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The dynamics of bush thickening by Acacia mellifera in the arid Namibian Highland Savanna was investigated. First, a conceptual state-and-transition model was developed, based on preliminary findings, personal observations and resultant insights. In this model it was proposed that two main states exist, an open, grassy state and a bush-thickened state. Each of these is subdivided into other states. An unstable transitional state with A. mellifera seedlings within the grass sward is a crucial juncture between the grassy and bush thickened state. In the model, the transition to this unstable state occurs after at least two, but more likely three, consecutive years of well above-average annual rainfall through seed production followed by germination and establishment. Only an interruption by fire, which has a high probability of coinciding with this establishment if the grass sward is lightly utilised, prevents a further transition to a bush thickened state. Fire returns the vegetation to a grassy state by causing an almost 100 % mortality of seedlings. If fire is absent through a lack of fuel (overgrazing) or fire is deliberately excluded, the transition to a bush-thickened state is a fait accompli, but may take decades to reach. Transitions from the bush-thickened state to a grassy state require drought and the associated fungal dieback, which accelerates the senescence of mature shrubs. The model proposes that a transition towards the unstable transitional state occurs rarely, due to the rarity of suitable climatic conditions (protracted period of consecutive years of above-average annual rainfall). The mechanisms of two key transitions were tested. Firstly, the transition to an unstable state through the en masse production of seeds followed by the successful establishment of seedlings after a protracted period of well above average rainfall was tested during a nine-year period (late 1998 to early 2007). Secondly, the transition back to an open grassy state during a potential establishment event, through the mortality of seedlings after a fire, was tested experimentally (2008 and 2009). Both of these studies confirmed the predictions of the model and the mechanisms proposed for these transitions. Preliminary evidence suggests that browsing by small herbivores, in particular lagomorphs, thins resultant thickets out through herbivory. Preliminary evidence also suggests that competition between grasses and seedlings does not directly stop the transition to a bush thickened state but may prolong the window of opportunity for a fire to be effective, through reducing the growth rate of seedlings and saplings. The findings are of relevance to management, and thus an expert system for rangeland management, with emphasis on bush thickening, was developed, based largely on the findings of this research. Preliminary historical evidence casts doubt upon the prevailing perception that bush thickening is mostly a phenomenon of the last half century, and, consequently, that bush thickening is the primary cause of the loss of rangeland productivity in the arid rangelands of Namibia during this period. The study suggests that fire in arid savannas is as important as it is in mesic savannas. A general principle could be stated as follows: The importance of the timing of fire in savannas increases with increasing aridity, whilst the importance of the frequency of fire in savannas decreases with increasing aridity.

