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Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy

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Allelopathy is a common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms. These biochemicals are known as allelochemicals and have beneficial or detrimental effects on target organisms. Plant allelopathy is one of the modes of interaction between receptor and donor plants and may exert either positive effects (e.g., for agricultural management, such as weed control, crop protection, or crop re-establishment) or negative effects (e.g., autotoxicity, soil sickness, or biological invasion). To ensure sustainable agricultural development, it is important to exploit cultivation systems that take advantage of the stimulatory/inhibitory influence of allelopathic plants to regulate plant growth and development and to avoid allelopathic autotoxicity. Allelochemicals can potentially be used as growth regulators, herbicides, insecticides, and antimicrobial crop protection products. Here, we reviewed the plant allelopathy management practices applied in agriculture and the underlying allelopathic mechanisms described in the literature. The major points addressed are as follows: (1) Description of management practices related to allelopathy and allelochemicals in agriculture. (2) Discussion of the progress regarding the mode of action of allelochemicals and the physiological mechanisms of allelopathy, consisting of the influence on cell micro- and ultra-structure, cell division and elongation, membrane permeability, oxidative and antioxidant systems, growth regulation systems, respiration, enzyme synthesis and metabolism, photosynthesis, mineral ion uptake, protein and nucleic acid synthesis. (3) Evaluation of the effect of ecological mechanisms exerted by allelopathy on microorganisms and the ecological environment. (4) Discussion of existing problems and proposal for future research directions in this field to provide a useful reference for future studies on plant allelopathy.
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REVIEW
published: 17 November 2015
doi: 10.3389/fpls.2015.01020
Edited by:
Richard Sayre,
New Mexico Consortium at Los
Alamos National Labs, USA
Reviewed by:
Shan Lu,
Nanjing University, China
Bala Rathinasabapathi,
University of Florida, USA
*Correspondence:
Zhihui Cheng
chengzh@nwsuaf.edu.cn
Specialty section:
This article was submitted to
Plant Physiology,
a section of the journal
Frontiers in Plant Science
Received: 14 July 2015
Accepted: 04 November 2015
Published: 17 November 2015
Citation:
Cheng F and Cheng Z (2015)
Research Progress on the use
of Plant Allelopathy in Agriculture
and the Physiological and Ecological
Mechanisms of Allelopathy.
Front. Plant Sci. 6:1020.
doi: 10.3389/fpls.2015.01020
Research Progress on the use
of Plant Allelopathy in Agriculture
and the Physiological and Ecological
Mechanisms of Allelopathy
Fang Cheng and Zhihui Cheng*
College of Horticulture, Northwest A&F University, Yangling, China
Allelopathy is a common biological phenomenon by which one organism produces
biochemicals that influence the growth, survival, development, and reproduction of
other organisms. These biochemicals are known as allelochemicals and have beneficial
or detrimental effects on target organisms. Plant allelopathy is one of the modes of
interaction between receptor and donor plants and may exert either positive effects
(e.g., for agricultural management, such as weed control, crop protection, or crop re-
establishment) or negative effects (e.g., autotoxicity, soil sickness, or biological invasion).
To ensure sustainable agricultural development, it is important to exploit cultivation
systems that take advantage of the stimulatory/inhibitory influence of allelopathic
plants to regulate plant growth and development and to avoid allelopathic autotoxicity.
Allelochemicals can potentially be used as growth regulators, herbicides, insecticides,
and antimicrobial crop protection products. Here, we reviewed the plant allelopathy
management practices applied in agriculture and the underlying allelopathic mechanisms
described in the literature. The major points addressed are as follows: (1) Description
of management practices related to allelopathy and allelochemicals in agriculture. (2)
Discussion of the progress regarding the mode of action of allelochemicals and the
physiological mechanisms of allelopathy, consisting of the influence on cell micro-
and ultra-structure, cell division and elongation, membrane permeability, oxidative and
antioxidant systems, growth regulation systems, respiration, enzyme synthesis and
metabolism, photosynthesis, mineral ion uptake, protein and nucleic acid synthesis.
(3) Evaluation of the effect of ecological mechanisms exerted by allelopathy on
microorganisms and the ecological environment. (4) Discussion of existing problems and
proposal for future research directions in this field to provide a useful reference for future
studies on plant allelopathy.
Keywords: allelochemical, allelopathy, agriculture practice, physiological mechanism, ecological mechanism,
microorganism, agricultural sustainable development
INTRODUCTION
Allelopathy is a sub-discipline of chemical ecology that is concerned with the effects of chemicals
produced by plants or microorganisms on the growth, development and distribution of other plants
and microorganisms in natural communities or agricultural systems (Einhellig, 1995). The study
of allelopathy increased in the 1970s and has undergone rapid development since the mid-1990s,
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
becoming a popular topic in botany, ecology, agronomy, soil
science, horticulture, and other areas of inquiry in recent
years. The allelopathic interaction can be one of the significant
factors contributing to species distribution and abundance within
plant communities and can be important in the success of
invasive plants (Chou, 1999; Mallik, 2003; Field et al., 2006;
Inderjit et al., 2006; Zheng et al., 2015), such as water hyacinth
(Eichhornia crassipes Mart. Solms) (Jin et al., 2003; Gao and Li,
2004), spotted knapweed (Centaurea stoebe L. ssp. micranthos)
(Broeckling and Vivanco, 2008) and garlic mustard (Alliaria
petiolata M. Bieb) (Vaughn and Berhow, 1999). Allelopathy is
also thought to be one of the indirect causes of continuous
cropping obstacles in agriculture. As a result of the in-depth
study of allelopathy, strategies for the management of agricultural
production and ecological restoration involving the application
of allelopathy and allelochemicals are improving. The main
purposes of this review are to present conclusions regarding
the application of allelopathy in agricultural production, to
highlight the physiological and ecological mechanisms underlying
plant allelopathy, to illustrate the effect of allelopathy on
soil microorganisms and to discuss key points for further
research.
ALLELOPATHY AND ALLELOCHEMICALS
The definition of allelopathy was first used by Molish in 1937 to
indicate all of the effects that directly and indirectly result from
biochemical substances transferred from one plant to another
(Molisch, 1937). Almost half a century later, the accepted targets
of allelochemicals in the plant kingdom include algae, fungi
and various microorganisms. The term was refined by Rice
(1984) to define “any direct or indirect harmful or beneficial
effect by one plant (including microorganisms) on another
through production of chemical compounds that escape into the
environment” (Rice, 1984). In 1996, the International Allelopathy
Society broadened its definition of allelopathy to refer to any
process involving secondary metabolites produced by plants,
microorganisms, viruses and fungi that influence the growth and
development of agricultural and biological systems. In addition,
the allelopathic donor and receiver should include animals (Kong
and Hu, 2001).
Allelochemicals, which are non-nutritive substances mainly
produced as plant secondary metabolites or decomposition
products of microbes, are the active media of allelopathy.
Allelochemicals consist of various chemical families and are
classified into the following 14 categories based on chemical
similarity (Rice, 1974): water-soluble organic acids, straight-
chain alcohols, aliphatic aldehydes, and ketones; simple
unsaturated lactones; long-chain fatty acids and polyacetylenes;
Abbreviations: APX, ascorbic acid peroxidase; BNI, biological nitrification inhibition; BNIS, biological nitrification inhibition substances; BOA, 2(3H)-
benzoxazolinone; C4H,cinnamate-4-hydroxylase; CAT, catalase; COMT,caffeic acid O-methyltransferases; DEP, diethyl phthalate; DIBOA, 4-dihydroxy-
1,4(2H)-benzoxazin-3-one; DTD, [4, 7-dimethyl-1-(propan-2-ylidene)-1, 4, 4a, 8a-tetrahydronaphthalene-2, 6(1H, 7H)-dione]; F5H,ferulic acid 5-hydroxylase;
GR, glutathione reductase; GS, glutamine synthetase; HHO, [6-hydroxyl-5-isopropyl-3, 8-dimethyl-4a, 5, 6, 7, 8, 8a-hexahydronaphthalen-2(1H)-one]; ISR,
induced systemic resistance; MDA, malondialdehyde; NiR, nitrate reductase; NIS, nitrification-inhibiting substances; PA, pyrogallic acid; PAL, phenylalanine
ammonialyase; PDMS, polydimethylsiloxane; PGPR, plant growth-promoting rhizobacteria; POD, peroxidase; PPO, polyphenol oxidase; QTL, quantitative trait
locus; RAPD, random amplification of polymorphic DNA; ROS, reactive oxygen species; SDH, succinodehydrogenase; SOD, superoxide dismutase; STEM,
silicone tubing microextraction.
benzoquinone, anthraquinone and complex quinones; simple
phenols, benzoic acid and its derivatives; cinnamic acid and
its derivatives; coumarin; flavonoids; tannins; terpenoids and
steroids; amino acids and peptides; alkaloids and cyanohydrins;
sulfide and glucosinolates; and purines and nucleosides. Plant
growth regulators, including salicylic acid, gibberellic acid and
ethylene, are also considered to be allelochemicals. The rapid
progress of analysis technology in recent years has made it possible
to isolate and identify even minute amounts of allelochemicals and
to perform sophisticated structural analyses of these molecules.
The structures of some allelochemicals produced by plants are
shown in Figure 1.
