7.1 Introduction
Sweetpotato (Ipomoea batatas [L.] Lam.) is the third important root crop grown by 98% of subsistence farmers in all agro-ecological zones of Rwanda. Rwanda experiences severe drought spells during the off season (April/May to September/October) for crop production owing to the limited rainfall that falls from June to September. Most rural communities face periodic hunger during October to January every year in the country. The critical problem is that there is a limited number of drought tolerant varieties of major food crops, including sweetpotato. Consequently, drought tolerant sweetpotato varieties are required for sustainable sweetpotato production and to ensure food security in Rwanda. The value of a new sweetpotato variety depends on farmers’ and end users’ preferences. Sweetpotato storage roots with high dry matter content might be adopted by farmers and subsequently by consumers and processors. Therefore, breeding for drought tolerance and high dry matter content is an overriding consideration for enhancing sweetpotato production in Rwanda.
This section presents the thesis overview and summarizes the research objectives and keys findings of the study.
The specific objectives of this study were:
1. To assess farmers’ perception, production constraints, preferences, and breeding priorities of sweetpotato in selected agro-ecologies of Rwanda.
2. To characterise and identify breeding parents among 54 sweetpotato genotypes grown in Rwanda, East and Central Africa.
3. To select drought tolerant sweetpotato genotypes under managed drought conditions using greenhouse and in-vitro screening techniques with early and late developmental traits.
4. To determine general combining ability (GCA), specific combining ability (SCA), maternal effects and heritability of drought tolerance, yield and yield components of among newly developed sweetpotato clones.
5. To determine genotype x environment interaction and yield stability of sweetpotato breeding clones recently bred in Rwanda and to identify promising genotypes.
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7.2 Summary of major findings
The first study focused on a survey involving 168 farmers and 14 focus group discussions across eight representative districts. The study identified key production and productivity constraints, preferences and breeding priorities of sweetpotato farmers in Rwanda. The specific outcome of this study showed that:
• Drought stress, unavailability of improved cultivars and planting material, and pest and disease damage are the five main constraints limiting sweetpotato production.
• High yield, early maturity, drought tolerance, disease and pest tolerance, and good culinary taste are the most important traits of a good sweetpotato cultivar.
• High dry matter content, good culinary taste, good shape, root size, and sweetness are main characteristics of good storage roots identified by farmers.
• Each agro-ecological zone had its own specific sweetpotato production constraints and farmers’ preferences.
The second study characterized the phenotypic diversity of 54 sweetpotato genotypes grown in Rwanda, east and central Africa. Field trials were conducted using a 9 x 6 unbalanced alpha lattice design with three replications at the Karama and Rubona Research stations of Rwanda Agriculture Board. The main findings of this study were the following:
• Genotypes K513261, Kwezikumwe, 8-1038 and 2005-110 flowered the most.
• Genotypes K513261, Purple 297, Kwezikumwe and New Kawogo were identified as producing high storage root yields.
• Genotypes Ukerewe, 2005-103, Meresiyana and Mvugamo showed the highest mean dry matter content.
• Among 26 phenotypic traits used to characterize sweetpotato genotypes, only 19 traits were identified as the most influential characters for effective genetic diversity studies of sweetpotato.
• Overall, the following genotypes: K513261, Kwezikumwe, 2005-020, Otada 24, SPK004, Ukerewe, 2005-110 and 2005-034 were identified as potential breeding parents of sweetpotato for Rwanda with superior storage root yield and dry matter content.
171 The third study compared the use of greenhouse and in-vitro screening techniques of drought tolerance using 54 sweetpotato genotypes at the Rubona Research Station of the Rwandan Agricultural Board. The main findings of this study included:
• Positive correlations were observed between vine yield and fresh weight gain (FWG) and between total biomass and FWG, during greenhouse and in-vitro experiments, respectively.
