that allelochemicals present in the vicinity of plant
roots or induced by neighboring plants can induce
secondary oxidative stress on plants. These phenom-
ena increase reactive oxygen species, activating the
cellular antioxidant system and disrupting hormonal
balance (Maqbool and Abdul 2013). Allelopathic
effects can result from the direct release of chemical
compounds from the donor plant and from the induc-
tion of the release of biologically active compounds
by a third species. These phenomena can also be indi-
rectly influenced by degraded or transformed prod-
ucts of released compounds resulting from abiotic
and biotic influences on soil or water (Inderjit and
Keating 1999).
Environmental factors such as climate, soil struc-
ture, soil nutrient content, water properties (physi-
cal, chemical, and biological), and agricultural
practices are responsible for the concentration of
allelochemicals in the plant (Maqbool and Abdul
2013). Under stress conditions, plants produce alle-
lochemicals consisting mainly of phenolic acids
and terpenoids. Allelopathic activity under dry soil
conditions is higher than in well-irrigated areas for
plant species such as cassava (Manihot esculenta
Crantz), sorghum (Sorghum bicolor), sunflower
(Tithonia diversifolia) and walnut (Cyperus rotun-
dus). Under drought conditions elevated levels of
cyanogenic glycosides were recorded for the species
mentioned above and increased amounts of ferulic
acid in wheat (Maqbool and Abdul 2013). Addi-
tionally, an increase in barley autotoxicity under
drought conditions has been observed, and rice
responses to drought and salinity have also been
demonstrated through the production of momilac-
tones A and B as a defense mechanism (Scavo and
Mauromicale 2021). An increase in the production
of phenolic compounds has been noted as a result
of changes in soil characteristics, such as conduc-
tivity, pH, organic carbon content, and nutrient
content (Nornasuha and Ismail 2017). The secre-
tion of phenolics influences nutrient availability,
the dynamics of organic matter, and nitrogen min-
eralization. For example, scopoletin and chloro-
genic acid concentrations increased several-fold in
all tobacco and sunflower plants under potassium,
sulfur, and nitrogen-deficient conditions. Similarly,
scopoletin was shown to help tobacco plants survive
in soil deficient in boron, magnesium, calcium, and
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phosphorus (Maqbool and Abdul 2013). Zobel and
Clarke (1999) had already observed increased syn-
thesis of allelochemicals under high heavy metal
content in the soil. Other researchers later con-
firmed that severe metal stress increased momilac-
tone B levels in rice crops (Kato-Noguchi 2009).
In response to pathogenic organisms, plants acti-
vate defense mechanisms that increase the produc-
tion of secondary metabolites with defensive effects
(Nornasuha and Ismail 2017). Disposal of plant res-
idues can generate biotic stress for newly growing
plants. The microorganisms use energy, some min-
eral elements, water, and oxygen during the decom-
position of residues, which, if insufficient in the soil
environment, would result in competition for lim-
ited resources. Some products of decomposed resi-
dues are of nutritional value for new vegetation oth-
ers are phytotoxic and interfere with soil microbes
(Gawronska and Golisz 2006). Another example
of biotic stress is competition in plants living in
unsuitable conditions competing for the resources
necessary for growth. Harmful allelopathic inter-
ference can also occur between plants of the same
species. The occurrence of autointoxication poses a
problem when transplanting fruit trees in orchards,
shrubs, and perennial plantations, as well as field
plants and crops that need to be transplanted every
year (Gawronska and Golisz 2006).
In a complex ecosystem, different stress fac-
tors act synergistically to increase allelochemical
synthesis. This causes difficulties in manipulat-
ing stress induction to obtain adequate amounts of
compounds with the allelopathic potential to use for
bioherbicide production (Scavo and Mauromicale
2021). However, tremendous advances in molecular
biology, metabolomics, genomics, and proteomics,
as well as modern biotechnological techniques, have
contributed to a better understanding of such com-
plex interactions. At the same time, it is possible
to link genes involved in the production of second-
ary metabolites and modify their composition by
employing genomic approaches and genetic engi-
neering. Allelopathic interactions induced by biotic
and abiotic stresses trigger processes at all levels of
trophic relationships, which can be used to develop
biocontrol measures in organic farming and also
in ecosystem management (Gawronska and Golisz
2006).
5 Allelochemicals asplant secondary metabolites
5.1 Nitrogen-containing compounds
Alkaloids are plant-derived heterocyclic com-
pounds and are among the largest group of second-
ary metabolites, with over 27,000 currently listed in
the Dictionary of Natural Products (Parthasarathy
etal. 2021). Alkaloids are mainly amino acid deriv-
atives but can also be synthesized via terpenoids or
formed from polyketide pathways (Macías et al.
2019). They can be differentiated based on their bio-
synthetic origin into several groups: indole alkaloids
derived from tryptophan, pyrrolizidine alkaloids from
ornithine or arginine, and quinolizidine alkaloids
from lysine (Latif etal. 2017). They are synthesized
in the cytoplasm, vesicles, or chloroplasts but can
also be stored in vacuoles due to their water-soluble
abilities (Macías et al. 2019). The purine alkaloid
caffeine causes autotoxicity in coffee and tea planta-
tions, whereas gramine and nicotine affect seed ger-
mination and shoot growth (Bachheti et al. 2020).
The activity of these compounds occurs in relatively
high concentrations (> 0.1%), compared to phenolic
compounds, which are toxic already in concentra-
tions of 10–200ppm (Yoneyama and Natsume 2010).
Alkaloids induce plant growth inhibition by inter-
fering with DNA, causing changes in enzyme activ-
ity, protein metabolism, and cytoplasmic membrane
integrity, amongst other things. This is confirmed, for
example, by studies in which quinolizidine alkaloids
produced by legumes, such as lupanine and spartenin,
impaired membrane permeability and inhibited pro-
tein synthesis. Plant alkaloids are widely prevalent
in four plant families, including Asteraceae, Apoc-
ynaceae, Boraginaceae, and Fabaceae (Latif et al.
2017).
The production of plant alkaloids with significant
allelopathic effects is still fully unconfirmed, how-
ever, compounds such as morphine, berberine, ergot-
amine, allyl isothiocyanate, quinine, and colchicine
have been confirmed to exhibit phytotoxicity, inhibit
germination and/or seedling growth of neighboring
plants. In some experiments, scopolamine and hyos-
cyamine were isolated from the soil where colcora
(Datura stramonium) was growing and showed an
inhibitory effect on Helianthus annuus seedlings that
lasted for eight months (Haig 2008). A research study
was also conducted in which the alkaloid fraction of
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Crotalaria retusa was collected and tested at various
concentrations for allelopathic potential in Phaseolus
vulgaris. With increasing concentration, allelochemi-
cals inhibited bean seed germination through induced
oxidative stress (Bachheti etal. 2020). The chemical
structures of selected compounds with allelopathic
potential are presented in Fig.3.
Benzoxazinoids, which show significant allelo-
pathic potential, are another group of compounds
that are abundant in the world’s largest field crop
species such as rye, wheat, corn, and rice (Hus-
sain et al. 2022). They have been thoroughly tested
for allelopathic efficacy and phytotoxicity on
weeds. Various benzoxazinoids and hydroxamic
acids, including benzoxazolin-2(3H)-one (BOA),
benzoxazinones 2,4-dihydroxy-7-methoxy-(2H)-
1,4-benzoxazin-3(4H)-one (DIMBOA), 2-hydroxy-
7-methoxy-1,4-benzoxazin-3-one (HMBOA),
2-hydroxy-1,4-benzoxazin-3-one (HBOA), 6-meth-
oxy-benzoxazolin-2-one (MBOA) and 2,4-dihy-
droxy-(2H)-1,4-benzox-azin-3(4H)-one (DIBOA),
are synthesized in cereals and released from the plant
tissues and residues by decomposition and through
root exudation (from root hairs or secondary roots)
Fig. 3 Examples of natural products with allelopathic actions
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into the surrounding soil solution (Reiss etal. 2018).
It has been shown that after production and release,
they undergo physicochemical and microbiological
changes, thus causing changes in phytotoxicity medi-
ated by microorganisms (Hussain etal. 2022).
Benzoxazinones are stored in the glucosidic form
in vacuoles, however, as a result, external triggers are
released into the cytoplasm, and they are hydrolyzed
by the β-glucosidases increasing their reactivity and
biological activity (Gawronska and Golisz 2006). The
resulting unstable benzoxazinone aglucones (DIBOA
and DIMBOA) are toxic, and their benzoxazolinone
degradation products (MBOA and BOA) are consid-
ered to be less bioactive than the starting molecules.
