Biostimulant Effects of Bacillus strains on wheat from in vitro towards field conditions are modulated by nitrogen supply

Bacillus velezensisstrains, belonging to plant growth-promoting rhizobacteria (PGPR), areincreasingly used as microbial biostimulant. However, their field application to winter wheatunder temperate climate remains poorly documented. Therefore, threeB. velezensisstrainsIT45, FZB24 and FZB42 were tested for their efficacy under these conditions. Two biological in-teraction systems were firstly developed under gnotobiotic and greenhouse conditions combinedwith sterile or non-sterile soil, respectively, and finally assayed in the field during two yearscoupled with different N fertilization rates. Under gnotobiotic conditions, all three strains signifi-cantly increased root growth of 14 d-old spring and winter wheat seedlings. In the greenhouseusing non-sterile soil, only FZB24 significantly increased root biomass of spring wheat (+31%).The three strains were able to improve nutrient uptake of the spring wheat grown in the green-house, particularly for the micronutrients Fe, Mn, Zn, and Cu, but the observed increases innutrient uptake were dependent on the organs and the elements. The root biomass increases ininoculated plants coincided with lowered nutrient concentrations of P and K. In 2014, under fieldconditions and absence of any N fertilizer supply, FZB24 significantly increased grain yields by983 kg ha–1, or 14.9%, in relation to non-inoculated controls. The three strains in the 2015 fieldtrial failed to confirm the previous positive results, likely due to the low temperatures occurringduring and after inoculations. The Zeleny sedimentation value, indicative of flour quality, wasunaffected by the inoculants. The results are discussed in the perspective of bacterial applicationto wheat under temperate agricultural practices