Improvement in nitrogen-use efficiency (NUE) on maize is among the best strategies to mitigate the problems of poor soil fertility in tropical conditions. The objectives of this study were: i) to quantify the genetic variability for NUE-components and agronomic traits in a set of tropical maize inbred lines; ii) to study the genetic divergence among tropical maize inbred lines under contrasting nitrogen (N) levels; iii) to identify the secondary traits associated with NUE in tropical maize inbred lines; and iv) to identify maize inbred lines efficient in NUE and its components. Sixty-four tropical maize inbred lines were evaluated in the field under low- and high-N conditions for NUE-components and agronomic traits. Genetic variability for NUE-components and agronomic traits was found; lines in eight different groups for each N condition were allocated, and N-efficient inbred lines were identified in different groups. Furthermore, we suggest flowering time traits and kernel number as great secondary traits for selecting tropical maize inbred lines for NUE under both N conditions, and chlorophyll content for selecting for NUE under N stress.

Brazil has two growing seasons for maize (Zea mays L.) production: summer and winter. The additional maize produced in the winter and the high-yielding opportunities in the summer season make it important to understand responses of maize hybrids to row spacing and plant population across the two annual Brazilian growing seasons. In the 2015 to 2016 summer season and the 2016 winter season, maize response to plant population, ranging from 60,000 to 90,000 plants ha -1 , was evaluated for three hybrids in 0.45-, 0.60-, 0.75-, and 0.90-m row spacing. Plant architecture traits, grain yield and yield components did not differ with row spacing, and their responses to plant population were not affected by hybrid type and/or row spacing. The DKB390PRO2 hybrid (single cross) demonstrated better grain yield performance in the summer, whereas the BG7049YH hybrid (three-way cross) had the highest yield in the winter. Plant and ear height increased linearly in response to plant population in the summer season, whereas other plant architecture traits, ear leaf chlorophyll concentration, and grain yield components decreased linearly with increasing plant populations in both seasons. There was a quadratic response in maize grain yield to plant population, and it was maximized with 78,688 plants ha -1 in the summer, and 71,206 plants ha -1 in the winter. Our results show preliminary evidence that regardless of row spacing, maize grain yield can be maximized with DKB390 hybrid (single cross) with 78,500 plants ha -1 in the summer, and BG7049YH hybrid (three-way) with 71,000 plants ha -1 in the winter season.

Major locus for spontaneous haploid genome doubling detected by a case-control GWAS in exotic maize germplasm Key messageA major locus for spontaneous haploid genome doubling was detected by a case-control GWAS in an exotic maize germplasm. The combination of double haploid breeding method with this locus leads to segregation distortion on genomic regions of chromosome five.

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Background The characterization of genetic diversity and population differentiation for maize inbred lines from breeding programs is of great value in assisting breeders in maintaining and potentially increasing the rate of genetic gain. In our study, we characterized a set of 187 tropical maize inbred lines from the public breeding program of the Universidade Federal de Viçosa (UFV) in Brazil based on 18 agronomic traits and 3,083 single nucleotide polymorphisms (SNP) markers to evaluate whether this set of inbred lines represents a panel of tropical maize inbred lines for association mapping analysis and investigate the population structure and patterns of relationships among the inbred lines from UFV for better exploitation in our maize breeding program. Results Our results showed that there was large phenotypic and genotypic variation in the set of tropical maize inbred lines from the UFV maize breeding program. We also found high genetic diversity (GD = 0.34) and low pairwise kinship coefficients among the maize inbred lines (only approximately 4.00 % of the pairwise relative kinship was above 0.50) in the set of inbred lines. The LD decay distance over all ten chromosomes in the entire set of maize lines with r2 = 0.1 was 276,237 kb. Concerning the population structure, our results from the model-based STRUCTURE and principal component analysis methods distinguished the inbred lines into three subpopulations, with high consistency maintained between both results. Additionally, the clustering analysis based on phenotypic and molecular data grouped the inbred lines into 14 and 22 genetic divergence clusters, respectively. Conclusions Our results indicate that the set of tropical maize inbred lines from UFV maize breeding programs can comprise a panel of tropical maize inbred lines suitable for a genome-wide association study to dissect the variation of complex quantitative traits in maize, mainly in tropical environments. In addition, our results will be very useful for assisting us in the assignment of heterotic groups and the selection of the best parental combinations for new breeding crosses, mapping populations, mapping synthetic populations, guiding crosses that target highly heterotic and yielding hybrids, and predicting untested hybrids in the public breeding program UFV.