ABSTRACT Plant-growth-promoting bacteria of the genus Azospirillum and Herbaspirillum can improve crop yields of maize in tropical environments. The main mechanism proposed for its promotion is related to the auxins produced by these species. The aim of this study was to compare the maize growth response of inoculation using two species, Azospirillum baldaniorum Sp245 (Ab-Sp245) and Herbaspirillum seropedicae ZAE94 (Hs-ZAE94) during the initial growth under controlled conditions. A pot experiment was conducted with five harvests measuring bacterial counts, root morphology, biomass, and nitrogen content, and soluble metabolites for 38 days. Depending on the harvest period, a differential growth response between the two strains test was compared to the control. Plants use the bacterium applied to improve nitrogen acquisition and transform it into higher biomass and nitrogen accumulation, especially for Ab-Sp245. On the other side, Hs-ZAE94 altered the soluble metabolites resulting in higher NO3-, NH4+, and N-amino and lower soluble fractions compared to Ab-Sp245 and the control. The observed plant-bacteria interaction is closely fine-tuned and regulated by the strain used and differences in growth promotion effects cannot solely attributed to the root morphology.

ABSTRACT Inoculating corn with diazotrophic bacteria as growth promoters has been demonstrated to be an efficient agricultural practice in Brazil, mainly due to the root stimulation they provide to plants. This study investigates the corn (Zea mays L.) root architecture in a greenhouse assay where A. baldaniorum Sp245 and H. seropedicae ZAE94 strains were inoculated and evaluated for 22 days under two N levels: 0.6 and 6 mmol L-1 of N. Short-term bioassays were conducted to assess the plant’s response to the addition of indole-acetic acid, two bacterial populations, and two corn genotypes, utilizing image capture software WinRhizo Pro. The growth and distribution of tips, crossing, and length of fine roots were determined to be the most sensitive aspects to inoculation and indole-acetic acid induction. These responses were influenced by the genotype and the number of bacterial cells present. Biomass accumulation analyses quantified these modifications after a 22-day period. Additionally, the growth response was found to be more significant when applying the Hs-ZAE94 strain to plants fertilized with a higher dose of nitrogen (6.0 mmol L-1), and this response was positively correlated with bioassay data. Selected strains used as an inoculant can improve root architecture and, consequently, the N use efficiency.

Abstract The objective of this work was to evaluate inoculation response to Azospirillum baldaniorum strain Sp245 in 14 Urochloa cultivars. The evaluated cultivars were: Marandú, Xaraés, Paiaguás, Piatã, Basilisk, B140, Comum, Tupi, H47, Llanero, Ruziziensis, Ipyporã, Mulato, and Convert HD364. Three experiments were conducted to measure root traits: two using soil and one a sterile substrate. Compared with the control, a single application of A. baldaniorum Sp245 increases dry matter accumulation and number of roots in cultivars Marandú, Llanero, Ruziziensis, and Convert HD364. However, after inoculation, cultivars Xaraés and Ipyporã show some root traits with lower values.

Resumo O objetivo deste trabalho foi avaliar a resposta à inoculação da estirpe Sp245 de Azospirillum baldaniorum em 14 cultivares de Urochloa. As cultivares avaliadas foram: Marandú, Xaraés, Paiaguás, Piatã, Basilisk, B140, Comum, Tupi, H47, Llanero, Ruziziensis, Ipyporã, Mulato e Convert HD364. Foram conduzidos três experimentos para mensuração de parâmetros radiculares: dois em solo e um em substrato estéril. Em comparação ao controle, a aplicação única de A. baldaniorum Sp245 aumenta o acúmulo de massa seca e a quantidade de raízes das cultivares Marandú, Llanero, Ruziziensis e Convert HD364. No entanto, após a inoculação, as cultivares Xaraés e Ipyporã apresentam alguns parâmetros radiculares com valores menores.

ABSTRACT This work aimed to isolate and characterize plant growth promoting rhizobacteria (PGPR) from 10 Paspalum genotypes and evaluate the effect of their inoculation on P. regnellii, P. atratum, and P. malacophyllum genotypes. The bacterial population ranged from undetectable to 107 bacterial cells per gram of fresh matter in the Paspalum genotypes. Initially, we isolated 164 bacteria from rhizospheric soil and roots of the Paspalum genotypes using media N-free LG agar plate, semi-solid NFb, and LGI. The isolates were characterized genetically and physiologically. The sequencing of 16S rRNA showed the presence of many genera, and some are new in association with Paspalum. The most common was Bacillus followed by Rhizobium, Paraburkholderia, Enterobacter, Cupriavidus, Pseudomonas, Dyadobacter and Acinetobacter. Thirty-eight per cent of isolates produced siderophores, 25 % produced solubilized phosphate, and only 9 % produced indolic compounds. Three greenhouse experiments were performed in randomized blocks with six replicates using representative bacterial strains isolated from P. regnellii, P. malacophyllum and P. atratum cv. Pojuca. We also included strain Sp245 (Azospirillum baldaniorum), uninoculated control, and nitrogen control (150 kg N ha−1). There was an increase of up to 53 % in shoot dry matter in P. regnellii inoculated with strain Sp245 and the shoots accumulated more N. In contrast, only small effects were observed for the other Paspalum genotypes inoculated with PGPR from the host genotypes. This study shows a high diversity of diazotrophic rhizosphere bacteria and suggests no strain specificity between the bacterial isolates and the Paspalum genotypes.

