Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: The high cost of herbicides for weed control in crop fields in the Western Cape is a major cause of reducing farmers' net income. As chemical weed control became more difficult and expensive, it became necessary to focus on the technique of reducing weed impact, which does not only involve herbicide usage. Aspects such as tillage method, sowing date of crops, crop rotation, weed ecology and germination requirements, amongst other, may play a role in reducing weed impact. The first experiment was done to determine whether temperature and light had an effect on the germination of six selected weed species, namely Arctotheca calendula, Avena fatua, Bromus diandrus, Emex australis, Lolium temulentum and Raphanus raphanistrum. Seeds of the aforementioned weeds were collected from Langgewens during 2000 and stored at room temperature before being used in this study. The seeds were germinated in a germination cabinet at three temperature regimes namely 5 "C /15 "C, 10°C / 15°C and 10°C / 20°C. Most seeds showed a positive germination response at the 10°C / 15°C treatment, except for Emex australis and Lolium temulentum, which was believed to germinate throughout the year under favourable conditions. Among the germinated weed species, Avena fatua germinated best with a cumulative value of 90% as compared to Raphanus raphanistrum, which germinated least with a cumulative value of 12%. The second experiment was done to evaluate the effect of three growth regulators, namely gibberellic acid, hydroxylamine (auxin), and kinetin (cytokinin) to break dormancy and enable simultaneous germination of the six weed species mentioned above. Weed seeds were germinated in a germination cabinet at 20 °C using the test solutions of the aforementioned growth regulators. The germination was assessed after 3, 7, 10 and 14 days of incubation and the tetrazolium test for the viability of seeds was done for ungerminated seeds. The result obtained showed that no chemical/concentration proved to be successful in stimulating the germination of all species tested. As for example a high concentration of hydroxylamine increased germination of Emex australis to nearly 100% and inhibited germination of Raphanus raphanistrum to less than 12.5% at all hydroxylamine concentrations. The third experiment was conducted with the aim of determining the competitiveness of the six weed species mentioned above when grown together with wheat in order to decide when weed control will be cost-effective. An additive series experiment was conducted in a glasshouse. The influence of weed species on wheat plant height, tiller number at different growth stages, vegetative dry mass and gram mass was determined by using different varying weed densities. Weed densities, through reduction in number of tillers, reduced wheat grain yield. By increasing the density of wheat this competitive effect could be reduced. Different weed densities caused a significant difference in wheat plant height, tiller number, dry mass and grain mass. The results showed that species had a significant effect (p < 0.05) on wheat plant height during tillering, stem elongation and heading growth stages. Total wheat plant above-ground dry mass was significantly (p < 0.05) reduced by an increase in weed density. / AFRIKAANSE OPSOMMING: Die hoë koste van onkruiddoders vir onkruidbeheer in graangebiede van die Wes- Kaap is een van die hooffaktore wat graanprodusente se netto inkomste verlaag. Weens moeiliker en duurder chemiese onkruidbeheer en om die impak van onkruide te verlaag, het dit noodsaaklik geword om op die tegnieke te fokus wat nie die gebruik van chemikalieë insluit nie. Faktore wat in rol kan speel sluit bewerkingspraktyke, saaidatum van gewasse, wisselbou, ekologie en ontkiemingsvereistes van onkruide in. Die eerste eksperiment is gedoen om te bepaal of temperatuur en lig 'n effek het op die ontkieming van ses gekose onkruide, naamlik Arctotheca calendula, Avenajatua, Bromus diandrus, Emex australis, Lo/ium temulentum en Raphanus raphanistrum. Sade van die voorgenoemde onkruide is in 2000 op Langgewens versamel en by kamertemperatuur geberg voordat dit in hierdie studie gebruik is. Die sade is in 'n ontkiemingskabinet geïnkubeer by drie temperatuurreekse naamlik 5 oe / 15 oe, 10 oe / 15 oe en 10 oe / 20 oe. Die meeste sade het 'n positiwe ontkiemingsreaksie getoon na die 10 oe / 15 oe behandeling, met die uitsondering van Emex australis en Lolium temulentum, wat oënskynlik heeljaar salontkiem onder gunstige omstandighede. Avena fatua het die hoogste ontkiemingspersentasie oor alle spesies getoon met 'n kumulatiewe waarde van 90% en Raphanus raphanistrum die minste met 'n kumulatiewe waarde van 12%. Die tweede eksperiment is gedoen om die invloed van drie groeireguleerders, naamlik gibberelliensuur, hidroksielamien (ouksien) en kinetin (sitokinien), op die opheffmg van saadrus te ondersoek en om ook die gelyktydige ontkieming van bogenoemde ses onkruidspesies moontlik te maak. Onkruidsade is in 'n ontkiemingskabinet by 20 oe ontkiem deur gebruik te