Genetics of Tolerance to Drought and Low- Soil Nitrogen in Iita and CIMMYT Early Yellow Maize Inbreds.

Recurrent drought and low levels of soil nitrogen (Low N) constitute major constraints to maize production in the savannas of West Africa (WA).  Presently, only a few early maturing maize hybrids have been commercialized in the sub region despite the availability of numerous early inbreds in the IITA maize program and the increasing demand for hybrid seed. Hybrid combinations between the IITA inbreds and selected elite CIMMYT inbreds from environments similar to WA could produce outstanding hybrids. One hundred and thirty-six single-cross hybrids derived from a diallel cross of eleven IITA and six CIMMYT early yellow inbreds plus four hybrid checks were evaluated under drought, low N and optimum conditions at four locations in Nigeria between 2010 and 2012. The objectives were to examine the combining ability of the inbreds for grain yield and other traits, group the inbreds and identify the best testers under the contrasting environments. Results revealed that the general combining ability (GCA) effects for all traits were greater than specific combining ability (SCA) effects under drought, low N, optimum and across environments suggesting that additive gene action was more important than the nonadditive in the set of inbred lines. The inbred lines were classified into four heterotic groups each across environments based on the heterotic groups’ specific and GCA (HSGCA), and the SNP-based genetic distance methods while the GCA effects of multiple traits of the inbred lines  ((HGCAMT) method placed them into three groups. There was close correspondence between the classifications based on the three grouping methods, indicating that they were all effective in classifying the inbreds into heterotic groups. However, the HGCMAT was the most efficient method followed by the HSGCA and then the SNP-based genetic distance methods across research environments because it was the only method that had significant linear contrasts for all possible group comparisons. HGCAMT identified CIMMYT inbreds ENT 17, ENT 15 and ENT 8 as the best testers for heterotic groups 1, 2 and 3, respectively across research environments. ENT 13 had significant and high positive GCA effects for grain yield, and EPP under all environments except EPP under drought, suggesting that the inbred could be invaluable source of favorable alleles for the traits for improving IITA germplasm. Similarly, the IITA inbreds TZEI 17 and TZEI 16 had positive and significant GCA effects for grain yield under low N, optimum and across research environments. Furthermore, TZEI 16 had significant and positive GCA effects for ASI under drought, low N, and optimum environments and negative and significant GCA effects for stay green characteristic under drought and could be used to improve the CIMMYT germplasm for drought tolerance and the stay green characteristic. Hybrids TZEI 17 x ENT 15, and TZEI 149 x ENT 15 were  the highest yielding and most stable across test environments and should be promoted for commercialization in WA.