It has been suggested that the Category 1 weed, Campuloclinium macrocephalum (Less) D.C has allelopathic potential, which would, at least partially, explain its apparent success as an alien plant in South Africa. Studies were done on the plant’s root, stems and leaves to determine where the strongest allelopathic potential can be found. Once it was determined that the leaves held the strongest potential, bioassay studies were conducted on lettuce (Lactuca sativa), Eragrostis tef, Eragrostis curvula and Panicum maximum with positive results found for C. macrocephalum’s allelopathic potential. Electron microscopy was performed to determine whether allelopathic substances originate and/or are stored on the surfaces of the leaf. Positive results proved that there are possible sources of allelochemicals on both adaxial and abaxial surfaces of young and mature leaves. A dipping experiment involving dichloromethane then followed to determine the solubility of the contents of the glands found on the leaf surfaces. It can be deduced from results of all of the experiments performed that C. macrocephalum is potentially allelopathic to dicotyledonous species and to grasses. Structures found on the leaves of the plant could possibly contain the allelochemicals used by the plant to ensure its successful invasive growth habits in South Africa. The allelopathic effects that this weed will have on desirable species should be considered within the broader context of its ability to interfere with those species. In this regard its competitive ability should also be studied. Campuloclinium macrocephalum is fast invading susceptible areas of South Africa; if continuous research on control and eradication of this plant is not carried out soon, the country could suffer grave economic losses. Copyright / Dissertation (MInstAgrar)--University of Pretoria, 2009. / Plant Production and Soil Science / unrestricted

Kenaf (Hibiscus cannabinus L.) was introduced in 2005 as a fibre crop on a commercial scale in the KwaZulu-Natal Province of South Africa. No herbicides have yet been registered for use in this crop. The purpose of this study was to determine the tolerance of kenaf to a total of five pre-emergence and four post-emergence herbicides under semi- and fully controlled conditions. The herbicides were chosen based on their potential safety for use in Hibiscus spp. as well as on the weed spectra they are registered for in other crops. Several additional factors were also taken into consideration, such as: temperature, soil depth and timing of herbicide application. Four pot trials were conducted to determine the separate and combined effects of herbicide, temperature, planting depth and application timing. During the first trial the effects of five pre-emergence herbicides and four post-emergence herbicides were researched. The pre-emergence herbicides were: S-dimethenamid, imazethapyr, fluometuron/prometryn, pendimethalin, S- metolachlor and the post-emergence herbicides were: bentazone, 2,4-DB, monosodium methanearsonate and pyrithiobac sodium. The trials were conducted under either semi-controlled conditions in a glasshouse or in growth cabinets under fully controlled conditions at the Hatfield Experimental Farm of the University of Pretoria. All experiments were conducted with a Hutton soil with 22% clay. Each trial lasted about 40 days to allow for maximum phytotoxicity damage manifestation on the kenaf seedlings. Measurements that were taken included plant height, herbicide damage, weed control efficiency, fresh plant weight, dried plant weight, and dried root weight. The data were subjected to analysis of variance (ANOVA) to determine the statistical likelihood of damage to plants from the herbicides. In Trial 2, 3 and 4 the interaction effects of herbicide and plant depth, herbicide and temperature, and herbicide and application timing were researched respectively. Neither planting depth nor application timing affected the kenaf seedlings negatively, but low temperature in combination with the application of herbicides during germination of seed and seedling emergence had serious deleterious effects on the young kenaf seedlings. Based on the findings the majority of the herbicides can be included in further field trials on Hibiscus cannabinus L. with the exception of S-dimethenamid and fluometuron/prometryn which caused substantial injury to the kenaf seedlings. Copyright / Dissertation (MInstAgrar)--University of Pretoria, 2011. / Plant Production and Soil Science / unrestricted