RESUMO Aumentar a produtividade de milho em condições de clima e solo do nordeste brasileiro podem ser atingida com a utilização de insumos biológicos acoplado a aplicação de menores doses de N -fertilizante. O objetivo deste trabalho foi avaliar o rendimento de grãos de milho inoculado com duas espécies de bactérias diazotróficas combinadas com doses de N-fertilizante durante três anos consecutivos em um Cambissolo no estado de Sergipe, Brasil. Duas espécies de bactérias foram utilizadas: Azospirillum brasilen se estirpe BR11005 (Sp245) e Herbaspirillum seropedicae BR11147 (ZAE94) e as doses crescentes de ureia de 0, 100, 200 e 300 kg ha-1. Sob condições de campo houve limitação do rendimento dependendo do ano agrícola e regime hídrico. A inoculação, associada a uma dose de 100 kg ha-1 N com as estirpes BR11147 e BR11005, aumentou a produtividade em 1.230 e 614 kg ha-1, respectivamente; sendo 37% e 19% maior que o controle não inoculado. Na ausência do fertilizante também foi observado incremento da produtividade no milho inoculado com BR11147 de 18% e no teor de N na folha índice pela inoculação da BR11145 em 10% comparado ao controle.

ABSTRACT Increasing the crop yield of maize planted in the climate and soil conditions of the northeast region of Brazil can be accomplished with the use of biological inputs coupled with the application of lower doses of nitrogen (N) fertilizer. The objective of this work was to evaluate the yield of maize inoculated with two diazotrophic bacterial species under different N rates, during three consecutive years in an Inseptisol of Sergipe state, Brazil. The two bacterial species used were Azospirillum brasilense BR11005 (Sp245) and Herbaspirillum seropedicae BR11147 (ZAE94). N was applied in the form of urea at 0, 100, 200, and 300 kg ha-1. Under field conditions and depending on the year, the crop yield was limited by the rainfall regime. The inoculation associated with the 100 kg ha-1 N treatment with BR11147 and BR11005, increased productivity by 1,230 kg ha-1 and 614 hg ha-1, respectively. This corresponded to a 37% and 19% productivity increase, respectively, compared to the productivity of the uninoculated control. At zero N, inoculation increased grain yield of maize plants inoculated with BR11147 by 18%. Additionally, the N content of the index leaf inoculated with BR11005 increased by 10% over that of the uninoculated control.

Resumo: O objetivo deste trabalho foi avaliar o efeito de nitrogênio e da inoculação de Herbaspirillum seropedicae sobre a produtividade de milho (Zea mays) e os teores de nutrientes nos grãos. Os híbridos simples BRS 1030 e BRS 1060 receberam inoculação da estirpe BR 11417, na presença ou não de doses de adubação nitrogenada, em Argissolo Vermelho-Amarelo distrófico, durante os períodos de entressafra (plantio em maio de 2012) e safra (plantio em outubro de 2012). Em cada época, os seguintes tratamentos foram avaliados: controle absoluto, sem adubação nitrogenada nem inoculação; controle com inoculação; doses de 50 e 100 kg ha-1 de N, sem inoculação; e dose de 50 kg ha-1 de N mais inoculação. BRS 1030 produziu 1.157 kg ha-1 a mais de grãos que BRS 1060, na análise conjunta dos dois cultivos. Para o primeiro genótipo, a dose de 50 kg ha-1 de N, com inoculação, incrementou em 2% a produtividade na entressafra e em 4,5% na safra. A inoculação de H. seropedicae, estirpe BR 11417, favorece o acúmulo de P nos grãos, mas tem efeito positivo sobre a produtividade somente em combinação com doses de N mineral, o que indica que seu efeito é mais destacado na promoção do crescimento do milho do que na fixação biológica de N.

Abstract: The objective of this work was to evaluate the effect of nitrogen and inoculation with Herbaspirullum seropedicae on maize (Zea mays) yield and nutrient contents in grains. The BRS 1030 and BRS 1060 simple hybrids received inoculation with the BR 11417 strain in the presence or absence of nitrogen-fertilizer levels in a Typic Hapladult, during the periods of off-season (sowing in May 2012) and season (sowing in October 2012). In each season, the following treatments were evaluated: absolute control, without nitrogen fertilization or inoculation; inoculated control; doses of 50 and 100 kg ha-1 N, without inoculation; and dose of 50 kg ha-1 N plus inoculation. BRS 1030 produced 1,157 kg ha-1 more grains than BRS 1060, in the joint analysis of both crop seasons. For the first genotype, the dose of 50 kg ha-1 N with inoculation increased crop productivity by 2% in the off-season and by 4.5% in the season. The inoculation with H. seropedicae, BR 11417 strain, favors P accumulation in grains, but it has positive effect on yield only in combination with mineral N levels, which indicates that its effect is more important for maize growth promotion than for biological N fixation.