Thesis (M.Sc. Agriculture (Crop Science)) -- University of Limpopo, 2011 / The nature and magnitude of the genotype by environment interactions is important to identify superior and stable genotypes under the target environments. This will assist to maximize specific adaptation and to speed up the transfer of new cultivars to growers. The objective of this study was to determine the stability of selected soybean genotypes with regards to the agronomic traits, high yield and nodule formation. Field experiments were conducted under dryland conditions during the 2007/2008 and 2008/2009 growing seasons at the University of Limpopo’s experimental farm (Syferkuil) and at a farmer’s field at Gabaza community, Mopani District near Tzaneen. Ten selected soybean cultivars were evaluated under a randomised complete block design with three replications. Stability was assessed via joint regression and superiority analyses. Significant differences were found for genotypes, environments and genotype by environment interactions. Stability analysis after Eberhart and Russell’s model suggested that the genotypes showed marked differences to environmental changes. The cultivar superiority measure for seed yield indicated that variety Clark was the most stable genotype with an average yield of 5235 kg/ha, followed by L81-4858 and Barc-2 that provided average yield of 4839 kg/ha and 4582 kg/ha, respectively. In terms of number of nodules Magoye was observed to be stable with average of five nodules per plant. Cultivar Barc-2 was found stable for number of active nodules with an average of 3.17 active nodules per plant. Most of the genotypes performed better at Syferkuil than at Gabaza. In general Barc-2 was found stable for yield and other agronomic traits considered in this study. This variety could be suitable for large scale production in these or other similar environments in Limpopo Province.

A cytokinin habituated soybean callus has been isolated, utilizing the cytokinin soybean bioassay. The habituated callus line was subsequently characterized with a non-habituated callus line in relationship to levels of endogenous growth substances, ultrastructure, nitrogen metabolism and pattern of gene expression. The cytokinin habituated soybean callus contained a higher level of endogenous cytokinin-like activity in comparison to the non-habituated callus. This higher level of cytokinin present is in part due to a lower rate of degradation. The habituated callus tissue produced very low levels of ethylene, while the non-habituated callus produced ethylene at a much higher rate (57 fold higher), than the habituated callus. In contrast to what was found in habituated sugarbeet callus, only low levels of putrescine could be detected in both callus types. The putrescine content of habituated callus tissue was lower than that of non-habituated callus tissue. The ultrastructure of habituated callus cells exhibited several differences to what was observed in the non-habituated callus. Habituated callus cells appeared to have a thinner cell wall than that of the non-habituated callus cells. The cristae of the mitochondria in habituated cells were thicker than that of the non-habituated callus cells, indicating a lower metabolic activity. On day 14 of the growth period the nuclei of habituated callus demonstrated active RNA synthesis as indicated by the presence of several vacuolated nucleoli. Although no significant differences between proline levels of habituated callus and proline levels of non-habituated callus were observed, it was demonstrated that there was a difference in proline metabolism between the habituated and non-habituated calli. Utilizing an inhibitor of OAT, gabaculine, it was shown that in habituated callus tissue proline originated from ornithine during the first 14 days of growth. During the second half of the growth period, which characteristically consists of tissue with low biosynthetic activity, proline originated from glutamate. The production of proline in habituated callus from ornithine also corresponded to a period of high NH₄⁺ content in both callus types, while the production of proline from glutamate corresponded to a period of low NH₄⁺ content in the cells of both callus types. No such correlation was observed in proline metabolism of non-habituated callus. A similar turning point was observed in the activity of OAT of both callus types. Although the specific activity of OAT in both callus types mirrored their changes in RNA concentration, the percentage inhibition of OAT by gabaculine was not significant from day 14 in both callus types. This may indicate a change in the catalyzing properties of OAT in both callus types. It was further demonstrated that the non-habituated callus tissue contained some inhibitor inactivating OAT activity. With the use of gabaculine it was further shown that, in contrast to what was found in other habituated calli, there is no metabolic link between proline metabolism and putrescine synthesis. Both the habituated callus and the non-habituated callus exhibited a high nitrogen influx during the first 14 days of the growth period. The low NH₄⁺ content present in both callus types during the second half of the growth period coincided with higher levels of amino acids present in both callus types. The levels of precursor amino acids (glutamate, aspartate and alanine) did not fluctuate during the growth period, indicating a tight control on amino acid pools. Levels of amino acids further down the path of metabolism did not fluctuate drastically and there appeared to be very little difference between the levels of different amino acids measured in the habituated and nonhabituated calli. Serine was the dominant amino acid in both callus types. Total RNA concentrations of habituated callus were low in comparison to that of the non-habituated callus, except for a striking 12 fold increase on day 14 of the growth period. RNA concentrations of non-habituated callus increased gradually during the growth period and the highest concentration was recorded 21 days after subculturing. Several polypeptides were observed in the habituated callus that were not present in the non-habituated callus, utilizing IEF. Three polypeptides exhibited a change in concentration from day 6 to day 14 of the growth period in both the habituated and nonhabituated callus. These polypeptides appeared to decrease in nonhabituated callus, while they increased in the habituated callus. A complete cDNA library was constructed for both of the habituated and nonhabituated callus lines. Six different clones, that were over expressed in the habituated callus tissue, were isolated via subtractive techniques. One clone was characterized and showed homology to the glutamate/aspartate transport protein, the membrane component, of E. coli. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1998.