Despite this, it has been shown that the glucosides
of DIBOA and DIMBOA with their respective agly-
cones and degradation products, suppress weeds
such as barnyard grass, crabgrass, or redroot pigweed
(Gawronska and Golisz 2006).
Allelochemicals from various wheat genotypes
have also been shown to mainly inhibit the growth of
various weed species, including Bromus japonicus,
Chenopodium album, Portulaca oleraceae, Avena
fatua, and Lolium rigidum (Hussain etal. 2022). The
soil microflora is responsible for the transformation
of benzoxazinones into more potent bioherbicide
metabolites. Benzoxazinones may be useful as weed
control agents due to their phytotoxicity, specific
activity, and limited soil persistence. Such research
can be used to develop modern benzoxazinone-rich
wheat breeding for sustainable weed control pro-
grams (Hussain etal. 2022).
5.2 Phenolic compounds
Plant phenolics are a diverse group of organic com-
pounds, their common feature consists of an aromatic
ring possessing at least one hydroxyl group. Phenolic
compounds are synthesized via the phenylpropanoid
and shikimic acid pathways, whereas the combina-
tion of the shikimate pathway with the mevalonate
pathway leads to the synthesis of flavonoids. These
processes are all presented in Fig.4. Phenolic com-
pounds can be generally classified into phenolic
acids, flavonoids, coumarins, lignins, tannins, and
stilbenes (Latif etal. 2017). Phenolic compounds can
exist in free form, conjugated with sugars or proteins,
as esters, and by polymerization and condensation
from tannins, lignans, lignin, cutin, and suberin (Bor-
relli and Trono 2016).
Benzoic and cinnamic acid derivatives are the
most common plant-originated allelochemicals,
Fig. 4 General scheme of the relationships between primary metabolism and the major pathways for the synthesis of secondary
metabolites (created with https:// www. biore nder. com/)
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for example, p-hydroxybenzoic acid, salicylic acid,
gallic acid, vanillic acid, p-coumaric acid, caf-
feic acid, ferulic acid, and chlorogenic acid, which
structures are presented in Fig. 5 (Yoneyama and
Natsume 2013). Among the cinnamic acids, caffeic
acid and its esterified derivatives are the most abun-
dant in fruit, whereas derivatives of ferulic acid are
the most abundant in cereal grains. Flavonoids are
among the most powerful antioxidants that have
been obtained from plants, and their effects are due
to the presence of hydroxyl groups in positions 3’
and 4’ of ring B, which participate in electron delo-
calization and stabilize the radical that is formed,
and of a double bond between the C2 and C3 car-
bons in ring C, together with a carbonyl group at
the C4 carbon, which makes delocalization of an
electron from ring B possible, (Fig.5) (Borrelli and
Trono 2016).
The mechanism of allelopathy associated with
phenolic compounds includes interfering with hor-
mone activity, membrane permeability, photosyn-
thesis, respiration, and synthesis of organic com-
pounds in susceptible plants (Latif etal. 2017). Free
phenolic compounds affect plant growth because
they accumulate in rhizosphere soils, thereby influ-
encing the accumulation and availability of soil
nutrients (Li et al. 2010). Examples are the main
allelochemicals in the rhizosphere soil of Agera-
tum conyzoides L., which were isolated and iden-
tified as p-coumaric acid, gallic acid, ferulic acid,
p-hydroxybenzoic acid, and anisic acid, with allelo-
pathic interference on rice (Oryza sativa). Research
by Batish et al. (2009) showed that root exudates
and residues of A. conyzoides released inhibitory
substances in the soil rhizosphere, and these phyto-
toxins negatively affected rice growth. The phenolic
Fig. 5 Structures of
phenolic compounds with
allelopathic potential (Bor-
relli and Trono 2016)
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content of the phytotoxins isolated from the soil
solution was nearly six times higher compared to
the control soil solution (Batish etal. 2009).
The allelopathic effect is the strongest when a
combination of multiple phenolic compounds are pre-
sent. For example, one study investigated the effect of
an aqueous extract of Delonix regia on the growth of
lettuce (Lactuca sativa) and Chinese cabbage (Bras-
sica chinensis) (Li et al. 2010). Chlorogenic acid,
protocatechuic acid, gallic acid, 3,4-dihydroxyben-
zaldehyde, p-hydroxybenzoic acid, caffeic acid, and
3,5-dinitrobenzoic acid were identified in this extract.
This combination of compounds with allelopathic
potential inhibited the growth of the neighboring
plant, and this effect increased with increasing con-
centrations of these compounds (Li etal. 2010).
Some of the potential phenolic allelochemicals
were identified in the leachates of bark, fresh leaves,
and leaf litter of Eucalyptus tereticornis, E. cama-
ldulensis, E. polycarpa, and E. microtheca. Investi-
gations showed the presence of p-coumaric, gallic,
gentisic, p-hydroxybenzoic, syringic, and vanillic
acids and catechol, which have harmful effects on
the crops in the ecosystem, for example, black gram
(Phaseolus mungo L.), resulting in the reduction and
delaying of germination, mortality of seedling and
reduction in growth and yield (Li etal. 2010). Other
reports are available that detail the Eucalyptus spe-
cies allelopathic effect. The allelopathic activity was
investigated against seed germination of Abutilon
theophrasti, Asclepias syriaca, and Chenopodium
album. Crude wheat and corn straw extracts com-
pared to fermented extracts show more inhibition of
seed germination and seedling growth of Abutilon
theophrasti, Asclepias syriaca, and Chenopodium
album (Bachheti etal. 2020). It is also worth noting
that some researchers observed that the application of
a bur cucumber seed extract and its phenolic chemical
(2-linoleoyl glycerol) triggers abscisic, salicylic, and
jasmonic acid accumulation, and inhibits the gibber-
ellin pathway, so seed germination of lettuce will be
halted (Kumar etal. 2020). A large number of reports
on the discovery of phenolic allelochemicals from
plants testifies to the huge and almost unexploited
reservoir of compounds that can potentially be used
for pest and weed control in the field of agricultural
production. Accurate identification and quantitative
analysis of allelopathic phenolic compounds are the
basis for developing toxicologically benign weed
management and pest control tools with high nutra-
ceutical value (Kumar etal. 2020).
Coumarins and their glucosides are abundant and
commonly present phytochemicals in plants in par-
ticular families Apiaceae, Rutaceae, Asteraceae,
and Fabaceae (Bachheti etal. 2020). A C7 hydroxyl
group in the coumarin structure seems to contribute
significantly to the herbicidal activity of this family
of compounds, therefore accordingly, 7-hydroxycou-
marins, such as umbelliferone, esculetin, and sco-
poletin, (Fig.3) have received interest as eco-friendly
herbicides and are known for their allelopathic effect
(Galán-Pérez etal. 2022).
As for the apparent additive effect already noted
among the phenolic acids, coumarin effects can simi-
larly be enhanced by the presence of phenolic acid
(Haig 2008). For example, Korableva et al. (1969)
report that scopoletin is more effective as a growth
retardant when used in combination with caffeic acid
than when used alone, and Einhellig (1996) reports
that a combination of coumarin (umbelliferone), phe-
nolic acid (salicylic acid), and flavonol (rutin) also
possesses a phytotoxic effect in the mixture. While
the phenolic acids do not show evidence of being
able to influence cell division, compounds such as
scopoletin and coumarin have been reported to have
an effect in decreasing mitosis, and also inhibiting
certain enzyme actions (Haig 2008). Scientists after
most in-depth analysis observed that scopoletin, one
of the most common coumarins in higher plants, dis-
played strong phytotoxicity on Arabidopsis thaliana
seedlings, and showed its herbicidal activity towards
the parasitic weed Orobanche crenata (Galán-Pérez
etal. 2022).
Misra et al. (2020), in their carefully designed
study, employed non-targeted metabolomics to inves-
tigate the short-term metabolic changes induced in
wheat seedlings by the allelochemical umbelliferone.
The study clearly showed the system-wide metabo-
lomic changes in wheat seedlings in response to
umbelliferone treatment. The approach was novel due
to the initial short-term experiment using sub-lethal
concentrations. This non-targeted metabolomics
approach allowed the identification of system-wide
metabolic responses activated by the plants to deal
with this phytotoxic compound. It has been shown
that umbelliferone induced the dysregulation of
metabolites involved in the shikimate pathways, as
well as in tryptophan and tryptamine metabolism.
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This experiment provides new insights into the early
response of plants to this specialized allelopathic
metabolite and can inform the design of new organic
herbicide approaches (Misra etal. 2020).
