Symbiotic relationships and biological effects of inoculation with plant growth-promoting bacteria in forage species

Abstract

Brazil has the largest commercial cattle herd and it is the largest commercial exporter in the world, an important characteristic of Brazilian cattle raising is having most of its herd raised to pasture. Due to the high cost of chemical fertilizers, the interest for bacterial inoculants has grown. The objective of this work was to identify forage specimens with better association with different bacterial strains in order to promote plant growth. Experiments, with a completely randomized design, where three forage species were evaluated: white oat (Avena sativa L.), brachiaria ruziziensis (Urochloa ruziziensis) and Millet (Pennisetum glaucum), inoculated with four bacterial strains: Azospirillum brasilense, Herbaspirilum seropedicae, Ensifer adherens and Leclercia adecarboxylata. Seed asepsis was performed with acidified hypochlorite and afterward the seeds were transferred to agar / water medium and incubated for 48 h at 32 ° C in the dark. The pre-germinated seeds were placed in test tubes containing 25 mL of MS medium and inoculated with 106 CFU.mL-1 of each of the strains, except for E. adherens which was inoculated with 104 CFU.mL-1. After 21 days of in vitro culture, the following factors were evaluated: length, fresh and dry weight of shoot and root biomass, gross protein and total nitrogen quantification, root morphology by staining with 1% methylene blue, stoma morphology and microbial counting (epiphytic and endophytic). Data were submitted to ANOVA and compared by the Tukey test (p >5%). The different forage species presented different behavior in relation to the association with the bacterial strains. Oats and Brachiaria were considered as more responsive to the bacterial association than Millet because they presented a larger increment in the size of central / lateral roots and root area. Although Oat presented significant results for BP and TN, Brachiaria had a larger epi and endophytic population, indicating that it was preferential for bacteria.