Combining ability between drought and heat stress tolerant donors and adapted CIMMYT Zimbabwe maize inbred lines
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Drought and heat stress are major abiotic stresses limiting maize production in Zimbabwe and Africa at large. It is of great importance to evaluate the breeding value of combined drought and heat stress donor parents for development of new and locally adapted maize hybrids. A North Carolina Design II (NCDII) mating scheme was used for crossing 10 combined drought and heat stress tolerant donor lines and six adapted CIMMYT Zimbabwe lines. The cross combinations that were successfully pollinated resulted in a total of 30 single cross hybrids and five stress tolerant donor parents were dropped from the evaluations as they did not have all cross combinations with the testers. These single cross hybrids were evaluated under optimum, sandy and managed drought conditions using a 0.1 alpha lattice design with two replications in the 2013-14 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 CIMMYT Zimbabwe adapted maize inbred lines, (ii) to classify the stress tolerant donor lines into heterotic group A and B using CML312 and CML444 as testers and (iii) to evaluate GXE interaction of the single crosses developed. For grain yield and other secondary traits evaluated across environments, significant GCA and SCA effects indicated the importance of both additive and non-additive gene effects in the expression of these traits. Additive gene action contributed more to genotypic variation amongst testcrosses for 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 this trait. The basis used for tester identification was good GCA effects for grain yield. Lines CL1215159, CL133480, CML395and CML444 showed good GCA. For heterotic grouping using CML312 and CML444 as testers, lines CL1215159, VL062656 and CL1215158 were classified in heterotic group A and CL1215157 and CL133480 were classified in group B. In heterotic group A, the single cross CL1215159 x CML312 was identified and in heterotic group B, CL133480 x CML444 was identified as potential single cross testers. This study was therefore able to identify genotypes to be incorporated in stress breeding programmes.