Sweetpotato (Ipomoea batatas (L.) Lam.) is an important food crop in East Africa including
Kenya. The crop incurs high yield losses in production due to biotic (insect pests and diseases)
and abiotic (drought, and heat) constraints. Among abiotic constraints, drought is the most
important. Prolonged periods of drought in arid and semi-arid areas of Kenya have led to
reduced quantity and quality of sweetpotato storage roots and in severe cases caused total crop
failure. The objectives of this study were to: 1) determine sweetpotato production system
constraints and farmers’ coping strategies; 2) evaluate sweetpotato clones for yield performance
and drought tolerance; 3) analyse genotype x environment interaction and stability for storage
root yield of selected clones, 4) determine mechanisms of drought tolerance in sweetpotato, and
5) determine combining ability and heterosis for yield and drought tolerance traits under
managed drought stress conditions.
To determine the production constraints and farmers’ coping strategies, a survey was
conducted in central, eastern and western Kenya. Out of 345 farmers interviewed, 60% were
women, and 40% men. Farm sizes ranged from 0.4-0.8 ha, with 90% of sweetpotato cultivated
on 0.2 ha or less. The main sweetpotato varieties were Vitaa, Kabonde and Bungoma and the
majority of farmer’s used their own conserved planting material which was conserved by leaving
them in the field after harvest. About 35% of the farmers identified weevils as the major pest,
and sweetpotato virus disease (SPVD) as the major disease, while 28% of the farmers identified
drought as a major constraint. The farmers used clean seed, high yielding varieties, high
planting density, and manure application as the main strategies to cope with sweetpotato
production constraints.
Eighty four sweetpotato clones were evaluated under managed drought stress environments at
KARI-Kiboko and KARI-Thika. Drought reduced the fresh weight of storage roots (FSR)
(72.5%), fresh biomass weight (FB) (74.0%), marketable fresh storage root (MFSR) (80.7%),
number of storage roots (NSR) (24.5%), days to permanent wilting point (DPWP) (0.3%), but
seemed to increase percent root dry matter (% RDM) (-4.7%), harvest index (HI) (-2.6%), and
chlorophyll content (CC) (-2.7%). Across the environments, genotypes 194555.7 (1.06), 421066
(1.05), Chingovu (0.94), 420014 (0.91), Excel (0.9), 199062.1 (0.87) and Unawazambane06-01
(0.81) gave higher FSR yields (kg plant-1) than the local checks. Genotypes Nyarmalo and
Polista were among the lowest yielding in the irrigated and non-irrigated conditions. Clones
W119, 441725, and Xiadla-xa-kau were the highest yielding under the drought stress
conditions.
The performance of 24 improved clones was evaluated in replicated trials at KARI-Thika and
KARI-Kiboko using 24 sweetpotato clones grown under managed drought stress conditions for
two seasons. AMMI, GGE biplots and regression analyses were conducted to determine
stability of the clones. Mean FSR was significantly different (P < 0.001) in the two research sites
with the environment contributing to 92.7% of the total variation, genotype 1.8%, and
interactions 0.4%. AMMI and GGE biplots, and regression indicated the most stable clones to
be 441725, Unawazambane06-01 and 189150.1, while Xiadla-xa-kau was the least stable.
To gather more information on drought tolerance mechanisms expressed in the sweetpotato
genotypes, an experiment was conducted in the greenhouse using clones expressing different
levels of drought tolerance in the field. The results showed more under-developed roots (pencil
roots) in the drought stressed regimes than in the irrigated regimes and in the drought
susceptible genotypes. Drought tolerant genotypes produced more FSR and NSR. The number
of vine branches (NVB), vine tip pubescence (VTP) and mature leaf pubescence (MLP)
increased with reduction of drought stress however, drought tolerant clones had reduced NVB.
Drought stressed clones had shorter basal vine length (BVL), reduced CC, and reduced leaf
growth (LG). Drought stress reduced growth of vines in terms of internodes length, internodes
diameter, vine length, petioles length, and leaf CC. Overall, for the first time, the study
demonstrated that drought stress in the first three months after planting, leads to the
proliferation of non-edible pencil roots, which do not become edible storage roots even when
drought stress is removed. Therefore, water is critical during this period for improved
sweetpotato storage root yield, probably as a drought tolerant mechanism.
Combining ability for yield and drought tolerance of 15 F1 sweetpotato families generated
through a half diallel mating of six parents was evaluated at KARI-Kiboko in 2012. Significant
(P≤0.05) general combining ability (GCA) and specific combining ability (SCA) effects were
recorded for root yield in both drought stress and no stress conditions, indicating that both
additive and dominance gene effects were important in the inheritance of resistance to drought
stress. Progenies from families G2, G5, G7, G8, G10, G12 and G15 had good SCA for fresh
storage root yield, total biomass, number of days to permanent wilting point (DPWP), harvest
index, and drought stress index (DSI). Progeny 8 from family G4, 5 and 8 from G15, had the
highest mid and best parent heterotic effect (117-270%) for fresh storage root yield in both
drought stress and no stress conditions. Progeny 6, and 7 from family G10, had the highest mid
and best parent heterotic effect (165-234%) for fresh total biomass yield under no drought
stress conditions. In conclusion, the progenies from families G2, G5, G7, G8, G10, G12 and
G15 that had high yield and biomass specific combining ability under drought and no drought
stress indicated that they could be having drought tolerant genes, and therefore could be
incorporated into advanced drought screening trials with the aim of releasing the best
performing drought tolerant varieties. Secondly, the findings in this study lay a foundation for
sweetpotato breeding programmes on drought tolerance. Thirdly, for the first time, this study
uniquely combines yield performance, combining ability estimates, days to permanent wilting
point and heterosis under contrasting moisture regimes to unmask the gene action of drought
tolerance in sweetpotato, a milestone in science. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10941 |
| Date | January 2013 |
| Creators | Kivuva, Benjamin Musembi. |
| Contributors | Mwangi, Stephen Githiri., Yencho, George Craig., Sibiya, Julia. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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