Abstractallelopathy refers to both inhibitory and stimulatory reciprocalbiochemical interactions between plants including microorganisms. A largenumber of compounds such as phenolic acids, fatty acids, indoles and terpeneshave been identified in rice root exudates and decomposing rice residues, asputative allelochemicals which can interact with surrounding environment. Sincethese allelopathic interactions may be positive, they can be used as effectivecontributor for sustainable and eco-friendly agro-production system. Geneticmodification of crop plants to improve their allelopathic properties andenhancement of desirable traits has been suggested.
Development of crops withenhanced allelopathic traits by genetic modification should be done cautiously,keeping in view of the ecological risk assessment (non-toxic and safe forhumans and ecosystem, crop productivity, ratio of benefit and cost, etc.).Allelopathy is a sub-discipline of chemical ecologythat is concerned with the effects of chemicals produced by plants ormicroorganisms on the growth, development and distribution of other plants andmicroorganisms in natural communities or agricultural systems (Einhellig, 1995).
The study ofallelopathy increased in the 1970s and has undergone rapid development sincethe mid-1990s, becoming a popular topic in botany, ecology, agronomy, soilscience, horticulture, and other areas of inquiry in recent years. Theallelopathic interaction can be one of the significant factors contributing tospecies distribution and abundance within plant communities and can beimportant 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 oneof the indirect causes of continuous cropping obstacles in agriculture. As aresult of the in-depth study of allelopathy, strategies for the management ofagricultural production and ecological restoration involving the application ofallelopathy and allelochemicals are improving.
The main purposes of this revieware to present conclusions regarding the application of allelopathy inagricultural production, to highlight the physiological and ecological mechanismsunderlying plant allelopathy, to illustrate the effect of allelopathy on soilmicroorganisms and to discuss key points for further research.Agriculturalpractices and allelopathyInderjit and Weiner (2001) suggested that allelopathy is not justplant-plant interference but also involves soil-mediated chemical intervention.Allelopathy of soil may get influenced by many factors (physical, chemical, andbiological), including the climatic conditions and presences of other plantspecies in the vicinity.In production systems with no-tillage or conservation tillage, the cropresidues are buried in the soil and thus the release of allelochemicals fromboth the growing plants and residue decomposition might act synergistically.The trend in certain regions towards no- or minimum- tillage cropping systemhas developed curiosity to determine the allelopathic effect of crop residueson seed germination of weeds and on production of the successive crop (Moyer and Huang, 1997). The allelochemicals released fromcereal residues are reported to have inhibitory effect on seed germination ofsurface weeds (Jung et al., 2004).Cover crops and mulches prevent weed growth either throughallelochemicals, competition or other mechanisms that include stimulation ofmicrobial allelochemicals, physical barriers such as obstructing lightpenetration and transforming soil characteristics (Hobbs et al.
, 2008). Cover crops have several advantages,however, if not judiciously selected and used, they can lead to significantproblems in seeding of the next crop and stimulation of the pests that may ruinthe following cash crop (Snapp et al., 2005). Recently, Kim et al (2013) studied the effects of winter covercrop on rice yield and total global warming potential (GWP) and suggested thatcover crops with low C/N ratio, such as vetch, may be more desirable greenmanures 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 Viciavillosa), are mainly used. The leguminous crops can increase the soil Ncontent through symbiotic N fixation (Na et al.
, 2007), while non-leguminous ones havecomparatively higher biomass productivity (Zhang et al., 2007). Some volatile allelochemicals fromcrucifer green manures like glucosinolates, the breakdown epithinitriles,nitriles, isothiocyanates and ionic thiocyanates have fungicidal and herbicidalactivities (Vaughan and Boydston, 1997) Reductionof Nitrogen Leaching and Environmental Pollution Nitrogen leaching is a severe ecological problem due towater pollution. Mineralization of soil organic nitrogen, especially thenitrification of nitrogen fertilizer, is one of the main reasons for theenrichment of nitrogen in the soil.
Biological nitrification inhibition (BNI)has gradually become the main target in investigating the effect of plants onsoil nitrification. In recent years, studies have proven thatnitrification-inhibiting substances (NIS) produced by plants are the firstchoice for soil nitrification management. For example, biological nitrificationinhibition substances (BNIS) are allelochemicals that are able to inhibit soilnitrification. Wheat allelochemicals, such as ferulic acid, p-hydroxybenzoicacid and hydroxamic acid, can act on soil microbes to inhibit soilnitrification, reduce the emission of N2O, improve the utilizationrate of nitrogen fertilizer and reduce pollution to the environment (Ma, 2005). Dietz et al. (2013) found that the allelopathic plantain (Plantagolanceolata L.) plant has inhibitory effects on soil nitrogenmineralization, suggesting that plantain could be utilized to reduce soilnitrogen leaching.Influenceon Water and Nutrient UptakeMany allelochemicals affectnutrient absorption in plant roots or induce water stress through long-terminhibition of water utilization.