The industrial application of Wacker oxidation of terminal olefins in aqueous aerobic mixtures with PdCl2 and CuCl/CuCl2 has largely been limited to shorter chain alkenes, that is, ethylene. As the alkene chain length increases, so do the challenges that render the reaction inapplicable for large scale production. Longer chain alkenes tend to isomerize due to the limited solubility in organic-aqueous mixtures. More so, the use of co-oxidants such as CuCl or CuCl2 in stoichiometric amounts results in the formation of toxic chlorinated by-products which make the system corrosive. Pd0 aggregation from the PdII active state, is also pertinent in these reactions hence the use of large amounts of a co-oxidant. Small TONs and TOFs have subsequently been reported. As one of the approaches to curb these challenges, ligand support and modification has recently been viewed with interest because it promises efficient stabilization of Pd0, wherein the efficiency of O2 to re-oxidize the Pd0 species is relied upon thereby avoiding Pd0 aggregation. Ligand support can also be used to alter the electronic environment of the PdII centre thereby affecting its activity and selectivity. The application of phosphorus-palladium complexes in this study is not only a new approach in Wacker oxidation but the utilization of the Ï-accepting and or Ï-donating abilities of phosphorus compounds was also advantageous in altering the PdII electronic environment. No co-oxidants were used in this study w.r.t. the oxidation of 1-octene and the complexes evaluated were comparable to those reported in literature with PdCl2/DMA systems under similar conditions. Since oxygen is the preferred oxidant in all oxidation reactions because of its natural abundance, its reported enhanced selectivity and ease of separation from products, it was decided to evaluate the utilization of this reagent as first choice in the current investigation of ligand supported palladium catalysts in the Wacker oxidation. Due to the fact that the phosphite based palladium catalyst, PdCl2[P(OPh)3]2, is readily soluble in DMA, it was determined that no pre-stirring as for PdCl2 was required for this catalyst. In order to obtain the optimum reaction conditions for oxygen as oxidant with this catalyst, conditions like solvent, reaction temperature, O2 pressure and water, catalyst, and substrate concentration were varied. The optimized conditions were determined to be 0.5 mol% of catalyst in DMA:H2O (6:1) under 9 atm of O2 at 80°C, while the optimum substrate concentration was found to be 0.2M. PdCl2[P(OPh)3]2 showed the highest activity of the catalysts evaluated and gave a TOF of >1370 (mol/mol/hr), which compared favourably with other known catalysts like PdCl2, PdCl2(CH3CN)2 Pd(OAc)2, and Pd(CF3SO3)2 where TOFâs of 1429, 1420, 817 and 524 respectively, were obtained under the conditions optimized for PdCl2[P(OPh)3]2. While the palladium metallocycle [Pd(u-Cl)(C6H4O)(OC6H6)2]2 gave TOFâs (1380 mol/mol/hr) virtually the same as PdCl2[P(OPh)3]2, total conversion for the latter catalyst was only 93%, so it can be regarded as the second best of all the catalysts evaluated. The monomers thereof, PdCl[(C6H4O)(C6H6O)2P(OPh3)] and PdCl[(C6H4O)(C6H6O)2(PPh3)], revealed the least basic P(OPh3) to be more reactive (TOF >900 mol/mol/hr) than the TPP containing analogue, where the latter showed no activity within the first hour of reaction. While all the active catalysts showed good selectivities of >80%, the metallocycle [Pd(u-Cl)(C6H4O)(OC6H6)2]2 proved to be the best with a selectivity of 89%. Catalyst recyclability was also observed to at least 3 cycles, with selectivities maintained above 80%. No Pd0 âfall-outâ or aggregation was observed with any of the catalysts evaluated. For the palladium phosphinite catalysts 1,2-Ph(OPPh2)2PdCl2 and 1,3-Ph(OPPh2)2PdCl it was found that both were active in the Wacker oxidation of 1-octene albeit with very low rates for the latter complex (1,3-Ph(OPPh2)2PdCl). The low reactivity of 1,3-Ph(OPPh2)2PdCl was similar to that of the phosphines (PPh3)2PdCl2 and (3,5-CF3-PPh2Cl)2PdCl2 where (PPh3)2PdCl2 showed some conversion only after 3 hours and (3,5-CF3-PPh2Cl)2PdCl2 gave only 53% conversion after an hour. Through a comparison of the reactivity of 1,2-Ph(OPPh2)2PdCl2 with that of the hydrolyzed equivalent [μ-ClPd(PPh2OH)(PPh2O)]2, it seemed as if the phosphinite catalysts are prone to hydrolysis under the prevailing conditions as the final conversion of both these catalysts were almost the same (85 and 79% respectively). Hydrogen peroxide and tert-butylhydroperoxide (TBHP) were also evaluated as alternative oxidants with PdCl2[P(OPh)3]2 as catalyst and H2O2 was found to be the better of the two oxidants with conversion (99%), selectivity (86%), and TOF (1220) almost as good as those found for oxygen (100, 82% and 1370 respectively). In addition, the catalyst could also be recycled three times although degradation of the H2O2 was observed and additional peroxide (12 eq.) had to be added with each cycle of substrate. TBHP, however, suffered from moderate selectivities of only 60-65%, while the catalysts was deactivated during the first oxidation cycle and could therefore not be recycled at all. Although all phosphite catalysts promoted isomerization to internal 1-octene isomers to some extent, the cyclopalladated [Pd(u-Cl)(C6H4O)(OC6H6)2]2 catalysts proved to be the best in this aspect of the reaction w.r.t. oxygen as oxidant and led to very low quantities of isomerised products being observed (3 - 4%). It was also evident that the type and amount (for H2O2 and TBHP) of oxidant played a crucial role in enhancing or suppressing isomerization and hydrogen peroxide (at only 2% isomerization) was found to be the best oxidant in this regard followed by oxygen (13%).