Juglone (5-hydroxy-1,4-naphthalenedione),
(Fig.3) is a phenolic compound well known to have
a negative impact on the growth of other plants, pro-
duced by the black walnut (Juglans nigra L.). Juglone
is a strong inhibitor of hydroxyphenylpyruvate dioxy-
genase (HPPD), the key enzyme in plastoquinone
biosynthesis, and also inhibits photosynthetic and
respiratory electron transport systems (Yoneyama and
Natsume 2013).
Leptospermone (1-hydroxy-2-isovaloryl-4,4,6,6-
tetramethyl cyclohexen-3,5-dione), (Fig.3), is a natu-
ral triketone produced by the roots of the bottlebrush
(Callistemon citrinus Curtis) and its herbicidal activ-
ity is due to its inhibition of p-hydroxyphenylpyru-
vate dioxygenase. These disorders lead to disruption
in carotenoid biosynthesis and loss of chlorophyll. Its
herbicidal action at high doses excludes commercial
development. However, the structure of leptosper-
mone was used as a basis for developing synthetic
analogs applied for the control of broadleaved weeds
in maize, for example (Soltys et al. 2013). Dayan
etal. (2011) draw attention to manuka oil, with the
principal active ingredient leptospermone which can
be used to potentiate the herbicidal activity of other
herbicidal essential oils. Manuka oil (1%) applied as
a post-emergence spray, significantly decreased the
growth and dry weight of redroot pigweed, barnyard
grass, velvetleaf, and hairy crabgrass. Scientists have
proven that manuka oil can be used to potentiate the
herbicidal activity of other herbicidal essential oils
due to additive or synergistic action. This kind of
application poses another possibility of usage for this
allelopathic compound in its natural form without
chemical modification of the structure (Soltys etal.
2013).
5.3 Terpenes
Terpenoids are classified based on the number of
isoprene units in their carbon skeleton, e.g. monoter-
penes, sesquiterpenes, diterpenes, sesterpenes, and
triterpenes (Nair etal. 2022). Two distinct pathways
are involved in the production of the basic isoprene
units—isopentenyl diphosphate (IDP) and dimethy-
lallyl diphosphate (DMADP)—required for terpenoid
synthesis. In plastids, the methylerythritol phos-
phate (MEP) pathway, while in the cytosol, endo-
plasmic reticulum, and peroxisomes the mevalonic
acid (MVA) pathway takes place, as shown in Fig.4
(Corso et al. 2021). Terpenoids have multiple bio-
logical activities in plants as photoprotective agents,
mediators of polysaccharide assembly, reproductive
hormones, allelochemicals, and agents in communi-
cation and defense. Several studies confirm the allel-
opathic nature of terpenoids, causing inhibitory and
autotoxic effects on seedling germination and growth.
Such effects are due to characteristic interactions such
as disruption of ATP formation and endocrine activ-
ity, complexation with proteins, and obstruction of
respiration (Bachheti etal. 2020). Considering these
properties terpenoids have a high potential as polli-
nator attractants, in the defense of plants against her-
bivores and microbial pathogens (Borrelli and Trono
2016).
Monoterpenes, such as 1,4-cineole and 1,8-cineole,
are the main constituents of plant essential oils and
are widely known for their strong inhibitory effects
on plant growth and seedling germination (Latif etal.
2017). Monoterpenes and sesquiterpenes have been
quite well characterized in terms of their phytotoxic
potential. For example, the sesquiterpene b-caryo-
phyllene is present in numerous plant volatiles and
has an inhibitory effect on germination and seedling
growth of Brassica napus L. and Raphanus sativus
L. even at very low concentrations (Latif etal. 2017).
However, in other studies, allelopathic monoterpe-
nes such as p-menth-2-en-1-ols thymol, carvacrol,
1,8-cineole, α-pinene, and β-pinene were isolated
from Eucalyptus species. Furthermore, sesquiterpe-
nes such as spathulenol and α-, β-, and γ-eudesmols
were detected in Eucalyptus in other experiments
(Bachheti etal. 2020).
Asteraceae plants such as asters, daisies, and sun-
flowers are another example of a natural source of
allelopathic sesquiterpenes and sesquiterpene lac-
tones (Araújo etal. 2021). Araújo etal. (2021) show
that monoterpenes can act by inhibiting the enzyme
asparagine synthase, thus preventing growth. Their
action may also include impairing mitochondrial cell
respiration of organelles and a slow release of pro-
teins in the plasma membrane.
Momilactones belong to a group of naturally
occurring diterpenes known as (9β-H)-pimaranes,
which are characterized by a β-orientation of the
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proton on carbon-9 of the pimarane skeleton (Yoney-
ama and Natsume 2013). Momilactone B (Fig. 3)
has allelopathic properties, such as antifungal activ-
ity against the pathogen Piricularia oryzae, which
causes a devastating disease leading to 10–30% loss
in total rice yields, as well as production losses in
wheat, barley and millet crops (Zhao et al. 2018).
Momilactone B was found in shoots and roots of rice
plants over their entire life cycle and its highest level
in this part of plants at the day of flowering initiation
was 245 and 64.1nmol g−1 fresh weight, respectively.
When converted to 1kg of rice shoots and roots, this
plant mass was found to be able to release 245 and
64.1μmol of momilactone B into the soil or neigh-
boring environment by decomposition of their resi-
dues (Kato-noguchi and Ino 2005). Momilactone B
inhibits the growth of typical rice weeds like Echi-
nochloa crus-galli and E. colonum at concentrations
greater than 1μmol/L (Amb and Ahluwalia 2016) so
this concentration may be sufficient to cause growth
inhibition of their neighboring or successional plants
(Kato-noguchi and Ino 2005). This evidence sup-
ports the idea that allelochemicals can be important
tools for weed management, helping to address the
challenges of environmental pollution and herbicide
resistance.
6 Allelopathic potential intheworld’s most
important crop cultivation
Some crop accessions: wheat (Triticum sp.), rye
(Secale cereale), maize (Zea mays), rice (Oryza
sativa), and sorghum (Sorghum bicolor), have been
shown to possess strong allelopathic potential against
the growth of certain weed species (Schandry and
Becker 2020). However, the exploitation of the poten-
tial of these crops in agriculture depends on the sta-
bility of the allelochemicals and their environmental
fate, as well as their presence in biologically active
concentrations. The role of the biotic soil environ-
ment is fundamental in assessing allelopathic traits of
crops for agricultural strategies such as weed manage-
ment (Schandry and Becker 2020).
Implementation of suitable weed management
practices is critical in crop production to minimize
competition for water, nutrients, space, and light
between weeds and economically important crops.
Over the past several years attempts have been made
to enhance the allelopathic properties of crops by
variety selection and conventional breeding (Moham-
madi 2013). Allelopathy is a cost-effective and envi-
ronment-friendly approach to replacing synthetic
herbicides therefore, keeping in mind the role of this
phenomenon in weed management, many scientists
are involved in designing strategies to manipulate the
allelopathic traits of globally cultivated crops. Sev-
eral studies support the thesis that many crops exhibit
allelopathic effects relative to other crops grown
simultaneously or downstream. Therefore, many
crops have been considered for allelopathic activity
against other plants or weeds (Chung et al. 2018).
Sustainable food security requires abundant yields
from a limited land area. A major challenge is maxi-
mizing crop productivity under changing climatic
conditions to meet global food security challenges.
6.1 Rice (Oryza sativa L.)
Of the species with allelopathic character, special
mention should be made of rice (Oryza sativa L.)—
one of the most important crops worldwide, with
more than 1.5 × 108 hm2 of land being cultivated for
its production (Amb and Ahluwalia 2016). Although
rice is cultivated on a massive scale—globally, rice
provides about 20% of the caloric intake for more
than 50% of the world’s population—its yields are
at risk of significant losses due to disease, pests, and
weeds. To minimize losses, three million tons of her-
bicides are currently used annually worldwide to con-
trol paddy weeds in agricultural systems (Chung etal.
2006). Considering that rice is a staple food for most
of the world’s population, controlling weeds in rice
agriculture is of utmost importance. Extensive use
of herbicides in rice cultivation has led to the devel-
opment of herbicide resistance in a wide range of
weeds, thus in countries such as China, Korea, Japan,
the USA, and India, extensive research has been initi-
ated to exploit the allelopathic potential of this plant
(Mushtaq etal. 2020).
Fields of rice have ecotones that encompass
aquatic habitats as well as drylands and therefore
harbor biodiverse plant communities (Chung et al.