• Genotypes such as 2005-146, 4-160, 8-1038, Karibunduki, Kwezikumwe, Purple 4419, NASPOT 9 O, Nsasagatebo, Karebe, IMBY 3102, Mwanakumi, 97-062 and Matembere were selected with comparatively high drought tolerance using two screening procedures.
The fourth study investigated the genetic basis and heritability of drought tolerance; yield and yield components. This was carried out using 12 genotypes selected for superior performance for the traits of storage root yield, dry matter content or drought tolerance. The clones were crossed using a full diallel mating design. Families were field evaluated at three sites to determine general combining ability (GCA), specific combining ability (SCA), maternal effects and heritability of drought tolerance, yield and yield components. The main outcomes of this study were:
• Broad sense heritability estimates were 0.95, 0.84, 0.68, 0.75, 0.50 and 0.58 for canopy temperature (CT), canopy wilting (CW), root yield, storage root dry matter content, vine yield and total biomass, respectively.
• The GCA effects of parents and SCA effects of crosses were significant (P<0.01) for CT, CW, storage root, vine and biomass yields, and dry matter content of storage roots.
• The ratio of GCA/SCA effects for CT, CW, yield of storage roots and dry matter content of storage roots were higher than 50%, suggesting the preponderance of additive over non-additive gene action in the expression of these traits. This indicates that the mating design should therefore be to enhance recurrent selection.
• The best general combiners for drought tolerance were the parents 8-1038, Otada 24 and 4-160 with the lowest CT and CW and relatively high yields.
• Best combiners for high storage root yields were the parents, Nsasagatebo, K513261 and Ukerewe, while Nsasagatebo, 2005-034 and Ukerewe were the best combiners for high dry matter content of storage roots.
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• Maternal effects were significant (P<0.05) among families for CT, CW, flesh color and dry matter content, vine yield and total biomass.
• Based on reduced CT and CW, the best families with significant SCA effects were 4- 160 x Nsasagatebo, 4-160 x Ukerewe, Otada 24 x 4-160, Nsasagatebo x 2005-020, Otada 24 x Nsasagatebo, 4-160 x K513261 , 513261 x 4-160, 8-1038 x 4-160, 4-160 x 8-1038, 8-1038 x 2005-020 and Nsasagatebo x Ukerewe, which were selected for breeding for drought tolerance.
• Across sites, the best five selected families with significant SCA effects for storage root yields were Nsasagatebo x Otada 24, Otada 24 x Ukerewe, 4-160 x Nsasagatebo, K513261 x 2005-034 and Ukerewe x K513261 with 11.0, 9.7, 9.3, 9.2, 8.6 t/ha, respectively.
• The best families with high dry matter content of storage roots were Ukerewe x 2005- 034, 4-160 x Nsasagatebo, 2005-034 x Ukerewe, 2005-034 x K513261 and 2005- 020 x Ukerewe.
The fifth study tested genotype x environment interaction and yield stability of 45 selected sweetpotato breeding clones using field trials established across six environments in Rwanda. The study revealed that:
• Interaction effects of clone x site x season, clone x site and clone x season on yields and dry matter content of storage roots were significant (p=0.001).
• Site x season interaction had significant effects on dry matter content of storage roots only.
• Candidate clones designated as clone 21 (4-160 x 2005-020), 137 (K513261 x 2005- 034) and 22 (4-160 x 2005-020) had the highest storage root yields of 38.23, 23.41 and 20.79 t ha-1, respectively.
• The highest dry matter content of storage roots of 40.58, 35.92 and 32.89% were recorded in the following clones: 21, 137 and 259 (2005-034 x 8-1038), respectively.
• AMMI stability values (ASV) revealed that genotypes, Nsansagatebo and clones 210 (8-1038 x 4-160), 2005-110 and 456 (SPK004 x K513261) were the most stable for storage root yields and Clones 46 (Kwezikumwe x 2005-020), 509 (Ukerewe x Kwezikumwe), Mugande, and 358 (Ukerewe x 8-1038) were stable for dry matter content of storage roots.