Bacterial canker is among the most common bacterial diseases affecting the production of stone fruit trees in South Africa. As reported in previous studies, the causal agents associated with bacterial canker of stone fruit trees include Pseudomonas syringae pv. syringae and P. syringae pv. morsprunorum. Since these studies date back to the1980s, a current study is required to investigate which causal agents are responsible for bacterial canker of stone fruit trees in South Africa. In addition, plum tree decline was reported in the Western Cape late 2015. The cause of the tree deaths was unknown as dying plum trees were of different ages and cultivars. The assumption was that several factors were responsible for the plum tree mortality including bacterial canker, drought and ring nematodes. Anecdotal evidence from a report in Franschhoek in the previous year suggested that ring nematodes were linked to a bacterial canker outbreak which resulted in plum tree mortality. Therefore, the overall objective in this study was to determine the effect of drought and ring nematode stresses in predisposing plum trees to bacterial canker infection. Firstly, the causal agents of bacterial canker of stone fruit trees in the Western Cape were identified using a multilocus sequence typing approach based on four housekeeping loci. The strains isolated from symptomatic plant material of stone fruit trees were delineated into two phylogenetic groups, namely, P. syringae sensu stricto and P. viridiflava. The delimitation into the two respective genetic clusters was further corroborated by whole genome-based analyses. Furthermore, the Pseudomonas strains in the present study were found to have a relatively increasing pangenome comprising a small set of core genes and a large accessory genome. These strains also consisted of a small T3SE repertoire complemented by detection of additional virulence factors, such as biosynthetic clusters of three phytotoxins, i.e. phaseolotoxin, syringomycin and syringopeptin. Most of the Pseudomonas strains comprised a locus encoding an insecticidal toxin indicating a potential entomopathogenic ability in these strains that is yet to be proven. A disease survey was also conducted in 12 major plum-producing farms in the Western Cape evaluating the link between high ring nematode densities and bacterial canker outbreaks. The findings from the survey revealed a weak to no association between high ring nematode densities and bacterial canker infection. On the other hand, drought was shown to significantly influence both the ring nematode densities and the occurrence of bacterial canker infection. In contrast, the greenhouse study undertaken in this study showed that drought did not influence the occurrence of bacterial canker infection as lesion development of inoculated plum seedlings of two cultivars did not significantly progress under soil water deficit. However, the physiology of these plum seedlings was altered under water stress as illustrated by reduced net rate of CO2 assimilation, which reduces the photosynthetic rate in these seedlings. This study has shown that drought and ring nematode had very little impact on bacterial canker infection. It is now apparent that additional factors also contributed to the plum tree decline. / Thesis (PhD)--University of Pretoria, 2020. / NRF / Hortgro / UP / Microbiology and Plant Pathology / PhD / Restricted

The world food crisis, Britain's reliance on imported food and feedstuffs and balance of payments difficulties were some of the factors which lent weight to the call for increased self-sufficiency in Britain's agriculture in the 1970s. This project considers two main areas: an investigation of the impact of radical agricultural change, designed to increase self-sufficiency, on the balance of payments; and, an appraisal of the potential role of the food industry within a radically different food system" The study proceeded by: an examination of the principles of agricultural policy and its development in Britain; an overview of the mechanism and meaning of the balance of payments; a consideration of the debate on agricultural import saving; the construction of radical agricultural strategies; the estimation of effects of the strategies , particularly to the balance of payments the role of the food industry and possible innovations within the strategies; a case study of textured vegetable proteins and" the wider implications of implememtation of radical agricultural alternatives. Two strategies were considered a vegan system, involving no livestock and an intermediate system including some livestock and dairy cattle. The study concludes that although agricultural change could principle make a contribution to the balance of payments, implementation of agricultural change cannot be justified for this purpose alone. First, balance of payments problems can be solved by more appropriate methods. Second, the UK' s balance of payments problem haa disappeared for the time being owing to North Sea oil and economic recession. Third, the political and social consequences of the changes investigated would be unacceptable. Progress in UK food policy is likely to be in the form of an integrated food and health policy.