MANAGEMENT OF PLANT ALLELOPATHY
IN AGRICULTURE
Allelopathy is a natural ecological phenomenon. It has been
known and used in agriculture since ancient times (Zeng, 2008,
2014). Allelochemicals can stimulate or inhibit plant germination
and growth, and permit the development of crops with low
phytotoxic residue amounts in water and soil, thus facilitating
wastewater treatment and recycling (Macias et al., 2003; Zeng
et al., 2008). They are a suitable substitute for synthetic herbicides
because allelochemicals do not have residual or toxic effects,
although the efficacy and specificity of many allelochemicals
are limited (Bhadoria, 2011). The main purposes of research on
allelopathy include the application of the observed allelopathic
effects to agricultural production, reduction of the input of
chemical pesticides and consequent environmental pollution, and
provision of effective methods for the sustainable development
of agricultural production and ecological systems (Macias et al.,
2003; Li et al., 2010; Han et al., 2013; Jabran et al., 2015).
The use of allelopathic crops in agriculture is currently being
realized, e.g., as components of crop rotations, for intercropping,
as cover crops or as green manure (Cheema and Khaliq, 2000;
Singh et al., 2003; Cheema et al., 2004; Khanh et al., 2005;
Reeves et al., 2005; Yildirim and Guvenc, 2005; Iqbal et al.,
2007; Mahmood et al., 2013; Wortman et al., 2013; Farooq
et al., 2014; Silva et al., 2014; Wezel et al., 2014; Haider
et al., 2015). The applications of allelopathy in crop production
in Pakistan are successful examples in recent years (Cheema
et al., 2013). The suitable application of allelopathy toward the
improvement of crop productivity and environmental protection
through environmentally friendly control of weeds, insect pests,
crop diseases, conservation of nitrogen in crop lands, and the
synthesis of novel agrochemicals based on allelochemicals has
attracted much attention from scientists engaged in allelopathic
research.
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
FIGURE 1 | Structures of some of the allelochemicals produced by
plants.
ARRANGEMENT OF CROPPING SYSTEMS
Competition is one of the main modes of interaction between
cultivated crops and their neighboring plants (Inderjit and Moral,
1997; Xiong et al., 2005; He et al., 2012b; An et al., 2013).
Allelopathy is a chemical mechanism that provides plants with
an advantage for competing for limited resources (Singh et al.,
1999; He et al., 2012b; Gioria and Osborne, 2014). The ability of
plants to suppress weeds is thus determined by crop allelopathy
and competitiveness. Crop allelopathy can be effectively used to
control weeds in the field, to alleviate allelopathic autotoxicity
and reduce inhibitory influence among allelopathic crops (Iqbal
et al., 2007; John et al., 2010; Farooq et al., 2013; Andrew et al.,
2015), to improve the utilization rate of land and to increase the
annual output of the soil by establishing reasonable crop rotation
and intercropping systems. For example, Odeyemi et al. (2013)
reported relative abundance and population suppression of plant
parasitic nematodes under Chromolaena odorata (L.) (Asteraceae)
fallow in a field study conducted over 2 years, and suggested
that the use of bush fallow with C. odorata might become an
integrated management practice in the management of nematode
pests in crop production in south-western Nigeria. Intercropping
is a common practice among farmers in developing countries
for maximizing land resources and reducing the risks of single
crop failure. Weed population density and biomass production
can be markedly reduced using crop rotation and intercropping
systems (Liebman and Dyck, 1993; Narwal, 2000; Nawaz et al.,
2014; Jabran et al., 2015). Intercropping of sorghum (Sorghum
bicolor L.), sesame (Sesamum indicum L.) and soybean (Glycine
max L.) in a cotton (Gossypium hirsutum L.) field produced
greater net benefits and a significant inhibition on purple nutsedge
(Cyperus rotundus L.) in comparison with the cotton alone in a 2-
year experiment (Iqbal et al., 2007). Recently, Wang et al. (2015)
reported that eggplant/garlic relay intercropping is a beneficial
cultivation practice to maintain stronger eggplant growth and
higher yield. However, the allelopathy between different species
may cause promontory or inhibitory effects. Farooq et al. (2014)
reported that when grown in rotation with tobacco (Nicotiana
tabacum L.), the stand establishment and growth of maize
(Zea mays L.) were improved compared to mung bean (Vigna
radiata L.), whereas mungbean stand establishment and growth
were suppressed. Therefore, the allelopathic nature of crops
must be considered in crop rotation, intercropping and stalk
mulching (Xuan et al., 2005; Cheng et al., 2011; Cheng and Xu,
2013).
STRAW MULCHING
In conventional agriculture, weed control using herbicides is
not only an expensive practice; it is also harmful to the
environment. Allelopathic applications, such as straw mulching,
provide sustainable weed management (Jabran et al., 2015),
further reducing the negative impact of agriculture on the
environment (Cheema and Khaliq, 2000; Cheema et al., 2004).
Using allelopathic plants as ground cover species provides an
environmental friendly option (Dhima et al., 2006; Moraes
et al., 2009; Wang et al., 2013a). The allelochemicals from
decomposed straw can suppress weed growth in farmlands, and
reduce the incidence of pests and diseases. Moreover, straw
mulch can improve the soil organic matter content and increase
soil fertility. However, it may also have negative effects by
increasing the C: N ratio of the soil. Research has shown that
green wheat (Triticum aestivum L.) straw inhibits the growth
of Ipomoea weeds in corn (Zea mays L.) and soybean fields,
thereby reducing the need for herbicide application. Rye (Secale
cereale L.) mulch significantly reduced the germination and
growth of several problematic agronomic grass and broadleaf
weeds (Figure 2;Schulz et al., 2013). The transformation reactions
of rye allelochemicals, i.e., benzoxazinoids, in soil led primarily
to the production of phenoxazinones, which can be degraded by
several specialized fungi via the Fenton reaction. Because of their
selectivity, specific activity, and presumably limited persistence
in the soil, benzoxazinoids or rye residues are suitable means for
weed control (Schulz et al., 2013). Furthermore, Tabaglio et al.
(2008) found that the allelopathic inhibition effects on weeds
differ between different cultivars of rye straw used for mulching.
Xuan et al. (2005) concluded that the application of allelopathic
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
FIGURE 2 | Field trial on rye mulch preceding a tomato crop in a biological farm (Schulz et al., 2013). Left, test plot with rye mulch left on the soil surface,
showing the good weed suppression ability. Right, control plot without rye mulch, split into two treatments: left side, untreated sub-plot in which tomato plants are
almost completely overgrown by weeds; right side, sub-plot with mechanical control by cultivation, in which tomato plants grow as well as those in the test plot.
plant materials at 1–2 tons ha1could reduce weed biomass by
approximately 70%, and increase rice (Oryza sativa L.) yield by
approximately 20% in paddy fields (1998–2003) compared with
the respective controls. In the southeastern region of Brazil, coffee
(Coffea arabica) fruit peels, which contain allelochemicals such
as phenols, flavonoids and caffeine, are often used as an organic
amendment in agricultural practice to control weeds (Silva et al.,
2013). An et al. (2013) found that switchgrass (Panicum virgatum
L.) plants and residues reduced the biomass and density of
associated weeds, and their research provided weed management
strategies in agroecosystems and important information for the
introduction of switchgrass into new ecosystems. Water extracts
of Conyza bonariensis (L.) Cronquist, Trianthema portulacastrum
L., and Pulicaria undulata (L.) C. A. Mey. can be applied at a
concentration of 10 g L1to manage the weed Brassica tournefortii
Gouan by inhibiting germination and seedling growth (Abd El-
Gawad, 2014). Moreover, some soybeans induce the germination
of sunflower broomrape (Orobanche spp.), a noxious parasitic
weed, which suggests that soybean has the potential to be used as a
trap crop to reduce the seed bank of sunflower broomrape (Zhang
et al., 2013b).
DEVELOPING ENVIRONMENTALLY
FRIENDLY AGROCHEMICAL AND
MICROBIAL PESTICIDES
Allelochemicals with negative allelopathic effects are important
components of plant defense mechanisms against weeds and
herbivory. The technology that modifies allelochemicals for the
production of environmentally friendly pesticides and plant
growth regulators allows the effective management of agricultural
production and confers few environmental problems in the soil
due to the fairly high degradability of allelochemicals (Bhadoria,
2011; Ihsan et al., 2015). Uddin et al. (2014) revealed that
sorgoleone, a hydrophobic compound found in Sorghum bicolor
(L.) root exudates, was more effective in inhibiting weed growth
after formulation as a wettable powder, while crop species
were tolerant to it. Some microorganisms are capable of using
sorgoleone as a carbon source. Sorgoleone can be mineralized
via complete degradation to CO2in soil, although the different
chemical groups of the molecule were not mineralized equally
(Gimsing et al., 2009). The strong weed-suppressive ability of
formulated sorgoleone raised interest as an effective, natural,
environmentally friendly approach for weed management. Plant
growth-promoting rhizobacteria (PGPR) include a wide range
of beneficial bacteria that confer positive effects on plants, such
as eliciting induced systemic resistance (ISR), promoting plant
growth and reducing susceptibility to diseases caused by plant
pathogens (Kloepper et al., 1980, 2004). Allelopathic bacteria can
achieve the same function in mixtures of bacteria that exhibit
PGPR attributes and activity against allelopathic weeds, which
reduces the inhibitory effect on susceptible plants caused by
allelopathic weeds (Kremer, 2006; Mishra and Nautiyal, 2012).
