Breeding of sweetpotato (Ipomoea batatas (L.) Lam) for drought tolerance and high dry matter content in Rwanda.
Sweetpotato is the third most important root crop next to cassava and potato in Rwanda. Drought stress remains the leading abiotic constraint to sweetpotato production in the Southern and Eastern Provinces of the country. Therefore, development and release of improved sweetpotato varieties incorporating end users’ preferences such as high storage root yields and dry matter content under limited water conditions remains important for sustainable production. The specific objectives of this research were: (i) to assess farmers’ perception, production constraints, preferences, and breeding priorities of sweetpotato in selected agro-ecologies of Rwanda, (ii) to characterise and identify breeding parents among 54 sweetpotato genotypes grown in Rwanda, East and Central Africa, (iii) to select drought tolerant sweetpotato genotypes under managed drought conditions using greenhouse and in-vitro screening techniques with early and late developmental traits, (iv) to determine general combining ability (GCA), specific combining ability (SCA) and maternal effects and heritability of drought tolerance and yield components of among newly developed sweetpotato clones and (v) to determine genotype x environment interaction and yield stability of sweetpotato breeding clones recently bred in Rwanda and to identify promising genotypes. A participatory rural appraisal (PRA) study was carried out involving 495 farmers in eight representative districts to identify farmers’ perception, production constraints, preferences, and breeding priorities of sweetpotato in the Eastern, Southern and Northern Provinces of Rwanda. Sweetpotato ranked among the five important food crops for food security and income generation. Drought stress, unavailability of improved cultivars and planting material, and pest and disease damage were perceived to be the five main constraints limiting sweetpotato production. The most important sweetpotato cultivar traits were high yield, early maturity, drought tolerance, disease and pest resistance, and good culinary taste. The characteristics of good storage roots identified by farmers included, high dry matter content, good culinary taste, good shape, root size, and sweetness. Each agro-ecological zone has its own specific sweetpotato production constraints and farmers’ preferences, necessitating targeted breeding of different sweetpotato cultivars for each agro-ecological zone for enhanced productivity and successful adoption of cultivars. Fifty four sweetpotato genotypes grown in Rwanda, East and Central Africa were field evaluated to identify breeding parents. Genotypes K513261, Kwezikumwe, 8-1038 and 2005-110 had the highest flowering rate of 44.97, 20.63, 19.05, and 14.82%, respectively. Suitable genotypes such as K513261, Purple 297, Kwezikumwe and New Kawogo were identified with high storage root yields at 31.9, 28.6, 28.2 and 27.1 t ha-1, respectively. Genotypes Ukerewe, 2005-103, Meresiyana and Mvugamo showed the highest mean dry matter content at 36.5, 35.5, 35.3 and 34.0%, respectively. Greenhouse and in-vitro screening methods were compared for effective selection of drought tolerance using 54 diverse sweetpotato genotypes. Positive correlations were observed between vine yield and fresh weight gain; and between total biomass and fresh weight gain, during greenhouse and in-vitro studies, respectively. The genotypes 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 the two screening procedures. The genotypes 2005-020, K513261, Kwezikumwe and Otada 24 were selected for high yield of storage roots, while 2005-034, 2005-110, SPK004 and Ukerewe were selected for high dry matter content, and 4-160, 8-1038, Nsasagatebo and Purple 4419 selected for high drought tolerance. The selected genotypes have a high flowering rate and are potential parents to breed for high yield and dry matter content of storage roots and drought tolerance. Twelve genotypes selected for their high yield, dry matter content or drought tolerance were crossed using a full diallel mating design. Families were field evaluated at Masoro, Karama and Rubona Research Stations of Rwanda Agriculture Board to determine general combining ability (GCA), specific combining ability (SCA), maternal effects and heritability of drought tolerance, yield and yield components. The GCA effects of parents and SCA effects of crosses were significant (P<0.01) for canopy temperature (CT), canopy wilting (CW), storage root, vine and biomass yields, and dry matter content of storage roots. 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 roots yield were the parents Nsasagatebo, K513261 and Ukerewe, while Nsasagatebo, 2005-034 and Ukerewe were the best combiners for high dry matter content. 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 36.1, 35.1, 34.3, 34.0, and 33.9% were Ukerewe x 2005-034, 4-160 x Nsasagatebo, 2005-034 x Ukerewe, 2005-034 x K513261, 2005-020 x Ukerewe, in that order. The selected families are valuable genetic resources for sweetpotato breeding for drought tolerance, yield and yield components. Genotype by environment interaction and yield stability of 45 selected sweetpotato breeding clones were evaluated across six environments in Rwanda. 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 38.2, 23.4 and 20.8 t ha-1, respectively. The highest dry matter content of storage roots of 40.6, 35.9 and 32.9% were recorded in clones 21, 137 and 259 (2005-034 x 8-1038), respectively. AMMI stability values (ASV) revealed the following most stable genotypes: Nsansagatebo, 210 (8-1038 x 4-160), 2005-110 and 456 (SPK004 x K513261), for storage root yields and clones 46 (Kwezikumwe x 2005-020), 509 (Ukerewe x Kwezikumwe), and 358 (Ukerewe x 8-1038) for dry matter content of storage roots. The study identified high yielding and stable candidate sweetpotato clones such as 21, 137 and 22 (4-160 x 2005-020) for their high yields and dry matter content of storage roots. These clones are recommended for direct production or sweetpotato breeding programmes in Rwanda and similar environments.