Combining ability among tropical maize lines in different environmental conditions

A análise dialélica é um dos métodos mais usados para a caracterização e seleção de genitores superiores em um programa de melhoramento de milho. O emprego desse método permite estimar a capacidade de combinação entre genitores e avaliar o desempenho deles em cruzamentos. Dessa forma, o objetivo deste trabalho foi identificar linhagens promissoras para formação de uma população base para fins de desenvolvimento de híbridos de milho para condições de estresses. Para isso, avaliou-se 190 combinações híbridas (CH), provenientes de um dialelo completo entre 20 linhagens de milho e seis híbridos comerciais (testemunhas) de milho em cinco ambientes de Minas Gerais, na safra 2018/2019. O delineamento experimental utilizado foi o de blocos incompletos (látice quadrado 14 x 14) com duas repetições. Cada unidade experimental foi constituída de uma linha de quatro metros, espaçadas em 0,8 metros. Os caracteres avaliados foram: dias até o florescimento masculino (FM, dias) e feminino (FF, dias), altura de planta (AP, cm) e espiga (AE, cm) e produtividade de grãos (PG, kg ha -1 ). Os dados foram submetidos à análise dialélica pelo método 4 de Griffing. O coeficiente de variação experimental variou de 1,84% (FF, em Coimbra 2, sob condições normais de N) a 32,21% (PG, em Coimbra 1, em baixo nível de N). As estimativas de capacidade geral de combinação (CGC) foram significativas para quase todos os caracteres avaliados, exceto para PG em Viçosa 2. Esse fato viabiliza a seleção de linhagens superiores para serem usadas como genitoras na formação de população base. As estimativas de capacidade específica de combinação (CEC) foram significativas para alguns caracteres avaliados em alguns ambientes. Na análise conjunta, a interação CGC x ambientes foi significativa para todos os caracteres pelo teste qui quadrado (P < 0,05). Já a interação CEC x ambientes foi significativa pelo teste qui quadrado (P < 0,05) para PG. As linhagens que apresentaram maiores estimativas de CGC para PG ao longo dos ambientes foram VML124, VML090 e VML083. Dentre as combinações híbridas, a VML022xVML115 e VML003xVML016 se destacaram por apresentarem alta estimativa de CEC, além de seus parentais serem de grupos heteróticos distintos. Conclui-se que existem genótipos que apresentam estimativas desejáveis da CGC e da CEC, o que indica que podem ser selecionados para a obtenção de população base de melhoramento, associado à avaliação em vários ambientes devido à interação desses efeitos com o ambiente. Palavras-chave: Zea mays L.. Dialelo. Terras baixas. Estresse abiótico.The diallelic analysis is one of the most used methods for the characterization and selection of superior parents in a maize breeding program. The use of this method makes it possible to estimate the combining ability between parents and to evaluate their performance at crossings. Thus, our objective was to estimate the combining ability among maize inbred lines under multiple environmental conditions. For this, 190 hybrid combinations (CH) derived from a diallel design of 20 inbred lines and six commercials maize hybrids were evaluated in five environments in Minas Gerais State, 2018/2019 season. The trial design used was an incomplete block (square lattice 14 x 14) with two replications. Plots were 4 m long, with one row spaced 80 cm apart and plot size of 3.2 m2. We evaluated five traits: days to pollen shedding (DTP, days), days to silking (DTS, days), plant height (PH, cm), ear height (EH, cm), and grain yield (GY, kg ha-1). The data were subjected to a diallelic analysis by method 4 of Griffing. The coefficient of variation ranged from 1.84% (DTS, in Coimbra 2) to 32.21% (GY, in Coimbra 1, at a low N). The estimates of general combining ability (GCA) were significant for almost all traits evaluated, except for GY in Viçosa 2. Thus, it is possible to select superior inbred lines for forming a synthetic population that can be used as a base population to develop new inbred lines. The estimation of specific combining ability (SCA) was significant for some traits evaluated in some environments. In the combined analysis, the GCA x environment interaction was significant for all characters by the chi-square test (P < 0.05). The SCA x environment interaction was significant by the chi-square test (P < 0.05) for GY. The lines that had the highest GCA to GY estimates across environments were VML124, VML090, and VML083. Among the hybrid combinations, VML022xVML115 and VML003xVML016 stood out for presenting a high estimate of SCA, in addition to their parents being from different heterotic groups. In conclusion, some genotypes present desirable estimates of GCA and SCA, which indicates that they can be selected to obtain a breeding base population, associated with evaluation in various environments due to the interaction of these effects with the environment. Keywords: Zea mays L. Diallel. Lowlands. Abiotic stress

Genotypic variation and relationships among traits for root morphology in a panel of tropical maize inbred lines under contrasting nitrogen levels

A strategy to increase nitrogen (N) use efficiency in maize is the genetic improvement of N acquisition through root morphology. Here, we quantified the genetic variation of 150 tropical maize inbred lines for root morphology and shoot traits and investigated the relationships among traits. We evaluated the inbred lines at the seedling stage in a greenhouse experiment under two treatments: high N and low N supply. A mixed model approach was used to estimate variance components. Canonical correlations were estimated between root- and shoot-related groups of traits, and the genetic diversity among inbred lines was determined. Our inbred line panel showed huge genetic variability for all traits and presented large genetic diversity under both N levels. Root dry weight (RDW) was associated with shoot dry weight (SDW) at high N, and RDW and total root length (TRL) were positively associated with SDW at low N. Based on SDW, RDW and TRL, we selected a set of the top 15 maize inbred lines to be used in maize breeding programs focusing on N-use efficiency. We therefore concluded that there is a significant diversity in tropical maize inbred lines, which have the genetic potential to produce N-efficient hybrids and maize breeding populations for N stress conditions