The consistently low yield turnout of maize on farmers’ fields owing to drought and the nutritional challenges attributable to the consumption of white endosperm maize pose a major threat to food and nutritional security in Sub-Saharan Africa (SSA). The objectives of this study were to assess the performance of newly developed extra-early maturing orange hybrids under managed drought and well-watered conditions, compare the outcomes of multiple-trait base index and multi-trait genotype–ideotype distance index selection procedures, and identify drought-tolerant hybrids with stable performance across contrasting environments for commercialization in SSA. One hundred and ninety orange hybrids and six checks were evaluated under managed drought and well-watered conditions at Ikenne for two seasons between 2021 and 2023. A 14 × 14-lattice design was used for the field evaluations under both research conditions. Drought stress was achieved by the complete withdrawal of irrigation water 25 days after planting. Results revealed significant differences among the hybrids under drought and well-watered conditions. Grain yield, ears per plant, and plant aspect under managed drought were correlated to the same traits under well-watered conditions, suggesting that the expression of these traits is governed by common genetic factors. Twenty-nine hybrids were identified as top-performing drought-tolerant hybrids by the multiple-trait base index and the multi-trait genotype–ideotype distance index. Of the selected outstanding 29 hybrids, 34% were derived from crosses involving the tester TZEEIOR 197, demonstrating the outstanding genetic potential of this inbred line. Further analysis of the 29 selected hybrids revealed TZEEIOR 509 × TZEEIOR 197 as the hybrid that combined the most drought-tolerant adaptive traits. However, the hybrids TZEEIOR 526 × TZEEIOR 97, TZEEIOR 384 × TZEEIOR 30, TZEEIOR 515 × TZEEIOR 249, TZEEIOR 510 × TZEEIOR 197, TZEEIOR 479 × TZEEIOR 197, and TZEEIOR 458 × TZEEIOR 197 were identified as the most stable hybrids across drought and well-watered conditions. These hybrids should be extensively tested in multi-location trials for deployment and commercialization in SSA.

Background: Orange maize genotypes are sources of provitamin A (PVA) carotenoids, which are precursors of vitamin A. PVA deficiency and drought constitute major challenges causing increasing food and nutritional insecurity in sub-Saharan Africa (SSA). Breeding of drought-tolerant provitamin A hybrid maize can mitigate these challenges. This study was undertaken to determine the combining ability of newly developed extra-early orange inbreds for grain yield and related traits under managed drought stress and rain-fed conditions, determine the mode of gene action conditioning the inheritance of the traits, and classify the inbreds into heterotic groups.Methodology: One hundred and ninety-six extra-early orange hybrids comprising 180 testcrosses, 10 single crosses, and 6 commercial checks were evaluated under managed drought and rain-fed conditions at Ikenne. In addition, 41 inbreds comprising 36 orange lines and 5 PVA testers involved in the hybrid development were assessed under drought and rain-fed conditions.Results: The means square for general combining ability (GCA) and specific combining ability (SCA) were significant for grain yield and most other traits under both growing conditions. The contributions of GCA to performance were larger than SCA in each growing condition. Broad-sense and narrow-sense heritability estimates for grain yield were 66% and 37% under managed drought and 88% and 32% under rain-fed conditions, respectively. Mid-parent heterosis and better-parent heterosis for grain yield were 338% and 247% under managed drought, while 173% and 137% under rain-fed conditions. Significant positive correlations existed among grain yield of hybrids, heterosis, and specific combining ability under managed drought. The 41 inbred lines were classified into three heterotic groups under both growing conditions. Sixteen testcross hybrids out-yielded the best commercial check under managed drought.Conclusion: The testcross hybrids have great potential for commercialization to address the problem of drought and PVA deficiency in SSA. Inbred TZEEIOR 510 showed desirable GCA effects for grain yield and 04 other traits under drought.

Maize is one of the most important cereals in the world. To maximize the potential of hybrids from heterosis, knowledge of genetic diversity is crucial. This study aimed to examine the population structure and genetic diversity of new extra-early elite orange maize inbred lines of the IITA-Maize Improvement Program using phenotypic and genome-wide SNP markers. One hundred and eighty-seven orange inbred lines sourced from four populations were phenotyped under well-watered conditions and genotyped using the high-density DArTseq platform. Examination of the genetic dissimilarity heatmap based on the Gower matrix derived from phenotypic data showed low variability among the inbred lines, while moderate variability existed based on the identical-by-state (IBS) matrix from SNP markers. Gower matrix (Phenotypic data) assigned the 187 inbred lines into two distinct groups, while IBS matrix (SNP marker data) assigned the inbred lines into four groups. The cophenetic correlation between the two genetics groups (Gower and IBS) was low, indicating a lack of concordance. A joint matrix derived from the Gower and IBS matrices assigned the 187 inbred lines into three groups. Mantel correlation of the combined matrix showed 0.81 and 0.68 magnitudes with the Gower and IBS matrix, respectively. The outcome of this study provided new insights into the genetic diversity and population structure of newly developed extra-early orange inbred lines. By facilitating the optimal use of heterosis in hybrid maize breeding, these findings will contribute to the development of high-yield potential and provitamin A-rich varieties, ultimately improving food security and nutrition in the region.

