Genetic Control of Sugars, Cry Matter, and Beta-Carotene in Sweetpotato (Ipomoea batatas [L.] Lam)
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Date
2013-12
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University of Ghana
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 β-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 β-carotene. Low broad sense heritability (H 2 ) of 27%
was recorded for yield but high estimates between 70% and 86% were obtained for quality traits.
Narrow sense heritability (h 2 ) 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 h 2 estimate of 0.80 was
observed for β-carotene using variance method, while a moderate estimate of 0.40 was obtained
using regression of offspring values on mid-parent values. The moderate h 2 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
h 2 and low genetic advance. All other quality traits including total sugars and β-carotene, had
moderate to high h 2 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 (σ gca2 / σ sca2 ; between 2.7 – 21.8) confirm the preponderance of additive over
non-additive gene action in the expression of the traits. Twenty-eight F 1 clones with desired
characteristics have been selected for further evaluation.
Description
Thesis (PhD) - University of Ghana, 2013