2006). Additionally, the paddy field ecosystem main-
tains nutrient recycling, trophic structure balance, and
water recharge. The ability of rice to grow in water
has been used as part of weed control, as weeds are
less likely in this medium, but growing rice in water
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is a labor-intensive process and irrigation measures
are becoming scarce with time (Mushtaq etal. 2020).
Several rice varieties are known to release biocidal
allelochemical compounds which might affect micro-
bial and pathogenic diversity and soil characteristics
(Mushtaq et al. 2020). Since these interactions may
be positive, it is advantageous to use this as an effec-
tive contributor to a sustainable and eco-friendly sys-
tem (Chung etal. 2006).
Based on many studies, researchers have con-
cluded that plants having higher yield potential,
strong competitive ability against weeds, adequate
plant height and sufficient leaf area may have height-
ened allelopathic potential (Mushtaq et al. 2020).
Rice cultivars and lines have been analyzed for the
effects of this crop on the growth of common weeds.
A wide range of chemical compounds produced by
this plant have been isolated, including momilactone
A and B, resorcinols, cyclohexanones, flavones, ben-
zoxazolinones, and glycoside derivatives. Secondary
plant metabolites such as phenols, coumarins, terpe-
noids, steroids, alkaloids, and indoles have also been
identified (Gniazdowska 2007).
Several reports confirm a key role in rice allelo-
pathic activity of momilactone A and B, which are
secreted by rice roots into the rhizosphere over the
entire life cycle. Allelopathic varieties of these plants
can release up to 2–3μg of momilactone B per day
(Soltys etal. 2013). These allelochemicals inhibited
the growth of typical weeds in rice, e.g. awnless barn-
yard grass (Echinochloa colona Link.). In addition
the phytotoxic abilities of momilactones were also
demonstrated on livid pigweed (Amaranthus lividus
L.), hairy crabgrass, and annual bluegrass (Poa annua
L.) at specific concentrations (Soltys etal. 2013).
Momilactones were first isolated as growth inhibi-
tors, but were later, also found as phytoalexins
(Soltys etal. 2013). Momilactone A and B, consid-
ered unique to rice, are diterpenoid phytoalexins,
and are antimicrobial secondary metabolites that are
produced in response to signaling molecules termed
biotic elicitors. Recently, these compounds have also
been found in a taxonomically distinct plant, the moss
(Hypnum plumaeforme Wils.), but although they have
a proven ability to inhibit plant growth, the mecha-
nism of their action in plants is still unrecognized
(Soltys etal. 2013).
Diterpenoid momilactones are an example of nat-
ural compounds for which correlative biochemical
evidence for a role in allelopathy has been obtained.
Researchers used reverse genetics, using knock-out
of the respective diterpene synthases (copalyl diphos-
phate synthase 4 (OsCPS4) and kaurene-like synthase
4 (OsKSL4)), aimed at providing evidence that rice
momilactones are responsible for allelopathy, mainly
inhibiting the growth of the widespread rice weed
Echinochloa crus-galli. These conclusions furnish
a molecular target for breeding and metabolic engi-
neering and the intriguing possibility of momilactone
biosynthesis insitu-produced herbicides in rice crops
(Xu etal. 2012).
Under field conditions, almost all rice cultivars
indicate allelopathic potential against ducksalad,
barnyard grass, redstem, and monochoria, which rep-
resent real rice–weed interaction and show more than
40% inhibition of spinach growth (Mushtaq et al.
2020). The finding that more than one allelochemical
is responsible for the allelopathic effect is based on
laboratory and field testing showing that rice cultivars
can inhibit both monocotyledonous and dicotyledon-
ous weeds (Mushtaq etal. 2020). Many allelochemi-
cals were isolated from the roots of different rice cul-
tivars, but none of these was able to break the growth
effects of weeds alone. Further experiments made
between weed plants and rice, and also the research of
more genetic and morphological characteristics using
more rice varieties are needed (Ahn etal. 2005). The
combination of plant allelopathy with existing agro-
technical practices can be an ecological approach to
increase agricultural yields through sustainable weed
management. Understanding the factors influencing
allelochemical production in rice and the mechanisms
of their phytotoxic effects can help develop novel
weed control tactics while enabling farmers to man-
age weeds in an environmentally friendly manner
(Rahaman etal. 2022).
6.2 Wheat (Triticum aestivum L.)
Wheat (Triticum aestivum L.), is another crop spe-
cies with high allelopathic potential, where natu-
rally produced allelochemicals can be manipulated
to eliminate weeds and ensure an environmentally
friendly and sustainable agricultural production sys-
tem. A variety of parameters affect wheat’s biological
potential, like that of all other allelopathic crops, such
as its age, soil pH, carbon and nitrogen concentra-
tion, and soil water content. A variety of phytotoxic
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compounds suspected of inducing allelopathic effects
in wheat have been found and categorized into three
major allelochemical categories: phenolic acids,
hydroxamic acids, and fatty acids (Haig 2008). The
presence of these secondary metabolites depends on
plant family, species, and chemotype. The Labia-
tae family is notably rich in terpenoids and phenolic
acids, the Apiaceae family coumarins and furanocou-
marins, while hydroxamic acids and benzoxazinoids
being abundant in the Poaceae family (Haig 2008).
Benzoxazinoids, including benzoxazinones and
benzoxazolinones, are unique bioactive metabo-
lites generated by some species of Poaceae, includ-
ing wheat, that are allelopathic interference agents
(Mwendwa etal. 2021). The most abundant of these
acids in wheat is 2,4-dihydroxy-7-methoxy-1,4-ben-
zoxazin-3-one (DIMBOA), which, together with
2,4-dihydroxy-2H-1,4-benzoxazin-3-one (DIBOA)
and its associated microbial conversion products,
are compounds with strong allelopathic effects on
numerous broadleaf weeds (Latif et al. 2017). The
highest amounts of benzoxazinoids are detected in
young tissues of the roots and shoots, where they are
glucosylated and stored in vacuoles or extruded by
the roots. The soil is the primary medium through
which allelochemicals, soil microorganisms, and
their target plants interact, therefore its parameters
such as organic matter, reactive mineral surfaces, ion
exchange capacity, and inorganic ions have the main
impact on allelochemical effects (Mwendwa et al.
2021).
Propionic acid, one of the fatty acids found in
wheat residues, decreased the germination and devel-
opment of annual ryegrass considerably. Allelopathic
activity of p-coumaric acid and propionic acid in
wheat accessions was associated with a decrease in
germination in both resistant and susceptible annual
ryegrass ecotypes. These effects have been reported
in a variety of wheat tissues or organs, including
shoots and roots; moreover, roots and their extracts
have been proven to be more phytotoxic than other
plant tissues (Hussain etal. 2022). Wheat also con-
tains long-chained carboxylic acids such as oleic, lin-
oleic, and stearic acids. These fatty acids decreased
Leptochloa chinensisgermination but did not affect
the weed’s root elongation (Haig 2008).
There are many reports that there is a strong
genetic basis for conferring allelopathic potential in
wheat. Wu and co-workers reported that there are
substantial genetic variations in allelopathic activ-
ity in wheat, thus providing a sufficient gene pool
for the development of allelopathic wheat cultivars
to suppress weeds (Wu etal. 2000). In their study,
the authors showed that the normal distribution of
allelopathic activity in the collection of 453 wheat
accessions was similar to that reported in rice, indi-
cating that wheat allelopathic activity is a quantita-
tive trait (Wu etal. 2000).
6.3 Maize (Zea mays L.)
Zea mays is the third most cultivated food crop after
rice and wheat. Apart from its consumption applica-
tions, it is an important resource to produce plas-
tics, dyes, and packaging materials (Ahmed etal.
2022). It is now one of the driving models for plant
utilitarian genomics and, because of its high phyto-
chemical content, it is used to treat a variety of ail-
ments (Ahmed etal. 2022). As early as 1983, it was
shown that allelochemicals in maize are produced in
all parts of the plant and are released into the envi-
ronment because of root exudate seeping into the
soil during rain leaching. Even maize pollen when it
falls on plants growing nearby (such as bottle gourd,
watermelon, etc.) causes a decrease in their fruiting
(Mushtaq etal. 2020). The allelopathic potential of
extracts from different plant parts of Zea mays was
evaluated, and together with sorghum, it showed a
pronounced inhibitory effect on germination and
growth of wild barley (Hordeum spontaneum) (Al-
Tawaha and Odat 2010). The compared allelopathic
effects of several varieties of sunflower against
problematic weed species in wheat were also eval-
uated (Alsaadawi etal. 2011). Scientists grew the
allelopathic sunflower cultivars in a mixture with
weeds or applied the residues of sunflower cultivars
to the wheat crop and its weeds. It was observed
that the sunflower varieties tested differed in their
allelopathic effect and reduced total weed density
by 10–87% and total weed biomass by 34–81%. The
maize cultivars with a strong suppressive effect not
only inhibited the growth of companion weeds but
also reduced the population density and biomass of
the weeds when their residues were introduced into
the soil (Alsaadawi etal. 2011).