Thesis (M.S.)--University of Missouri-Columbia, 2005. / "May 2005" The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Includes bibliographical references.

<p dir="ltr"><a href="" target="_blank">Shoot architecture in plants refers to the spatial layout of the above-ground organs, which develops through complicated networks of genetic and environmental interactions. Modification of shoot architecture has been a major driver of yield increases in many crop species, but knowledge of the genetic control of shoot architectural traits in soybeans remains incomplete. Chapter 1 provides an overview of soybean shoot architecture traits, encompassing stem growth habit, plant height, branch number, branch angle, petiole angle, leaf size, and leaf shape. The review not only delves into the genetic basis of these traits but also underscores their importance, identifies knowledge gaps, and outlines avenues for future research leveraging cutting-edge technologies in gene editing, phenomics, and genomics. Chapter 2 describes the identification and mapping of a novel locus modulating semideterminate and indeterminate stem growth habits, <i>dt3</i>, on chromosome 10. Allelic and haplotypic analysis of the USDA soybean germplasm collection was conducted to find semideterminate soybean accessions which did not carry known stem termination alleles at the <i>dt1</i> and <i>Dt2 </i>loci. Mapping populations were developed by crossing several of these accessions to indeterminate cultivars, and initial mapping revealed a region on chromosome 10 common to all populations. <i>dt3</i> is a recessive mutation resulting in semideterminate growth habits, and this locus displays a unique pattern of inheritance compared with known stem growth habit genes in soybean or other plant species. Chapter 3 describes the identification of <i>Dt4</i>, a novel semideterminacy allele of <i>FT5a</i> originating from wild soybean. <i>Dt4</i> was identified by quantitative trait locus (QTL) mapping using a population developed by crossing LD00-3309, an indeterminate cultivar with a semideterminate recombinant inbred line (RIL1890) originating from a cross between wild and domesticated soybean. A combination of fine mapping and candidate gene expression analysis pinpointed the allele of the floral inducer <i>FT5a</i> in RIL1890 as <i>Dt4 </i>for semideterminacy. Intriguingly, when the <i>Dt4 </i>allele was transformed into LD00-3309, it resulted in not only semideterminate stem growth habit but also narrowed leaf shape. Chapter 4 describes the identification and mapping of <i>GmBa1</i>, a novel locus specifying soybean branch angle, in which wide branch angle is completely dominant over narrow branch angle. This locus was identified in two distinct biparental mapping populations. The findings described in this dissertation deepen our understanding of genetic mechanisms underlying shoot architecture traits and provide a valuable resource for breeders looking to modify these traits for soybean improvement.</a></p>