Allelochemicals can inhibit the activities ofNa+/K+-ATPase involved in the absorption and transport ofions at the cell plasma membrane, which suppresses the cellular absorption of K+,Na+, or other ions.Bergmarket al. (1992) found that ferulic acid (250 ?M)inhibited ammonium and NO3– uptake in cornseedlings, although ammonium uptake was less sensitive to this treatment thanNO3–. Ferulic acid also inhibits Cl– uptakeand increases the initial net K+ loss from roots exposed to alow K ammonium nitrate solution and delays recovery that results in a positivenet uptake. Yuanet al. (1998) showed that the effects ofallelochemicals, such as ferulic acid, benzaldehyde and 4-tert-butylbenzoicacid, on nitrogen absorption in wheat seedlings are negatively correlated, butthe negative effects of NH4+-N on nitrogen absorptionwere stronger than those of NO3–-N.
Yuand Matsui (1997) observed that cinnamic acid and theroot exudates of cucumber inhibited the uptake of NO3–,SO42–, K+, Ca2+, Mg2+,and Fe2+ by cucumber seedlings. Through further study, Lv et al. (2002) foundthat cinnamic acid and p-hydroxybenzoic, the main allelochemicals found incucumber root exudates, strongly inhibited the activities of root dehydrogenase,root-combined ATPase and nitrate reductase in cucumber, thus inhibiting theroot uptake of K+, NO3–, and H2PO4–.
Sorgoleone and juglone significantly inhibited H+-ATPase activityand the proton-pumping function across the root cell plasmalemma, which affectedsolute and water uptake in peas (Pisum sativumL.), soybeans and corn (Hejl and Koster, 2004a,b). Abenavoli et al. (2010) foundthat the allelochemicals trans-cinnamic, ferulic acid and p-coumaric acidinhibited net nitrate uptake and plasma membrane H+-ATPase activity inmaize seedlings, while umbelliferone and caffeic acid had no effect on H+-ATPaseactivity. Sunflower (Helianthus annus L.) residues negativelyaffected plant development, the efficiency of translocation of assimilates andnutrient accumulation in radish plants (Barrosde Morais et al., 2014).
The effects ofallelochemicals on ion uptake are closely related to allelochemicalconcentrations and classifications. For example, a low concentration of dibutylphthalate 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 Nand K but inhibits the absorption of P by tomato roots (Geng et al., 2009).Effectsof Allelochemicals on Microorganisms and the Ecological EnvironmentResearchershave found that there are significant relationships between crop growth andsoil microbes under the application of allelochemicals or in the presence ofallelopathic plants (Figure ?(Figure3;3; Barazani andFriedman, 1999; Bais et al., 2006; Mishra et al., 2013).
Recentstudies demonstrated that indirect effects of allelopathy as a mediator ofplant–plant interactions were more important than the direct effects of aninhibitor (Zeng, 2014).Chemical-specific changes in soil microbes could generate negative feedbacks insoil 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 allelopathicpotential of plants through positive feedback (Inderjit et al., 2011; Zuo et al.
, 2014; Wu et al., 2015). Bacteriacan help to increase inhibition by activating a non-toxic form of anallelochemical (Macias et al.
, 2003). Forexample, non-glycosylated compounds may be modified after release from plantsand become more toxic (Tanrisever et al.,1987; Macias et al., 2005a). However,bacteria can also help susceptible plants to tolerate biotic stress associatedwith weeds, and to decrease the allelopathic inhibition of weeds by causingalterations in the expression patterns of some genes that might be responsiblefor different functions but ultimately lead to a self-defense process (Mishra and Nautiyal, 2012).In addition, the microbial degradation/transformation of allelochemicals insoil affects the effective dose of allelochemicals that can cause plantinhibition (Mishra et al., 2013; Li et al., 2015).
