Masters Degrees (Plant Sciences)
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Browsing Masters Degrees (Plant Sciences) by Advisor "Cawood, M. E."
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Item Open Access Influence of abiotic stress on allelopathic properties of Amaranthus cruentus L.(University of the Free State, 2016-06) Allemann, Ingrid; Cawood, M. E.; Allemann, J.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.Item Open Access Initiation of defence responses by plant extracts and their insecticidal role against the Russian wheat aphid in wheat(University of the Free State, 2015-11) Saba, Lubabalo; Cawood, M. E.; Pretorius, J. C.The Russian wheat aphid (Diuraphis noxia: Kurdjumov) is considered a harmful pest in South Africa and many other parts of world. The application of insecticides to host plants poses two unique problems as firstly it is too costly and the health risks associated with synthetic applications is usually met with consumer resistance. Plant species like Artemisia afra, Agave attenuata and Zanthoxlum capense are rich in bioactive compounds which are widely used in different industries. The bioactive compounds in the plant species and their possible role in providing protection against aphids served as the rationale for the basis of this study. In this study, the essential oil of Ar. afra and polar and non-polar extracts from the three plant species mentioned, were investigated for their repellent and insecticidal abilities towards the Russian wheat aphid (RWA) and their ability to induce a defence response in wheat. Water, methanol and dichloromethane (DCM) crude extracts were prepared from each plant using dried ground leaf material. The essential oil of Ar. afra was obtained through hydro-distillation from fresh leaf material. The identification of compounds present in the different plants through thin layer chromatography revealed the presence of terpenes and phenolics in Ar. afra extracts. In A. attenuata, steroids and saponins were highlighted as being present, while Z. capense extracts showed the presence of steroids, flavonoids and other phenolic compounds. Essential oil of Ar. afra and DCM extracts of the three plant species were analysed using gas chromatography coupled to mass spectrometry. In the essential oil, compounds found in high abundance were eucalyptol (12.87%), camphor (15%), α–thujone (29%), β-thujone (12%) and 1,8-cineole (7.42%). Alpha-thujone (29.47%) and β-thujone (37.36%) were identified as the major constituents in the DCM extract of Ar. afra. Compounds present in A. attenuata DCM extract revealed the presence of methyl jasmonate (6.4%) and 2,5-dimethyl-3,4-hexanediol (23%). The DCM extract of Z. capense consist of, among others, 2,4-dimethylhex-1-ene (25.98%) and E – (β)-farnesene (2.41%) . Laboratory bioassays were carried out to evaluate the insecticidal activity of the essential oil and other crude extracts against the RWA by spraying the insects directly with a 10 mg mL-1solution. Mortality was recorded at 1 h and 24 h after treatment. All the plant extracts and essential oil caused significant mortality of 50% or more after 1 h and at least 70% at the end of 24 hours. Ar. afra essential oil proved to be the most toxic with a LC50 – value of 0.396 mg mL-1. Initial screening for repellency of the essential oil and extracts was performed using a four arm olfactometer. Both polar and non-polar extracts repelled the RWA. However, the mean duration recorded in repeated 10 min cycles showed the The DCM extract and essential oil of Ar. afra, water extract of A. attenuata and DCM extract of Z. capense were highly repellent. The repellent properties and abilities of the essential oil and plant extracts to limit RWAs settling on susceptible and resistant wheat leaves, ensuing lowered disease symptoms after 7 days, were exploited in a leaf settling bioassay under glasshouse conditions. At 2 h.p.i, the essential oil and polar extracts of Ar. afra and DCM extract of A. attenuata reduced aphid settling significantly when compared to the control in the susceptible cultivar. The water extract treatment of Ar. afra, DCM extract treatment of A. attenuata maintained lower number of aphid settling on wheat leaves even after 72 h.p.i in the susceptible cultivar. In the resistant cultivar, no significant effects were observed, however the DCM extract treatment of A. attenuata still maintained to lower aphid settling after 72 h.p.i. The symptom analysis revealed that the extracts mentioned above also lowered the prevalence of disease in wheat after 7 days and the number of live aphids were less. The intiation of defence responses by the extracts of Ar. afra and A. attenuata was comparable to what was observed in the leaf settling bioassay. β-1,3-Glucanase and peroxidase activity increased with the application of the water and essential oil extracts of Ar. afra in both uninfested and infested plants during a 144 h period. Furthermore, an increase in peroxidase activity was observed when wheat was treated with the DCM extract of A. attenuata. The results obtained in this study, indicate that the plant extracts contain bioactive compounds that possess the ability to aid in the control of the RWA through various mechanisms and their integration in pest management strategies may prove to be beneficial.