maak van toetsoplossings van bogenoemde groei-reguleerders. Die ontkieming van die sade is na inkubasie periodes van 3, 7, 10 en 14 dae geëvalueer en die tetrazoliumtoets VIr saadkiemkragtigheid is toegepas vir onontkiemde sade. Die verkreë resultate het getoon dat geen chemiekalie / konsentrasie die ontkieming van alle spesies suksesvol kon stimuleer nie. Hoë konsentrasies hidroksielamien het die ontkieming van Emex australis tot byna 100% verhoog en het die ontkieming van Raphanus raphanistrum geïnhibeer tot minder as 12.5% by alle hidroksielamien konsentrasies. Die derde eksperiment is uitgevoer met die doelom die kompetisievermoë van dieselfde ses onkruidspesies te bepaal as dit saam met koring groei en om te besluit wanneer onkruidbeheer koste effektief sal wees. 'n Additiewe reeks eksperiment wat uit vier herhalings bestaan het, is in plastiekpotte in 'n glashuis uitgevoer. Die invloed van die onkruidspesies op koring planthoogte, halmgetal by verskillende groeistadiums, droë massa en graanmassa is bepaal deur onkruiddigthede te gebruik. Onkruide het koring opbrengs verlaag deur die vermindering van halmgetal en hierdie kompeterende effek kon verminder word deur 'n toename in die plantestand van koring. Verskillende onkruiddigthede het 'n betekenisvolle invloed op koring planthoogte, halmgetal, droë massa en graanmassa gehad. Die resultate het getoon dat spesies 'n betekenisvolle effek (p< 0.05) op koring planthoogte, stem verlenging en aarverskyning gehad het. Die totale bogrondse droë massa van koring is betekenisvol (p< 0.05) verminder deur 'n verhoging in onkruiddigtheid.

The cochineal insect Dactylopius austrinus De Lotto, which was released on jointed cactus, Opuntia aurantiaca Lindley, in South Africa in 1932, failed to bring the weed under an acceptable level of control in spite of the encouraging results during the first few years after release. The reasons for this apparent failure were never clearly understood. In 1957 the State embarked on an intensive herbicidal control programme which is still in force today. This sustained and expensive programme has undoubtedly reduced the density of the weed in most areas but has failed to solve the problem and the plant continues to expand its range. The biological control of o. aurantiaca in South Africa has been fundamentally influenced by this chemical control campaign. The relationship between chemical and biological control methods is reported in this study.

The alien invader plant Parthenium hysterophorus is a Category 1 weed in South Africa, where it poses a serious threat to indigenous vegetation in particular, and to biodiversity in general. In addition to its competitive ability, it is hypothesized that the successful invasiveness of P. hysterophorus is linked to the allelopathic potential of the plant. One compound in particular, parthenin, is alleged to play a major role in this allelopathic potential. Interference between P. hysterophorus and three indigenous grass species (Eragrostis curvula, Panicum maximum, Digitaria eriantha) was investigated on a site with a natural parthenium infestation at Skukuza, Kruger National Park. The trial was conducted over two growing seasons on exclosure plots which eliminated mammal herbivory. P. maximum displayed best overall performance and was eventually able to completely overwhelm P. hysterophorus. Eragrostis curvula and D. eriantha grew more favourably in the second season after becoming better established but were clearly not well adapted to the trial conditions. Although P. maximum was the supreme interferer, all grasses were able to significantly interfere with P. hysterophorus growth in the second season. The ability of P. maximum to interfere with P. hysterophorus growth so efficiently that it caused mortalities of the latter species, indicates that P. maximum exhibits high potential for use as an antagonistic species in an integrated control programme. An investigation on the production dynamics of parthenin in the leaves of P. hysterophorus indicated that high levels of this compound are produced and maintained in the plant up until senescence. The high resource allocation priority of the plant towards this secondary metabolite even in the final growth stages may indicate the use of residual allelopathy to inhibit or impede the recruitment of other species. Studies on the persistence of parthenin in soil revealed that parthenin is readily degraded in soil and that microbial degradation appears to play a predominant role. Significant differences between parthenin disappearance-time half-life (DT50) values were observed in soils incubated at different temperatures and in soils with different textures. Exposure of the three grass species to pure parthenin showed that, in terms of their early development, the order of sensitivity of the grasses was: Panicum maximum>Digitaria eriantha>Eragrostis curvula. It may therefore prove challenging to establish P. maximum from seed in P. hysterophorus stands during the execution of an integrated control programme due to the sensitivity of this grass species to parthenin. From the research findings it appears possible that P. hysterophorus can inhibit or impede the recruitment of indigenous vegetation under natural conditions. At least one mechanism through which this alien species can exert its negative influence on other plant species is the production and release of parthenin. / Dissertation (MSc (Agric) Agronomy)--University of Pretoria, 2007. / Plant Production and Soil Science / unrestricted