There are some organic herbicides or plant growth inhibitors
that have been manufactured from allelopathic plant materials
to inhibit weed growth in fields (Guillon, 2003; Ogata et al.,
2008; Miyake, 2009). Ogata et al. (2008) manufactured a type
of herbicide comprised of a mixture of components extracted
from pine (Pinus L.), hinoki (Chamaecyparis obtusa Endl.), or
Japanese cedar (Cryptomeria japonica D. Don) and bamboo
(Bambusoideae; Poaceae) vinegar, which provided a practical
method of utilizing plant allelopathy in paddy fields.
REDUCTION OF NITROGEN LEACHING
AND ENVIRONMENTAL POLLUTION
Nitrogen leaching is a severe ecological problem due to water
pollution. Mineralization of soil organic nitrogen, especially
the nitrification of nitrogen fertilizer, is one of the main
reasons for the enrichment of nitrogen in the soil. Biological
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
nitrification inhibition (BNI) has gradually become the main
target in investigating the effect of plants on soil nitrification.
In recent years, studies have proven that nitrification-inhibiting
substances (NIS) produced by plants are the first choice for soil
nitrification management. For example, biological nitrification
inhibition substances (BNIS) are allelochemicals that are able to
inhibit soil nitrification. Wheat allelochemicals, such as ferulic
acid, p-hydroxybenzoic acid and hydroxamic acid, can act on
soil microbes to inhibit soil nitrification, reduce the emission
of N2O, improve the utilization rate of nitrogen fertilizer and
reduce pollution to the environment (Ma, 2005). Dietz et al.
(2013) found that the allelopathic plantain (Plantago lanceolata
L.) plant has inhibitory effects on soil nitrogen mineralization,
suggesting that plantain could be utilized to reduce soil nitrogen
leaching.
BREEDING OF ALLELOPATHIC CULTIVARS
Allelopathic cultivars, which have great potential to minimize
the introduction of refractory chemicals and effectively control
weeds in farmland ecosystems, represent the most promising
application of allelopathy (Mahmoud and Croteau, 2002; Weston
and Duke, 2003; Fragasso et al., 2013). Both conventional breeding
methods and those developed using transgenic technology can
be applied in the breeding of allelopathic cultivars. Successful
cultivars must also combine a weed suppression ability with high
yield potential, disease resistance, early maturity and quality traits
(Gealy and Yan, 2012). Rondo, a rice cultivar that combines
a high yield potential with rice blast resistance and weed
suppression ability, has been grown in a commercial organic
rice production operation in Texas and its weed-suppressive
ability is superior to that of many commercial cultivars (Yan and
McClung, 2010; Gealy and Yan, 2012). Huagan 3, a particularly
promising F8generation line derived from crosses between the
local rice cultivars N9S and PI 312777, is considered to be
the first commercially acceptable weed-suppressive cultivar in
China (Kong et al., 2011). Bertholdsson (2010) bred spring wheat
for improved allelopathic potential by conventional breeding.
The material used originated from a cross between a Swedish
cultivar with low allelopathic activity and a Tunisian cultivar
with high allelopathic activity. The result from the field study
was a 19% average reduction in weed biomass for the high
allelopathic lines. However, a negative effect was that the grain
yield was reduced by 9% in the high allelopathic lines. In
this research, the high allelopathic lines showed a lower early
biomass compared with the control. If the early biomass of the
allelopathic wheat had also been improved, the weed biomass
should have been much lower (Bertholdsson, 2004). Putative
genes related to the weed competition ability of wheat have been
found on chromosomes 1A, 2B, and 5D via quantitative trait locus
(QTL) identification, which might be helpful for the breeding
of allelopathic wheat (Zuo et al., 2012a). However, until now, a
successful allelopathic wheat cultivar has not been obtained. To
increase crop resistance to continuous cropping obstacles and
autotoxicity and in the selection of crop successions, species’
detoxification potential should be considered as an important
indicator of breeding.
MECHANISMS UNDERLYING
ALLELOPATHY
Allelopathy has been studied for quite some time, and many
aspects of plant physiological and biochemical processes have
been proved to be affected by allelochemicals (Zeng et al., 2001;
Gniazdowska and Bogatek, 2005). A series of physiological and
biochemical changes in plants induced by allelochemicals are
detailed as follows.
CHANGES IN THE MICRO- AND
ULTRA-STRUCTURE OF CELLS
The shape and structure of plant cells are affected by
allelochemicals. Volatile monoterpenes, eucalyptol and camphor
can widen and shorten root cells, in addition to inducing nuclear
abnormalities and increasing vacuole numbers (Bakkali et al.,
2008; Pawlowski et al., 2012). Cruz Ortega et al. (1988) found that
a corn pollen extract reduced mitotic activity by more than 50%,
induced nuclear irregularities and pyknotic nuclei, and inhibited
radicle and hypocotyl growth in watermelon (Citrullus lanatus
var. lanatus). Upon exposure to hordenine and gramine, which
are allelochemicals from barley (Hordeum vulgare) roots, the
radicle tips of white mustard (Sinapis alba L.) exhibited damaged
cell walls, increases in both the size and number of vacuoles,
disorganization of organelles, and cell autophagy (Liu and
Lovett, 1993). Likewise, cinnamic acid significantly deformed the
ultrastructure of cucumber chloroplasts and mitochondria (Wu
et al., 2004). After treatment with benzoic acid, mustard (Brassica
juncea L.) roots displayed irregularly shaped cells arranged in a
disorganized manner and disruption of cell organelles (Kaur et al.,
2005). Allelochemicals from Convolvulus arvensis L. and catmint
(Nepeta meyeri Benth.) can alter the random amplification of
polymorphic DNA (RAPD) profiles of receiver plants (Kekec
et al., 2013; Sunar et al., 2013). Citral is a volatile essential oil
component of lemongrass (Cymbopogon citrates) and other
aromatic plants that has been suggested to have allelopathic
traits (Dudai et al., 1999). It was reported that citral can cause
disruption of microtubules in wheat and Arabidopsis thaliana L.
roots, where the mitotic microtubules were more strongly affected
than the cortical microtubules (Chaimovitsh et al., 2010, 2012).
Moreover, citral has a strong long-term disorganizing effect
on the cell ultra-structure of A. thaliana seedlings, thickening
the cell wall and reducing intercellular communication and the
formation of root hairs (Grana et al., 2013).
INHIBITION OF CELL DIVISION
AND ELONGATION
Allelochemical monoterpenoids (camphor, 1,8-cineole, beta-
pinene, alpha-pinene, and camphene) affected cell proliferation
and DNA synthesis in plant meristems (Nishida et al., 2005);
2(3H)-benzoxazolinone (BOA) inhibited the mitotic process,
especially the G2-M checkpoint of lettuce (Sanchez-Moreiras
et al., 2008); and sorgoleone reduced the number of cells in
each cell division period, damaging tubulins and resulting in
polyploid nuclei (Hallak et al., 1999). Burgos et al. (2004)
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
argued that the rye allelochemicals BOA and 2, 4-dihydroxy-
1,4(2H)-benzoxazin-3-one (DIBOA) significantly inhibited the
regeneration of cucumber root cap cells and thus inhibited
growth. Following the treatment of soybean with aqueous leaf
extracts from Datura stramonium L., Cai and Mu (2012) found
that higher concentrations of the extracts inhibited primary
root elongation and lateral root development, decreased root
hair length and density, inhibited cell division in root tips and
increased the chromosomal aberration index and micronucleus
index. Teerarak et al. (2012) suggested that the ethyl acetate
fraction of Aglaia odorata Lour. leaves inhibited mitosis and
induced mitotic abnormalities in Allium cepa roots by damaging
chromatin organization and the mitotic spindle in roots exposed
to the allelochemicals.
IMBALANCES IN THE ANTIOXIDANT
SYSTEM
The generation and clearing of reactive oxygen species (ROS)
and the balance of the redox state in the cell play an important
role in allelopathic effects. After exposure to allelochemicals,
the recipient plants may rapidly produce ROS in the contact
area (Bais et al., 2003; Ding et al., 2007), and alter the activity
of antioxidant enzymes such as superoxide dismutase (SOD),
peroxidase (POD; Zeng et al., 2001; Yu et al., 2003) and
ascorbic acid peroxidase (APX; Zuo et al., 2012b) to resist
oxidative stress. Batish et al. (2008) argued that caffeic acid
induces significant changes in the activities of proteases, PODs,
and polyphenol oxidases (PPOs) during root development and
decreases the content of total endogenous phenolics in hypocotyl
cuttings from mung bean (Phaseolus aureus). Shearer et al.
(2012) found that allelopathic interactions led to changes in
signal transduction and an imbalance between the production
of reactive oxidant species and antioxidant capabilities within a
coral holobiont. This oxidative imbalance resulted in rapid protein
degradation and ultimately, apoptosis or necrosis of the coral
Acropora millepora when compensatory transcriptional action by
the coral holobiont insufficiently mitigated the damage caused
by allelochemicals produced by Chlorodesmis fastigiata (Shearer
et al., 2012).
INCREASES IN CELL MEMBRANE
PERMEABILITY
Many studies have shown that allelochemicals significantly inhibit
the activity of antioxidant enzymes and increase free radical
levels, resulting in greater membrane lipid peroxidation and
membrane potential alteration, which diminish the scavenging
effect on activated oxygen and damage the whole membrane
system of plants (Lin et al., 2000; Zeng et al., 2001; Lin, 2010;
Harun et al., 2014; Sunmonu and Van Staden, 2014). The
growth of Hordeum spontaneum,Avena ludoviciana, and wild
mustard seedlings were found to be inhibited by an aqueous
extract of barley aerial parts through increasing lipid peroxidation
(Farhoudi et al., 2012; Farhoudi and Lee, 2013). Zuo et al.
