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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufh/vital:28088 |
| Date | January 2017 |
| Creators | Mandumbu, Ronald |
| Publisher | University of Fort Hare, Faculty of Science & Agriculture |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 210 leaves, pdf |
| Rights | University of Fort Hare |
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