The predictive values of standard and non-standard germination temperatures, standard accelerated ageing (AA) and saturated salt accelerated ageing (SSAA) tests were investigated. Germination tests were performed at standard and non-standard temperatures of 10°, 20°, 20°/30° and 30°C for cabbage and tomato, and 10°, 15°, 20° and 30°C for onion. The AA and SSAA tests were conducted using four relative humidities: standard AA (RH100), NaCl (RH75), Ca(NO3)2 (RH43) and MgCl2 (RH32). Emergence trials were also conducted at a range of temperatures (winter, 15°/25° and 30°C) and media (Hygromix and soil) using seedling trays. Correlations were calculated to evaluate the relationship between laboratory and emergence test results. Three onion seed lots that have different vigour levels were primed with PEG or NaCl and were germinated and planted under varying temperatures and levels of salinity. There were vigour differences among different seed lots of cabbage, onion and tomato seeds used in this study. Using the different laboratory tests, seed lots were distinguished as high, medium and low vigour seed lots. The low temperature germination test (10°C) for cabbage and onion; and 20°C for tomato were found to be effective for separation of seed lots according to their vigour levels. All SSAA vigour tests were also useful in differentiating seed lots based on their physiological stage of deterioration. Using the standard AA (RH100) the seed moisture content was high (29 to 45%), resulting in fungal growth. However, in the SSAA test the moisture content was below 14% and no fungal growth was observed. Ageing of seed lots using NaCl and MgCl2 resulted in a low narrow range of moisture contents, but using Ca (NO3)2 the range of the moisture content between seed lots was higher. For all crop seeds, there was no significant difference in the percentage emergence between the two growth media but larger seedlings were observed from the commercial growth medium (Hygromix). For cabbage, highly significant correlations were obtained between germination percentage at 10°C and RH32 and all emergence trials. In onion, the correlations were not consistent; highly significant correlations were observed from germination capacity (GC), 15°C, 20°C, 30°C, RH75 and RH32; and emergence parameters at specific conditions. However, the germination percentage from 30°C correlated significantly with the final emergence percentage and seedling vigour index. Germination of tomato seeds at 20°C and all SSAA had highly significant correlation with all emergence parameters. The germination rate parameters: MGT (mean germination time) and GI (germination index) were valuable in categorising seed lots with moderate vigour levels, however, except GE in cabbage and onion, all other germination rate characters failed to correlate with any emergence parameters. The radicle percentage germination (viability or GC) and final emergence percentage (FEP) were not enhanced by priming in all seed lots in the germination and emergence trials at all temperatures. The percentage of normal seedling was increased due to priming when seeds were subjected to low temperature and higher salinity levels. Priming was effective in improving the rate, uniformity of germination/emergence; and increased the seedling dry mass of onion seeds when grown under relatively cold environmental conditions. Priming was more beneficial for low vigour seed lots (seed lot A) than for high vigour seed lots (B, C). Priming also improved germination rate and uniformity at all salinity levels, but faster rate of emergence, more uniform and higher seedling dry mass were only observed when seeds were watered with low salinity levels (25 mM of NaCl). No significant difference was observed using NaCl or PEG as priming agent. Onion seeds are tolerant to salinity during germination and the effect of priming was more beneficial at emergence. / Dissertation (MSc (Agric) Horticulture)--University of Pretoria, 2007. / Plant Production and Soil Science / unrestricted

<p>U radu su sprovedena ispitivanja kvaliteta semena samooplodnih linija kukuruza stvorenih u Institutu za kukuruz &bdquo;Zemun Polje&ldquo; razliĉitih grupa zrenja i genetiĉke osnove. U laboratorijskim i poljskim uslovima utvrĊena je klijavost i vigor semena iz 2011. nakon ĉetiri godine ĉuvanja u kontrolisanim uslovima (18 &deg;C i 60% RH) i semena iz 2014. nakon godinu dana ĉuvanja na 5 &deg;C i 60% RH.<br />Cilj je bio utvrditi tolerantnost odabranih linija na niske temperature prilikom klijanja i nicanja, dugoveĉnost semena u kontrolisanim uslovima ĉuvanja, genetiĉku divergentnost linija primenom proteinskih markera i proceniti uticaj genetiĉke osnove na kvalitet semena.<br />U laboratoriji za ispitivanje semena praćeni su sledeći parametri kvaliteta: vitalnost, klijavost, energija klijanja, dužina klijanaca, vigor ispitan tetrazolium testom, hladnim testom, testom ubrzanog starenja, testom pojave primarnog korena i testom provodljivosti.<br />Primenom vigor testova na semenu dva nivoa starosti omogućena je bolja klasifikacija linija u pogledu kvaliteta, nego standardnim testom klijavosti. Rezultati rada ukazuju da su hladni test i test ubrzanog starenja najpogodniji za procenu vigora semena u laboratoriji, s tim da je hladni test bolji pokazatelj nicanja u polju.