ABSTRACT Maize plants can be N-use efficient or N-stress tolerant. The first have high yields in favorable environments but is drastically affected under stress conditions; whereas the second show satisfactory yields in stressful environments but only moderate ones under optimal conditions. In this context, our aim was to assess the possibility of selecting tropical maize lines that are simultaneously N-stress tolerant and N-use efficient and check for differences between simultaneous selection statistical methods. Sixty-four tropical maize lines were evaluated for Nitrogen Agronomic Efficiency (NAE) and Low Nitrogen Tolerance (LNTI) response indices and two per se selection indices, Low Nitrogen Agronomic Efficiency (LNAE) and Harmonic Mean of Relative Performance (HMRP). We performed eight selection scenarios: LNAE; HMRP; Additive index; Mulamba-Mock index; and Independent culling levels. The last three was predicted by REML/BLUP single-trait and multi-trait using genotypic values of NAE and LNTI. The REML/BLUP multi-trait analysis was superior to the single-trait analysis due to high unfavorable correlation between NAE and LNTI. However, the accuracy and genotypic determination coefficient of NAE and LNTI were too low. Thus, neither single- nor multi-trait analysis achieved a good result for simultaneous selection nor N-use efficiency nor N-stress tolerance. LNAE obtained satisfactorily accurate values and genotypic determination coefficient, but its performance in selection gain was worse than HMRP, particularly in terms of N-use efficiency. Therefore, because of the superior performance in accuracy, genotypic determination coefficient and selection, HMRP was considered the best simultaneous selection methodology of the scenarios tested for N-use efficiency and N-stress tolerance.

RESUMO O Brasil apresenta uma demanda anual de mais de 2,83 milhões de toneladas de fertilizantes fosfatados. Parte disto se deve a baixa eficiência no uso do fósforo (EUP) pelas plantas, especialmente nos atuais cultivares de milho. Assim, objetivou-se foi elaborar índices que permitam a seleção acurada de genótipos de milho eficientes no uso de P para condições de baixa e alta disponibilidade deste nutriente. O experimento foi conduzido em casa de vegetação (20º45'14"S; 42º52'53"W), na Universidade Federal de Viçosa, em outubro de 2010. Foram avaliadas 39 combinações híbridas experimentais e 14 linhagens de milho, divergentes para a eficiência no uso de P, em duas condições de disponibilidade deste nutriente. Foi realizada a análise de importância relativa dos caracteres estudados, estimada por meio do método dos componentes principais, a análise de fatores e confecção dos índices de seleção. Para obtenção de genótipos responsivos a alta disponibilidade de fósforo é recomendado o índice ISAP = 0,3985 MRS + 0,3099 MPS + 0,5567 CRLAT + 0,2340 EAbP - 0,1139 SER. Para obtenção de genótipos tolerantes a baixa disponibilidade deste nutriente é sugerido o índice ISBP = 0,3548 MRS + 0,3996 CRLAT + 0,3344 MSP + 0,0041 PA/SR - 0,1019 SER.

ABSTRACT Brazil generates an annual demand for more than 2.83 million tons of phosphate fertilizers. Part of this is due to low P use efficiency (PUE) by plants, particularly in current maize cultivars. Thus, the aim of this study was to create indexes that allow accurate selection of maize genotypes with high PUE under conditions of either low or high P availability. The experiment was conducted in a greenhouse (20º45'14"S; 42º52'53"W) at the Universidade Federal de Viçosa in October 2010. We evaluated 39 experimental hybrid combinations and 14 maize inbred lines with divergent PUE under two conditions of P availability. The relative importance of the traits studied was analyzed and estimated by principal component analysis, factor analysis, and establishment of selection indexes. To obtain genotypes responsive to high P availability, the index SIHP (selection index for high phosphorus) = 0.3985 RDM + 0.3099 SDM + 0.5567 RLLAT + 0.2340 PUEb - 0.1139 SRS is recommended. To obtain genotypes tolerant to low P availability, the index SILP (selection index for low phosphorus) = 0.3548 RDM + 0.3996 RLLAT + 0.3344 SDM + 0.0041 SH/RS - 0.1019 SRS is suggested.

Vários trabalhos de avaliação de eficiência no uso de nitrogênio (EUN) já foram feitos utilizando estádios precoces. Contudo, não há relatos científicos sobre o estádio ideal e, neste, quais caracteres que apresentam maior correlação com a EUN. Assim, o objetivo foi identificar os estádios fenológicos e os caracteres secundários que maximizam a acurácia na seleção precoce e indireta para a EUN em milho. Para isso, foram avaliadas três linhagens endogâmicas de milho em delineamento inteiramente casualizado, com cinco repetições, em esquema fatorial triplo (linhagens x níveis de N x estádio fenológico), em duas doses contrastantes de nitrogênio - baixo e alto nitrogênio. As plantas foram avaliadas em cinco estádios fenológicos sendo eles: nove (V9) e 14 folhas completamente desenvolvidas (V14), pendoamento (VT), florescimento (R1) e maturidade fisiológica (R6). Foram avaliados os caracteres: eficiência na utilização, na absorção, no uso e na translocação de nitrogênio, atividade da nitrato redutase e da glutamina sintetase, comprimento de raiz lateral e axial, área de raiz específica, teor de clorofila, número de folhas, altura de planta, diâmetro de colmo, teor de fósforo e potássio. Considerando os ganhos diretos e indiretos estimados, conclui-se que as atividades da glutamina sintetase, nos estádios V9 e V14, permitem a seleção precoce e indireta para eficiência no uso de nitrogênio em milho, nas condições de baixa e alta disponibilidade de N, respectivamente.

