The structures of three polysaccharides have been investigated, pectin and reduced pectin from cured leaf stems of Bright Virginia tobacco, and pectin from fresh leaves of Virginia tobacco plants. The hydrolysis products from cured tobacco pectin have been examined and the main components characterised and determined quantitatively. This pectin has an anhydrogalacturonic acid content of more than 90%. The polysaccharide has been subjected to periodateoxidation and to partial hydrolysis with a pectinase preparation. The latter yielded di-, tri- and tetra-galacturonic acids and the linkages in these compounds have been shown to be alpha-1,4. Diborane has been used to reduce the polysaccharide and the resultant material has been methylated and hydrolysed and the products characterised by vapour phase chromatography. The anhydrouronic acid content of pectin from fresh leaves is much lower (ca. 20%) and unlike the "cured pectin" readily yields aldobiouronic acids on partial hydrolysis with dilute mineral acids. The structures of three of these hydrolysis products have been examined. The chromatographic behaviour of D-galacturonic acid, the effect of sodium borohydride on two uronic acid derivatives and the effect of bromine and sulphuric acid on tartaric acids have been examined inrelation to the analytical work which was carried out with the pectins.

Thesis (MSc. (Agriculture)) --University of Limpopo, 2011 / Maize is the most important cereal crop grown in South Africa. This crop is produced throughout the country under diverse environments. The study only focuses on the technical efficiency because it is an important subject in developing agriculture where resources are limited, but high population growth is very common. Technical efficiency is the ability of a farmer to obtain output from a given set of physical inputs. Farmers have a tendency of under and/or overutilising the factors of production. The main aim of this study was to analyse the technical efficiency of small-scale maize producers in Ga-Mothiba rural community of Limpopo Province. The objective of the study was to determine the level of technical efficiency of smallscale maize producers and to identify the socio-economic characteristics that influence technical efficiency of small-scale maize producers in Ga-Mothiba. Purposive and Snowball sampling techniques were used to collect primary data from 120 small-scale farmers. Cobb-Douglas production function was used to determine the level of technical efficiency and Logistic regression model was used to analyse the variables that have influence the technical efficiency of maize production. Cobb-Douglas results reveal that small-scale farmers in Ga-Mothiba are experiencing technical inefficiency in maize production due to the decreasing return to scale, which means they are over-utilising factors of production. Logistic regression results indicate that out of 13 variables included in the analysis as socio-economic factors, 10 of them (level of education, income of the household on monthly basis, farmer`s farming experience, farm size, cost of tractor hours, fertiliser application, purchased hybrid maize seeds, membership to farmers` organisation, is maize profitable) were found to be significant and 3 (gender, age and hired labour) are non-significant. However, farm size was found to be the vi most significant variable at 99% level, showing a positive relationship to smallscale maize producer`s technical efficiency. Therefore, it is recommended that government should do the on-farm training since farmers mainly depend on trial and error and farmers` should have access to enough arable land and tractor services. However, farmers need to be trained on matters relating to fertiliser application, on the amount of seeds a farmer should apply per ha, and the importance of using hybrid seed.

Les effets de faibles doses de clomazone 2-(2-chlorobenzyl)-4,4-dimethyl-1,2-oxazolidin-3-one, un herbicide non-sélectif de la famille des isoxazolidinones, ont été étudiés chez Nicotiana tabacum L. De faibles concentrations de clomazone (1, 0.1, 0.01, 0.001, 0.0001 et 0.00001 µM), ont été appliquées chez deux variétés de tabac (Nicotiana tabacum L. cv. Xanthi; Nicotiana tabacum L. cv.Virginie vk51) cultivées en hydroponie. Le transport d'électrons photosynthétiques a été étudié par l'analyse de la cinétique rapide de fluorescence chlorophyllienne et par la mesure de l'évolution de l'oxygène chloroplastique. Les résultats ont montré que le clomazone à des concentrations de 1, 0.1, 0.01, 0.001et 0.0001 µM, affecte les paramètres JIP (Fv/Fm, PIabs, 1–VJ, ABS/RC, TR0/RC, DI0/RC, ET0/RC et ET0/ABS). Le clomazone à une concentration de 1 µM, conduit à une importante accumulation de H2O2 chez Virginie induisant la mort cellulaire programmée (PCD). Nos résultats montrent que la variété Xanthi présente une tolérance plus importante au clomazone que la variété Virginie, ce qui peut s'expliquer par une meilleure efficacité du transport d'électrons au delà de QA, ainsi qu'une amélioration du fonctionnement du système antioxydant enzymatique (APX et MDHAR). Le prétraitement des plantules de la variété Virginie par une alternance de cycles courts de lumière/obscurité (16 min/ 8min) pendant trois jours lorsque les plantules sont au stade de trois feuilles, a amélioré leur tolérance face à la toxicité du clomazone. Ce prétraitement induit l'activation des systèmes de détoxification des ERO via l'augmentation des activités des enzymes antioxydants et la biosynthèse de composés antioxydants tels que l'ascorbate et les composés phénoliques et réduit le niveau de mort cellulaire programmée, induit par le stress photooxydatif. Il est supposé que la stimulation des voies alternatives de dissipation des électrons, telle que la photorespiration, ainsi que les modifications du cycle circadien, soient prioritairement impliquées dans l'acquisition de la tolérance. Cet effet dit de "priming" se maintient tout au long de la croissance du tabac et se manifeste par une augmentation de la croissance et de la photosynthèse nette qui perdure jusqu'au stade de la floraison. / The effect of low doses of clomazone 2-(2-chlorobenzyl)-4,4-dimethyl-1,2-oxazolidin-3-one, a non-selective herbicide of the isoxazolidinone family, has been already determined in Nicotiana tabacum L. The plantlets of two varieties of tobacco (Nicotiana tabacum L. cv. Xanthi; Nicotiana tabacum L. cv. Virginie vk51) were subjected to low concentrations of clomazone (1, 10−1, 10−2, 10−3, 10−4 and 10−5) µM in a hydroponic system. The photosynthetic electron transport has been studied by analyzing the rapid kinetics of chlorophyll a fluorescence and measuring changes in the chloroplast oxygen evolution. The results showed that clomazone at concentrations of 1, 10−1, 10−2, 10−3 and 10−4 µM affected several parameters of chlorophyll a fluorescence and OJIP test (Fv/Fm, PIabs, 1-VJ, ABS/RC, TR0/RC, DI0/RC, ET0/RC and ET0/ABS). Clomazone at 1 µM led to a large accumulation of H2O2 in Virginie variety inducing programmed cell death. Our results show that Xanthi variety has a higher tolerance to clomazone as compared to Virginie, this can be explained by increasing the efficiency of electron transport beyond QA− (1–VJ) as well as improving the function of antioxidant enzymes (APX and MDHAR). The pretreatment of Virginie plantlets (three leaf stage) with alternation of short cycles (16 min/ 8min) of light/dark (AL) for three days, has improved its tolerance to clomazone toxicity. This pretreatment stimulated ROS-detoxification systems by increasing the activity of antioxidant enzymes and the biosynthesis of antioxidant compounds such as ascorbate acid and phenolic compounds, and consequently reduced the level of programmed cell death (PCD) provoked by the stress photooxydatif. It is suggested that the stimulation of alternative pathways of electron dissipation, such as the photorespiration, as well as changes in the circadian cycle, are primarily involved in the acquisition of the tolerance. This effect so-called "priming" has maintained during the growth of tobacco and has been revealed by increases in the growth and net photosynthesis until the flowering stage.