<br />Ispitivane samooplodne linije pokazale su dobru sposobnost oĉuvanja kvaliteta semena kao i visok nivo toleratnosti na niske temperature u toku klijanja i ranog porasta biljaka. Kontrolisani uslovi ĉuvanja na 18 &deg;C omogućili su da se klijavost i vigor semena u ispitivanom periodu, održi na visokom nivou.<br />Dužina vegetacije samooplodnih linija nije imala znaĉajan uticaj na klijavost i vigor semena. Kvalitet semena ispitivanih samooplodnih linija je prvenstveno bio pod uticajem genetiĉke osnove. Linije poreklom od Lancaster populacije imale su slabiji vigor u odnosu na linije BSSS i Iowa Dent porekla.<br />U radu je potvrĊeno da vigor testovi imaju važnu ulogu u ispitivanju kvaliteta semena samooplodnih linija kukuruza. Dobijeni rezultati direktno se mogu iskoristiti u oplemenjivaĉkim programima u cilju pobolj&scaron;anja klijavosti i vigora semena. Ostvareni rezultati ukazuju da su informacije o genetiĉkom poreklu samooplodnih linija važan indikator vigora semena. Pored podataka o genetiĉkom poreklu linija veliki znaĉaj ima karakterizacija putem biohemijskih markera.</p> / <p>The extensive seed quality evaluations of maize inbred lines developed at the Maize Research Institute Zemun Polje of different maturity groups and genetic background were conducted. In laboratory and field conditions germination and vigour of seed produced in 2011 and 2014 was determined, after four years of storage at 18 &deg;C, 60% RH and one year of storage at 5 &deg;C, 60% RH, respectively.<br />The objective of this research was to determine the chilling tolerance of selected inbred lines during germination and emergence, seed longevity under controlled storage conditions, genetic divergence determined by biochemical markers and to assess the impact of the genetic background on seed quality.<br />Following seed quality parameters were tested: viability, germination, seedling length and vigour examined by cold test, accelerated ageing test, tetrazolium test, radicle emergence test and conductivity test.<br />Vigour tests provided a better classification of inbred lines of different seed age in terms of quality than standard germination test. Obtained results show that cold test and accelerated aging test are the most suitable for assessing seed vigour in the laboratory, but cold test is a better indicator of field emergence.<br />Tested maize inbred lines showed good seed storability and high level of chilling tolerance during germination and early growth of plants. Controlled storage conditions at 18 &deg;C, have enabled that germination and seed vigor remains at the high level in the examined period.<br />Seed germination and vigour of maize inbred lines were not affected by length of vegetation. Seed quality was primarily influenced by genetic background of tested inbred lines. Inbred lines originating from Lancaster populations had lower vigour compared to lines with BSSS and Iowa Dent origin.<br />This research confirmed that vigor tests play an important role in seed quality testing of maize inbred lines. Results of this work can be directly used in breeding programs to improve germination and seed vigour. Achieved results indicate that information about the genetic origin of inbred lines is an important indicator of seed vigour. Finally, data emphasized the importance of inbred line characterization by biochemical markers in addition to genetic origin information.</p>

Actuellement, dans les plateformes aérospatiales, le nombre et le besoin d’intégration des équipements électriques et électroniques sont grandissant du fait que leurs fonctions nécessitent de plus en plus de puissance. L’objectif de minimisation des coûts et surtout la disponibilité des dispositifs électroniques forcent les concepteurs et les fabricants de ces plateformes à s’orienter vers des produits commerciaux (dits grand public). La fiabilité des boîtiers des composants de puissance doit être évaluée dans les environnements sévères des applications aérospatiales. Une dizaine de composants électroniques de puissance a été sélectionnée en fonction de leur disponibilité et l’adéquation de leurs performances électriques et thermiques aux exigences des applications aérospatiales. Ces composants intègrent différents types de semi-conducteurs tels que le silicium, le carbure de silicium et le nitrure de gallium. Tout d’abord, une étude a été menée sur les potentiels modes et mécanismes de défaillance de ces composants électroniques de puissance dans ces environnements. Elle a permis de mettre en place plusieurs procédures de vieillissement accéléré ainsi que le développement de deux bancs de tests pour suivre électriquement le vieillissement de ces composants. Ces tests ont été menés sur deux diodes Schottky SiC, commercialisées par deux fabricants, regroupant les technologies des boîtiers des composants électroniques de puissance. Les analyses de défaillance ont tout d’abord mis en évidence une immaturité de la technologie de la jonction Schottky des puces SiC de l’une des deux diodes soumis à une tension inverse. Ces défaillances sont attribuées à la destruction partielle de la structure Schottky et indique une reproductibilité non maitrisée de la fabrication des puces de ce composant. Ensuite, ces analyses ont mis en évidence plusieurs mécanismes de vieillissement lors de tests simulant des régimes « On-Off » des applications (cycles thermiques