Genetic and path coefficient analyses and heterotic orientation of maize germplasm under combined heat and drought stress in sub-tropical lowland environments.
Crop failures due to simultaneous occurrence of drought spells and heat waves have become a common phenomenon in tropical and subtropical environments as a consequence of climate change. This phenomenon has raised lots of concerns among farmers and triggered serious debate among governments and scientists. There are no practical agronomic measures to control high temperatures in large open fields for crop production and investment in irrigation is unaffordable for the majority of the farmers in developing countries. Therefore, breeding for combined heat and drought stress tolerance is crucial in order to increase or stabilise maize productivity in tropical and sub-tropical regions. The present research was designed to, firstly, assess genetic variability for combined heat and drought stress tolerance in maize germplasm; secondly, investigate the level of relationship between maize traits correlated with grain yield in inbred lines per se versus hybrids under stressed and non-stressed conditions; thirdly, study gene action controlling maize grain yield and other agronomic traits under isolated heat, drought and combined heat and drought stress conditions; and finally, determine the heterotic orientation of thirty selected maize inbred lines towards three drought-tolerant and one high yield potential inbred tester lines. A hundred and eight inbreds per se were evaluated under severe heat and drought stress, moderate heat-drought stress, random drought stress and non-stressed conditions to assess genetic variability for combined heat and drought stress tolerance. Results revealed existence of wide genetic variability for combined heat and drought stress tolerance among maize inbred lines available in Mozambique but superior genotypes under severe combination of heat-drought stress were not exactly the same under the rest of the growing conditions of this study. However, the study identified 15 out of 108 inbred lines (14%) as the most promising genotypes under severe heat and drought as well as under moderate heat and drought stress environments. The superior lines can be employed in the future breeding programmes for combined heat-drought stress tolerance. Ten inbred lines, including two of the superior entries identified in the genetic variability study, were randomly assembled from the available maize germplasm in Mozambique and used to generate forty-five crosses in a half diallel mating design. The purpose was to study gene action controlling grain yield and other traits under combined heat and drought stress conditions. The diallel cross hybrids were evaluated together with three genetic testers under combined heat and drought stress, drought stress alone, heat stress alone and non-stressed conditions. The yield reduction due to heat stress alone was 19% of the non-stressed experiment while reductions due to drought alone and the combined stresses were 41 and 59%, respectively, indicating that the combined stress condition was more detrimental than the individual stresses. For grain yield, additive gene action was predominant over non-additive and the magnitude of its predominance was larger under combined stress compared to individual stresses and non-stressed conditions. For the other traits, additive gene action was predominant regardless of the environment. The results imply that improvement of tropical maize for tolerance to combined heat and drought stress is possible and it can be faster when selections is conducted under combined stress conditions than either under heat or drought separately. Thirty superior inbred lines (28 from the genetic variability study plus two other elite lines) were selected and crossed as female parents with four other lines as males in a line × tester mating design to assess heterotic orientation of the female parents towards the four male testers under stressed and non-stressed conditions. The resulting 120 testcrosses were evaluated under combined heat and drought stress and non-stressed conditions. Both general combining ability (GCA) due to lines and testers, and specific combining ability (SCA) due to line × tester mean squares were significant under the two water regimes of the study. The proportion of SCA effects was bigger than the total GCA effects under full-irrigation and equal under combined stress environment, indicating that SCA effects were more important than GCA effects under favourable conditions with the importance of GCA effects increasing under combined stress conditions. Combination of the new approach “heterotic group's specific and general combining ability” (HSGCA) and the traditional yield-SCA method successfully associated the thirty female lines to the four testers. It was found that heterotic orientation changed significantly with change in environmental conditions. Twenty inbred lines (67%) changed from one tester to another when experimental conditions changed from fully-irrigated to stressed conditions. Under full irrigation, tester N3 was related with 11 lines (37%) while under stressed condition only seven (23%) were found related with this tester. On the other hand, only five lines (17%) were assigned to tester CML444 under fully-irrigated condition but nine lines (30%) were assigned to it under stressed experimental condition. Testers CML312 and CML445 were related with seven lines (23%) under fully-irrigated condition and they only changed to more and less one line under stressed condition. The results suggest that appropriate tester must be identified and used for specific stress category. Data from the line per se and diallel hybrid trials were used to investigate the level of relationship between grain yield (GY) and other traits under stressed and non-stressed conditions. Genetic correlation coefficients were partitioned into direct and indirect effects following path coefficient analysis. In general, genetic correlation and path coefficients analyses revealed positive and significant relationship between GY and number of ears per plant (EPP) and ear aspect (EA) under stressed and non-stressed environments in both the inbred line per se and hybrid trials. This implies that EPP and EA can be used as indirect selection traits when breeding maize for both stressed (heat and drought) and non-stressed environmental conditions. The study also identified direct positive contribution of shorter anthesis-silking intervals (ASI) to GY under severe stresses but only indirectly through number of grains per ear (NGPE) under moderate stress environments. The NGPE had strong positive direct effect on GY while 100-grain weight contributed only indirectly through NGPE in hybrids. It can, therefore, be concluded from this study that EPP, EA, ASI and NGPE would be useful as secondary traits for maize GY selection under combined heat and drought stress conditions.
Doctor of Philosophy in Plant Breeding. University of KwaZulu-Natal, Pietermaritzburg 2016.
Theses - Plant Breeding.