Influence of abiotic stress on allelopathic properties of Amaranthus cruentus L.

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Date
2016-06
Authors
Allemann, Ingrid
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University of the Free State
Abstract
English: Allelopathic plants disrupt germination and growth of agricultural crops which will lead to a lower yield production. However, allelopathy is also important in an environmentally friendly approach to control weeds and reduce chemical herbicide usage. Allelochemicals are secondary metabolites, released into the environment through leaching, volatile emissions, root exudation and the decaying of plant residues. The isolation and identification of allelochemical compounds involved in the plant-plant interactions are of great importance. Amaranthus cruentus is used for both its grain and leaves for consumption. This plant is now being considered at as a new agricultural crop; therefore it is important to understand how this plant will interact within a changing environment and how this will influence other plants. It is known that amaranth plants have the ability to synthesize allelochemicals in order to protect itself from different types of stress. In this study, the leaf litter as well as the polar and non-polar extracts of the leaves of A. cruentus from three different temperature regimes were investigated for their in vitro allelopathic properties towards tomato, pepper, cucumber and lettuce. The chemical compounds induced during each temperature treatment were also compared. Methanol-water and dichloromethane (DCM) crude extracts were prepared from each of the temperature treated amaranth plants using dried ground leaf litter. The identification of compounds present in the different treated plants was done through thin layer chromatography (TLC), high pressure liquid chromatography (HPLC) and gas chromatography coupled to mass spectrometry (GC-MS). TLC indicated the presence of phenolics, terpenes and flavonoids in all A. cruentus extracts. Differences were seen in the number and type of compounds between the temperature treated plant extracts. Methanol-water extracts of the three plant treatments were analysed using HPLC with specific consideration to phenolic compounds. The optimal treatment showed the most peaks followed by the cold and lastly the heat treatment that showed the least amount of peaks. Compounds identified in the optimal and cold treatments included catechin and rutin, while the cold treatment also contained quercetin. The heat treatment contained none of the compounds that were used as standards. Dichloromethane (DCM) extracts of the three plant treatments were analysed using GC-MS. In the optimal treatment, the compound found in the highest abundance was phosphine imide, P, P, P-triphenyl (38.59%), the cold treatment contained neophytadiene (27.35%) and α-linolenic acid (24.41%). Dichloroacetic acid (29.68%) was identified as the major compound in the DCM extract of A. cruentus heat treated plant litter. Laboratory bioassays were carried out to evaluate the in vitro allelopathic activity of different concentrations of the stressed amaranth plant litter or methanol-water and DCM crude extracts, against four vegetables (tomato, lettuce, cucumber and pepper) by means of the Sandwich Method. Germination, hypocotyl and radicle length were adversely influenced by the plant litter for all the temperature treatments. The inhibition caused by the litter was dependant on concentration and each vegetable species showed different levels of sensitivity. The extracts indicated that both types of extracts and all temperature treatments had a negative influence on the germination of all the vegetables. The methanol-water and DCM extracts adversely affected organ length for all the vegetables at the highest concentration for each of the treatments. The severity of inhibition was dependent on both the concentration and temperature treatment. Each vegetable species showed different levels of sensitivity against the different extract treatments. The results obtained in this study, demonstrated that cold, hot and optimal temperature treatments induced the expression of different chemical compounds in the leaves of A. cruentus plants. All temperature treatments however affected germination and growth of some vegetables, exhibiting allelopathic properties. Therefore, plant litter and both polar and non-polar extracts of all the treated amaranth plants, contained compounds responsible for allelopathy. Furthermore, the results indicated that vegetables reacted differently towards the treatments of the leaf litter and extracts, showing variation in sensitivity against various compounds. Thus, abiotic stress does have an influence on both the chemical composition and allelopathy of A. cruentus, but it was likewise proven that plants grown under optimal growth conditions also produced both polar and non-polar compounds with allelopathic properties which may have a negative impact in crop rotation systems.
