Breeding investigations for fresh root yield and nutritional quality traits for sweetpotato (Ipomoea batatas [L.] Lam.) germplasm.

Abstract

Sweetpotato (Ipomoea batatas (L.) Lam.) is an important crop for smallholder farmers in sub-Saharan Africa (SSA), providing food, income, and nutritional security. In Zimbabwe, white-fleshed sweetpotato (WFSP) varieties are favored for their high dry matter content, but they lack β-carotene, limiting their ability to combat vitamin A deficiency. Despite this, Zimbabwe lacks a systematic breeding programme targeting both yield and β-carotene enhancement in WFSP. This study aimed to establish a sweetpotato breeding programme with the following objectives: (1) assess genotypic variation in fresh storage root yield and micronutrient content (β-carotene, Fe, Zn); (2) identify high-yielding, nutrient-rich genotypes; (3) evaluate genotype × environment interactions (GEI) for target traits; (4) assess genetic diversity using single nucleotide polymorphism (SNP) markers; and (5) determine combining ability for yield and β-carotene improvement. A multi-location trial of 22 genotypes (including CIP-derived lines and Zimbabwean landraces) revealed significant differences (p < 0.01) in β-carotene, Fe, and Zn contents, root yield, and the number of commercial roots per plot. Genotype O-105 recorded the highest β-carotene content (12.5 mg/100 g), while Zn and Fe ranged from 12 to 20.5 ppm and 45.2 to 118.5 ppm, respectively. Genotype O-104 combined high yield with superior levels of all three nutrients. A strong positive correlation (r = 0.80; p < 0.01) between storage root yield and number of commercial roots suggests effective selection using conventional breeding methods. Genotype-byenvironment interaction analysis across eight environments revealed significant genotypic differences (p < 0.001) for storage root yield and micronutrient content (p < 0.001). Commercial storage root yield ranged from 1.02 to 8.38 t/ha. Genotypes G3, G19, G16, G22, and G2 were identified as stable and high yielding. Genotypes G19, G12, and G16 had the highest β-carotene levels, while G12 and G18 excelled in Fe and Zn, respectively. Broadsense heritability was high (H² > 0.65) for all traits, supporting genetic gains through selection. The 22 genotypes evaluated in the multi-location trials represent a carefully selected subset derived from the broader germplasm base of 327 accessions analysed for genetic diversity. The diversity study using 30 SNP markers on these 327 accessions provided insights into the overall genetic variability, population structure, and parental diversity within the sweetpotato germplasm. Genetic diversity analysis using 30 SNP markers on 327 accessions revealed moderate diversity (mean GD = 0.36). The mean polymorphism information content (PIC) was 0.29, and the minor allele frequency (MAF) averaged 0.34. A high fixation index (FST = 0.68) and low heterozygosity (mean = 0.12) suggest limited parental diversity. Population structure analysis identified two subpopulations, indicating significant genetic differentiation among groups. These results are valuable for guiding germplasm conservation and improvement. Combining ability analysis using a North Carolina Design II with eight parents and 16 crosses, evaluated across three locations, revealed significant differences in number of commercial roots per plant and β-carotene content. Crosses Kau 4 × O-105, Magutse × O-105, and Dube × O-105 yielded up to 26.97 t/ha, while Dube × O-104 and Dube × O-105 showed high β-carotene levels (up to 11.49 mg/100 g). Most parents, particularly O-105 and Dube, had strong general combining ability (GCA). Broad-sense heritability was high for both traits (0.85–0.93), indicating both additive and non-additive gene effects, supporting the use of both recurrent selection and hybridization in breeding. Parental lines O-104, O-105, and P-201, and their top-performing crosses, are recommended for further breeding to enhance sweetpotato yield and nutritional quality in Zimbabwe.