Weed control is a major cost for growers in the sugarcane industry, especially for monocotyledonous species such as Cynodon and Rottboellia spp. The introduction of imazapyr-tolerant sugarcane would be advantageous as this herbicide has shown to be effective against the above-mentioned weeds but it also kills sugarcane. In a previous study in our laboratory, several sugarcane putative-mutant lines of variety N12 were generated by in vitro exposure of embryogenic callus to 16 mM ethyl methanesulfonate (EMS), followed by selection on imazapyr-containing medium. Tolerance to a low dose of imazapyr was confirmed in seven of those lines when the herbicide was applied (182 g a.i. ha-1) to 3 month-old plants in pots. The aim of the present study was to identify which of the seven herbicide mutant lines had agronomic characteristics at least equivalent to un-mutated N12. The objectives were to: 1) confirm tolerance to increased rate (312 and 625 g a.i. ha-1) of imazapyr in field plants; 2) measure the agronomic characteristics of these lines; 3) determine the effect of residual soil herbicide activity on germination of sugarcane setts. The seven mutant lines (Mut1-Mut7) and un-mutated N12 were clonally propagated in vitro by shoot multiplication followed by rooting and planted in three plots (untreated, sprayed with 312 or 625 g a.i. ha-1 imazapyr), in the field, in a randomized complete block design. In the untreated control plot there were no significant differences between the control and the mutant plants for agronomic traits (tiller number/plot, stalk height and stalk diameter) or estimated yield (kg/plot) after 10 months, indicating that the mutation process had no effect on general plant phenotype. In the sprayed (312 and 625 g a.i. ha-1) plots, Mut1, Mut4, Mut5, Mut6 and Mut7 plants showed tolerance to imazapyr as the leaves remained green compared with Mut2, Mut3 and N12 control plants, which displayed chlorotic leaves and eventually died in the plot sprayed with 625 g a.i. ha-1. Post-herbicide application, the yields of Mut5, Mut6 and Mut7 (52.33, 43.43 and 41.43 kg/plot, respectively) from the 312 g a.i. ha-1 plot were not significantly different from that of N12 control (53. 61 kg/plot) in the untreated plot. However, in the 312 g a.i. ha-1 plot, the yield and agronomic trait measurements of the untreated N12 control were significantly higher than those of the herbicide-susceptible plants Mut2 and Mut3. Similarly, in the 625 g a.i. ha-1 plot, the recorded yields for Mut4, Mut6 and Mut7 were 41.60, 43.44 and 36.30 kg/plot, respectively, indicating that their imazapyr tolerance and yield characteristics were comparable to the untreated N12 control. Imazapyr is conventionally applied to a fallow field 3-4 months prior to planting sugarcane as there is residual herbicide activity in the soil that suppresses sugarcane germination and growth. Therefore, in order to establish if the herbicide-tolerant mutants could germinate in iii an imazapyr-treated field, 3-budded setts of the mutant lines (Mut1-Mut7) and N12 control were planted in two plots, one unsprayed and one sprayed with 1254 g a.i. ha-1 imazapyr, 2 weeks previously. Germination was calculated after 3 weeks as the number of germinated setts in each plot/no. germinated setts in unsprayed plot x100. In the sprayed plot, the setts from Mut1, Mut4 and Mut6 displayed the highest germination percentages (60, 71 and 74%, respectively) compared with Mut2 (24%), Mut3 (46%), Mut5 (34%), Mut7 (40%) and the N12 control (12%). The in vitro acetolactate synthase (ALS) enzyme activity of 10 month-old plants from the untreated plot was assessed in the presence of 0-30 μM imazapyr to determine the herbicide concentration that inhibited ALS activity by 50% (IC50). The IC50 values for the mutated lines were between 3 and 30 μM, i.e. 1.5-8.8 times more tolerant to imazapyr than the N12 control plants, with Mut6 displaying the highest IC50 value (30 μM). On the basis of the results, it was concluded that Mut1, Mut6 and Mut7 lines were more tolerant to imazapyr than N12 and the other tested lines. Future work includes phenotypically assessing these lines for traits including sucrose content, fibre content, actual yield (tons cane ha-1) and altered pest and disease resistance. Once isolated and sequenced, the ALS gene conferring imazapyr tolerance can be used in genetic bombardment in the genetic modification approach as the gene of interest or as a selectable marker. In addition, the imazapyr-tolerant line can be used for commercial purposes in the field and as the parent plant in the breeding programme. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Durban, 2013.

