**1. Introduction**

Wheat (*Triticum aestivum* L.) is an annual cycle plant, considered among the cool season cereal, one that has greater economic importance, with large grain yield capacity [1].

The final crop yield is defined according to the cultivar used, the amount of agricultural supplies, and management techniques employed. The increasing use of high-yield potential wheat has implicated in more frequent use of agricultural supplies, among which nitrogen fertilization shown to be important in defining the grain yield [2]. Therefore, there is a need to study wheat cultivars verifying their response to the uptake and utilization of nutrients in the soil and their performance and cultural practices in different environments [3].

Nitrogen fertilization is one of the highest costs of the production process of nonleguminous crops [4]. Wheat, corn, and rice crops utilize approximately 60% of the N fertilizer produced in the world [5]. The use of N fertilizer must be carefully controlled to ensure good yield and manage N in the soil; N fertilizer increases production costs for farmers [6].

Several authors reported a positive response of nitrogen fertilization on grain yield of wheat [3, 7–11]. Due to the high cost of fertilizers and awareness in support of sustainable agriculture and less polluting, in which the research is growing, one possibility would be to use inoculants containing bacteria that promote growth and increase the productivity of plants. Studies on biological nitrogen fixation (BNF) by *Azospirillum* in grass have been carried out in Brazil. Until recently, no commercial inoculants with these bacteria are available in the country [12].

Although the plant genotype performs an essential role in the colonization of bacteria, cultivars with high and low potential of association exist [13]. Several studies have been published confirming that *Azospirillum* produces phytohormones that stimulate root growth in many plant species. The components released by *Azospirillum brasilense* responsible for stimulating root growth are indoleacetic acid (IAA), gibberellins, and cytokinins [14]. Inoculation with *Azospirillum* can improve the leaf photosynthetic parameters, including chlorophyll content and stomata conductance, greater proline content in shoots and roots, improvement in water potential, an increase in water content in the apoplast, more elasticity of the cell wall, more biomass production, and greater plant size which were reported by Barassi et al. [15]. Increases in photosynthetic pigments such as chlorophyll a and b and auxiliary photoprotective pigments, such as violaxantine, zeaxantin, ateroxantine, lutein, neoxanthin, and beta-carotene, which result in greener plants without water-related stress, were verified by Bashan et al. [16].

In addition, the increase in root development caused by inoculation with *Azospirillum* is involved with several other effects. Increases in water and mineral uptake have been reported, as well as greater tolerance to stresses, such as salinity and drought, resulting in a more vigorous and productive plant [17, 18]. According to Dobbelaere et al. [19], positive responses to inoculation with *A*. *brasilense* are obtained even when the crops are grown in soils with high N content available, which indicates that the plant responses occur not only due to the N2 fixed but also mainly depending on the production of phytohormone growth promoters such as cytokinin, gibberellin, and indoleacetic acid. Lemos et al. [20], studying five wheat cultivars, found a positive interaction of *A*. *brasilense* and nitrogen fertilization only for one wheat cultivar (CD 150). Increases in nitrogen fertilization efficiency associated with inoculation with *A*. *brasilense* were reported by Galindo et al. [21] but in the grain yields of corn in the Brazilian Cerrado.

Considering the benefits attributed to several crops by inoculation with *A*. *brasilense*, with emphasis on biological nitrogen fixation, greater development of the root system, and, consequently, greater absorption of water and nutrients, therefore, the inoculation can improve crop performance allowing greater efficiency of nitrogen fertilization. Thus, research that evaluates the nutritional status and wheat yield, in terms of nitrogen rates in association with inoculation with *A*. *brasilense*, is important.