(2012b) argued that the combination of non-sterile shoots of
wheat and Alopecurus aequalis weeds led to the accumulation of
oxygen radical species, such as the superoxide radical O2
anion,
H2O2and malondialdehyde (MDA) in the leaves of transgenic
(with Cu/ZnSOD and APX genes) and non-transgenic potato
(Solanum tuberosum L.) seedlings, in addition to increasing
membrane permeability and altering the activities of SOD and
APX. Poonpaiboonpipat et al. (2013) found that lemongrass
(Cymbopogon citratus) essential oil damages the membrane
system of barnyard grass (Echinochloa crus-galli L.), causing lipid
peroxidation and electrolyte leakage. Sun et al. (2014) investigated
the generation of ROS induced by pyrogallic acid (PA) in
Microcystis aeruginosa. They found O2
to be the precursor of
H2O2and showed that the hydroxyl radical OH·was generated at
significant levels, demonstrating that PA caused oxidative stress
in M. aeruginosa and that futile redox cycling of PA was the main
source of excessive intracellular O2
and consequent H2O2and
OH·production.
EFFECT ON THE PLANT GROWTH
REGULATOR SYSTEM
Allelochemicals can alter the contents of plant growth regulators
or induce imbalances in various phytohormones, which inhibits
the growth and development of plants, for example, with
respect to seed germination and seedling growth. Most phenolic
allelochemicals can stimulate IAA oxidase activity and inhibit
the reaction of POD with IAA, bound GA or IAA to influence
endogenous hormone levels (Yang et al., 2005).
Leslie and Romani (1988) found that salicylic acid inhibited
the synthesis of ethylene in cell suspension cultures of pear
(Pyrus communis). Through treatment of wheat seedlings with
high concentrations of ferulic acid (2.50 mM), Liu and Hu
(2001) found that the growth of wheat seedlings was inhibited by
the accumulation of IAA, GA3, and CTK, with a simultaneous
increase in ABA. An aqueous extract from rice was shown
to significantly stimulate IAA oxidase activity in barnyard
grass and reduce IAA levels, thereby damaging the growth
regulation system and inhibiting seedling growth (Lin et al.,
2001). Yang et al. (2008) investigated the mechanisms of
two allelochemicals: DTD [4, 7-dimethyl-1-(propan-2-ylidene)-
1, 4, 4a, 8a-tetrahydronaphthalene-2, 6(1H, 7H)-dione] and
HHO [6-hydroxyl-5-isopropyl-3, 8-dimethyl-4a, 5, 6, 7, 8,
8a-hexahydronaphthalen-2(1H)-one], isolated from Ageratina
adenophora Sprengel weeds. DTD at a higher concentration
(1.5 mM), significantly increased the ABA content in the roots of
rice seedlings, but this decreased sharply after 96 h of treatment.
HHO also significantly enhanced the ABA content for 48 and
96 h. However, the application of DTD or HHO decreased
the IAA and ZR contents in rice roots. The IAA/ABA and
ZR/ABA ratios decreased quantitatively in response to higher
concentrations of DTO or HHO. These results suggest that
the endogenous hormones might have dependent as well as
interactive effects on the responses of rice seedlings and their
adaptability to DTD or HHO stress. Moreover, the results from
another study indicated that cyanamide (1.2 mM) caused an
imbalance of plant hormone (ethylene and auxin) homeostasis
in tomato (Solanum lycopersicum L.) roots (Soltys et al.,
2012).
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
EFFECT ON THE FUNCTIONS AND
ACTIVITIES OF VARIOUS ENZYMES
Allelochemicals exert different effects on the synthesis, functions,
contents and activities of various enzymes. Previous studies
have shown that the key enzyme λ-phosphorylase involved in
seed germination might be inhibited by chlorogenic acid, caffeic
acid and catechol (Rice, 1984; Einhellig, 1995). Additionally,
POD, CAT, and cellulase can be suppressed by tannic acid,
which can also reduce the synthesis of amylase and acid-
phosphatase in the endosperm. Phenolic acids can increase
the activity of phenylalanine ammonialyase (PAL) and β-
glucosidase, while reducing the activity of phenol-β-glucose
transferase, thus inhibiting root growth. In addition, protease,
invertase and succinodehydrogenase (SDH) can be suppressed by
allelochemicals.
Lin et al. (2001) argued that caffeic acid, gallic acid and phenols
regulate phenylalanine metabolism by suppressing the activities
of PAL and cinnamic acid-4-hydroxylase. An aquatic extract of
the above-ground parts and rhizospheric soil of chrysanthemum
(Chrysanthemum indicum L.) inhibited the activities of root
dehydrogenase and nitrate reductase (NiR), reduced the contents
of soluble sugar and soluble protein, and inhibited the root growth
of stem cuttings of the same species (Zhou et al., 2010). Cheng
(2012) investigated the effects of diethyl phthalate (DEP) on
the enzyme activity and polypeptide accumulation of glutamine
synthetase (GS) in greater duckweed (Spirodela polyrhiza L.) and
found that DEP is toxic to this species due to the inhibition of GS
isoenzymes in nitrogen assimilation and antioxidant enzymes.
INFLUENCE ON RESPIRATION
Allelochemicals affect plant growth by influencing different stages
of respiration, such as electron transfer in the mitochondria,
oxidative phosphorylation, CO2generation and ATP enzyme
activity. These chemicals can reduce oxygen intake, which
prevents NADH oxidation, inhibits ATP synthesis enzyme
activity, reduces ATP formation in mitochondria, disturbs plant
oxidative phosphorylation and ultimately inhibits respiration; on
the other hand, they can stimulate the release of CO2, which
promotes respiration.
Cruz Ortega et al. (1988) found that an ethanol extract from
corn pollen acted as an inhibitor of the electron pathway and
decreased oxygen consumption; the specific inhibition site was
most likely located upstream of cytochrome c. Rasmussen et al.
(1992) found that sorgoleone interfered with the function of
mitochondria isolated from etiolated soybean and corn seedlings
by blocking electron transport at the b-c1complex. Moreover,
Hejl and Koster (2004b) observed that juglone could reach the
mitochondria in the root cells of corn and soybean seedlings,
thereby disrupting root oxygen uptake. Alpha-pinene, camphor,
limonene and other monoterpenes significantly affect radicle
and hypocotyl mitochondrial respiration in soybean and corn,
but their targets are different. Alpha-pinene acts under at least
two mechanisms: uncoupling of oxidative phosphorylation and
inhibition of electron transfer. Alpha-pinene strongly inhibits
mitochondrial ATP production, decreases the mitochondrial
transmembrane potential and impairs mitochondrial energy
metabolism. Camphor causes uncoupling of mitochondria.
Limonene inhibits coupled respiration but does not affect basal
respiration, and inhibits ATP synthetase and the activities of
adenine nucleotide translocase complexes at concentrations of 1.0
and 5.0 mM (Abrahim et al., 2003a,b).
EFFECT ON PLANT PHOTOSYNTHESIS
The impacts of allelochemicals on plant photosynthesis mainly
involve inhibition of or damage to the synthesis machinery and
acceleration of the decomposition of photosynthetic pigments.
Consequently, photosynthetic pigment contents are decreased,
which blocks energy and electron transfer, reduces ATP synthesis
enzyme activity, inhibits the synthesis of ATP, and affects stomatal
conductance and transpiration, which inhibit the photosynthetic
process (Meazza et al., 2002; Yu et al., 2003, 2006; Wu et al., 2004).
Allelochemicals affect photosynthesis mainly by influencing the
function of PS II (Weir et al., 2004; Wang et al., 2014). For example,
sorgoleone inhibits the decay of variable fluorescence, blocks
the oxidation of the PSII-reduced primary electron acceptor,
Q
A, by the PSII secondary electron acceptor and that of QBby
displacing QBfrom the D1protein, thus inhibiting photochemical
effects (Gonzalez et al., 1997). Similarly, Shao et al. (2009)
demonstrated that the D1protein is an important target in the
damage caused to Microcystis by pyrogallol. Moreover, Uddin
et al. (2012) found that sorgoleone reduced the Fv/Fm of weeds
and inhibited weed growth. By studying the inhibitory effect
of the dried macroalga Gracilaria tenuistipitata (Rhodophyta)
on the microalga Phaeodactylum tricornutum,Ye et al. (2013)
found a decrease in the number of active reaction centers and
blockade of the electron transport chain. Poonpaiboonpipat et al.
(2013) observed that a high concentration of essential oil from
lemongrass (Cymbopogon citratus) leaves significantly decreased
the chlorophyll a and b and carotenoid contents of barnyard
grass and affected alpha-amylase activity in seeds, indicating that
essential oil interferes with photosynthetic metabolism. However,
aqueous extracts of leaves from Trema micrantha (Ulmaceae), an
allelopathic plant, did not lead to inhibition of the synthesis of
photosynthetic pigments in radish (Raphanus sativus L.) (Borella
et al., 2014).
INFLUENCE ON WATER AND NUTRIENT
UPTAKE
Many allelochemicals affect nutrient absorption in plant roots
or induce water stress through long-term inhibition of water
utilization. Allelochemicals can inhibit the activities of Na+/K+-
ATPase involved in the absorption and transport of ions at the
cell plasma membrane, which suppresses the cellular absorption
of K+, Na+, or other ions.
