Breeding of sweetpotato for improvement of root dry matter and β-carotene contents in Ethiopia.

Abstract

Sweetpotato [Ipomoea batatas (L.) Lam] is the second major crop among the root and tuber crops in Ethiopia contributing to food and nutrition security. Especially, the orange fleshed sweetpotato (OFSP) is a low-priced and sustainable source of vitamin A. It is useful to combat the problem of vitamin A deficiency (VAD) prevailing in Ethiopia and most sub-Saharan African countries. However, acceptance and adoption rate of the OFSP varieties by farmers is low due to the low root dry matter content (RDMC) of the released OFSP varieties. Thus, improving the RDMC of the OFSPs through breeding is vital to increase the adoption rate and thereby to improve vitamin A intake by the rural communities. Therefore, the objectives of the studies were: (1) to assess and document the major constraints affecting production, pre- and post-harvest handling, and farmers’ preferences for sweetpotato in Ethiopia; (2) to determine the combining ability, type of gene action and heritability of RDMC and β-carotene content, and yield related traits of selected sweetpotato clones, for further evaluation and breeding; (3) to estimate the magnitude of genotype-by-environment (G x E) interactions and to select stable and high yielding candidate sweetpotato clones for RDMC, β-carotene content and fresh root yield, and to identify the most discriminating and representative test environments in Ethiopia; (4) to assess associations between yield and yield related traits, and to identify the most efficient yield-predicting traits in sweetpotato for effective selection; and (5) to determine the nutritional value of newly developed OFSP clones and to establish the associations between β-carotene content and micro-nutrients for targeted large scale production to alleviate nutrient deficiencies. Firstly, a participatory rural appraisal (PRA) study was conducted in six selected districts from three major sweetpotato growing administrative zones in southern Ethiopia involving 183 farmers. PRA techniques including semi-structured interview, focus group discussions and discussion with key informants were used to collect data. The results indicated that sweetpotato is among the major crops grown in the study areas. According the respondents, the major pre-harvest constraints were heat and drought (21.6% respondents), shortage of planting materials (20.1%), shortage of land (15.7%), diseases (10.0%), insect pests (9.4%), a lack of draft power (8.1%) and shortage of money to cover input costs (7.9%) were the major pre-harvest production constraints. The major post-harvest constraints were poor access to markets (22.6% of respondents), poor market prices (19.1%), low yields (14.2%), low root dry matter content (13.6%), a lack of knowledge on processing (11.7%), a lack of processing equipment (11.1%) and transportation problem (7.7%). The primary criteria for sweetpotato variety selection by farmers were identified as resistance to heat and drought (19.6% of respondents), RDMC (16.4%), taste (14.3%), root yield (13.6%) and, resistance to diseases and insect pests (13.3%). Secondly, seven genotypes selected for their high RDMC, β-carotene content or fresh root yield were crossed using a half diallel mating design. A total of 28 genotypes: 21 crosses and 7 parents, were evaluated at four locations in Ethiopia using a 7 x 4 alpha lattice design with two replications. Significant differences (p < 0.01) were observed among genotypes across the four test environments for the following traits: RDMC, β-carotene, SPVD, fresh root yield and harvest index (HI). The general combining ability (GCA) to the specific combining ability (SCA) variance ratios were 0.96, 0.94, 0.74, 0.96 and 0.97 for RDMC, β-carotene content, SPVD, fresh root yield and HI, respectively, indicating that the inheritance of these traits was controlled mainly by additive genes. The following parents were considered good general combiners for the three traits: Ukrewe and PIPI for RDMC; Ukrewe, Resisto and Ejumula for β-carotene content and; Resisto and NASPOT-1 for fresh root yield. Good specific combiners were the families belonging to Ukrewe x Resisto, Resisto x Ogansagan, Ejumula x PIPI and NASPOT-1 x Temesgen with high RDMCs of 36.6, 37.5, 38.2 and 37.2%; β-carotene contents of 9844.7, 10590.3, 4685.6 and 5153.4 μg 100 g-1, respectively. Also, these crosses had medium to high mean fresh root yields. Clones from these families were selected for advanced selection and breeding for high RDMC, β-carotene content and root yield. Thirdly, G x E interaction and stability analyses were conducted across six environments (Halaba, Kokate, Areka, Arbaminch, Hawassa and Dilla) in southern Ethiopia. A total of 24 experimental clones selected based on their specific combining ability effects for RDMC, β- carotene content and root yield along with one local check variety were evaluated. Superior and stable candidate clones, and suitable environments were identified using Additive Main Effects and Multiplicative Interaction (AMMI) and Genotype Main Effect and Genotype by Environment Interaction (GGE) biplot analyses. Accordingly, four candidate clones designated as G1 (Ukrewe x Ejumula-10), G6 (Resisto x Ejumula-7), G19 (Resisto x Ogansagen-23) and G20 (Ejumula x PIPI-10) with above average RDMCs of 31.82, 32.60, 33.09 and 30.06%; high β-carotene contents of 12.48, 14.27, 16.30 and 13.99 mg 100 g-1; and, stable and high fresh root yields of 25.09, 26.92, 21.30 and 25.46 t ha-1, in that order, were selected for finishing off and recommendation. Among the sites covered in this study, Arbaminch was identified as the best environment for sweetpotato testing or production in the southern Ethiopia with a high mean RDMC of 32.9%, mean β-carotene content of 7.2 mg 100 g-1 and the highest mean fresh root yield of 37.1 t ha-1. Fourthly, correlation and path-coefficient analyses were conducted involving 24 newly developed sweetpotato genotypes and one check variety using 15 quantitative traits. Root yield showed significantly positive correlations with most traits studied, indicating that component characters should be simultaneously selected for sweetpotato improvement. β- carotene content and root flesh colour showed a highly positive correlation (r = 0.76), suggesting the importance of root flesh colour as an indirect selection criterion of high β- carotene content in sweetpotato. With path-coefficients of 0.821, 0.776, 0.276, and 0.410, individual root weight, number of roots per plant, RDMC and above ground fresh weigh had high positive direct effects on fresh root yield, respectively. These traits can be used for indirect selection to improve root yield. Finally, eight nutritional traits: β-carotene, protein, iron, zinc, starch, fructose, glucose and sucrose of the 25 newly developed sweetpotato genotypes were examined. The genotype designated as G8 (Resisto x PIPI-2) was the most promising with the highest contents of β- carotene (20.01 mg 100 g-1), protein (7.08%), iron (2.55 mg 100 g-1), zinc (1.42 mg 100 g-1), fructose (4.45%), glucose (5.34%) and sucrose (16.20%) followed by the genotypes G15 (Resisto x Temesgen-23) and G19 (Resisto x Ogansagen-23). This shows the potential of developing OFSP varieties enriched with these important micro-nutrients. The present study revealed the possibility of breeding sweetpotato varieties that combined high RDMC, moderate β-carotene content, and high fresh root yields with wide or specific adaptation for large scale production in Ethiopia. Overall, the study assessed the major sweetpotato production constraints, developed valuable sweetpotato families with high combining ability and heterosis for RDMC, β-carotene content and fresh root yield. Four traits, including individual root weight, number of roots per plant, RDMC and above ground fresh weight, were identified for indirect selection to improve root yield. The newly developed candidate OFSP clones are good sources of vitamin A, iron and zinc with high levels of protein and soluble sugars including sucrose, glucose and fructose.