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6.4 Rye (Secale cereale L.)
Another example of a crop with high allelopathic
activity is rye, whose most important compounds
responsible for this effect are benzoxazinones
[2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA)
and 2(3H)-benzoxazolinone (BOA)]. The researchers,
after an extensive review of the allelopathic potential
of rye, recorded 16 allelochemicals present in this
plant, including β-phenyllactic acid, protocatechuic
acid, DIBOA (glucoside), vanillic acid, apigenin-
glycosides, syringic acid, luteolin-glucuronides,
p-hydroxybenzoic acid, p-coumaric acid, benzoxa-
zolinones BOA, cyanidin glycosides, β-hydroxybutric
acid, isovitexinglucosides, DIMBOA (glucoside),
gallic acid, and ferulic acid/conjugates (Schulz etal.
2013).
6.5 Sorghum (Sorghum bicolor L.)
Extensive literature explains also the allelopathic
potential of sorghum and its implications in different
cropping. Sorghum produces a variety of allelopathic
compounds, the most important of which are hydro-
phobic p-benzoquinone (sorgoleone), phenolics, and
acyanogenic glycoside (Abbas et al. 2021). Sorgo-
leone and its 1,4-dihydroxy form (resorcinol) repre-
sent 90% of the compounds present in sorghum root
exudates (Głąb etal. 2017). A precise determination
of the mechanism of sorghum root trichome forma-
tion can be used for the targeted use of sorghum as
a mulch or cover crop for effective control of germi-
nating weed seedlings. The action of sorgoleone can
be compared to the action of the soil herbicide pen-
dimethalin. A multidisciplinary approach to sorghum
cultivation represents a promising prospective treat-
ment using secondary metabolites that can also serve
as lead compounds in herbicide discovery programs
(Hussain etal. 2021).
6.6 Plant extracts as a source of allelochemicals
Another approach to weed control is using extracts
from certain allelopathic plants, which reduce the
occurrence of weeds by inhibiting germination and
seedling growth. There are several papers in which
the allelopathic potential of aqueous plant extracts
under field conditions in the most significant crops
such as wheat, maize, or cotton has been investigated,
presenting them with phytotoxic results expressed in
terms of weed density and biomass reduction (Scavo
and Mauromicale 2021). Examples from the literature
are presented in Table1, which was prepared based
on articles Scavo and Mauromicale (2021), Cheema
etal. (2013), Farooq etal. (2011). Among the most
common aqueous plant extracts used for weed control
are sorghum and sunflower, which in concentration at
12 L ha −1 are efficient in limiting the dry weight of
wild oats (Avena fatua L.) and canary grass (Phalaris
minor Retz.), while at 6 L ha−1 is the most economi-
cally viable treatment. The authors of these applica-
tions recommended applying the extract directly to
the soil or growing media to mitigate phytotoxicity on
the above-mentioned crops (Muhammad etal. 2009).
Summarizing the chemical effects induced in plants
by the treatment of these extracts, the authors high-
lighted that there was a significant increase in reactive
oxygen species (ROS), suppression of the gibberellic
pathway and accumulation of abscisic, salicylic and
jasmonic acids, changes in cell membrane perme-
ability and deregulation of nutrient uptake as well as
modification of photosynthesis and respiration. Allel-
opathic plant extracts and synthetic herbicides can
be used together to reduce the application doses of
these harmful ones (Scavo and Mauromicale 2021).
Selected examples of this combination are sum-
marized in Table2. An example of this approach is
a study that used aqueous extracts of sorghum, sun-
flower, brassica (Brassica compestris L.), and mul-
berry (Morus alba L.) with a reduced dose of atra-
zine. It was concluded that a dose of atrazine alone
and half of the dose of atrazine in combination with
allelopathic plant aqueous extracts of sorghum, sun-
flower, brassica, and mulberry improved the reduction
in weed density and dry biomass, thereby enhancing
the grain yield and net income. Therefore, it may be
concluded that the use of allelopathic plant extracts
can effectively reduce herbicide usage rates for weed
control (Khan etal. 2012). Encouraging results have
been obtained in wheat, maize, cotton, and rice cul-
tivation, but there are still not enough studies con-
ducted to assess synergism between bioherbicides
and synthetic herbicides. For more widespread appli-
cation, it is crucial to understand the specific role of
environmental factors on the bioavailability and effec-
tiveness of allelochemicals. Environmental elements,
mainly air and soil, act as carriers of allelochemicals,
so pedoclimatic conditions can greatly influence the
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Table 1 Effects of aqueous allelopathic plant extracts on weed suppression in field crops
Allelopathic cover crop Dose
(Extract concen-
tration)
Crops ben-
efited
Yield
increase
Targetweeds Weed control (%) References
Sunflower 0.1 L m‒2
(10%)
Wheat – Chenopo-
diumalbumL.
‒ 70%ofbiomass Anjum and
Bajwa
(2007)
Chinesecabbage 0.002L m‒2
(10%)
Mungbean – Trianthema
portulacastrumL.,
CyperusrotundusL
‒ 14.6% of
density and dry
weight
Ullah etal.
(2020)
Treewormwood 4L m‒2
(18.82%)
Wheat − 52.9% Severalmonocotsand
dicots,mainlyA.fatua
andP.paradoxa
~ 30%ofweed
suppression
Carrubba
etal. (2020)
Siciliansumac 4L m‒2
(8.75%)
Wheat + 9% 50.8%ofweed
suppression
Commonthyme 4L m‒2
(22.33%)
Wheat − 7.2% ~ 35%ofweed
suppression
Commonlantana 4L m‒2
(6.14%)
Wheat + 16.5% 16%ofweedsup-
pression
Mediterraneanspurge 4L m‒2
(2.27%)
Wheat − 2.3% ~ 40%ofweed
suppression
Treeofheaven 0.001‒0.002gL
−1
(20%)
Sage,rose-
mary,
carnation
–LepidiumsativumL.,
RaphanussativusL.
0%weedpresence
in sage androse-
mary, ~ 24%in
carnation
Caser etal.
(2020)
Sorghum 12 L ha−1 Cotton + 45.5% Trianthema portulac-
astrum L.
Reduction in total
weed density
(47%) and dry
weight (29%)
Cheema etal.
(2002)
Sorghum 12 L ha −1 Wheat + 11% Fumaria indica
Hauskn., Reduction in total
weed density
(21.6%) and dry
weight (35.4%)
Cheema and
Khaliq
(2000)
Sorghum 12 L ha−1 Sunflower + 7.7% Cyperus rotundus L., Reduction in total
weed density
(19.3%) and dry
weight (27.2%)
Nawaz etal.
(2001)
Sun-
flower + Rice + Bras-
sica
4.5 t ha−1 each Maize 41.0 Trianthema portulac-
astrum
60.1% weeds dry
weight reduction
Khaliq etal.
(2010)
Cotton + Sorghum - Maize 23.7 Trianthema portulac-
astrum, Convolvulus
arvensis
92.0% weeds dry
weight reduction
Iqbal etal.
(2007)
Sorghum 0.0006L m‒2Wheat + 39% AvenafatuaL., 21–41% weeds
dry weight
reduction
Rehman etal.
(2010)
Phalaris minorRetz. 23–41% weeds
dry weight
reduction
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transport and retention of these compounds (Scavo
and Mauromicale 2021). Regarding weed manage-
ment in wheat crops, it was proven that the dose of
isoproturon could be reduced by as much as 60% fol-
lowing the application of a mixture of an aqueous
extract of sorghum (Farooq etal. 2011). It was also
discovered that the combination of an aqueous extract
of sorghum and pedimentalin at a dose equivalent to
one-third of the standard amount resulted in a higher
cotton yield than the application of the full dose of
synthetic herbicide, even though weed suppression
was relatively lower. Reduced doses of pedimenta-
lin were also combined with aqueous extracts from
sorghum, sunflower, brassica, and rice demonstrat-
ing that 50–67% less herbicide in combination with
allelopathic aqueous extracts can be effective in con-
trolling weeds and increasing oilseed canola yields
(Farooq etal. 2011).