Bergmark et al. (1992) found that ferulic acid (250 µM)
inhibited ammonium and NO3
uptake in corn seedlings,
although ammonium uptake was less sensitive to this treatment
than NO3
. Ferulic acid also inhibits Cluptake and increases
the initial net K+loss from roots exposed to a low K ammonium
nitrate solution and delays recovery that results in a positive
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
net uptake. Yuan et al. (1998) showed that the effects of
allelochemicals, such as ferulic acid, benzaldehyde and 4-tert-
butylbenzoic acid, on nitrogen absorption in wheat seedlings
are negatively correlated, but the negative effects of NH4+-N
on nitrogen absorption were stronger than those of NO3
-
N. Yu and Matsui (1997) observed that cinnamic acid and
the root exudates of cucumber inhibited the uptake of NO3
,
SO42, K+, Ca2+, Mg2+, and Fe2+by cucumber seedlings.
Through further study, Lv et al. (2002) found that cinnamic
acid and p-hydroxybenzoic, the main allelochemicals found in
cucumber root exudates, strongly inhibited the activities of root
dehydrogenase, root-combined ATPase and nitrate reductase in
cucumber, thus inhibiting the root uptake of K+, NO3
, and
H2PO4
. Sorgoleone and juglone significantly inhibited H+-
ATPase activity and the proton-pumping function across the root
cell plasmalemma, which affected solute and water uptake in
peas (Pisum sativum L.), soybeans and corn (Hejl and Koster,
2004a,b). Abenavoli et al. (2010) found that the allelochemicals
trans-cinnamic, ferulic acid and p-coumaric acid inhibited net
nitrate uptake and plasma membrane H+-ATPase activity in
maize seedlings, while umbelliferone and caffeic acid had no
effect on H+-ATPase activity. Sunflower (Helianthus annus L.)
residues negatively affected plant development, the efficiency of
translocation of assimilates and nutrient accumulation in radish
plants (Barros de Morais et al., 2014).
The effects of allelochemicals on ion uptake are closely related
to allelochemical concentrations and classifications. For example,
a low concentration of dibutyl phthalate increases the absorption
of N but decreases that of P and K. However, a high concentration
of this chemical inhibits the absorption of N, P and K. Similarly, a
low concentration of diphenylamine stimulates the absorption of
N and K but inhibits the absorption of P by tomato roots (Geng
et al., 2009).
INFLUENCE ON PROTEIN AND NUCLEIC
ACID SYNTHESIS AND METABOLISM
Most alkaloids show allelopathic potential. Some can closely
integrate with DNA and increase the temperature of DNA
cleavage, while some can inhibit DNA polymerase I and prevent
the transcription and translation of DNA, whereas others can
inhibit protein biosynthesis (Wink and Latzbruning, 1995).
Allelochemicals can also inhibit amino acid absorption, in
addition to transport, thus interfering with protein synthesis,
which affects cell growth (Abenavoli et al., 2003). All phenolic
acids can affect the integrity of DNA and RNA. Ferulic acid
and cinnamic acid as well as many phenols and alkaloids can
also inhibit protein synthesis (Baziramakenga et al., 1997; Zeng
et al., 2001; Li et al., 2010). This suggests that the observed
allelopathic phenomenon is partly a result of the interaction of
the allelochemicals with these basic targets, such as DNA, RNA,
protein biosynthesis and related processes.
By analyzing the gene expression profile of A. thaliana after
treatment with fagomine, gallic acid, and rutin, which are
allelochemicals found in buckwheat (Fagopyrum esculentum
Moench), Golisz et al. (2008) observed that genes that reacted to
the allelochemicals mainly fell into several functional categories:
interaction with the environment, subcellular localization,
proteins with a binding function or cofactor requirement, cell
rescue, defense and virulence, or metabolism. The plant response
to allelochemicals was similar to the response to biotic or abiotic
stress. This indicated that allelochemicals might have relevant
functions in the cross-talk between biotic and abiotic stress
signaling, as they generate ROS (Bais et al., 2003; Baerson et al.,
2005; Golisz et al., 2008, 2011). Shao et al. (2009) found that
the allelochemical pyrogallol affects the expression of psbA,
mcyB,prx, and faab( in Microcystis aeruginosa, and indicated
that membranes are the first target in the damage of Microcystis
caused by pyrogallol. Guo et al. (2011) showed that HHO affected
the expression of CHS, which is associated with the synthesis
of various amino acids in Eupatorium adenophorum roots.
Cyanamide alters the expression of the expansin genes, LeEXPA9
and LeEXPA18, which are responsible for cell wall remodeling
after cytokinesis, thereby inhibiting the formation of tomato root
(Soltys et al., 2012). In a recent study, Fang et al. (2015) found
that the expression levels of miRNAs relevant to plant hormone
signal transduction, p53 signaling pathways, nucleotide excision
repair and the peroxisome proliferator-activated receptor were
enhanced in barnyard grass co-cultured with allelopathic rice or
treated with rice-produced phenolic acids. Kato-Noguchi et al.
(2013) reported that the rice allelochemicals momilactone A and
B might inhibit the germination of Arabidopsis seeds by inhibiting
the degradation process of the storage proteins cruciferin and
cruciferina.
Allelochemicals produced by donor plants act on receiver
plants, while the receiver plants will react to the donor
plants by inducing changes in gene expressions. The up-
regulated expression of PAL,cinnamate-4-hydroxylase
(C4H), ferulic acid 5-hydroxylase (F5H), and caffeic acid
O-methyltransferases (COMT), which are involved in the
biosynthesis of phenolic compounds in rice, is consistent with
their inhibitory effects on barnyard grass, while barnyard grass
induces the expression of genes related to the synthesis of
phenolic compounds in allelopathic rice (He et al., 2012a).
EFFECTS OF ALLELOCHEMICALS ON
MICROORGANISMS AND THE
ECOLOGICAL ENVIRONMENT
Researchers have found that there are significant relationships
between crop growth and soil microbes under the application of
allelochemicals or in the presence of allelopathic plants (Figure 3;
Barazani and Friedman, 1999; Bais et al., 2006; Mishra et al., 2013).
Recent studies demonstrated that indirect effects of allelopathy as
a mediator of plant–plant interactions were more important than
the direct effects of an inhibitor (Zeng, 2014). Chemical-specific
changes in soil microbes could generate negative feedbacks in soil
sickness and plant growth (Stinson et al., 2006; Huang et al., 2013;
Zhou et al., 2013; Li et al., 2014). Meanwhile, the rhizosphere soil
microbes contribute to the allelopathic potential of plants through
positive feedback (Inderjit et al., 2011; Zuo et al., 2014; Wu et al.,
2015). Bacteria can help to increase inhibition by activating a
non-toxic form of an allelochemical (Macias et al., 2003). For
example, non-glycosylated compounds may be modified after
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
FIGURE 3 | A schematic diagram showing the various roles of microbes in modulating the interaction of allelopathic donor-receiver species
(Barazani and Friedman, 1999; Bais et al., 2006; Mishra et al., 2013). Red arrows with double lines indicate the phenomenon of allelopathy, and blue arrows
with single lines indicate the involvement of various microbial processes in reducing/enhancing allelopathic inhibition by soil microorganisms. This figure explains that
beneficial rhizobacteria can minimize the phytotoxicity of the allelopathic donor toward the allelopathic receiver by using various rhizospheric processes such as
rhizosphere colonization, biofilm formation, and degradation/transformation of toxic allelochemicals or modulation of the defense mechanism in receiver species by
inducing the expression of stress responsive genes or the activity of antioxidant enzymes. Furthermore, microbes also can play an important role in the activation of
allelochemicals, e.g., through the release of non-toxic glycosides followed by microbial degradation to release the active allelochemical.
release from plants and become more toxic (Tanrisever et al.,
1987; Macias et al., 2005a). However, bacteria can also help
susceptible plants to tolerate biotic stress associated with weeds,
and to decrease the allelopathic inhibition of weeds by causing
alterations in the expression patterns of some genes that might
be responsible for different functions but ultimately lead to a
self-defense process (Mishra and Nautiyal, 2012). In addition,
the microbial degradation/transformation of allelochemicals in
soil affects the effective dose of allelochemicals that can cause
plant inhibition (Mishra et al., 2013; Li et al., 2015). Bacterial
biofilms in rhizospheric regions can protect colonization sites
from phytotoxic allelochemicals and can reduce the toxicity of
these chemicals by degrading them (Mishra and Nautiyal, 2012;
Mishra et al., 2012). Microorganisms have the ability to alter
the components of allelochemicals released into an ecosystem,
highlighting their key role in chemical plant–plant interactions
and suggesting that allelopathy is likely to shape the vegetation
composition and participate in the control of biodiversity in
ecology (Fernandez et al., 2013). Some sesquiterpenoid lactones
and sulfides are antimicrobial and can disrupt the cell walls
of fungi and invasive bacteria, while others can protect plants
from environmental stresses that would otherwise cause oxidative
damage (Khan et al., 2011; Chadwick et al., 2013). Zhang et al.
(2013a) found that antifungal volatiles released from Chinese
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
chive (Allium Tuberosum Rottler) helped to control Panama
disease (Fusarium wilt) in banana (Musa spp.) and showed
that intercropping/rotation of banana with Chinese chive could
control Panama disease and increase cropland biodiversity.
