Genetic improvement of beta carotene in cassava (Manihot esculenta Crantz) landraces
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Date
2020-01
Authors
Peprah, Bright Boakye
Journal Title
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Publisher
University of the Free State
Abstract
The aim of this study was to identify farmers’ adoption challenges, perceptions and preferences of yellow-flesh cassava. Combining ability and stability of these genotypes were also determined. Total carotenoid content (TCC), proximate values and hydrogen cyanide (HCN) of the yellow-flesh cassava were measured and the retention of carotenoids in boiled biofortified cassava was determined. This information will help breeders to identify genotypes with the best nutritional quality across the tested locations for planting and promotion in Ghana also could provide a basis for implementing a recurrent selection scheme for developing cassava varieties with high levels of carotenoids and dry matter. In all the locations visited, farmers’ knowledge on the improved cassava varieties (white flesh) and the yellow-flesh cassava were generally poor among the men and women interviewed, due to their inability to access planting materials, which could be improved by strengthening the cassava seed system for awareness, and increased availability of the varieties to farmers. Very few men and women cultivated improved varieties and yellow-flesh cassava. The young adults, who are the future of the agricultural sector, lacked access to improved varieties and they must be given extra attention to understand the activities of cassava breeding programmes, to empower them to make use of these materials. The general combining ability (GCA) was larger than specific combining ability (SCA) for cassava mosaic disease (CMD), harvest index (HI) and TCC, with predictability ratios (0.98, 0.88 and 0.92 respectively) close to one. Hence, there is a possibility for improvement of the characteristics by selection. Positive significant correlation between pulp colour and TCC (r=0.59) and pulp colour and cortex colour (r=0.58) were observed. Negative significant correlation were seen between CGM and HI (r=-0.50), CMD and RTN (r=-0.45), and HI and RTN (r=-0.51). It implies that these key traits could be effectively combined in a breeding program. In particular, breeders can rapidly screen for high TCC by visually assessing the pulp colour in addition selection for CMD symptoms (in a high disease pressure zone). The selected individuals for pulp colour at early stage screening can then be quantified for carotenoids at later stages, to save cost. Some of the yellow-fleshed genotypes (progenies) displayed comparable dry matter content (DMC) values as their white-flesh elite parents and were selected for multilocational trial testing towards commercial release in Ghana. Findings of this study demonstrated that it is possible to simultaneously select for yield and quality traits, such as DMC, at seedling stage. It was shown that the yellow flesh cassava varieties could be used in a hybridization scheme with local material to combine both TCC and DMC traits with high yield in a CMD resistance background. Carotenoid-rich varieties also showed variation for important characteristics, which are key drivers of variety adoption in Ghana. In view of this, some cassava varieties, such as IBA090151 and IBA083774, are proposed for release in Ghana. The HCN content of the cultivars varied from location to location and the values observed were below 50 µg g-1 and hence can be classified as sweet cultivars (low HCN). The cultivars that were sweet were, however, above the range of the maximum acceptable HCN limit recommended by the World Health Organisation (WHO) and for that reason need to be processed before consumption (for example as fufu, konkonte, gari). Finally, it is recommended that cassava breeders review their breeding objectives to reflect the preferred traits of end-users, and pay attention to stakeholders’ perceptions of the yellow flesh cassava to develop demand driven varieties that will serve the need of end-users. Education to create awareness on the potential advantages and diverse uses of the improved biofortified cassava is also needed.
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Keywords
Thesis (Ph.D. (Plant Sciences (Plant Breeding))--University of the Free State, 2020, Combining ability, Cyanide, Farmer-preferred traits, Nutritional value, Provitamin A, Yellow flesh cassava