Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.Wheat (Triticum aestivum L.) is the third most important cereal crop after rice and maize globally. Dryland wheat production in South Africa is challenged by recurrent drought leading to low profitability for farmers. Development of drought tolerant wheat genotypes presents the most sustainable strategy to mitigate the effects of drought stress associated with climate change. In an attempt to develop drought tolerant wheat genotypes, the wheat research group at the University of KwaZulu-Natal (UKZN) in collaboration with the Agricultural Research Council-Small Grain Institute (ARC-SGI) developed a breeding population and advanced it to the F2 generation. The breeding population was developed through crosses involving selected promising parents with local drought susceptible cultivars. The F2 families need to be advanced to the F3 generation and selected for genetic advancement with regards to drought tolerance and important agronomic traits. Therefore, the overall objective of this study was to select superior drought tolerant bread wheat families at the F3 generation for further screening in advanced generations. The specific objectives of the study were: 1) to undertake early generation selection of wheat genotypes for drought tolerance and agronomic traits for genetic advancement, 2) to determine the combining ability effects and the mode of gene action that controls yield and yield components in selected wheat genotypes under drought-stressed and non-stressed conditions, and 3) to assess the association between yield and yield-components in wheat and identify the most important components to improve grain yield and drought tolerance.
Seventy-eight genotypes consisting of 12 parents and their 66 F3 families were evaluated using a 13 x 6 alpha-lattice design with two replications in two contrasting water regimes under greenhouse and field conditions in the 2017/2018 growing season. The following agronomic traits were assessed: number of days to heading (DTH), days to maturity (DTM), plant height (PH), productive tiller number (TN), spike length (SL), spikelets per spike (SPS), kernels per spike (KPS), thousand kernel weight (TKW), fresh biomass (BI) and grain yield (GY). Highly significant differences (P<0.05) were observed for the assessed traits among the genotypes under the two water regimes. Variance components and heritability estimates among agronomic traits and yield showed high values for days to heading and fresh biomass under drought stress. Genetic advance values of 29.73% and 37.61% were calculated under drought-stressed and non-stressed conditions, respectively, for fresh biomass. The families LM02 x LM05, LM13 x LM45, LM02 x LM23 and LM09 x LM45 were relatively high yielding in both stressed and non-stressed conditions and are recommended for genetic advancement.
The above data were subjected to combining ability analysis to discern best combiners. Significant general combining ability (GCA) effects of parents were observed for DTH, PH and SL in both the greenhouse and the field under drought-stressed and non-stressed conditions.
The specific combining ability (SCA) effects of progenies were only significant for DTH under all testing conditions. The heritability of most traits was low (0 < h2 < 0.40) except for SL which showed moderate heritability of 0.41 under drought-stressed condition. The GCA/SCA ratio was below one for all the traits indicating the predominance of non-additive gene action. Low negative GCA effects were observed for DTH, DTM and PH on parental line LM17 in a desirable direction for drought tolerance. High positive GCA effects were observed on LM23 for TN and SL, LM04 and LM05 (for SL, SPS and KPS), LM21 (TKW), LM13 and LM23 (BI) and LM02, LM13 and LM23 for GY. Families LM02 x LM05 and LM02 x LM17 were the best performers across the test conditions.
Significant correlations (P<0.05) were observed between GY with PH, TN, SL, KPS, TKW and BI under both drought-stressed and non-stressed conditions. Partitioning of correlation coefficients into direct and indirect effects revealed high positive direct effects of KPS and BI on GY under drought-stressed conditions. Among all the assessed traits, BI had significant simple correlations of 0.75 and 0.90, and high direct effects of 0.76 and 0.98 with grain yield under drought-stressed and non-stressed conditions, in that order. The top yielding genotypes such as LM02 x LM05, LM02 x LM23 and LM13 x LM45, showed high mean values for KPS, TKW and BI. The overall association analyses indicated that the latter three traits had significant influence on grain yield performance and are useful for selection of drought tolerant breeding populations of wheat.
Overall, the present study identified promising families including LM02 x LM05, LM02 x LM23, LM09 x LM45 and LM13 x LM45 that have drought tolerance and suitable agronomic traits. These families can be advanced using the single seed descent selection method for further characterisation of end-use quality traits and comparison with local checks or commercial cultivars