Table 1 (continued)
Allelopathic cover crop Dose
(Extract concen-
tration)
Crops ben-
efited
Yield
increase
Targetweeds Weed control (%) References
Sorghum + sunflower 0.0006L m‒2
each
Wheat + 62% AvenafatuaL., 24–39% weeds
dry weight
reduction
Muhammad
etal. (2009)
Phalaris minorRetz. 30–35% weeds
dry weight
reduction
Sorghum + sunflower 0.00012L m‒2
each
Wheat + 53.5% AvenafatuaL., 42–62% weeds
dry weight
reduction
Phalaris minorRetz. 36–55% weeds
dry weight
reduction
Sorghum + eucalyptus 0.0006L m‒2
each
Wheat + 47.5% AvenafatuaL., 28–3% weeds dry
weight reduction
Phalaris minorRetz. 13–28% weeds
dry weight
reduction
Sorghum + sesame 0.00006L m‒2
each
Wheat + 44% AvenafatuaL., 21–24% weeds
dry weight
reduction
Phalaris minorRetz. 19–24% weeds
dry weight
reduction
Sorghum + tabacco 0.0006L m‒2
each
Wheat + 18.5% AvenafatuaL., 14% weeds dry
weight reduction
Phalaris minorRetz. 10–14% weeds
dry weight
reduction
Sorghum + brassica 0.00006L m‒2
each
Wheat + 19% AvenafatuaL., 18–24% weeds
dry weight
reduction
Phalaris minorRetz. 21–27% weeds
dry weight
reduction
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Table 2 Weed control through a combination of allelopathic aqueous extracts with a reduced dose of herbicides
Crop Allelopathic extract
(Rate)
Herbicide
(Rate)
Weed species Weed control [%] References
Standard dose
herbicide
Herbicide (1/2
dose) + allelopathic
extract
Rice
(Oryza sativa L.)
Sorghum + Sun-
flower + Rice
(15 L ha−1 each)
Butachlor
(1200g a.ia. ha−1)Echinocloa crusgalli WDb 80% WD 80% Rehman etal. (2010)
DWc 79% DW 66%
Cyperus iria WD 79% WD 67%
DW 74% DW 71%
Dactyloctenum
aegyptium WD 76% WD 74%
DW 80% DW 76%
Sorghum + Sun-
flower + Rice
(15 L ha−1 each)
Pretilachlor
(625g a.i. ha−1)Echinocloa crusgalli WD 82% WD 76%
DW 73% DW 60%
Cyperus iria WD 83% WD 66%
DW 75% DW 60%
Dactyloctenum
aegyptium WD 82% WD 74%
DW 85% DW 81%
Sorghum + Sun-
flower + Rice
(15 L ha−1 each)
Ethoxysulfuronethyl
(30g a.i. ha−1)Echinocloa crusgalli WD 81% WD 72%
DW 73% DW 62%
Cyperus iria WD 79% WD 69%
DW 75% DW 64%
Dactyloctenum
aegyptium WD 85% WD 75%
DW 82% DW 69%
Sorghum
(15 L ha−1 each)
Penaxolam
(30mL a.i. ha−1)Echinocloa crusgalli DW 26% DW 35% Wazir etal. (2011)
Echinocloa colonum
Cyperus rotundus
Cyperus iria
Dactyloctenum
aegyptium
Wheat
(Triticum aestivum L.)
Sorghum
(12 L ha−1 each)
Isoproturon
(1kg a.i. ha−1)Phalaris minor WD 94%
DW 79%
WD 94%
DW 65%
Cheema etal. (2003)
Melilotus parviflora
Coronopus didymus
Sorghum + Sunflower
(18 L ha−1 each)
Bensulfuron + Iso-
proturon
(1,050g a.i. ha−1)
Phalaris minor WD 85% WD 89% Razzaq etal. (2010)
DW 93% DW 95%
Coronopus didymus WD 82% WD 88%
DW 64% DW 87%
Metribuzin
(175g a.i. ha−1)Phalaris minor WD 92% WD 69%
DW 46% DW 83%
Coronopus didymus WD 91% WD 88%
DW 77% DW 98%
Metribuzin + phe-
noxaprop
(190g a.i. ha−1)
Phalaris minor WD 89% WD 83%
DW 92% DW 97%
Coronopus didymus WD 87% WD 87%
DW 91% DW 94%
Mesosulfuron + ido-
sulfuron
(120g a.i. ha−1)
Phalaris minor WD 42% WD 89%
DW 85% DW 98%
Coronopus didymus WD 91% WD 87%
DW 74% DW 93%
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7 Biotechnology tools forunderstanding
allelopathic interactions
Many studies have pointed out the high dependence
of allelopathic potential on genotype, as there are
differences between varieties in allelochemical con-
centrations and allelopathic activity. Rice, wheat,
rye, barley, and sorghum are the most researched
allelopathic crops, with considerable allelochemi-
cal differences depending on the cultivar (Scavo and
Mauromicale 2021). A novel approach to plant cul-
tivation uses the biotechnological transfer of allelo-
pathic traits between cultivars of the same species
or between species. This concept is based on selec-
tive and efficient screening of the occurrence of
allelopathic characteristics using several molecular
markers (Amb and Ahluwalia 2016).
Modern techniques make it easier to locate genes
conferring allelopathic traits. These genes can be
cloned and incorporated into current commercial
varieties and other plants with competitive compo-
nents (e.g., early vigor, wide leaf area, fast seed-
ling emergence, root development, plant height,
and tillering). This approach would be a milestone
toward the further development of sustainable crop
production systems with less dependence on her-
bicides (Wu et al. 2002). Conventional breeding
methods are more easily introduced for plant breed-
ers because of the public concerns arising from
transgenic crops (Wu etal. 2002). It is essential to
ensure that genetic modifications with allelochemi-
cals do not damage the environment, humans, ani-
mals, and other non-target organisms, as well as
monitor for potential effects on different varieties or
weed species (Wu etal. 2002).
However, polygenicity and the low economic
added value make breeding methods very difficult
(Scavo and Mauromicale 2021). Allelopathy is a
polygenetic trait poorly connected to yield, neces-
sitating the modification of several genes to encode
the synthesis of allelochemicals. This has been found
in the case of benzoxazinoids such as DIMBOA and
DIBOA in Poaceae members, for example (Scavo
and Mauromicale 2021). The allelochemicals’ fate is
mainly determined by the developmental stages of the
plant and the conditions of the external environment.
a a.i.—active ing redient
b D—weed density
c DW—dry weight
Table 2 (continued)
Crop Allelopathic extract
(Rate)
Herbicide
(Rate)
Weed species Weed control [%] References
Standard dose
herbicide
Herbicide (1/2
dose) + allelopathic
extract
Maize (Zea mays L.) Sorghum + Bras-
sica + Sun-
flower + Mulberry
(20 L ha−1 each)
Atrazine (500g a.i.
ha−1)Trianthema portulac-
astrum L
WD 38% WD 36% Khan etal. (2012)
DW 94% DW 90%
Sorgaab Furamsulfuron Convolvulus arvensis
L
DW 64% DW 57% Latifi and Jamshidi
(2011)
Amaranthus retro-
flexus
Sunflower
(Helianthus annuus
L.)
Sorghum + Sunflower
(15 L ha−1 each)
Pendimethalin
(825mL a.i. ha−1)Chenopodium album
Melitotus indica WD 95% WD 84% Awan etal. (2009)
DW 86% DW 67%
Canola
(Brassica napus L.)
Sorghum + Brassica
(15 L ha−1 each)
Pendimethalin
(1.2kg a.i. ha−1)Trianthema portulac-
astrum WD 100% WD 91 Jabran etal. (2008)
DW 100% DW 94
Cyperus rotundus WD 32% WD 43%
DW 6% DW 38%
Chenopodium album WD 78% WD 74%
DW 83% DW 62%
Coronopus didymus
L
WD 39% WD 66%
DW 37% DW 71%
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Genetic modification can modulate the production
and release of allelochemicals, and the potential for
crop weed control can be improved (Wu etal. 2002).
However, the challenge for scientists is to deter-
mine the mechanism of inheritance of allelopathic
agents and to isolate and quantify the compounds
responsible for these properties. A certain compli-
cation is a case when more than one gene encoding
special enzymes is required to increase the synthesis
of a given allelochemical. This situation occurred in
the case of the DIMBOA compound synthesized by
different grass species. In maize, for example, the
biosynthesis of this compound is determined by five
genes encoding three enzymes (Soltys et al. 2013).
The genes involved in synthesizing allelochemicals
and genetic engineering tools, such as recombinant
DNA, polymerase chain reaction, and metabolic engi-
neering, have been developed to understand meta-
bolic pathways better (Scavo and Mauromicale 2021).
