Abstract humans and ecosystem, crop productivity, ratio of


allelopathy refers to both inhibitory and stimulatory reciprocal
biochemical interactions between plants including microorganisms. A large
number of compounds such as phenolic acids, fatty acids, indoles and terpenes
have been identified in rice root exudates and decomposing rice residues, as
putative allelochemicals which can interact with surrounding environment. Since
these allelopathic interactions may be positive, they can be used as effective
contributor for sustainable and eco-friendly agro-production system. Genetic
modification of crop plants to improve their allelopathic properties and
enhancement of desirable traits has been suggested. Development of crops with
enhanced allelopathic traits by genetic modification should be done cautiously,
keeping in view of the ecological risk assessment (non-toxic and safe for
humans and ecosystem, crop productivity, ratio of benefit and cost, etc.).

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, 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.

practices and allelopathy

Inderjit and Weiner (2001) suggested that allelopathy is not just
plant-plant interference but also involves soil-mediated chemical intervention.
Allelopathy of soil may get influenced by many factors (physical, chemical, and
biological), including the climatic conditions and presences of other plant
species in the vicinity.

In production systems with no-tillage or conservation tillage, the crop
residues are buried in the soil and thus the release of allelochemicals from
both the growing plants and residue decomposition might act synergistically.
The trend in certain regions towards no- or minimum- tillage cropping system
has developed curiosity to determine the allelopathic effect of crop residues
on seed germination of weeds and on production of the successive crop (Moyer and Huang, 1997). The allelochemicals released from
cereal residues are reported to have inhibitory effect on seed germination of
surface weeds (Jung et al., 2004).

Cover crops and mulches prevent weed growth either through
allelochemicals, competition or other mechanisms that include stimulation of
microbial allelochemicals, physical barriers such as obstructing light
penetration and transforming soil characteristics (Hobbs et al., 2008). Cover crops have several advantages,
however, if not judiciously selected and used, they can lead to significant
problems in seeding of the next crop and stimulation of the pests that may ruin
the following cash crop (Snapp et al., 2005). Recently, Kim et al (2013) studied the effects of winter cover
crop on rice yield and total global warming potential (GWP) and suggested that
cover crops with low C/N ratio, such as vetch, may be more desirable green
manures to reduce total GWP per grain yield and to improve rice productivity.
In rice fields, two groups of cover crops with high biomass yield, i.e.
non-leguminous (Secale cerealis and Hordeum vulgare)
and leguminous crops (Astragalus sinicus and Vicia
villosa), are mainly used. The leguminous crops can increase the soil N
content through symbiotic N fixation (Na et al., 2007), while non-leguminous ones have
comparatively higher biomass productivity (Zhang et al., 2007). Some volatile allelochemicals from
crucifer green manures like glucosinolates, the breakdown epithinitriles,
nitriles, isothiocyanates and ionic thiocyanates have fungicidal and herbicidal
activities (Vaughan and Boydston, 1997)


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 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.

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.

et al. (1992) found that ferulic acid (250 ?M)
inhibited ammonium and NO3– uptake in corn
seedlings, although ammonium uptake was less sensitive to this treatment than
NO3–. Ferulic acid also inhibits Cl– uptake
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
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 sativumL.), 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).

of Allelochemicals on Microorganisms and the Ecological Environment

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 ?(Figure3;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 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 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.

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)


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.

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).




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.