Wang et al. (2013b) indicated that the shift in the microbial
community composition induced by barnyard grass infestation
might generate a positive feedback in rice growth and
reproduction in a given paddy system. The relative abundance
and population of plant parasitic nematodes were significantly
reduced in the presence of Chromolaena odorata (Asteraceae)
fallow (Odeyemi et al., 2013). Pearse et al. (2014) found that
radish soils had a net positive effect on Lupinus nanus biomass
and explained that radish might alter the mutualistic/parasitic
relationship between L. nanus and its rhizobial associates, with a
net benefit to L. nanus.Fang et al. (2013) indicated that inhibiting
the expression of the rice PAL gene reduced the allelopathic
potential of rice and the diversity of the rhizosphere microflora.
These findings suggested that PAL functions as a positive
regulator of the rice allelopathic potential.
PGPR, such as root-colonizing Pseudomonas,Paenibacillus
polymyxa, endophytes and Chryseobacterium balustinum Aur9,
have been shown to alter plant gene expression and regulate
plant allelochemical synthesis and signaling pathways to enhance
disease resistance, adaptability and defense capabilities in
response to biotic and abiotic stresses in plants (van Loon, 2007;
Dardanelli et al., 2010; Mishra and Nautiyal, 2012).
PROBLEMS AND FUTURE RESEARCH
DIRECTIONS
Allelochemicals mainly consist of secondary metabolites that are
released into the environment through natural pathways, such as
volatilization, leaf leaching, residue decomposition, and/or root
exudation. Therefore, it should first be noted how allelochemicals
are released into the environment (Inderjit and Nilsen, 2003).
The activity of allelochemicals varies with research techniques
and operational processes (Peng et al., 2004). The natural state of
allelochemicals may be changed somewhat during the process of
extraction (Li et al., 2002). Therefore, researchers must be careful
to determine whether a plant has allelopathic potential or separate
and identify allelochemicals using organic solvents and aqueous
extracts from plant tissues.
An allelochemical released into the environment is usually not
a single substance, and the amounts of allelochemicals released
under different conditions vary. Therefore, both the type and
amount of allelochemicals released by plants should be considered
when their allelopathic potential is investigated. Interactions such
as synergy, antagonism and incremental effects between different
allelochemicals should be evaluated because one allelochemical
may not show allelopathic activity as a single component in a
certain situation, but might increase allelopathy in association
with other allelochemicals (Albuquerque et al., 2010).
The type and amount of allelochemicals released into the
environment depend on the combined effects of the plant itself
(plant factors) and environmental factors, as shown in Figure 4
(Albuquerque et al., 2010). The plant factors include the species,
variety, growth stage and different tissues (Belz, 2007; Leao et al.,
FIGURE 4 | Induction of allelochemical production by the plant itself
and environmental factors (Part of this figure was modified from
Albuquerque; Albuquerque et al., 2010). The plant factors include species,
variety, growth stage, tissue type, etc. Environmental factors include abiotic
factors (irradiation, temperature, nutrient limitation, moisture, pH) and biotic
factors (plant competition, diseases, insects, animal attack and receptor
feedback regulation).
2012; Iannucci et al., 2013). Allelopathic effects vary between
varieties or genotypes (Li and Shen, 2006; Zhou et al., 2011;
Leao et al., 2012). Plants from the same environment or with
close taxonomic proximity do not necessarily display similar
production of secondary metabolites, and they may therefore
not secrete the same quantity and quality of allelochemicals
or have similar allelopathic effects (Chon and Nelson, 2010;
Hagan et al., 2013; Imatomi et al., 2013). Lin et al. (2000)
found that varietal differences in the allelopathic potential of
rice were related to the genetic background. Environmental
factors include both abiotic factors (e.g., irradiation, temperature,
nutrient limitation, moisture, pH) and biotic factors (e.g., plant
competition, diseases, insects, animal invasion, receptor feedback
regulation; Anaya, 1999). In a recent study, endogenous levels of
allelochemicals were used as indices of abiotic stress resistance.
Meanwhile, the exogenous application of allelochemicals has
been found to increase the endogenous level of the receivers,
with a simultaneous increase in growth and resistance against
abiotic stresses (Maqbool et al., 2013); consequently, appropriate
environmental conditions are necessary for allelopathic studies. It
has been noted that a stress environment can increase the release
of allelochemicals from allelopathic plants (Albuquerque et al.,
2010). Through studying the dynamic release of allelochemicals
under different stress environments, we can clarify the release
characteristics of allelochemicals and determine the conditions
required for allelochemical release, thereby revealing the nature
of allelochemicals.
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Cheng and Cheng Plant Allelopathy Application and Mechanisms
Allelochemicals can be degraded after they have been released
into the soil; the half-life of allelochemicals varies from a few hours
to a few months (Demuner et al., 2005; Macias et al., 2005b; Wang
et al., 2007; Barto and Cipollini, 2009; Bertin et al., 2009), and
this is mainly associated with the allelochemical concentration,
soil type, soil enzymes, and soil microbial population and
community structure (Macias et al., 2004; Understrup et al.,
2005; Kong et al., 2008; Gu et al., 2009). Previous studies
indicated that some allelochemicals had tremendous spatial and
temporal heterogeneity (Weidenhamer, 2005; Dayan et al., 2009;
Mohney et al., 2009; Weidenhamer et al., 2009, 2014), but these
characteristics of most allelochemicals have not been confirmed.
It was reported that polydimethylsiloxane (PDMS) microtubing
(silicone tubing microextraction, or STME) could be used as a
tool to provide a more finely resolved picture of allelochemical
dynamics in the root zone (Weidenhamer, 2005; Mohney et al.,
2009; Weidenhamer et al., 2009, 2014). Until now, much remains
unknown about the fate or persistence of allelochemicals in the
soil or their effects on soil chemistry or microflora (Belz, 2007).
Explaining how allelochemicals function is complicated due
to the many classes of chemicals and different structures that
have been identified as agents in allelopathy. There is no generic
allelochemical, and we should certainly anticipate different
mechanisms of action among allelopathic chemicals. Moreover, it
should be investigated in future studies whether allelochemicals
are absorbed through transport proteins or whether different
allelochemicals have the same molecular targets in different
species (Weston and Mathesius, 2013). The systematic study of
allelochemical detoxification mechanisms in differentspecies will
help reveal the differences in detoxification mechanisms between
plants and microbes.
Allelopathy is a complex process. Many allelochemicals
have been identified to date. Due to the different sensitivities
of different receptors to the same allelochemical and the
various allelopathic activities of different allelochemicals,
considerable further work is required in the field of allelochemical
research. Very little is known about the transportation and
biodegradation of allelochemicals in soil or the population
genetics of allelopathic species, the establishment of practical ways
of using allelochemicals in the field, the rapid adaptation of weeds
to avoid them, the diversity of the soil microbial community that
is maintained in their presence or the role of signal transduction
in herbivore defense. These areas should be the focus of future
investigations.
Considerable research has showed that allelopathy has good
application potential in agricultural production. Until now, many
allelopathic crops have been used in agricultural production, but
the applications are limited to small-scale and regional areas. The
structure and mode of action of many allelochemicals have been
deeply revealed in recent years, and this has laid a good foundation
for projects where allelochemicals are used to obtain the basic
structures or templates for developing new synthetic herbicides.
The commonly used methods of weed control (herbicide
application, mechanical weeding and hand weeding) are
effective in agricultural production. However, there are many
disadvantages associated with these methods, for example, the
evolution of herbicide resistance in weeds, the negative impacts
of herbicides on environmental, human and animal health,
the expense of herbicides, the losses in soil structure and the
enormous labor requirements. Many of the above problems can
be allayed by creating diversity in weed control practices with
the application of allelopathy. The combination of more than
one weed control method has been proved to be effective in
reducing the probability of herbicide resistance development in
weeds. Moreover, the combined application of reduced synthetic
herbicides dose and allelopathic extracts can provide control that
is as effective as that obtained from the standard dose of herbicides
(Farooq et al., 2011). Further, using diverse weed management
practices in certain fields can ensure sustainable and effective
weed control.
CONCLUSION
Allelopathy has been known and used in agriculture since
ancient times; however, its recognition and use in modern
agriculture are very limited. Allelopathy plays an important
role in investigations of appropriate farming systems as well as
in the control of weeds, diseases and insects, the alleviation of
continuous cropping obstacles, and allelopathic cultivar breeding.
Furthermore, allelochemicals can act as environmentally friendly
herbicides, fungicides, insecticides and plant growth regulators,
and can have great value in sustainable agriculture. Although
allelochemicals used as environmentally friendly herbicides has
been tried for decades, there are very few natural herbicides on
the market that are derived from an allelochemical. However,
there are a few research investigations testing natural-product
herbicides. With increasing emphasis on organic agriculture and
environmental protection, increasing attention has been paid
to allelopathy research, and the physiological and ecological
mechanisms of allelopathy are gradually being elucidated.
Moreover, progress has been made in research on the associated
molecular mechanisms. It is obvious that allelopathy requires
further research for widespread application in agricultural
production worldwide.
ACKNOWLEDGMENTS
The authors acknowledge the editors for providing us with this
opportunity to share our understanding of the practices of plant
allelopathy in agriculture and the physiological and ecological
mechanisms of allelopathy. This research and the writing of this
review were supported by a project of the National Natural Science
Foundation of China (No. 31471865).
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2015 Cheng and Cheng. This is an open-access article distributed under
the terms of the Creative Commons Attribution License (CC BY). The use, distribution
or reproduction in other forums is permitted, provided the original author(s) or
licensor are credited and that the original publication in this journal is cited, in
accordance with accepted academic practice. No use, distribution or reproduction is
permitted which does not comply with these terms.
