GENETIC CONTROL OF SUGARS, DRY MATTER AND BETA-CAROTENE IN SWEETPOTATO (Ipomoea batatas [L.] Lam), ABSTRACT
Sweetpotato has immense potential for food and nutrition security in Ghana. It is however, infrequently used in local cuisines. Breeders have attributed this low utilization to the fact that sweetpotato lacks quality characteristics that make it amenable to local food preparation. The present study was carried out to develop varieties that combine high dry matter, low sugar and high ?-carotene traits, to meet the needs of Ghanaian consumers.
A Participatory Rural Appraisal (PRA) and a survey were conducted to assess farmers? production constraints and quality characteristics that would enhance utilization of sweetpotato. Subsequently, 130 accessions were assembled and evaluated to assess variation and identify parental genotypes. Ten of these clones with good yield potential and desirable quality attributes were evaluated with a local check variety in four environments to assess genotype x environment interaction effect on expression of the quality traits under study. Eleven parental genotypes were identified as cross compatible and were used in a North Carolina mating design II for development of populations and to estimate genetic parameters for the desired traits. The production constraints listed by farmers during the PRA included, drought, pests and diseases, labour cost, declining soil fertility, low utilization, cost of vines, low market prices and lack of ready markets. Drought was ranked highest among the constraints. Farmer preferred quality attributes included low-sugar, high dry matter, and poundable varieties, with 70% of respondents indicating that the high sugar levels is a limiting factor to regular usage. The PRA also revealed farmers? willingness to use orange-fleshed sweetpotato with the desired quality attributes. A high diversity index of 0.85 was obtained for morphological traits using Rogers Tanimoto index while an index of 7.41 was obtained for quality traits using mean Euclidean dissimilarity index, indicating the presence of high diversity among the 130 genotypes. The G x E study revealed that genotype and environmental main effects and their interactions were significant for most traits. Regarding yield traits, G x E was more important while genotype main effects were more important than G x E for quality traits. The AMMI biplot and regression analysis revealed stable and specific adaptation for yield, dry matter, sugars, and ?-carotene. Low broad sense heritability (H2) of 27% was recorded for yield but high estimates between 70% and 86% were obtained for quality traits. Narrow sense heritability (h2) estimates for yield was 27% and 51% using parent-offspring analysis and variance components respectively. Parent-offspring regression gave moderate estimates of 0.38, 0.40, 0.39, 0.24 and 0.36 respectively, for dry matter, starch, fructose maltose and total sugars respectively while relatively high estimates of 0.56, 0.53, 0.45, 0.59, and 0.48 respectively were obtained with variance component method. Very high h2 estimate of 0.80 was observed for ?-carotene using variance method, while a moderate estimate of 0.40 was obtained using regression of offspring values on mid-parent values. The moderate h2 estimates for yield was associated with a moderate genetic advance implying that progress can be made through selection. With the exception of dry matter, starch sucrose and glucose were associated with low h2 and low genetic advance. All other quality traits including total sugars and ?-carotene, had moderate to high h2 that also associated with moderate genetic advance indicating that progress could be made through selection. General combining ability (GCA) for males and females were highly significant (P < 0.001) for most agronomic and quality traits. The presence of more significant GCA than Specific Combining Ability (SCA) effects and the substantially greater GCA than SCA (?gca2 / ?sca2; between 2.7 – 21.8) confirm the preponderance of additive over non-additive gene action in the expression of the traits. Twenty-eight F1 clones with desired characteristics have been selected for further evaluation.