Analysis of quantitative trait loci (QTLs) based on
amplified fragment length polymorphism (AFLP) and
restriction fragment length polymorphism (RFLP) is
used to identify genetic markers responsible for con-
ferring allelopathic traits (Scavo and Mauromicale
2021). These markers are used by scientists to sys-
tematically map and distinguish genes crucial in con-
ferring quantitative traits (Wu etal. 2002). Through
marker-assisted selection in crop breeding programs,
linkage analysis of genetic markers and QTLs may
increase genetic gains for allelopathic activity (Wu
et al. 2002). Given the potential benefits of allel-
opathy for the cultivation of commonly produced
crop plants such as rice, wheat, and sorghum, vari-
ous researchers have proposed improving crop cul-
tivar allelopathic qualities by traditional breeding or
genetic modification (Mohammadi 2013).
Rice diterpenoid momilactones are natural com-
pounds with correlative biochemical evidence sug-
gesting a function in allelopathy. Researchers apply
reverse genetics and knock-outs of the appropriate
diterpene synthases to prove that rice momilactones
are involved in allelopathy, such as reducing the
development of the typical rice paddy weed, barnyard
grass (Echinochloa crus-galli). The inducible nature
of momilactone formation in rice implies that similar
induction may enhance rice’s endogenous ability to
control weed development (Xu etal. 2012). Another
example is in screening for the alkaloids gramine,
hordenine, and its direct precursor N-methyltyramine
in barley (Scavo and Mauromicale 2021). The
researchers discovered a significant difference based
on plant parts between wild relatives and modern
genotypes, thus providing essential advance in breed-
ing this plant (Scavo and Mauromicale 2021).
8 Allelopathic practices used forweed control
Allelopathic interaction plays an important role in
agricultural ecosystems due to its influence on crop
plant development. Several variables need to be taken
into consideration when using allelopathy for weed
control, such as the weed species, climatic conditions,
type of agricultural practices, and economic aspects.
The phenomenon of allelopathy may provide a new
front in integrated weed control by including them
in rotational sequences or intercropping near a cash
crop, cover cropping as living or dead mulches, and
crop residue incorporation into the soil (Scavo and
Mauromicale 2021). There is a feasible perspective
of using the allelopathic mechanism as an environ-
mentally friendly tool for weed control in cropping
systems without dependence on chemical herbicides.
The incorporation of allelopathic plants into agricul-
tural strategies will improve sustainable crop produc-
tion due to the positive effects of these practices on
soil fertility, organic matter content, and ecosystem
biodiversity (Abbas etal. 2021).
Intercropping is the simultaneous cultivation of
different crops at the same time in the same field,
especially using allelopathic species, which have a
high potential to control weeds in an environmen-
tally friendly approach. The selective combination
of yield-improving crops promotes farm diversifica-
tion and provides economic benefits (Khamare etal.
2022). This type of cultivation is a great way to make
efficient use of natural resources, increase biodiver-
sity, control pests, and improve crop yield and qual-
ity, as well as natural soil fertility while using fewer
off-farm inputs (Glaze-Corcoran etal. 2020). Evaluat-
ing the effectiveness of intercropping main crops with
allelopathic plants, there is general agreement on the
potential for weed control through the release of allel-
ochemicals (Abbas etal. 2021). In a certain field trial,
the intercropping of legumes with wheat was evalu-
ated for weed suppression compared with the sole
wheat crop (Abbas etal. 2021). The intercrops in the
experiment included white clover (Trifolium repens
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L.), black medic (Medicago lupulina L.), alfalfa, and
red clover (Trifolium pratense L.). The intercrops
reduced weed density in the following crop, and red
clover was the most effective intercrop for suppress-
ing weeds in organically grown wheat. In a simi-
lar study, intercultivation of pea (Pisum sativum L.)
with barley (Hordeum vulgare L.) sorghum with pea
(Vigna unguiculata (L.) Walp.) wheat with rape and
wheat with chickpea (Cicer arietinum L.) was used
(Abbas et al. 2021). In all instances, intercropping
allelopathic plants with the main crop can reduce
weed intensity and improve yield gains. Properly car-
ried out intercropping is efficient in terms of the use
of natural resources and increases biodiversity as well
as crop quality.
Crop rotation is a cultivation strategy in which
different crops are grown in a specific order. A well-
designed crop rotation ensures the formation of an
unstable environment for weeds. Crop rotations
change growth conditions from year to year, creat-
ing a situation in which only a few weeds are readily
adaptable. Continuous monoculture exacerbates weed
and pathogen pressure and difficulties in maintaining
soil fertility. Crop rotations can be used by rotating
early, late and autumn crops; grasses, broadleaves,
and legumes; highly competitive crops with less com-
petitive crops; annual and perennial crops; alternating
between closed, dense crops which shade out weeds
and open crops (Mamolos 2008). In this cultiva-
tion method, allelopathic plants use allelochemicals
secreted by the roots and released by the decompo-
sition of residues from previous crops to suppress
weeds. These released allelocompounds also help to
improve soil organic matter and microorganisms and
increase soil fertility and yields (Scavo and Mauro-
micale 2021). An example of this is sunflower-wheat
rotation, which effectively reduced weed infestation
in the wheat crop after sunflower, as well as the wheat
crop after sorghum. The literature proved that includ-
ing canola in the rotation resulted in an approximately
40% reduction in weed density in the next crop in the
rotation (Abbas et al. 2021). A scheme of selected
agricultural practices using allelopathic potential is
presented in Fig.6.
In allelopathic mulching, the crop or weed resi-
dues are spread over the soil surface or incorporated
into the soil to suppress weeds (Abbas etal. 2021).
Several studies have proven that mulches with the
allelopathic crop can be a preventive weed control by
affecting the soil weed seedbank, weed emergence,
and establishment (Scavo and Mauromicale 2021).
Additional benefits of using allelopathic plant resi-
dues for sustainable agriculture include increased soil
fertility, increased soil organic matter, improved water
infiltration into the soil, regulating soil temperature,
and reduced soil erosion (Farooq etal. 2011). Farm-
ers generally use the economic parts of the crop and
incorporate the remaining parts of the crop into the
field as organic matter (Abbas etal. 2021). A study
showed that the use of sorghum straw as a mulch in
maize enabled 37% weed control, while in rice and
cotton, the use of sorghum mulch provided as much
as 50% and 60% weed control, respectively (Abbas
etal. 2021). The residues of various crops such as rye,
clover, rice, maize, and canola have been reported for
their potential as weed control (Abbas etal. 2021).
The application of mulches of allelopathic crops
including rice, maize, sorghum, and sunflower at 12
t ha−1 provided effective control of herbicide-resistant
Phlarus minor in wheat. The combination of differ-
ent allelopathic plant residues increases weed control
potential due to the synergistic effects of allelochemi-
cals. Another example of this approach could be the
combined use of canola, sunflower, and sorghum
mulches which provided more efficient weed control
in maize as compared to the sole use of individual
mulch material (Scavo and Mauromicale 2021).
9 Allelochemicals asbioherbicides
Changing consumer preferences and demand for
organically produced food make bioherbicides suit-
able alternatives to man-made herbicides. Intending
to reduce the use of synthetic herbicides, overcome
weed‐resistance phenomena and minimize their envi-
ronmental impact, plant‐based allelochemical bioher-
bicides are gaining in popularity (Scavo and Mau-
romicale 2021). Natural products degrade quickly,
making them much safer to use in the environment
(Lengai and Muthomi 2018). Most allelopathic com-
pounds are totally or partially water-soluble which
makes them easier to apply without additional sur-
factants. Allelochemicals have an environmentally
friendly chemical structure compared to synthetic
equivalents (Scavo and Mauromicale 2021). They
possess higher oxygen‐, nitrogen, and sp3 ‐hybrid-
ized carbon molecules with relatively few so-called
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‘heavy atoms’, a halogen substitute, and are charac-
terized by the absence of ‘unnatural’ rings. These
features reduce a compound’s environmental half-life,
preventing accumulation in soil and interference with
non-target plants and organisms (Soltys etal. 2013).
The development of bioherbicides is much more
complicated in contrast to synthetic ones because bio-
active substances must first be properly isolated from
plant extracts and the extractable mass of compounds
recovered is usually small compared to the simple
process of producing large quantities of synthetic
herbicides by chemical synthesis (Soltys etal. 2013).
Before an allelochemical can be used as an herbicide,
it must meet certain criteria. It is necessary to identify
its chemical structure, know its mechanism of action
in plants, determine its residence time in the soil, its
effect on the environment and non-target plants, its
possible toxicity to human health as well as toxicity
to terrestrial and aquatic lifeforms, as well as the prof-
itability of production on a commercial scale (Soltys
etal. 2013). The general steps of bioherbicide devel-
opment are shown in Fig.7.
