Most maize (Zea mays L.) in the tropics is grown under low‐nitrogen (N) conditions, raising the need to assess efficient breeding strategies for such conditions. This study assesses the value of low‐N vs. high‐N selection environments for improving lowland tropical maize for low‐N target environments. Fourteen replicated trials grown under low (no N applied) and high (200 kg N Ha−1 applied) N at CIMMYT, México, between 1986 and 1995 were analyzed for broad‐sense heritability of grain yield, genetic correlation between grain yields under low and high N, and predicted response of grain yield under low N to selection under either low or high N. Broad‐sense heritabilities for grain yield under low N were on average 29% sm ller than under high N because of lower genotypic variances under low N. Error variances were similar at low and highN. Genetic correlations between grain yields under low and high N were generally positive. They decreased with increasing relative yield reduction under low N, indicating that specific adaptation to either low or high N became more important the more low‐N and high‐N experiments differed in grain yield. Selection under high N for performance under low N was predicted significantly less efficient than selection under low N when relative yield reduction due to N stress exceeded 43%. Maize breeding programs targeting low‐N environments in the tropics should include low‐N selection environments to maximize selection gains.

Drought and low soil N cause significant yield reductions in maize (Zea mays L.) grown in the tropics. Understanding the genetic basis of hybrid performance under these stresses is crucial to designing appropriate breeding strategies. This study evaluates under optimal, drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the genetic control and modes of gene action for grain yield; and (iii) the relationship between line per se and hybrid performance. Seventeen lowland white‐grained tropical maize inbred lines were used in a diallel study. Lines and their hybrids were evaluated separately in trials under drought stress, low N, and optimal conditions in a total of 12 environments. The differences in grain yield between hybrids and inbreds (i.e., heterosis) increased with the intensity of drought stress. Significant interactions were observed for combining abilities under low and high N. The type of gene action appeared to be different under drought than under low N, with additive effects more important under drought and dominance effects more important under low N. The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parental lines to achieve acceptable hybrid performance under severe drought. Inbreds derived from the population ‘La Posta Sequía’ exhibited the highest GCA effects, stability coefficients, and frequency of dominant alleles for grain yield. Good performance across stress levels can be achieved in tropical maize hybrids.

Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non‐stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and nonstress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F1 performance (F1), SCA, midparent heterosis (MPH) and high‐parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F1, SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

of maize-based farming systems is to be sustained or increased. Drought and low soil N cause significant yield reductions in maizeMaize population improvement for drought tolerance (Zea mays L.) grown in the tropics. Understanding the genetic basis (DT) at flowering has been accomplished in source popof hybrid performance under these stresses is crucial to designing ulations by recurrent selection using managed drought appropriate breeding strategies. This study evaluates under optimal, stress (Bolañ os and Edmeades, 1993a; Edmeades et al., drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the 1999). Recurrent selection under drought was effective genetic control and modes of gene action for grain yield; and (iii) the at increasing yield across a range of drought stress levels relationship between line per se and hybrid performance. Seventeen in all populations under evaluation (Edmeades et al., lowland white-grained tropical maize inbred lines were used in a 1999). Gains under stressed conditions were signifidiallel study. Lines and their hybrids were evaluated separately in cantly greater than those observed in conventionally trials under drought stress, low N, and optimal conditions in a total selected counterpart populations without loss of yield of 12 environments. The differences in grain yield between hybrids potential (Byrne et al., 1995). Improvements were due and inbreds (i.e., heterosis) increased with the intensity of drought to a significant reduction in barrenness and increases stress. Significant interactions were observed for combining abilities in grain number per ear and harvest index, and were under low and high N. The type of gene action appeared to be different accompanied by a reduction in the anthesis-silking interunder drought than under low N, with additive effects more important val (ASI) and a delay in leaf senescence (Bolañ os and under drought and dominance effects more important under low N. , 1993b; Edmeades et al., 1999). Improvement Edmeades The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parentalfor drought tolerance also brought specific adaptation lines to achieve acceptable hybrid performance under severe drought. and improved performance under low N conditions sug-Inbreds derived from the population 'La Posta Sequía' exhibited the gesting that tolerance to either stress involves common highest GCA effects, stability coefficients, and frequency of dominant adaptive mechanisms (Bä nziger et al., 1999). alleles for grain yield. Good performance across stress levels can be Edmeades et al. (1997) showed that population imachieved in tropical maize hybrids.

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