1. Factors affecting the tolerance of Conyza bonariensis towards Glyphosate
Glyphosate is the active ingredient in many foliar, non-selective, systemic herbicide brands which offers broad spectrum control of weeds. Commercial use of glyphosate started in 1974 and the herbicide quickly became very popular because it is highly efficient in controlling weeds and environmentally friendly. Since then glyphosate became the most extensively used herbicide in the world. After the first 20 years of glyphosate application, no glyphosate-resistant weeds were reported which led many to believe that resistance was unlikely to develop. In 1996 glyphosate resistance was observed for the first time inLolium rigidum (Rigid Ryegrass) in Australia. Since then 23 other weeds over the world have evolved resistance.
In South Africa producers are experiencing problems with controlling Conyza bonariensis and therefore resistance to glyphosate is expected. The title of the study is “Factors affecting the tolerance of Conyza bonariensis towards glyphosate herbicide”. Seed from Conyza bonariensis have been collected at various locations all over the country in order to establish or confirm whether the plants from those locations are resistant. Pot-trials have been conducted where seed from the various locations were screened in order to identify resistant biotypes. Another experiment to test the influence of growth stage has also been conducted. Further experiments will focus on the effects of temperature and phosphate deficiency on glyphosate resistance in the plants.
2. Glyphosate resistance in Conyza bonariensis in the summer and winter rainfall regions of South Africa; A morphological, physiological, and molecular perspective
There has been a positive development in herbicide-tolerant crops and rapid adoption of glyphosate-tolerant (Roundup Ready®) crops and associated transgenic technology. However, overuse of this single weed management technology is jeopardizing this safe, highly effective, and economical tool due to the emergence of new weed species that are only poorly controlled by glyphosate and the evolution of Glyphosate tolerant weeds. Management practices like tillage methods and crop rotations effectively disrupt the use life cycle of herbicides, thereby reducing selection intensity. However, long-term effects of application of glyphosate technology on changes in weed density and diversity in crop rotation systems in South Africa are poorly known.
The main objective of this study will be to carry out a baseline survey study of fields with a long history of glyphosate use to identify trial sites for further tests. These trial sites will use the glyphosate technology with Roundup Ready® maize-soybean rotations with tests done on identification and characterization of the weed species, plant survival and visual phytotoxicity, level of glyphosate resistance and the cost-benefit analysis of the technology. Both laboratory and pot experiments will be conducted for verification of field experiment results.
3. Colorimetric determination and quantification of shikimic acid in glyphosate-susceptible and resistant maize (Zea mays), soybean (Glycine max) and Conyza bonariensis plants by making use of an enzymatic assay and High Performance Liquid Chromatography (HPLC)
Glyphosate dominates world herbicide usage due to its broad spectrum, ease of use and environmental attributes. Glyphosate kills plants by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), a key enzyme in the shikimate biosynthetic pathway which is necessary for the production of the aromatic amino acids, auxin, lignin and many other secondary products. In sensitive plants inhibition of the EPSPS enzyme leads to the deregulation of the pathway leading to an overproduction and accumulation of shikimate in especially meristematic tissue where the shikimate pathway is most active.
In this project we will build on earlier work by Monsato, to refine and test the ‘in-field’ use of assays developed to determine and measure the levels of shilimic acid build up in weeds after the application of glyphosate. In this way shikimic acid buildup will be used as an early detection of possible development of resistance as well as to evaluate the effectiveness of herbicide application procedures in weed control.
The aim and objectives of Johan’s research study is to quantify the amount/concentration of accumulated shikimic acid in glyphosate susceptible and resistant plants treated with the herbicide glyphosate by making use of HPLC and then to develop a quick field test kit which could be used to determine the degree of resistance of a plant (especially resistant weed biotypes) to the herbicide based on a colour change.