L'environnement géologique a-t-il accumulé et conservé les molécules liées à la biominéralisation des carbonates qui forment les dépôts calcaires ? Si oui, ces molécules ont-elles conservé une activité biologique? La sédimentologie montre que la Provence possédait au Crétacé il y a 120 millions d'années, un rivage tropical bordé d'une plateforme carbonatée où se sont accumulés des biocalcaires d'une grande pureté comme à Orgon. Sur ce site, le calcaire piège les molécules organiques au cours de la diagenèse sous forme d'inclusions dans la calcite des rudistes (principaux producteurs de carbonates) grâce à un processus de dissolution/recristallisation centrifuge. Plusieurs modes opératoires ont été testés afin d'extraire ces molécules. Ils permettent d'obtenir deux grands types de complexes organo-minéraux: un extrait plutôt lipidique, obtenu essentiellement à l'alcool et l'autre plutôt protéique obtenu à l'eau. Leur activité a ensuite été testée sur la peau et sur l'os. L'activité dermatologique est démontrée sur des explants de peau humaine maintenus en survie. On observe un renouvellement de l'épiderme dès 9 jours sans dérèglement, une restructuration de la jonction dermoépidermique et dans le derme un réseau de collagène densifié ainsi que la production d'acide hyaluronique. Sur l'os, les extraits ont stimulé la différenciation des cellules de la lignée ostéoblastique et également le dépôt d'hydroxyapatite à partir de culture de cellules souches. Ce travail démontre que des molécules vieilles de 120 millions d'années sont préservées, exploitables et qu'elles possèdent une série d'activités à visée thérapeutique sur la peau et sur l'os.

Recent studies have suggested that minimal actomyosin systems have the intrinsic property to oscillate whensubjected to an elastic load. A similar situation can be found in various biological systems, leading, both in-vivoand in-vitro, to spontaneous oscillations. In particular, muscular systems as well as mechanosensitive hair-cellbundles in the inner ear have been shown to oscillate spontaneously as the result of active force production by anacto-myosin protein complex. We attempt to shed light on the mechanism behind the oscillatory activity of theacto-myosin system, in particular by determining the parameters that control the frequency and amplitude ofoscillation. The stiffness of the system, the total force developed by the motors and the type of motors have beenproposed as being influential in this respect. To investigate this effect, we make use of a modified motility assayconsisting of a motor-driven stiff polarized actin bundle subjected to an elastic load provided by opticaltweezers. During the course of this work, we also characterized auto-assembled magnetic bead columns andassessed their viability as molecular force sensors to study the oscillations. The fact that they can easily beorganized into large arrays makes them interesting as potential 'high-throughput' force sensors

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This study examined the factors that influence minorities to enroll in agricultural science programs at Virginia Tech. Minorities have been and continue to be under-represented in most areas of the agricultural sciences. The presence of African-American and other minority students in the agricultural sciences is, therefore, very important. Enrollment numbers in programs across the country indicate that agriculture, does not appeal to many minority groups. Wardlow, Graham, & Scott (1995) noted that there is little doubt that minorities are underrepresented in academic departments at land-grant universities, professional roles in agricultural industries, and in governmental agencies such as the United States Department of Agriculture (USDA). The data collection in this study was through a survey developed by the researcher. The population in this study consisted of minority undergraduate and graduate students in the Minority Academic Opportunities Program (MAOP) and Minorities in Agriculture, Natural Resources, and Related Sciences (MANRRS) organization at Virginia Tech. The respondents in this study selected agricultural science as a major due to their personal decision, input form family members and friends, and the guidance form a high school or faculty member. They also noted there was opportunity for them in the agricultural sciences and felt that professors, administrators and students within the department were fair and supportive. Financial assistance was an important factor in the student's decision enrolling in the program, and the majority of the respondents were receiving financial assistance. However, the respondents noted that they would be enrolled if they were not receiving financial assistance and felt they had the opportunity to succeed in the agricultural sciences at Virginia Tech / Ph. D.

La voie de signalisation HH joue un rôle crucial dans le contrôle de la prolifération et la différenciation cellulaires. Le dérèglement de cette voie est responsable de nombreux cancers. J'ai ainsi mis en place les outils moléculaires nécessaires pour identifier des régions importantes dans les interactions protéines-protéines au sein du complexe intracellulaire de la voie qui vont permettre de mieux comprendre les mécanismes de régulation du facteur de transcription CI en fonction de l'état d'activation de la voie. Mes travaux ouvrent la voie à de nombreuses études structurales et fonctionnelles de ces protéines jusqu'ici peu étudiées.