de puissance). Celui considéré comme la cause de la défaillance est la fissuration de la soudure des fils d’interconnexion avec la puce. Une loi pouvant décrire la fissuration des interconnexions a été identifiée à la suite des évolutions des cycles thermiques de puissance à l’approche de la défaillance. L’étude de ces évolutions a permis de démarrer l’élaboration d’un modèle physique de défaillance adapté aux interconnexions de la puce en vue d’estimer la durée de vie des composants commerciaux. / Currently, in the aerospace platforms, the number and the need for integration of the electric and electronic equipment are growing because their functions require more and more power. The goal of cost minimization and mostly the availability of power electronic devices push the designers and the manufacturers of these platforms moving towards commercial products (Component Off-The Shelf). The packaging reliability of power electronic components must be assessed in harsh environments of aerospace applications. A dozen of power electronic components have been selected in accordance with their availability and with the adequacy of their electrical and thermal performances according to the requirements of aerospace applications. These components integrate different types of semi-conductors such as silicon, silicon carbide and gallium nitride. Foremost, a study has been leaded on the potential failure modes and mechanisms of these power electronic components in these environments. It has permitted to put in place several procedures of accelerated ageing and the development of two test benches to electrically monitor the ageing of these components. These tests have been carried on two SiC Schottky diodes, marketed by two manufacturers, gathering the technologies of the packaging of power electronic components. The failure analyses have first highlighted an immaturity of the Schottky junction technology of the SiC die of one of the two diodes subjected to a reverse voltage. These failures are attributed to the partial destruction of the Schottky structure and indicate a not mastered reproducibility of the die manufacturing of these components. Then, these analyses have highlighted several ageing mechanisms during tests simulating « On-Off » power of applications (power thermal cycles). One considered as the failure cause is the cracking of the welding of the wire bonding with the die. A behavioral law that can describe the cracking of the interconnections has been identified according to the evolutions of the power thermal cycles when near to failure. The study of these evolutions have permitted to start the elaboration of a physical model of failure adapted to the die interconnections in order to estimate the lifetime of commercial components.

Strategies to improve stem yield and juice quality in sweet sorghum were investigated in this study. Seed quality of sixty five accessions (landraces) from Botswana was investigated. Standard germination tests revealed that only 66% of the accessions had germination percentages in excess of 85%. The Accelerated Ageing test showed that only 50%of the 26 accessions had germination percentages above 80%. The results indicated that Botswana sweet sorghum seed quality is generally poor. Seed development and maturity observations demonstrated that maximum seed quality occurred 14 to 17 days after mass maturity (physiological maturity) and this coincided with maximum seed germination. These results suggest that harvesting sweet sorghum seed prior to mass maturity can lower seed quality. Farmers should, therefore be advised to select plants intended for seed harvesting and allow them to mature properly before the seeds are harvested. Differences in seed colour, shape and compactness of the inflorescences were observed amongst the 65 landraces collected from farmers in Botswana. Ten landraces were characterised and from the results it was evident that there was a range of genetic diversity which can be utilized in the improvement of the crop. Large panicles were characteristic of most sweet sorghum landraces, the effect of tiller, panicle and floret removal on juice quality was consequently studied. Removal of panicles and florets significantly improved juice quality whilst removal of tillers did not. Selection and breeding of genotypes with small panicles and male sterile varieties may improve juice quality and should be investigated. Effect of planting date, spacing and nitrogen were investigated. Early planting (October) resulted in increased stem yields but reduced juice quality. A 30 cm intra-row spacing resulted in high stem yields per plant and good juice quality. Nitrogen fertilisation increased stem yield and improved juice quality. On the bases of the results obtained from this study, early planting (October), application of 60 kg N ha-¹, and 30 cm intra-row spacing could be recommended for sweet sorghum production in pure stands. In pure stands yields of more than 37 000 stems (per hectare) of good quality can be attained. These could be sold at an estimated price of P2.00 (R2.25) per stem indicating the potential of sweet sorghum as a cash crop. However, its economic viability depends on the price elasticity in the supply - demand function. / Dissertation (PHD)--University of Pretoria, 2003. / Plant Production and Soil Science / unrestricted