Bacterial biofilms in rhizospheric regions can protect colonization sites fromphytotoxic allelochemicals and can reduce the toxicity of these chemicals bydegrading them (Mishra and Nautiyal, 2012; Mishra et al., 2012).Microorganisms have the ability to alter the components of allelochemicalsreleased into an ecosystem, highlighting their key role in chemical plant–plantinteractions and suggesting that allelopathy is likely to shape the vegetationcomposition and participate in the control of biodiversity in ecology (Fernandez et al., 2013). Somesesquiterpenoid lactones and sulfides are antimicrobial and can disrupt thecell walls of fungi and invasive bacteria, while others can protect plants fromenvironmental stresses that would otherwise cause oxidative damage (Khan et al.
, 2011; Chadwick et al., 2013). Zhang et al. (2013a) foundthat 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 couldcontrol Panama disease and increase cropland biodiversity.
Wanget al. (2013b) indicated that the shift in themicrobial community composition induced by barnyard grass infestation mightgenerate a positive feedback in rice growth and reproduction in a given paddysystem. The relative abundance and population of plant parasitic nematodes weresignificantly reduced in the presence of Chromolaena odorata (Asteraceae)fallow (Odeyemi et al., 2013). Pearse et al. (2014)foundthat radish soils had a net positive effect on Lupinus nanus biomassand explained that radish might alter the mutualistic/parasitic relationshipbetween L. nanus and its rhizobial associates, with a netbenefit to L. nanus.
Fanget al. (2013) indicated that inhibiting theexpression of the rice PAL gene reduced the allelopathicpotential of rice and the diversity of the rhizosphere microflora. Thesefindings suggested that PAL functions as a positiveregulator of the rice allelopathic potential.PGPR, such asroot-colonizing Pseudomonas, Paenibacillus polymyxa,endophytes and Chryseobacterium balustinum Aur9, have beenshown to alter plant gene expression and regulate plant allelochemicalsynthesis and signaling pathways to enhance disease resistance, adaptabilityand defense capabilities in response to biotic and abiotic stresses in plants (vanLoon, 2007; Dardanelli et al., 2010; Mishraand Nautiyal, 2012) Problemsand Future Research DirectionsAllelochemicals mainlyconsist of secondary metabolites that are released into the environment throughnatural pathways, such as volatilization, leaf leaching, residue decomposition,and/or root exudation. Therefore, it should first be noted how allelochemicalsare released into the environment (Inderjit and Nilsen, 2003).The activity of allelochemicals varies with research techniques and operationalprocesses (Penget al., 2004).
The natural state of allelochemicals maybe changed somewhat during the process of extraction (Liet al., 2002). Therefore, researchers must be careful todetermine whether a plant has allelopathic potential or separate and identifyallelochemicals using organic solvents and aqueous extracts from plant tissues.Allelochemicalscan be degraded after they have been released into the soil; the half-life ofallelochemicals 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 allelochemicalconcentration, soil type, soil enzymes, and soil microbial population andcommunity structure (Macias et al., 2004; Understrup et al.,2005; Kong et al., 2008; Gu et al., 2009). Previous studies indicated that someallelochemicals had tremendous spatial and temporal heterogeneity (Weidenhamer, 2005; Dayan et al.
, 2009; Mohney et al., 2009; Weidenhamer et al.,2009, 2014), but thesecharacteristics of most allelochemicals have not been confirmed. It wasreported that polydimethylsiloxane (PDMS) microtubing (silicone tubingmicroextraction, or STME) could be used as a tool to provide a more finelyresolved picture of allelochemical dynamics in the root zone (Weidenhamer, 2005; Mohney et al., 2009; Weidenhamer et al.,2009, 2014). Until now, muchremains unknown about the fate or persistence of allelochemicals in the soil ortheir effects on soil chemistry or microflora (Belz, 2007). ConclusionAllelopathyhas been known and used in agriculture since ancient times; however, itsrecognition and use in modern agriculture are very limited.
Allelopathy playsan important role in investigations of appropriate farming systems as well asin the control of weeds, diseases and insects, the alleviation of continuouscropping obstacles, and allelopathic cultivar breeding. Furthermore,allelochemicals can act as environmentally friendly herbicides, fungicides,insecticides and plant growth regulators, and can have great value insustainable agriculture. Although allelochemicals used as environmentallyfriendly herbicides has been tried for decades, there are very few naturalherbicides 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 thephysiological and ecological mechanisms of allelopathy are gradually beingelucidated. Moreover, progress has been made in research on the associatedmolecular mechanisms. It is obvious that allelopathy requires further researchfor widespread application in agricultural production worldwide.