The overall objective of this thesis was to improve our understanding of the light environment associated with two of the major deciduous competitors of commercial conifer seedlings in low elevation coastal British Columbia, and the physiological response of young conifers to specific environments and changes in the environment. Studies were conducted to investigate: 1) seasonal and herbicide-induced changes in the light regimes beneath the canopies of two brush species, 2) growth and physiology of seedlings of three conifer species in several different deciduous brush environments throughout the year, and 3) changes in the physiology and growth of suppressed conifer saplings when different methods and schedules were used to reduce shading from an overtopping red alder canopy. Seasonal changes in the light quantity and quality were documented within salmonberry (Rubus spectabilis Pursch) and below red alder (Alnus rubra Bong.) canopies. The prediction of light attenuation within salmonberry canopies using the Beer-Lambert law was good for foliated canopies using leaf area index, and defoliated canopies using stem area index. As salmonberry leaf area increased, the ratio of red to far-red light declined exponentially. Growth of grand fir (Abies qrandis (Dougl.) Forbes), western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-fir (Pseudetsuga menziesii (Mirbel) Franco) seedlings decreased with increases in the canopy density of overtopping deciduous species. Physiological differences between conifer species and differences between overtopping canopy treatments declined in the autumn and winter. Based on the physiological measurements, suppression of the shade intolerant Douglas-fir seedlings was greatly reduced during the period that overtopping deciduous canopies were leafless. Animal damage reduces growth and probably increases the duration seedlings are under overtopping canopies. Suppressed Douglas-fir saplings released from overtopping by red alder canopies showed the greatest growth response when the alder canopies were removed in the spring; complete removal of the alder canopy during the summer had a predictably deleterious effect on the sapling physiology and growth. Herbicide injections resulted in a slower but predictable increase in conifer growth. / Forestry, Faculty of / Graduate

Conyza bonariensis, flaxleaf fleabane, is a major weed threat on cultivated and non-cultivated lands, gardens, roadsides and waste places. The weed in South Africa is believed to have originated from South America, and the first herbarium sample is from a plant collected in May 1895 at Franschhoek. Adding to its problem status is the recent discovery that certain C. bonariensis biotypes in South Africa and other parts of the world are resistant to the herbicide glyphosate, and in certain cases to both glyphosate and paraquat. Despite its invasiveness and ability to compete severely with crops, the mechanisms of interference (= allelopathy + competition) employed by C. bonariensis are poorly understood and have not yet been thoroughly investigated. There is a need to expand on the knowledge of interference mechanisms of C. bonariensis in order to better understand its success as a weed, and to improve on knowledge for the successful management of this weed. In the present study, allelopathic potential of C. bonariensis was assessed, first by means of germination bioassays, followed by investigation employing hydroponics, leachate, and replacement series experiments. In a laboratory bioassay, the plant’s leaves and roots were extracted using two solvents, water and hexane, to which seeds of the test (acceptor) species lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum) were exposed in order to determine where the strongest allelopathic potential resides. Moreover, differential potency of crude extracts prepared with the two solvents (polar and non-polar) would at least provide some evidence on the nature of putative allelochemicals involved. Germination bioassays revealed that leaves harboured the strongest allelopathic potential (potency). Water extracts (infusions) caused greater growth inhibition of the test species than hexane extracts. Osmolalities of the water infusions were tested and found not to be inhibitory to germination and early seedling development of lettuce. Following on the germination bioassays, a hydroponic experiment was set up in a greenhouse in order to investigate whether C. bonariensis possesses and releases chemicals with allelopathic potential through its roots. Lettuce top and root growth was significantly reduced by all three populations of C. bonariensis (one from Pretoria; two from the Western Cape). No significant differences were observed in the degree of growth inhibition caused by the three weed populations on the growth of lettuce, except in the case of root dry mass results where the Hatfield population caused more damage (85% growth reduction). The leachate experiment was then performed to determine if leachate from C. bonariensis affected the growth of test species exposed to different leachate concentrations. Although there was no growth inhibition observed for both lettuce and tomato in this experiment, growth stimulation of tomato roots was observed at the highest leachate concentration (100%). Finally, in an attempt to simulate the allelopathic potential of C. bonariensis in a natural field situation, a replacement series experiment was conducted to determine the relative interference of Conyza bonariensis in relation to lettuce and tomato. Dry mass results showed that there was no growth inhibition of both crop species. RYT was > 1 at all weed: crop combinations, which implies that both crop species and C. bonariensis were less affected by interspecific interactions than in their respective monocultures. It is suggested that the results of this study can attributed to methodology and growth media. The results of this study represent the first step in showing that allelopathic potential C. bonariensis may contribute to the success of this weed as an invasive weed species and that this weed should not be allowed to attain significant biomass on crop field. Further research should include field trials that will yield a better understanding of the practical relevance of the allelopathic potential of C. bonariensis. Finally, crop producers and weed management practitioners should recognize that this important weed has the ability to interfere with the growth and development of a crop through two mechanisms, competition plus allelopathy. / Dissertation (MInst Agrar)--University of Pretoria, 2013. / gm2014 / Plant Production and Soil Science / unrestricted

