Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliographical references.

Sorghum is the fifth most important cereal in the world and a staple food for humans. It is also a source of food and fodder for animals. In addition to the abiotic stresses such as drought, parasitic weeds of the genus Striga cause losses in sorghum production in sub Saharan Africa. Striga asiatica is a parasitic weed that attacks cereals in low input agricultural systems and is distributed throughout semi- arid regions of Africa. Most sorghum producing farmers rely on tolerance for their harvests in Striga infested fields yet the stability of tolerance in the face of a changing climate (recurrent droughts), new farming systems (mulch based agriculture) and existence of various Striga strains needs further investigation. Reduced strigolactones production was also studied as a resistance mechanism. The first study was focused on the determination of tolerance of Striga asiatica infested sorghum under drought in a pot study. Five sorghum lines were subjected to infestation with Striga and some were not infested while watering was done at 50 percent field capacity (FC) and 100 percent FC. The results showed that the five sorghum lines differed significantly in chlorophyll content and Normalised Differential Vegetation Index (NDVI). Infection did not lower chlorophyll content when it co-occurred with drought across all sorghum lines. Drought and infestation had mutually exclusive effects on chlorophyll content and NDVI. Under infestation, internode length was similar both at 100 percent FC and at 50 percent FC while under uninfested conditions, 100 percent FC gave the longer internode compared to 50 percent FC. Both infestation and irrigation regime reduced the sorghum head weight, illustrating that the two effects have synergistic effects on sorghum head weight. The second study sought to determine the effects of mulching and infestation on sorghum spp tolerance to Striga asiatica. The experiments were carried out in the seasons 2013/14 and 2014/15 summer seasons. The results indicated that mulching increased chlorophyll content in the 2014/15 season which was a drier season compared to 2013/14. In the 2014/15 season, mulching increased chlorophyll content in all varieties except Ruzangwaya, Mukadziusaende and SC Sila. When the same varieties were infested under mulch and infested without mulch, the results showed that mulching overcomes the effects of infestation in some varieties. Mulch also negates the effect of Striga parasitism and results in yield maintenance in sorghum varieties. The third study sought to determine the stability of sorghum lines when exposed to two Striga asiatica lines sourced from two places which are 500 km apart in Zimbabwe. The two strains were termed the Chiundura and Rushinga strains, based on where they were sourced. The experiments were conducted at Henderson Research Station (HRS) at Mazoe and at Bindura University of Science Education (BUSE). The results showed differential virulence for some traits while the two strains were equally virulent for some traits. The two strains were equally virulent on all sorghum lines with respect to chlorophyll content. The different sorghum lines responded differently to the effects of the two strains. The effects of the two strains were generally similar for head index, root index, and leaf index at all sites. Generally the Chiundura strain was more virulent to sorghum lines compared to Rushinga strain, confirming the existence of physiological strains of Striga in Zimbabwe. Therefore physiological speciation of Striga asiatica exists and this adds a further dimension to the complexity of Striga management in the smallholder sector. Quantification of strigolactone production by different sorghum genotypes was conducted in the laboratory using the agar gel assay. The genotype Mukadziusaende produced significantly the least (P<0.01) quantities of strigolactones, as inferred from the maximum germination distance (MGD) from the sorghum root. The MGD was negatively correlated to tiller numbers illustrating that the more the strigolactones the less the tillering capacity. Tiller numbers and MGD can therefore be used to select for reduced strigolactones production.

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Typescript, etc. / Digitized by Kansas State University Libraries

Digitized by Kansas State University Libraries

Digitized by Kansas Correctional Industries

Digitized by Kansas Correctional Industries

Digitized by Kansas Correctional Industries

In sorghum, a predominantly self-pollinated crop, hybrid seed production relies exclusively on the cytoplasmic-genetic male sterility system. The system of hybrid development has caused sorghum breeding programs to develop two breeding groups: a male-parent group (R-line/ fertility-restorer) and a female-parent group (an A/B line, lacking the fertility-restoring gene of the A1 male-sterility system). These have served as heterotic groups in the absence of more information with reference to genetic diversity. Efforts to determine heterotic groups in sorghum have not been successful in clearly delineating any patterns. However, in a recent molecular marker-based study of 50 elite sorghum parental lines, groups similar to the working group system were observed, as was an absence of a consistent delineation, characteristic of heterotic groups, between the A/B- and R-lines. This study was conducted with the objective of evaluating the groups observed and assessing their potential as heterotic groups. Two parental lines from each of the five groups, and two lines from those not conforming to any group, were chosen and crossed in a half-diallel. The twelve parents, sixty-six diallel hybrids and three commercial hybrid checks were evaluated for grain yield and other agronomic traits in five environments' College Station, TX in 2003 and 2004, Weslaco, TX in 2003, and Halfway, TX in 2003 and 2004. Within-group crosses exhibited inferior heterotic expression, for grain yield and other traits, in comparison with across-group crosses. Furthermore, genetic similarity estimates for parental line pairs obtained from the molecular study were significantly correlated with specific combining ability and heterosis for yield of the corresponding hybrid combinations, revealing a pattern of correspondence between molecular data and heterosis. Hybrids made among R-lines and among B-lines were significantly lower in yield compared to AxR hybrids, likely to be a result of decades of breeding efforts to develop inbreds within the mutually isolated groups, rather than a consequence of phylogenetic divergence. An examination of the heterotic effects manifested in hybrid combinations reveals a pattern of interactions broadly in agreement with the molecular data, but differential responses between individual members of the proposed groups make it difficult to define distinct heterotic groups.