Biostimulant effects of rhizobacteria on wheat growth and nutrient uptake depend on nitrogen application and plant development

The capacity of plant growth-promoting rhizobacteria (PGPR) - Bacillus amyloliquefaciens GB03 (BamGB03), B. megaterium SNji (BmeSNji), and Azospirillum brasilense 65B (Abr65B) – to enhance growth and nutrient uptake in wheat was evaluated under different mineral N fertilizer rates, in sterile vs. non-sterile soils, and at different developmental stages. In gnotobiotic conditions, the three strains significantly increased plant biomass irrespective of the N rates. Under greenhouse conditions using non-sterile soil, growth promotion was generally highest at moderate N rate, followed by full N dose, while no significant effect of the inoculants was observed in the absence of N fertilizer. At 50N, plant biomass was most significantly increased in roots (up to +45% with Abr65B) at stem-elongation stage and in the ears (+19–23% according to the strains) at flowering stages. For some nutrients (N, P, Mn, and Cu), the biomass increases in roots and ears was paralleled with lowered nutrient concentrations in the same organs. Nevertheless, growth stimulation resulted in higher total nutrient uptake and nutrient uptake efficiency. Furthermore, Abr65B and BmeSNji counteracted the repression of root development caused by high N supply. Therefore, combining PGPR with a proper cultivated system, N rate, and plant stage could enhance their biostimulant effects.