Afrikaans: Allelopatiese plante ontwrig ontkieming en groei van landbougewasse wat kan lei tot laer opbrengste. Allelopatie is egter ook belangrik in 'n omgewingsvriendelike benadering vir onkruidbeheer en om die gebruik van chemiese onkruiddoders te verminder. Allelochemikalieë is sekondêre metaboliete, wat in die omgewing vrygestel word deur loging, vlugtige vrystellings, wortel afskeiding en die ontbinding van plantmateriaal. Die isolasie en identifisering van allelochemiese verbindings wat betrokke is by plant-plant-interaksies is daarom van groot belang. Beide die graan en blare van Amaranthus cruentus word gebruik. Hierdie plant word nou oorweeg as 'n nuwe landbougewas, daarom is dit belangrik om te verstaan hoe hierdie plant binne 'n veranderende omgewing sal reageer en hoe dit ander plante kan beïnvloed. Amarantus plante is daarvoor bekend dat hulle die vermoë het om allelochemikalieë te sintetiseer vir beskerming teen verskillende tipes stres situasies. In hierdie studie was A. cruentus onder drie verskillende temperatuur toestande gekweek en die in vitro allelopatiese eienskappe van die droë blaarmateriaal (residu), asook polêre en nie-polêre ekstrakte daarvan, teenoor tamatie, peper, komkommer en blaarslaai getoets. Die chemiese verbindings wat gedurende die verskillende temperatuur toestande gesintetiseer is, was ook vergelyk. Metanol-water en dichlorometaan (DCM) ekstrakte was berei uit die residu van die verskillende temperatuurstres behandelde amarantus plante. Identifisering van die verbindings in die verskillende plante is gedoen deur middel van dunlaagchromatografie, hoë druk vloeistofchromatografie en gaschromatografie gekoppel aan massaspektrometrie. Dunlaagchromatografie het die teenwoordigheid van fenole, terpene en flavonoïede in alle A. cruentus ekstrakte aangedui. Verskille in die aantal verbindings tussen die verskillende temperatuur- behandelde ekstrakte is opgemerk. Hoë druk vloeistofchromatografie, met spesifieke verwysing na fenoliese verbindings, is gebruik om die metanol-water ekstrakte van die drie temperatuur- behandelings te ontleed. Die optimale temperatuurbehandeling het die meeste pieke getoon, gevolg deur die koue- en laastens die hittebehandeling wat die minste pieke het. Verbindings in die optimale- en kouebehandelings het catechin en rutin ingesluit, terwyl die kouebehandeling ook quercetin bevat het. Die hittebehandeling het geen van die verbindings wat as standaarde gebruik is, bevat nie. Die dichlorometaan (DCM) ekstrakte van die drie temperatuurbehandelings is ontleed met behulp van gaschromatografie gekoppel aan massaspektrometrie. In die optimalebehandeling was die verbinding met die hoogste voorkoms fosfien imide, P, P, P-triphenyl (38,59%), terwyl die kouebehandeling neophytadiene (27,35%) en α-linoleensuur (24,41%) bevat het. Dichloroasynsuur (29,68%) is geïdentifiseer as die belangrikste verbinding in die DCM ekstrak van die hitte- behandelde A. cruentus plantmateriaal. Laboratorium analises is uitgevoer om die in vitro allelopatiese aktiwiteit van verskillende konsentrasies van die amarantus plantresidu en ekstrakte (metanol-water en DCM) teen vier groente (tamatie, blaarslaai, komkommer en peper) te evalueer deur gebruik te maak van die “Sandwich” metode. Ontkieming, hipokotiel en kiemwortel lengte was ongunstig beïnvloed deur die plant residu vir al die behandelings. Die remming wat deur die residu veroorsaak is, is afhanklik van die konsentrasie en elke spesie groente het verskillende vlakke van sensitiwiteit getoon. Die ekstrakte dui daarop dat beide tipes ekstrakte en al die temperatuur- behandelings 'n negatiewe invloed het op die ontkieming van al die groente. Die hoogste konsentrasies van die metanol-water en DCM ekstrakte vir elk van die behandelings, veroorsaak 'n afname in orgaan lengte in al die groente. Die mate van inhibisie was afhanklik van beide die konsentrasie en temperatuur- behandeling. Soos vir die plant residu, toon elke groente spesie verskillende vlakke van sensitiwiteit teenoor die verskillende ekstrakbehandelings. Die uitslag van hierdie studie, het getoon dat koue, warm en optimale temperatuur behandelings gelei het tot die induksie van verskillende chemiese verbindings in die blare van A. cruentus plante. Al die temperatuur behandelings het egter ʼn negatiewe invloed op ontkieming en groei van sekere groente gehad, wat op allelopatie dui. Dus, bevat die plant residu en beide polêre en nie-polêre ekstrakte van al die behandelde amarant plante, verbindings wat verantwoordelik is vir allelopatie. Verder toon die resultate dat groente verskillend reageer op die verskillende behandelings, wat dui op ʼn variasie in sensitiwiteit teenoor verskillende verbindings. Abiotiese stres het dus wel 'n invloed op die chemiese samestelling en allelopatiese eienskappe van A. cruentus, alhoewel dit ook bewys is dat plante wat onder optimale groeitoestande gekweek is, beide polêre en nie-polêre verbindings bevat wat allelopatie veroorsaak en 'n negatiewe impak op wisselboustelsels kan hê.
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Keywords
Allelopathy, Phenolic compounds, Extracts, A. cruentus, Vegetables, Germination, Hypocotyl, Radicle., Plants -- Effect of stress on, Dissertation (M.Sc. (Plant Sciences))--University of the Free State, 2016
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