Nickel (Ni)—a component of urease and hydrogenase—was the latest nutrient to be recognized as an essential element for plants. However, to date there are no records of Ni deficiency for annual species cultivated under field conditions, possibly because of the non-appearance of obvious and distinctive symptoms, i.e., a hidden (or latent) deficiency. Soybean, a crop cultivated on soils poor in extractable Ni, has a high dependence on biological nitrogen fixation (BNF), in which Ni plays a key role. Thus, we hypothesized that Ni fertilization in soybean genotypes results in a better nitrogen physiological function and in higher grain production due to the hidden deficiency of this micronutrient. To verify this hypothesis, two simultaneous experiments were carried out, under greenhouse and field conditions, with Ni supply of 0.0 or 0.5 mg of Ni kg−1 of soil. For this, we used 15 soybean genotypes and two soybean isogenic lines (urease positive, Eu3; urease activity-null, eu3-a, formerly eu3-e1). Plants were evaluated for yield, Ni and N concentration, photosynthesis, and N metabolism. Nickel fertilization resulted in greater grain yield in some genotypes, indicating the hidden deficiency of Ni in both conditions. Yield gains of up to 2.9 g per plant in greenhouse and up to 1,502 kg ha−1 in field conditions were associated with a promoted N metabolism, namely, leaf N concentration, ammonia, ureides, urea, and urease activity, which separated the genotypes into groups of Ni responsiveness. Nickel supply also positively affected photosynthesis in the genotypes, never causing detrimental effects, except for the eu3-a mutant, which due to the absence of ureolytic activity accumulated excess urea in leaves and had reduced yield. In summary, the effect of Ni on the plants was positive and the extent of this effect was controlled by genotype-environment interaction. The application of 0.5 mg kg−1 of Ni resulted in safe levels of this element in grains for human health consumption. Including Ni applications in fertilization programs may provide significant yield benefits in soybean production on low Ni soil. This might also be the case for other annual crops, especially legumes.

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RESUMO -Microrganismos do gêneroAzospirillum possuem o potencial de melhorar o desenvolvimento e a produção da cultura do milho. Porém as respostas dos híbridos de milho não são iguais frente à inoculação com estes microrganismos, devido a possível especificidade na interação entre genótipo e microrganismo e os diferentes manejos adotados. Desta forma, objetivou-se com este trabalho avaliar o desenvolvimento e a produtividade de dez híbridos a fim de indicar entre esses híbridos os mais responsivos quando submetidos à inoculação de Azospirillum brasilense e a diferentes doses de nitrogênio em cobertura. Vários caracteres agronômicos foram avaliados como: altura de plantas, altura de inserção da espiga, diâmetro de colmo, teor de clorofila total, tamanho de espiga, número de fileiras da espiga, números de grãos por fileira, massa de 100 grãos e produtividade de grãos. Além dos caracteres agronômicos avaliou-se a atividade da enzima redutase do nitrato em duas diferentes épocas (V4 -V6 e VT). Os híbridos mais produtivos foram o Biogene 7046, Dekalb 310 VT PRO™ e Agroeste 1596 VT PRO™. O incremento das doses de nitrogênio aumentou o diâmetro do colmo, teor de clorofila total, comprimento da espiga, grãos por fileira e a produção de grãos.Palavras-chave: adubação nitrogenada; bactérias diazotróficas; safrinha. MAIZE HYBRIDS INOCULATED WITH Azospirillum brasilense UNDER DIFFERENT DOSES OF NITROGENABSTRACT -Microorganisms of the genus Azospirillum have the potential to improve the development and production of maize. However, the response of corn hybrids is not equal under inoculation with these microorganisms due to the specificity in the interaction between genotype and microorganism and the use of different managements.Thus, the objective of this work was to evaluate the development and productivity of ten hybrids to indicate the most responsive when subjected to inoculation with Azospirillum brasilense and different doses of nitrogen fertilization.Several agronomic characters were evaluate as plant height, ear insertion height, stem diameter, total chlorophyll content, ear size, number of rows grains per ear, number of grains per row, mass of 100 grains and grain yield. In