Loblolly pine (Pinus taeda) and coppiced sweetgum (Liquidambar styraciflua) seedlings were grown in competition with a native weed community using soil and seed bank collected near Appomattox, Virginia. Seedlings and weeds were exposed to CO₂ (ambient and elevated) and water (water stressed and well watered) treatments for approximately one growing season in closed top chambers. Weed growth had an effect on tree growth, but the amount of variation in tree biomass explained by weed biomass was very low. It appears that the tree seedlings benefited more from available resources than the herbaceous weeds. The influence of competition with loblolly pine and elevated CO₂ did not have an influence on total weed biomass; however, it did favor C3 weed community development regardless of water availability. This suggests that weed community composition may shift toward C₃ plants in a future elevated CO₂ atmosphere. Loblolly pine height, diameter, needle, shoot and total biomass were significantly greater in the well watered treatment than the water stressed treatment. Pine root, needle, shoot and total biomass were significantly greater in the elevated treatment than the ambient treatment. While not significant, root biomass of water stressed pine seedlings was 63% greater in the elevated CO₂ treatment than the ambient treatment. There was a significant water and CO₂ interaction for pine root:shoot ratio. Under elevated CO₂, root:shoot ratio was significantly greater in the water stressed treatment than the well watered treatment. In contrast, root:shoot ratio in the ambient treatment was nearly identical under both water treatments. These results indicate that loblolly pine will respond favorably in an elevated CO₂ atmosphere, even under dry conditions. The coppiced sweetgum seedlings responded favorably to well watered conditions with significant increases in leaf area, specific leaf area, leaf, shoot and total biomass compared to water stressed conditions. Leaf, root, shoot+stump and total biomass of sweetgum significantly increased and specific leaf area decreased under elevated CO₂ compared to ambient CO₂, but differences were smaller than previous findings. This indicates that coppicing may dampen the growth response to elevated CO₂, at least in the initial growth stage after coppicing. / Master of Science

Phalaris arundinacea L. is an aggressive species that can dominate wetlands by producing monotypic stands that suppress native vegetation. In this study invasion windows were created for native species in monotypic stands of P. arundinacea with either fire or herbicide. Three native species groups, herbaceous plants, herbaceous seeds, and woody shrubs, were planted into plots burned or treated with herbicide in the early spring. Fire did not create an effective invasion window for native species; there was no difference in P. arundinacea root and shoot biomass or cover between burned and control plots (p ≥ 0.998). Herbicide treatment created an invasion window for native species by reducing P. arundinacea root and shoot biomass for two growing seasons, but that invasion window was fast closing by the end of the second growing season because P. arundinacea shoot biomass had nearly reached the shoot biomass levels in the control plots (p = 0.053). Transplant mortality, frost, and animal herbivory prevented the herbaceous species and woody seedlings from becoming fully established in the plots treated with herbicide during the first year of the experiment. Transplanted monocots had a greater survival than dicots. By the second growing season the herbaceous group had the greatest mean areal cover (5%), compared to the woody seedlings (3%) and seed group (0%). Long-term monitoring of the plots will determine whether the herbaceous transplants will compete effectively with P. arundinacea and whether the woody species will survive, shade the P. arundinacea, and accelerate forest succession.

Renewed interest in hemp (Cannabis sativa L.) for cannabinoid production has highlighted critical knowledge gaps for growers. The impacts of weed interference on hemp yield are largely unstudied despite causing serious economic losses in most cropping systems. Furthermore, understanding factors determining cannabinoid content of hemp will be crucial for effective production. To evaluate the effects of weed competition on hemp yield and cannabinoid content, hemp growing in plasticulture was subjected to competition from 5, 3, 1, or 0 waterhemp (Amaranthus tuberculatus (Moq.) Sauer) plants. Hemp biomass and cannabinoid content were not significantly affected by weed competition. Yields in weedy and weed-free treatments were generally comparable, although unexpectedly high variation limited the ability to detect subtle differences between treatments. Waterhemp biomass was significantly reduced by competition from hemp in comparison to hemp-free controls. Suppression of weed growth by hemp and lack of significant yield loss from weeds suggests that hemp can be highly competitive and grown successfully without herbicides in certain circumstances. Abundance and documented phytotoxicity of hemp secondary metabolites suggest a potential for allelopathic activity. While incorporated hemp residue was not more effective than a maize control at reducing plant growth, hemp residue and extracts effectively inhibit seed germination. A laboratory assay showed that crude extracts of hemp can reduce the germination of a Brassica napus L., a bioindicator. Furthermore, a greenhouse experiment showed that small amounts of hemp residue on the soil surface can effectively reduce and delay the germination of waterhemp. Together, these results show that hemp residue has the potential to be incorporated with a practice such as chaff-lining for enhanced control of germinating weed seeds.