Frontiers in Plant Science | www.frontiersin.org November 2015 | Volume 6 | Article 102016
... Allelopathy effects of VOCs have attained extensive importance during the last decade in biological weed management employing 'Bioherbicide' and/or 'Bio-fungicide' (Inderjit, 2005;Meiners et al., 2012;Awan et al., 2012;Bajwa et al., 2015;Tavella et al., 2017;Hunt et al., 2017;Bachheti et al., 2020), herbivores (Kim and Felton, 2013) and pathogens (Ameye et al., 2015;Siri-Udom et al., 2017). The Shikimate/phenylalanine, the mevalonic acid (MVA), the methylerythritol phosphate (MEP), and lipoxygenase (LOX) are four major synthesis pathways which facilitate the plants to emit terpenoids, phenylpropanoids/benzenoids, and fatty acid, possessing allelopathy effects on weeds and other non-crop plants (Cheng and Cheng, 2015;Xie et al., 2021). ...
... The plant defense systems react physiologically in response to biotic and abiotic stressors. The response in plants results in the emission of volatile organic compounds (pVOCs) and production of secondary metabolites (Cheng and Cheng, 2015). Development of sensitive detection devices, like the electronic-nose (e-nose), depend upon VOC profile libraries that have documented plant response and VOC emissions from a wide variety of crop plants and treatment conditions. ...
... Therefore, pVOC profiles provide a direct plant response of 'raw information' that can be used for the development of sensitive electronic detection devices (e.g. e-Nose, etc…) or biosensors for early diagnosis and detection of biotic and abiotic stresses, or various plant signaling (Balbi and Devoto, 2008;Dorokhov et al., 2014;Cheng and Cheng, 2015;Xie et al., 2021). ...
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Volatile organic compounds emitted by plants (pVOCs) protect themselves from abiotic and biotic stresses. Plants are under constant threats from biotic stress especially herbivores which facilitate plants to emit herbivore-induced plant volatiles (HIPVs), an inducible defense in plants against herbivores by communicating to herbivores’ natural enemies and neighboring plants. HIPVs are reported to act as feeding and/or oviposition deterrents to herbivores, belonging to four major groups including terpene/terpenoids, benzenoids and phenylpropanoids, the volatile fatty acid derivatives, and the volatile aminoacid derivatives. However, the plant volatile profiles induced by herbivores have been reported to be altered by silicon (Si)-fertilization, priming of plants with chemical elicitors and climate change which can induce plants to produce and emit novel plant volatiles that are not expressed by plants in response to damage by herbivory. Transgenic crops and/or sentinel crops with enhanced pVOCs emission profiles have been proposed to improve plant resistance, health and yields. Currently several technological advances in devices including field-portable electronic devices (e-Nose) and hand-held smartphone-based biosensors for in situ early detection of pVOCs profiles which are now available and supported by artificial intelligence (AI), to increase detection accuracy of these systems. The present review highlights the multi-dimension approaches emerging in pVOCs research for early and rapid detection of specific VOCs indicators as markers with associated plant biotic stressors.
... The secondary metabolites can play a main role in phytoimmuniy acting as phytotoxins for competing plants, pathogens, and herbivorous organisms (Mithöfer and Boland, 2012;Cheng and Cheng, 2015). The content of phenolic compounds can correlate with the bioactivity of N. nuda extracts. ...
... Our data on phytochemical potential of N. nuda provide additional information about the phytotoxic activity of N. nuda extracts ( Table 5). The inhibition of plant growth occurs upon competition for common resources, such as nutrients and light (Cheng and Cheng, 2015). Phenolic acids and coumarins act as direct allelopathic agents by inhibiting cell proliferation and the respective growth processes ( Table 5). ...
... Iridoids Konno et al., 1999 Allelopathy Water soluble allelochemicals N. nuda subsp. nuda water extract Dragoeva et al., 2017 Allelopathy Esculetin Reviewed in Cheng and Cheng, 2015 Allelopathy ...
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Nepeta nuda (catmint; Lamiaceae) is a perennial medicinal plant with a wide geographic distribution in Europe and Asia. This study first characterized the taxonomic position of N. nuda using DNA barcoding technology. Since medicinal plants are rich in secondary metabolites contributing to their adaptive immune response, we explored the N. nuda metabolic adjustment operating under variable environments. Through comparative analysis of wild-grown and in vitro cultivated plants, we assessed the change in phenolic and iridoid compounds, and the associated immune activities. The wild-grown plants from different Bulgarian locations contained variable amounts of phenolic compounds manifested by a general increase in flowers, as compared to leaves, while a strong reduction was observed in the in vitro plants. A similar trend was noted for the antioxidant and anti-herpesvirus activity of the extracts. The antimicrobial potential, however, was very similar, regardless the growth conditions. Analysis of the N. nuda extracts led to identification of 63 compounds including phenolic acids and derivatives, flavonoids, and iridoids. Quantification of the content of 21 target compounds indicated their general reduction in the extracts from in vitro plants, and only the ferulic acid (FA) was specifically increased. Cultivation of in vitro plants under different light quality and intensity indicated that these variable light conditions altered the content of bioactive compounds, such as aesculin, FA, rosmarinic acid, cirsimaritin, naringenin, rutin, isoquercetin, epideoxyloganic acid, chlorogenic acid. Thus, this study generated novel information on the regulation of N. nuda productivity using light and other cultivation conditions, which could be exploited for biotechnological purposes.
... Há relatos sobre o potencial alelopático de extratos de plantas contendo piplartina (SANTOS et al., 2013;LEI et al., 2015;AMRITA KUMARI, 2018), entretanto, há ausência de estudos desse potencial aplicado em plantas daninhas. INDERJIT et al., 2011;CHENG;CHENG, 2015;CHUNG et al., 2018;ZHIJIE ZHANG et al., 2020). ...
... Há relatos sobre o potencial alelopático de extratos de plantas contendo piplartina (SANTOS et al., 2013;LEI et al., 2015;AMRITA KUMARI, 2018), entretanto, há ausência de estudos desse potencial aplicado em plantas daninhas. INDERJIT et al., 2011;CHENG;CHENG, 2015;CHUNG et al., 2018;ZHIJIE ZHANG et al., 2020). ...
... Various studies deciding on the hereditary systems related to crop-weed interactions have indicated that the phytotoxicity impacts are profoundly intricate. ese have been classi ed into biological and physiological impacts, for example, hindrance of the division of a cell and elongation, antioxidant systems disturbance, rising cell lm penetrability, and impacts of allelochemicals of microbes and the prompt ecology [47][48][49][50]. e importance of laboratory, growth chamber, and greenhouse bioassays is clear to International Journal of Agronomy 3 address this allelopathic action in nature [5,46]. ...
... ey found that the use of sorgaab spray lowered the dry weight of the weed up to 49% and improved wheat crop by 21%. Sorghum stalk incorporation into the soil at 2, 4, and 6 Mg ha −1 decreased unwanted plants by 42,48, and 56%, correspondingly. ere was not much difference between one, two, or three sorgaab sprays at 1 : 10 and three sprays at 1 : 20 ratio at 90 days. ...
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Phytotoxicity including autotoxicity and allelopathy is the immediate or indirect biochemical impact of one organism on the germination, growth, survival, and reproduction of other organisms or improvement of neighbouring plant species through the arrival of substances into the environment. is biological phenomenon e ect might be either growth-enhancing (synergistic) or inhibiting (hostile), contingent upon the chemical substances delivered from donor plants and target species. Allelopathy has been viewed not just as a nature-accommodating way to control unwanted plant spices and biocidal products, but, additionally, a potential explanation for causing autotoxicity in yield. e application of chemical agents to reduce weed infestation may have negative consequences on human health as well as the environment. Plants with allelopathy activities derived from secondary metabolites could be an alternative strategy and have an expected function in sustainable weed biocontrol and boost global agricultural production and food security. us, protecting biodiversity, ensuring food safety, improving food, and nutrient quality, as well as crop production, are urgently needed as population and consumption are increasing. So, the objective of this study is to present recent advancements on phytotoxicity and allelopathic e ect of plant extracts (sorghum, sun ower, rice, and corn), for sustainable food and crop production in agroecosystems.
... On the one hand, weeds affect the crop yield and quality by competing for light, nutrients, water and living space (Gandia et al., 2021). Some weeds are allelopathic and have adverse effects on crops (Cheng & Cheng, 2015). Studies have indicated that the root exudates and decomposing substances of Chenopodium album and Capsella bursa-pastoris induced oxidative damage and cell damage, disturbed the expression levels of key genes related to photosynthesis, thus inhibiting the seed germination and seedling growth of recipient plants . ...
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Background Ziziphus jujuba Mill. cv. ‘LingwuChangzao’ is a traditional jujube cultivar in northwest China. It is of great significance to explore the weed community composition and environmental characterization for the ecological control and comprehensive management of weeds in jujube orchards. In this article, a total of 37 species were recorded in 40 sample plots (1 m × 1 m). Moreover, fourteen environmental indicators to characterize the spatial locations, climate and soil nutrient characteristics of the plant communities were adopted. Methodology Through the two-way indicator species analysis (TWINSPAN) quantity classification and canonical correspondence analysis (CCA) ranking methods, the types of weed communities in the main planting base of jujube ‘LingwuChangzao’ and the main environmental factors affecting the change and distribution of weed types were analyzed. Results The weed communities within the study area were divided into 15 types by the TWINSPAN classification. There were significant differences in soil factors to the species diversity indices of the weed communities, the diversity of weed communities was negatively correlated with available potassium, whereas positively correlated with soil water content. The CCA results showed that community structure and spatial distribution of weed communities were affected by soil water content, total potassium, soil organic carbon, total phosphorus, total nitrogen. Our results can be used as a reference for orchard weed management and provide a theoretical basis for weed invasion control and creating a higher biodiversity in arable land under the background of environmental change.