Several studies to evaluate nitrogen use efficiency (NUE) have been carried out using early growth stages. However, there are no scientific reports on the ideal stage for evaluation and on which characteristics have the highest correlation with the NUE at that stage. The aim therefore was to identify the phenological stages and secondary characteristics which maximize accuracy in early indirect selection for NUE in maize. To do this, three endogamic maize strains were evaluated in a completely randomised design with five replications, in a triple factorial scheme (strains x N levels x phenological stage), at two contrasting nitrogen levels: low and high nitrogen. The plants were evaluated at five growth stages: stage nine (V9), with 14 fully-developed leaves (V14), tasseling (VT), flowering (R1) and physiological maturity (R6). The following characteristics were evaluated: efficiency in the usage, absorption, use and translocation of nitrogen; activity of nitrate reductase and glutamine synthetase; length of the lateral and axial roots; specific root area; chlorophyll content; number of leaves; plant height; stem diameter; and the levels of phosphorus and potassium. Considering the estimated direct and indirect gains, it can be concluded that the activities of glutamine synthetase at the V9 and V14 stages permit early indirect selection for nitrogen use efficiency in maize under conditions of low and high N availability respectively.

As limitações do N no cultivo do milho relacionam-se ao fato de a eficiência da adubação nitrogenada no milho não ultrapassar os 50%, principalmente devido aos processos de perda por volatilização, desnitrificação ou lixiviação no solo. Nesse contexto, torna-se importante o desenvolvimento de cultivares eficientes no uso de N (EUN). Assim, o objetivo foi elaborar índices que permitissem a seleção acurada de genótipos de milho eficientes no uso de nitrogênio para as condições de alta e baixa disponibilidade desse nutriente. O experimento foi conduzido em casa de vegetação na Universidade Federal de Viçosa (20º45'14''S; 42º52'53''W) durante o mês de outubro de 2010. Foram avaliadas 39 combinações híbridas experimentais e 14 linhagens de milho divergentes para a eficiência no uso de N em duas condições de disponibilidade desse nutriente. Foram realizadas a análise de importância relativa dos caracteres estudados, estimada por meio do método dos componentes principais, a análise de fatores e a confecção dos índices de seleção. A seleção indireta e precoce para genótipos de milho tropical pode ser realizada por meio dos índices I AN = 0,022 MPS + 0,35 PASR + 0,35 CR AXI + 0,35 EUN, para ambientes de alta disponibilidade de nitrogênio, e I BN = -0,06 PSAR + 0,35 AS AXI + 0,35 CR AXI + 0,39 MPS, para condições de baixa disponibilidade de nitrogênio.

Nitrogen (N) limitation in maize crops is related to the fact that the efficiency of nitrogen fertilization in maize does not exceed 50%, primarily due to volatilization, denitrification and soil leaching. Therefore, the development of new nitrogen use efficient (NUE) cultivars is necessary. The aim of the present study was to develop indices for the accurate selection of NUE maize genotypes for use in conditions of both high and low N availability. The experiment was conducted in a greenhouse (20º45'14"S; 42º52'53"W) at the Federal University of Viçosa during October 2010. A total of 39 experimental hybrid combinations and 14 maize lines differing in NUE were evaluated under two N availability conditions. We determined the relative importance of the studied characters using principal component analysis, factor analysis and by developing efficient selection indices. We conclude that indirect and early selection of tropical maize genotypes can be performed using the indices I HN = 0.022 SDM + 0.35 RSDM + 0.35 RL A + 0.35 NUE for high N availability environments and I LN = -0.06 RSDM + 0.35 RSA A + 0.35 RL A + 0.39 SDM for low N availability environments.

When it comes to recommending sorghum (Sorghum bicolor) cultivars, it is essential to carry out a genetic evaluation of the agronomic traits of promising genotypes from several common environments where the crop is cultivated. This study consisted of a genetic evaluation of 52 experimental grain sorghum hybrids and eight commercial cultivars. Hybrids were evaluated in 19 experiments representing the most varied cultivation conditions in Brazil. Traits of agronomic interest such as grain yield, flowering and plant height were analysed. Genotypic evaluation was performed following the REML/BLUP (Restricted Maximum Likelihood/Best Linear Unbiased Predictor) procedure; the MHPRVG (Harmonic Mean of Relative Performance of Genotypic Values) method was also employed to study stability and adaptability. Hybrids which stood out in terms of highest grain yield based on genotypic values, stability and adaptability were 0306037, 1G150, DKB 599, 0306039, 1G282 and 0307671. Of these, only 1G282 showed restrictions as to plant height. For flowering, experimental hybrids showed shorter cycles than commercial cultivars, confirming the efficiency of genetic improvement for this trait. With the analysis of grain yield considering days to flowering and plant height as covariates, it was observed that most of the hybrids of greater performance, showed grain yield to be positively influenced by plant height and days to flowering.