The majority of South Africa’s fresh water (lotic and lentic), is eutrophic and this has resulted in water hyacinth, Eichhornia crassipes (C.Mart.) Solms. (Pontederiaceae) becoming South Africa’s most damaging aquatic macrophyte. Recently however, concerns have also been voiced over the presence of highly invasive submerged macrophyte species, such as Eurasian water-milfoil, Myriophyllum spicatum L. (Haloragaceae) in the Vaal River. Interaction studies between floating and submerged macrophytes have shown that floating macrophyte dominance restricts light penetration into the water column shading out submerged macrophytes while submerged macrophyte dominance reduces nutrient availability in the water column limiting floating macrophyte growth. This cycle ensures that these species cannot coexist in the same habitat for extended periods of time. The aims of this thesis were to: 1. Investigate changes in the historical and current macrophyte dominance in the Vaal River 2. Determine whether these changes could be attributed to stochastic events, such as floods and herbicide control measures. 3. The physio-chemical conditions of the water column, and whether pressure from herbivory by macroinvertebrates had possibly influenced Eurasian water-milfoil’s ability to dominate. Spatial and temporal analysis of satellite imagery revealed that water hyacinth and submerged macrophyte species dominated different regions of the study area over different periods of time from 2006 to 2010. This was significantly correlated with nitrate concentrations of the water column. One of the lower Vaal River Water Management Areas (WMA) had changed from a water hyacinth dominated state in 2006 to an alternative submerged macrophyte dominated stable state in 2008. It was concluded that this change could be attributed to: a stochastic flooding event in 2006; perturbation from integrated control measures implemented against water hyacinth; and low nitrate concentrations of the WMA. The lack of any substantial macroinvertebrate herbivory pressure or control measures implemented against Eurasian water-milfoil, compared to similar surveys conducted in the U.S.A. and its native range in Eurasia was shown to contribute to its dominance. Future successful integrated control programmes, including biological control against Eurasian water-milfoil, could provide the perturbation required to restore the ecosystem. However, without the reduction in nitrate concentration levels, water hyacinth will remain the dominant stable state of the rest of the Vaal River.

Interference of natural environment by invasive plants is a global concern. In South Africa and in particular Gauteng Province, interference of natural land by invasive plants that originated from other countries has been an endemic problem. These invasive plants pose a threat to biodiversity as a result of its wild and wide dispersion rate where it spreads into neighbouring Provinces such as Mpumalanga, Limpopo, North West and the Free State. Pompom weed is aggressive to control and can spread by means of both wind and water. This research project investigates problems associated with ineffective control of invasive plants in general and pompom weed in particular. State organs, Non Governmental Organisations (NGOs) and farming communities were identified as relevant respondents in this study. Three hundred (300) validated questionnaires were distributed to these stakeholders and 286 were adequately completed and received. These were analysed and the data interpreted. Results obtained showed that lack of coordination and teamwork from all stakeholders are responsible for ineffective control of invasive plants in the country. The use of biological control was recommended for the control and eradication of the invasive plants. / Environmental Sciences / M.A. (Environmental Management)

Interference of natural environment by invasive plants is a global concern. In South Africa and in particular Gauteng Province, interference of natural land by invasive plants that originated from other countries has been an endemic problem. These invasive plants pose a threat to biodiversity as a result of its wild and wide dispersion rate where it spreads into neighbouring Provinces such as Mpumalanga, Limpopo, North West and the Free State. Pompom weed is aggressive to control and can spread by means of both wind and water. This research project investigates problems associated with ineffective control of invasive plants in general and pompom weed in particular. State organs, Non Governmental Organisations (NGOs) and farming communities were identified as relevant respondents in this study. Three hundred (300) validated questionnaires were distributed to these stakeholders and 286 were adequately completed and received. These were analysed and the data interpreted. Results obtained showed that lack of coordination and teamwork from all stakeholders are responsible for ineffective control of invasive plants in the country. The use of biological control was recommended for the control and eradication of the invasive plants. / Environmental Sciences / M.A. (Environmental Management)