Allelopathic compounds could be used as tem-
plates to synthesize novel herbicides. For example,
several studies have been performed to develop new
herbicides using coumarins because of their bio-
activity, and the results showed that some of the
novel compounds had the same inhibitory effect
on weeds as the commercial herbicide acetochlor
(Zhao etal. 2021). In their latest study, Zhao etal.
(2021), designed and synthesized a series of new
phenoxypyridine derivatives containing the natural
product coumarin. These compounds showed excel-
lent herbicidal activity under greenhouse condi-
tions, similar to the commercial herbicide oxyfluor-
fen. The herbicidal activity of these compounds was
significantly affected by the types of substituents
introduced at the phenoxypyridine and coumarin
rings. The introduction of an electron-withdrawing
group on the first one increased the extent of activ-
ity. The visual injury and growth status of the test
crops were observed at regular intervals and the
crop selectivity tests showed that maize, cotton, and
soybean had excellent tolerance to the new com-
pound, but rice and wheat were damaged (Zhao
Fig. 6 Scheme of practical
applications of allelo-
pathic cover cropping by
intercropping, crop rotation,
and mulching crop residues
(created with https:// www.
biore nder. com/)
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et al. 2021). In a recent study, some researchers
examined how the addition of organic amendments
and organo-clays altered the sorption and persis-
tence of scopoletin in soil. Galán-Pérez etal. (2022)
showed that sorption and microbial activity changes
both appeared to contribute to increasing the per-
sistence of the allelochemical in amended soil. This
study has shed more light on the application of alle-
lochemical-based herbicides by modifying appro-
priate soil additives, to effectively use the bioactiv-
ity of allelochemicals for organic weed control in a
simple way (Galán-Pérez etal. 2022).
Bioherbicides are prepared from parts of plants
obtained from the environment or man-made—fully
synthetic, or created on the model of natural sub-
stances, which are cleaned of dirt or foreign materi-
als. The plant-based material prepared in this way
is then extracted using solvents or distilled to obtain
respective extracts or essential oils (Lengai and
Muthomi 2018). The quality of the extracts obtained
is influenced by the type of solvent used and the
extraction method. Dried plant parts are generally
preferred due to higher yields of the active ingredient.
Solvents with low toxicity, high capacity to dissolve
large amounts of compounds, easy evaporation, and
preservative properties should be used for extraction.
Organic solvents such as ethanol and methanol are the
most effective, compared to water which, although
a very versatile solvent, extracts significantly fewer
compounds (Lengai and Muthomi 2018).
Bioherbicides are formulated based on weed con-
trol’s most active botanical ingredients under field
conditions (Kremer 2019). These ingredients are
identified and evaluated for optimal formulation.
Bioherbicide formulations consist of an active ingre-
dient, a carrier, and excipients that create protection
during exposure to adverse environmental conditions
and support optimal weed control performance (Kre-
mer 2019). Stabilizers and carriers aim to increase the
durability and stability of the compound, whereas for-
mulation-type components ease handling and increase
efficiency and applicability. They also facilitate its
non-degradability when exposed to environmental
factors. The challenge of the widespread use of bio-
herbicides is to optimize formulation and application
methods that will allow the active agent to be evenly
introduced to the targeted area without excessive use
of the product. Precise application of the substance at
the site of the weed to be controlled without expos-
ing the crop significantly increases the effectiveness
of the bioherbicide (Kremer 2019). After many labo-
ratory and field trials using various combinations of
Fig. 7 Steps for producing commercial bioherbicides (created with https:// www. biore nder. com/)
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active ingredients and additives, reports are created as
the basis for submitting a product registration appli-
cation to the applicable state authorities (Lengai and
Muthomi 2018).
One should bear in mind that the cost of bioherbi-
cides must be competitive to become realistic alter-
natives to conventional herbicides. Thus, new plant
protection methods should be developed in a way that
minimizes the cost of crop production (Głąb et al.
2017). Given the above-stated reasons, the impera-
tive is finding an optimal solution that would simplify
the procedures and enable the development of new
bioherbicides with a competitive price acceptable to
agricultural producers and thus increase their applica-
tion (Šunjka and Mechora 2022).
However, the widely developed bioinformatics
and cheminformatics overcome these challenges by
supporting the discovery of new bioherbicides com-
pounds (Soltys etal. 2013). The precisely identified
and characterized chemical structure of allelochemi-
cals is the basis for designing products with analo-
gous properties using specialized computer programs.
Chemoinformatic tools can propose structures of sim-
ilar compounds and, after several alterations, lead to
improvements in their activity and stability. An exam-
ple of this was the modification of leptospermone
(Soltys etal. 2013). Owing to its herbicidal properties
leptospermone—a triketone identified in 1977—has
been used to create numerous highly active chemical
analogs such as Sulcotrione® and Mesotrione®. Other
products that are not herbicides per se, but have been
synthesized and produced based on natural molecules
are phosphinothricin (a biosynthetic version of glu-
fosinate) and bialophos (a microbial phytotoxic prod-
uct) (Cordeau etal. 2016).
While bioherbicides provide such advantages as
a safe environment and healthy food for human con-
sumption, some factors limit their full adoption in
weed control. The biodegradation, type, and concen-
tration of allelochemicals released into the environ-
ment depend on the combined effects of the plant
itself and environmental factors which are sometimes
difficult to control (Scavo and Mauromicale 2021).
The relatively short environmental half-life of alle-
lochemical substances is good from an environmen-
tal toxicology standpoint. However, a herbicide must
persist sufficiently long to have the desired effect and
to be effective (Motmainna etal. 2021). The design
of new bioherbicides also needs to be mindful that
allelopathic effects vary between varieties or geno-
types because it is not necessarily that plants with
close taxonomic proximity have similar allelopathic
effects (Motmainna et al. 2021). It is also worth
remembering that, compared to synthetic herbicide,
the botanical herbicide releases into the environ-
ment mixture of allelochemicals, with a qualitative
and quantitative composition that is difficult to pre-
cisely predict. A single allelopathic compound may
not show allelopathic activity individually in a certain
situation but might increase allelopathy in association
with other allelochemicals. It is therefore important
to evaluate interactions such as synergy, antagonism,
and incremental effects, between different allelo-
chemicals (Motmainna etal. 2021). Allelochemicals
have multi-site action in plants without the high
specificity which is achieved in the case of synthetic
herbicides and are also highly dose-dependent. Under
field conditions, high doses of allelochemicals are
necessary to achieve the desired efficacy, and the con-
centration of these bioactive components is dictated
by the environment under which they grow (Scavo
and Mauromicale 2021).
10 Conclusions
Although there are an increasing numbers of reports
on the use of allelopathy under field conditions, unfor-
tunately, most studies do not attempt to understand
the modes of action that drive these interactions and
there is a lack of knowledge on the ecotoxicological
impact of bioherbicides. This knowledge would opti-
mize the conditions for the use of allelochemicals and
encourage their use as bioherbicides. Allelochemicals
should not be excluded despite many limitations, as
they can contribute to towards improvement of crop
productivity and environmental protection through
eco-friendly control of weeds, crop diseases, and con-
servation of nitrogen in croplands.
The use of chemoinformatic methods to iden-
tify the chemical structures and mechanisms of
action of allelochemicals can be a starting point for
designing formulations with compound-like proper-
ties and the synthesis of novel agrochemicals based
on natural products. Further research and regula-
tion are needed to increase the number of effective
solutions in the global bioherbicide market (Soltys
et al. 2013). Therefore, a holistic approach to this
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remarkable phenomenon is proposed to design exper-
iments appropriate to the species and ecosystems
under investigation, using multidisciplinary programs
to implement allelopathy as a useful tool for weed
control.
Acknowledgements No funding was received to assist with
the preparation of this manuscript.
Author contributions MK-B designed the conception of the
review, conducted the literature research, and wrote the manu-
script. JP read the first draft of the article. HB critically revised
the manuscript. All authors read and approved the final version
of the manuscript.
Declarations
Competing interests All authors certify that they have no
affiliations with or involvement in any organization or entity
with any financial interest or non-financial interest in the subject
matter or materials discussed in this manuscript.
Open Access This article is licensed under a Creative Com-
mons Attribution 4.0 International License, which permits
use, sharing, adaptation, distribution and reproduction in any
medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Crea-
tive Commons licence, and indicate if changes were made. The
images or other third party material in this article are included
in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons licence and your
intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly
from the copyright holder. To view a copy of this licence, visit
http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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