... The influence of rye CCs, especially on fungi may explained by the production of allelochemicals. These compounds are mainly phytotoxins, some of which can be metabolized by specialized soil fungi and other microorganisms 37,38 . Rye also had the lowest aboveground N content and N uptake 26 , possibly leading to lower C:N inputs, and it was the only CC that overwintered, hence exposing soils to living roots for longer periods. ...
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Sustainable agricultural practices such as cover crops (CCs) and residue retention are increasingly applied to counteract detrimental consequences on natural resources. Since agriculture affects soil properties partly via microbial communities, it is critical to understand how these respond to different management practices. Our study analyzed five CC treatments (oat, rye, radish, rye-radish mixture and no-CC) and two crop residue managements (retention/R+ or removal/R−) in an 8-year diverse horticultural crop rotation trial from ON, Canada. CC effects were small but stronger than those of residue management. Radish-based CCs tended to be the most beneficial for both microbial abundance and richness, yet detrimental for fungal evenness. CC species, in particular radish, also shaped fungal and, to a lesser extent, prokaryotic community composition. Crop residues modulated CC effects on bacterial abundance and fungal evenness (i.e., more sensitive in R− than R+), as well as microbial taxa. Several microbial structure features (e.g., composition, taxa within Actinobacteria, Firmicutes and Ascomycota), some affected by CCs, were correlated with early biomass production of the following tomato crop. Our study suggests that, whereas mid-term CC effects were small, they need to be better understood as they could be influencing cash crop productivity via plant-soil feedbacks.
... Rashed et al. (2022) suggested that traditional medicine plays a valuable role in cancer treatment. Cheng & Cheng (2015) defined allelopathy as a normal biological phenomenon in which one organism produces biochemicals that impact Egypt. J. Bot. ...
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ETHNO-PHYTOTECHNOLOGY combines ethnobotany and biotechnology. This study evaluated the ethnobotanical role, anticancer potential, and allelopathy of Tribulus terrestris L. The ethnobotanical survey of twenty informants used an open-ended questionnaire. T. terrestris contains steroids, saponins, antioxidants, flavonoids, alkaloids, phenolics, proteins, and amino acids. The study investigated cytotoxic effects using six carcinoma cell lines. Hordeum vulgare and Lepidium sativum were used as recipient species in the allelopathy experiments. We found that 95% of the informants stated that T. terrestris is an aggressive species that injures livestock, reduces biodiversity, leads to soil dryness, consumes large amounts of space during the vegetative season, and affects soil pH and the absorption of minerals. Ethanolic extracts produced a significant effect on the prostate (PC3), breast (MCF 7), lung (A549), and liver (HEP-G2) carcinoma cell lines, with IC50 values of 19, 22, 33, and 33μg/mL, respectively. The intestinal carcinoma cell line (CAco2) had an IC50 60μg/mL. The colon (HCT) carcinoma cell line had an IC50 value of 68 μg/mL. Water extracts inhibited the seed germination, plumule length, radicle growth, and fresh and dry matter production of the recipient species. This study demonstrated that T. terrestris is potentially valuable as an anticancer agent and an herbicide against harmful weeds.
... e allelopathic effects of cover crop species are important during the germination and seedling growth (Cheng & Cheng, 2015;Shekoofa et al., 2020). e allelopathy phenomenon is understood as any influence of one plant on another plant through the release of chemical compounds (Jabran, 2017;Wu et al., 2020). ...
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The present study aimed to assess the allelopathic potential of four cereals: winter wheat (Triticum aestivum L.), triticale (×Triticosecale Wittm.), spelt wheat (Triticum spelta L.) and barley (Hordeum vulgare L.) through a completely randomized (CR) design. e allelopathic effects of water extracts of different parts of the cereal plants (stem, leaf, and spike) at different concentrations (0.5%, 1.0%, 2.0%, and 4.0%) were evaluated on the seed germination and seedling growth. e germination rate, length, and dry weight of the shoot and root of the seedlings were measured. Ferulic acid was detected in all the cereals. e water extracts at 2.0% and 4.0% concentration had an allelopathic effect on the germination rate, shoot and root length of seedlings of spelt wheat, barley, and triticale, and the stem and leaf extracts affected the root and shoot length of winter wheat. e allelopathic effect of the dried powder of the cereals were evaluated in pot experiments. Both spelt wheat and triticale powder treatment at elevated CO 2 levels increased the dry weight of the root, as well as the length of the shoot and root of winter wheat. Furthermore, treatment with 4.0 g of dry cereal powder combined with an elevated level of CO 2 increased the shoot length, whereas the root length of winter wheat was unaffected. In summary, the combination of dry cereal powder with elevated CO 2 stimulates the initial growth of winter wheat.
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The increasing use of synthetic herbicides in agriculture creates severe ecological and environmental threats. Several essential oils (EOs) (Thymol, Carvacrol, Cinnamaldehyde, and Eugenol) were investigated as natural herbicides, and their potential use as a substitute for synthetic and toxic chemicals for preventing roots intrusion in subsurface drip irrigation systems. To overcome their high volatility and to increase their thermal stability during processing, multiphase hybrid blends based on polymer/nanoclays (NCs) were prepared, enabling control of the EOs migration rate from the final active film. Germination experiments on mash bean seeds in open and closed systems have been conducted to evaluate the EOs efficacy as germination inhibitors. The amount of EO remaining in the films, after processing and for varying timepoints, was determined by UV–Vis spectroscopy through extraction. From these two experiment's results, we identified Thymol as the most effective herbicide. The effects of polymers/Thymol affinity and organoclay polarity were investigated to achieve a slow‐release effect. Linear low‐density polyethylene/polyamide 6 system showed better efficiency compared to the linear low‐density polyethylene in retaining Thymol during processing due to the thermodynamic affinity of the polyamide 6 phase with Thymol. NCs have been found to be nuclear foci during the first thermal process to obtain smaller highly surface voids allowing better absorption of the Thymol during the second thermal processing. NC Cloisite 15A showed better dispersion in the polymer matrix and improved chemical affinity between the nanocomposite and the Thymol. As a result, Thymol's desorption was delayed and a controlled release was obtained. Eventually, it was concluded that Thymol could be a natural and environmentally friendly alternative to the synthetic herbicides and use as root‐repellent agent.
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Vegetables are important worldwide, but their production faces problems of yield decline due to soil sickness and autotoxicity, when grown continuously for several years. Besides, allelopathic effects of other crops, weeds and trees also reduces yields. Thus the allelochemical interactions and their effects on vegetables are important in vegetable production. Although, research on various aspects of allelopathy in vegetable crops has been done but not compiled. Soil sickness is complex phenomenon due to several factors involved and autoxicity is major one. The autotoxic potential of certain vegetables has been discussed. In multi-storey cropping systems, where numerous crops and trees are grown together, vegetables are essential components and allelopathic interactions arise. Several vegetables possess antimicrobial principles and hence, allelopathically inhibit phytopathogenic fungi and bacteria. Certain vegetables possess nematicidal principles and therefore, offer immense potential for nematode control in their cultivation. Several studies have been done to elucidate the role of allelochemicals in vegetables across the world. The allelopathic interactions between the vegetables and other crops/weeds/trees and the potential of vegetables for pathogen and nematode management/control are reviewed in this paper. Future allelopathic research in vegetables should focus on (i) separating the allelopathic interference from competition in vegetable fields and vegetables based cropping systems, (ii) screening the germplasm/varieties of vegetable crops for allelopathic potential and later on do genetic manipulations to breed new varieties, (iii) exploiting the allelopathic potential of vegetable crops for weed control and plant protection, (iv) determining the critical concentrations of allelochemicals in each vegetable crop to express their inhibitory/stimulatory influences, (v) identifying the compatible and beneficial associations of vegetable crops with other crops and trees and (vi) determine the harmful and beneficial effects of allelopathy in vegetable crops in pot culture and field studies.
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This study aimed to evaluate the allelopathic effects of aqueous extracts of Trema micrantha leaves on seed germination and early growth of seedlings of radish (Raphanus sativus L.). Leaf extracts were prepared at concentrations of 2, 4 and 8%. pH and osmotic potential were also performed from the extracts. Germination bioassay consisted of five replicates of 25 seeds of radish distributed in Petri dishes with germitest paper and 7 mL of extract or water and kept at 25°C in B.O.D. for five days. Germination percentage (PG), germination speed (VG), germination speed index (IVG) and index of allelopathic effect (RI) were determined. For bioassay initial growth, seeds were germinated until they reach 2 mm protrusion radicle and transferred to Gerbox containing germitest paper and 15 mL of extract or water, kept seven days at 25°C in B.O.D. for evaluating length of the radicle and hypocotyl, fresh and dry mass, water content and content of chlorophyll (a, b and total). Leaf extracts from T. micrantha affected negatively all parameters examined for germination (PG, VG, IVG and RI). The extracts affected the initial growth, causing reduction of the radicle length and stimulating the growth of hypocotyls, but not interfered on fresh and dry weight and content of chlorophyll. Aqueous extracts of T. micrantha leaves exerted allelopathic action on germination and early growth of radish, but not caused inhibition of synthesis of photosynthetic pigments.