Drought and low N stresses are factors most frequently limiting maize production in the
tropics, where the vast majority of farmers have limited access to improved techno logy.
Previous studies confirmed good performance of the crop under these conditions by
improving tolerance to drought occurring at flowering, which is essential for resource
constrained farmers in countries like Ethiopia. In addition to screening at sites with rain
free seasons, the available genotypic variability for drought adaptive traits within adapted
maize populations is crucial for successful improvement of tolerance.
At Melkasa in Ethiopia, 196 randomly derived S1 lines from Population A-511 were tested
for high yield potential, and drought and plant density adaptive traits under two moisture
regimes, using two plant densities. Both plant density and drought stress significantly
affected the tested primary and secondary traits of the S1 lines. However, drought effects
were more effective in reducing grain yield and its components, as well as in increasing
anthesis-silking interval or in delaying days to 50% silking. Significant genotypic
variability within Population A-511 was observed for grain yield and most stress (drought
and high plant density) adaptive traits.
The association of yield with stress adaptive traits that included ears plant -1 , anthesis-silking
interval, kernels ear -1 , and kernels plant -1 increased with increased stress. In
contrast, the genotypic variance and heritability of grain yield and its components
increased with decreasing stress, except for ears plant -1 . Regarding the use of suitable
environments for drought tolerance screening in areas with unpredictable rainfall patterns,
indirect selection under drought stressed high plant density conditions and direct selection
under drought stressed normal density conditions were found to be equally efficient.
CIMMYT drought tolerant lines and their crosses were tested separately at two plant
densities under both drought stressed and well watered conditions managed by irrigation,
and also in rainfed environments. This was mainly to determine their potential in performance, combining ability and heterosis under both stress and reduced stress
environmental conditions. Considerable differences in performance were observed among
drought tolerant inbreds, and among their crosses under contrasting growing conditions.
However, most crosses from drought tolerant lines provided higher yields than local
hybrids, indicating their suitability for the environments in which they were tested.
In rainfed environments, higher yields were recorded for most genotypes at high plant
density (» 88 800 plants ha -1 ) than the recommended density (» 44 400 plants ha -1 ) for
local hybrids. The opposite held true when unimproved S1 lines derived from population
A-511 were tested in another experiment. This indicated the increased potential in
performance and tolerance to high plant density stress, mainly due to improvement made
in drought tolerance. CML442, Mex102, CML202 and Mex101 were superior lines in per
se performance and GCA effects, especially for yield. Crosses in which these lines were
involved were also superior in yield and most other traits as well as for the corresponding
SCA effects. Both additive (GCA) and non-additive (SCA) effects were important for
expression of the tested traits in rainfed environments. However, additive effects were
relatively more important than non-additive effects for expression of these traits. Drought
tolerant lines with high per se performance in yield across rainfed environments gave high
yielding hybrids under similar conditions, reflecting an association between crosses and
their parental lines.
Similarly, for drought tolerant genotypes evaluated across drought stressed and irrigated
environments, the highest grain yield was recorded under well- watered high plant density
conditions. On the contrary, under drought stressed high plant density conditions, the yield
of lines and their crosses was reduced by 73 and 56%, respectively. Furthermore, the
estimated heterosis increased with increased stress. These results confirmed increased
tolerance of the improved lines and their crosses to high plant density stress, and increased
tolerance of the crosses to drought stress compared to their parental lines. Combining
ability tested in contrasting environments showed a predominant role of additive (GCA)
effects for secondary traits and non-additive (SCA) effects for yield. For drought adaptive
traits and yield, lines CML202 and Ken were superior in GCA and per se performance across diverse environments, while Mex101 was superior under drought stress. Among
crosses, CML440 x Ken, CML440 x CML442, and Mex103 x CML202 were relatively
superior in each and across environments.
Drought tolerant genotypes that perform well in a range of soil N and moisture regimes are
expected to give better yields with reduced genotype by environment interaction (GEI)
across diverse environments. The AMMI 2 analysis model efficiently estimated the
drought tolerant GEI patterns over environments. Crosses developed from drought tolerant
lines showed variability in mean yield and GEI across environments. High yielding hybrids
such as Mex103 x CML442, Mex103 x CML202, CML440 x CML442, Mex101 x
CML445, CM202 x CML445, CML202 x Ken, CML440 x Ken and BH140 showed
minimum GEI, indicating their wide adaptation across stress and reduced stress
environments. In contrast, the unstable, high yielding hybrids like Mex101 x Mex102,
Mex101 x CML442 and Mex102 x CML442 were adapted to unfavourable environments
that included drought stress, and eroded topsoil. When compared to conventional hybrids,
22 and 26 crosses produced from drought tolerant parents were better than BH540 in mean
yield and stability, respectively. On the contrary, the other local hybrid, BH140 was
superior in terms of both parameters, due to one of its parents being improved for reduced
plant height by CIMMYT.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-01042006-073436 |
| Date | 04 January 2006 |
| Creators | Gebre, Gezahegn Bogale |
| Contributors | Prof CS van Deventer, Prof JBJ van Rensburg |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-01042006-073436/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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