Background: Understanding the genetic diversity and population structure of maize inbred lines is fundamental for effective hybrid development and strategic heterosis exploitation. The objective of this study was to refine and validate heterotic groups within a diverse collection of extra-early and early maturing tropical yellow and orange maize inbred lines. A total of 1,437 elite tropical yellow and orange maize inbred lines spanning early and extra-early maturing groups were genotyped using a panel of 3,305 DArTag SNP markers. In addition, the study evaluated 280 hybrids comprising 276 hybrids (from 214 inbred lines crossed with four standard testers) and four commercial checks. The yield performance trials used a 14 × 20 alpha-lattice design with two replicates over two years. Results: Marker-based diversity analysis revealed average marker polymorphism of 0.328, minor allele frequency of 0.265, and expected heterozygosity of 0.315. Principal component analysis (PCA), discriminant analysis of principal components (DAPC), and ancestral admixture analysis revealed three genetic clusters, while phylogenetic analysis supported the existence of two major clusters with groups one and three from DAPC and admixture methods combined into a single phylogenetic group. Several sub-groups were identified within the main genetic pools and maturity rather than kernel color was the major driver of these sub-groups. For operational ease in breeding applications, the lines were assigned to two major heterotic groups. The average performance of the hybrid for grain yield and heterosis estimates from between maturity-based heterotic subgroups were consistently and significantly ( p < 0.001) higher than those from within maturity-based heterotic subgroups. Conclusion: These results provide a robust framework for parental line selection, heterotic grouping, and hybrid breeding strategies, facilitating the development of extra-early and early superior yellow/orange maize hybrids adapted to tropical environments.

Maize is an important staple food crop in sub-Saharan Africa (SSA). However, the average grain yield on farmers’ fields falls considerably short of the demand of the region’s population. In addition, the normal endosperm maize widely consumed in SSA is deficient in vitamin A. Drought, a consequence of climate change, is a major constraint to maize production, resulting in seasonal low grain yield in farmers’ fields. The objectives of this study were to assess the performance of newly developed extra-early maturing provitamin A hybrids under managed drought and well-watered conditions, compare the outcomes of multiple trait base-index and multi-trait genotype-ideotype distance index selection procedures, and identify drought-tolerant hybrids with stable performance across contrasting environments for commercialization in SSA. One hundred and ninety provitamin A hybrids and six checks were evaluated under managed drought and well-watered conditions at Ikenne for two seasons between 2021 and 2023. A 14*14-lattice design was used for the field evaluations under both research conditions. Drought stress was achieved by the complete withdrawal of irrigation water 25 days after planting. Results revealed significant differences among the hybrids under drought and well-watered conditions. Grain yield, ears per plant, and plant aspect under managed drought were correlated to the same traits under well-watered conditions suggesting that the expression of these traits is governed by common genetic factors. Twenty-nine hybrids were identified as top-performing drought-tolerant hybrids by the multiple trait base index and multiple-trait genotype–ideotype distance index. Of the selected outstanding 29 hybrids, 34% were derived from crosses involving the tester TZEEIOR 197, demonstrating the outstanding genetic potential of this inbred line. Further analysis of the 29 selected hybrids revealed TZEEIOR 509 × TZEEIOR 197 as the hybrid that combined the most drought-tolerant adaptive traits. However, the hybrids TZEEIOR 526 × TZEEIOR 97, TZEEIOR 384 × TZEEIOR 30, TZEEIOR 515 × TZEEIOR 249, TZEEIOR 510 × TZEEIOR 197, TZEEIOR 479 × TZEEIOR 197, and TZEEIOR 458 × TZEEIOR 197 were identified as the most stable hybrids across drought and well-watered conditions. These hybrids should be extensively tested in multi-location trials for deployment and commercialization in SSA.

In order to meet the expected maize yield by 2050, breeders must work to improve breeding program efficiency by intensifying the implementation of new and improved technologies such as marker-assisted selection (MAS). Dissecting the genomic regions associated with drought tolerance is the first step forward in MAS program deployment for maize improvement under drought stress. Genome-wide association studies (GWAS) were used to investigate and identify quantitative trait loci (QTLs) associated with six traits under drought stress. One hundred and eighty-seven extra-early orange maize inbred lines were evaluated under managed drought stress at Ikenne, in Nigeria, during the 2022 and 2023 dry seasons. The materials were also genotyped using 9355 DArTseq SNP markers and analyzed using the enriched compressed mixed linear model (ECMLM). Enriched compressed mixed linear model was used for association-trait analysis. The ECMLM-based GWAS identified 45 candidate genomic loci associated with the six traits, including five for grain yield, with R² ranging from 8.79 to 25.3%. Independent validation using the multi-locus 3VmrMLM approach confirmed seven high-confidence genomic loci consistently detected by both methods across grain yield, anthesis-silking interval, ear aspect, and ears per plant, providing additional statistical support for these genomic regions. Candidate gene annotation identified biologically relevant genes underlying the validated loci, including Zm00001eb238250 (protein-serine/threonine phosphatase), Zm00001eb040940 (trehalose-phosphatase), Zm00001eb117820 (homeobox protein knotted-1-like 4), Zm00001eb145560 (zinc ion-binding protein), and Zm00001eb294180 (WRKY DNA-binding domain protein), suggesting their potential roles in drought adaptation and grain productivity. These findings improve our understanding of the genetic architecture of drought tolerance in extra-early orange maize and provide valuable genomic resources for accelerating drought-resilient maize breeding.