Doctoral Degrees (Agronomy)

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Browsing Doctoral Degrees (Agronomy) by browse.metadata.advisor "Reinhardt, Carl Frederick"

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    Factors influencing inhibition of glutamine synthetase enzyme in grass weeds by glufosinate ammonium under different temperatures
    (Stellenbosch : Stellenbosch University, 2019-12) Mucheri, Tendai; Pieterse, P. J.; Reinhardt, Carl Frederick; Kleinert, Aleysia; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Evolution of weed resistance emphasized the need to implement integrated weed management strategies, however, farmers still immensely rely on chemical weed management. Glufosinate ammonium is an alternative herbicide that can replace or be used in rotations with herbicides such as glyphosate and paraquat, but it poses a problem due to its inconsistencies in controlling weeds. Studies in this dissertation aimed to investigate the influence of temperature on glufosinate ammonium efficacy. Chapter 3 of this dissertation investigated the influence of temperature on ryegrass cuticle thickness, phenolic acid concentration and calcium accumulation, and subsequently, the effect of the afore-mentioned factors on glufosinate ammonium efficacy. Ryegrass was grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The grass was treated six weeks after planting and assessment was done four weeks after glufosinate application. Control of ryegrass decreased with increasing temperature. Results indicated that cuticle thickness and calcium content increased as temperatures increased, probably due to production of phenolic compounds responsible for plant defence mechanisms against herbicide stress, hence resulting in poor control of ryegrass under warmer temperatures. Chapter 4 investigated ammonia accumulation, glutamine synthetase, glutamate dehydrogenase, nitrate reductase activity and ryegrass photosynthesis in roots and leaves of control (0 L ha-1) and treated ryegrass (4.5 L ha-1) harvested 24 hours after glufosinate ammonium application. There was a significant increase in glutamine synthetase enzyme activity with increasing temperature after glufosinate ammonium application. Better control of ryegrass under cooler temperatures with glufosinate ammonium was mainly attributed to the plants’ inability to produce adequate glutamate and α-ketoglutarate, which form the carbon skeleton for transamination processes. Response of glutamine synthetase to glufosinate ammonium was significantly higher at warmer temperatures, such that the use of an alternative glutamate dehydrogenase pathway was not vital. The significant increase in glutamine synthetase activity in ryegrass under warm temperatures was able to circumvent photosynthetic inhibition. A comparative study to investigate the response of different grass weed species to glufosinate ammonium was conducted on ryegrass (Lolium spp.), ripgut brome (Bromus diandrus L.) and wild oats (Avena fatua L) in Chapter 5. The grasses were grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The study observed that glufosinate ammonium control differed among weed species. Control of ryegrass increased with decreasing temperature. Temperature had no effect on wild oats. Control of ripgut brome was initially poor at 10/15 °C then increased at 15/20 and 20/25 °C and finally decreased again at 25/30 °C. Such differences in the grass response to glufosinate ammonium, even after being grown under the same conditions, was attributed to their differences in morphological characteristics such as cuticle thickness, calcium accumulation and photosynthesis after herbicide application. Increase in cuticle thickness decreased mortality of all grasses. The study perceived that negative effects of calcium on mortality can only be noticed if the cytosolic and mitochondrial calcium is mobile and active, thus, allowing it to react with glufosinate ammonium. A possible solution to mitigate problems arising from calcium level, cuticle thickness and phenolic compounds was investigated in Chapter 6. The study investigated the role of adjuvants in increasing glufosinate ammonium efficacy. Ryegrass was grown at 20/25 °C and treated with 0, 1, 2, 3 and 4 L ha-1 glufosinate ammonium. Glufosinate ammonium was applied solo and in tank mixtures with Velocity Super™ (ammonium sulfate, L 9603), Summit Super (nitrogen solution/non-ionic surfactant, L 8539) and Class act NG™ (ammonium sulfate plus a non-ionic surfactant, L 10477). Better control of ryegrass was observed when treated with glufosinate ammonium in a tank mixture with Class act NG™ and Velocity Super™ than its solo application as well as in a tank mixture with Summit Super. Ammonium sulfate exhibits surfactant and humectant properties and it facilitates movement of glufosinate ammonium into the plant while non-ionic surfactants aim to reduce water surface tension only. This explains better control observed with glufosinate ammonium in tank mixture with adjuvants containing ammonium sulfate than with Summit Super. The study suggests that adjuvant Class act NG™ and Velocity Super™ can be used to mitigate the defensive response of phenolic compounds after glufosinate ammonium application, hence, increasing its efficacy. The practical relevance of glasshouse observations in Chapter 3, 4, 5 and 6 was confirmed in Chapter 7. The study was conducted under rainfed conditions at Langgewens and Roodebloem farms in 2018 and 2019. Glufosinate ammonium was applied at different times of the day (8:00 am, 12:00 pm and 5:00 pm). The dosage rates applied were 0, 2.5, 5 and 7.5 L ha-1. The study observed that morning (8:00 am) and evening (5:00 pm) applications showed better control of ryegrass than mid-day application provided relative humidity during application time was greater than 75%. Application at mid-day (when temperatures were higher than morning temperatures) showed good control only if relative humidity was recorded above 80%, however, higher dosage rates of 5 or 7.5 L ha-1 were required to achieve greater than 90% control.