Some of the factors influencing the performance of bromoxynil, 4(2,4-DB), or a companion crop for weed control in seedling alfalfa were studied in the greenhouse and at field locations in Farmington and Logan , Utah. The effect of application rate , stage of growth, temperature, and soil moisture on the phytotoxicity of bromoxynil and 4(2,4-DB) to alfalfa and weeds were studied. Alfalfa yields were increased by 4(2,4-DB) at all rates and stages of application. All rates and stages of bromoxynil treatment except the one-fourth pound per acre three to four trifoliate application resulted in alfalfa yields below that of the control. The use of a companion crop was not conducive to the growth and development of alfalfa although it did control the weeds . Bromoxynil at all rates and at both stages of application resulted in effective mustard control. Mustard control in the 4(2,4- DB) plots was excellent at the early stage of application but required three- fourths pound per acre for control at the later stage of weed growth . Pigweed control was rather ineffective in bromoxynil plots at Logan where moisture was optimum, but effective in plots at farmington where moisture was limited for 18-20 days following application. The density of the pigweed stand in bromoxynil treatments at Logan was attributed to an influx of weed growth following initial control of weeds . Control of pigweed by 4(2 , 4- DB) was in excess of 90 percent at the four to five leaf stage of weed growth.

The application of a sequential test, the sequential probability ratio test, for the tolerances of noxious weed seeds is studied. It is proved that the sequential test can give a similar power curve to that of the current fixed sample test if the test parameters are properly chosen. The average sample size required by a sequential test, in general, is smaller than that of the existing test. However, in some cases it requires relatively a larger sample than current test. As a solution to the problem a method of truncation is considered. A kind of mixed procedure is suggested. This procedure gives almost an identical power curve to the standard one with great savings in sample size. The sample size is always less than that of the current test procedure.

Weed management options in agronomic crop production have been severely limited by widespread populations of weeds resistant to herbicides, including waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] resistant to foliar applications of herbicides that inhibit protoporphyrinogen oxidase (PPO; EC 1.3.3.4) activity (PPO-R). Herbicides within this site of action (WSSA site of action #14) remain efficacious when soil-applied to PPO-R waterhemp populations. Therefore, the continued use of these herbicides for soil-residual control of PPO-R waterhemp, especially in soybean production, is paramount with limited postemergence herbicides that remain effective. An improved understanding of the selection for PPO-R waterhemp would provide information to help minimize future loss of residual PPO-inhibiting herbicide activity. Five studies, consisting of 14 experiments, were conducted to improve our understanding of the selection for herbicide-resistant individuals. Soil-residual herbicides have been suspected to select for herbicide-resistant individuals; however, this phenomenon has never been observed experimentally in field conditions. This dissertation provides direct evidence from greenhouse and field experiments that significant selection pressure can occur from soil-residual herbicides; however, this selection for resistance could be delayed when using full commercial herbicide rates and effective herbicides from multiple sites of action. Also, the frequency of heterozygous individuals (RS) and PPO-inhibiting herbicide efficacy on RS individuals is a factor in the selection for herbicide resistance; however, current information on the these topics is limited. To provide additional information on RS individuals, a large-scale genotypic and phenotypic screen of multiple PPO-R waterhemp populations was conducted. It was determined that RS individuals were less frequent than expected and PPO-inhibiting herbicide efficacy on RS individuals was population-dependent. Finally, the hormetic effects of soil-residual herbicides have been paradoxically implicated as a means of both mitigating and exacerbating the selection for herbicide resistant biotypes; however, limited information was available on the hormetic effects of soil-residual PPO inhibitors. Greenhouse and growth chamber experiments were conducted to improve understanding of hormesis and soil-residual PPO-inhibiting herbicides. Experiments indicated that PPO-inhibiting herbicides may exert a limited hormetic effect on waterhemp germination below doses that cause a phytotoxic effect of the emerging seedling, indicating this effect may exacerbate the issue of selection for PPO-R waterhemp. Overall, data presented in this dissertation provides important information on the under-studied interaction between PPO-inhibiting herbicides and PPO-R waterhemp to safeguard the sustained efficacy of herbicides within this site of action.