Spike morphology characteristics and the free-threshing habit of wheat have been extensively investigated because of their evolutionary significance and practical importance. Several genetic systems that govern these traits have been reported. Some studies suggest polygenic inheritance while others have identified major genes. This study was conducted to identify and locate quantitative trait loci (QTL) affecting the free-threshing habit and spike morphology characteristics in the International Triticeae Mapping Initiative (ITMI) recombinant inbred line (RIL) mapping population. The ITMI population was planted in three environments in 1999 and 2000. The ITMI RILs were evaluated for threshability and spike morphology characters. QTL analyses were performed using simple and composite interval mapping procedures. Two QTLs, one on chromosome lB and one on 4A, affecting spike length were identified. The QTL on chromosome 1B has not been described previously. One QTL controlling spikelet number was also detected on chromosome 4A. This QTL coincided in location with the QTL on chromosome 4A that affected spike length. One QTL controlling rachis internode length, a measure of spike compactness, was detected on chromosome 6A. The location of QTLs that affected spike length, spikelet number, and spike compactness did not coincide with the location of major genes (Q, C, S1, Ppd1, and Ppd2) known to affect these traits. Two QTLs, one on chromosome 2D and one on 4D, affecting threshability were identified. The QTL on chromosome 4D has not been described previously. A QTL that affected glume tenacity was also detected on chromosome 2D. Coincident QTLs on chromosome 2D that affected both threshability and glume tenacity are believed to correspond to Tg, a gene for tenacious glumes. In addition, an amplified fragment length polymorphism (AFLP) marker (XorstP3747207) that was putatively associated with Tg was identified using bulked segregant analysis. A QTL on chromosome 5A affecting glume tenacity was also identified. The QTL on chromosome 5A is believed to represent Q, a gene known to affect rachis fragility and glume tenacity. Information on the number, position, and effect of QTLs determining these traits and their associated molecular markers may facilitate their manipulation for wheat improvement purposes. / Graduation date: 2002

Thesis (PhD(Agric) (Agronomy))--Stellenbosch University, 2008. / The adoption of the no-till planting method brought about changes to the way the wheat crop is established in the Mediterranean climate of the Western Cape. Row widths have to increase from the normal narrow rows (170-180 mm) to at least 250 mm to allow for sufficient stubble handling. Furthermore, planters are designed to place seed accurately in the soil at uniform depth, which may increase seedling survival rates. The main objective of this study was to determine the influence of the use of wide row widths on yield, the components of yield and grain quality parameters and to revisit planting density recommendations to be used with the notill planting method. On-farm, producer managed trials which included cultivars, row widths and planting density treatments were planted at Riversdale, Swellendam and Caledon in the Southern Cape region and at Moorreesburg and Hopefield in the Swartland during the 2004 to 2006 production seasons. All trials were factorial RCB designs with split-split plot arrangements. Grain yield, grain protein, hectolitre mass (HLM) and the yield components, seedlings m-2, seedling survival (%), number of heads m-2, number of heads plant-1, number of kernels head-1 and thousand kernel mass (TKM) were determined at all sites in 2005 and 2006. Seedling survival rates of 80% were easily achieved in all trials with the exception of Caledon and Swellendam in 2005. The no-till planting method may be efficient to improve on survival rates of 50-70% found with the conventional planting methods. The yield component response that raised the most concern was the clear trend of the reduction in the number of heads m-2 as row widths increased, which was significant in eight out of the nine experiments. The number of heads plant-1 decreased significantly as planting density increased in all experiments. Cultivars differed in the grain quality parameters grain protein (%) and HLM but were influenced minimally by the other treatments. Reductions in grain yield occurred in three out of eight trials in the Southern Cape and in three out of six trials in the Swartland, with reductions of between 6.8% and 33% in some seasons. The risk of yield loss due to wide row widths could not be excluded by this study and therefore the row widths used by producers should remain as narrow as practically possible. Grain yield response to increasing planting density differed between the two regions. No significant yield benefits were found in any of these trials if planting densities were increased above 175 target plants m-2. Planting densities may be reduced to between 70 and 87.5 kg seed ha-1 to achieve this target if the crop is planted in time and seedling survival rates of at least 80% can be achieved.