Combining ability and heterosis of heat and drought stress tolerance in maize (Zea mays L.).
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Drought and heat stress are major abiotic stresses limiting maize production in Zimbabwe and Africa at large causing low yields. In order to bridge on this gap of low yields, within its Stress Tolerant Maize for Africa (STMA) program, CIMMYT-Zimbabwe breeders have resorted to look for other sources of heat and drought tolerance genes from exotic populations. Recently, CIMMYT-Zimbabwe breeders sourced 14 heat and drought inbred line donors from India and these were crossed with seven sub-tropically adapted CIMMYT elite lines, using the North Carolina Design II, during the 2017 winter season and 61 single cross hybrids were developed. These single cross hybrids were evaluated under optimum, high density, random stress, natural heat stress and managed drought conditions using a 0.1 alpha lattice design replicated twice in the 2017-18 summer and winter season. The objectives of this study were (i) to estimate combining ability effects among the drought and heat stress tolerant donors and tropically adapted CIMMYT elite inbred lines and identify the best combinations, (ii) to evaluate genotype x environment interaction (GEI) of the single crosses developed and identify high yielding and stable hybrids across all sites. All the data from each site was analyzed using the REML model in CIMMYT Fieldbook. Across site Analysis was done using Minitab Software and GEI was observed to be significant. Analysis of Genetic Designs in R (AGD-R) was used to analyze the combining ability of the inbred lines used in the single cross formation. For grain yield and other secondary traits evaluated across environments, significant general combining ability (GCA) and specific combining ability (SCA) effects indicated the importance of both additive and non-additive gene effects in the expression of these traits although additive gene effects was more prominent. Additive gene action contributed more to genotypic variation amongst testcrosses or the traits measured as evidenced by the higher mean squares for lines and testers than their interaction. For grain yield, additive gene action due to females had much contribution to the genotypic variation therefore highlighting the importance of maternal effects in the expression of grain yield. The best combination was reviewed as the cross between CML566 and CAL14140 that has the highest specific combining ability (SCA) estimates across all sites. CML566 and CML571 were the best females with highest GCA estimates hence recommended to be used as female testers in producing hybrids that are adapted for both stress and non-stress conditions. CAL1440 and CAL14138 were the best male exotic inbred lines with high GCA estimates. There was significant GEI across all the sites used and it was exploited using iii | P a g e Additive main effect and multiplicative interactions (AMMI) in the Genstat. It reviewed that entry 6 (CML566/CAL14138) was the most high yielding and stable across all sites. It was interesting to find out that best commercial checks were out competed by the best experimental hybrids under stress conditions; this shows the potential ability of the newly developed single hybrids to replace the current checks on the market.