Abstract

There is an increasing interest in using plant growth-promoting rhizobacteria (PGPR) to improve hybrid poplar performance under water stress conditions. We assessed the growth and leaf-level physiological responses of different hy-brid poplar clones to the inoculation of a microbial consortium and subjected to moderate water shortage. In a nursery experiment, growth, leaf gas ex -change, and biomass partitioning traits were assessed during one growing sea-son on twenty hybrid clones from interspecific crosses of Populus trichocarpa x P.deltoides and (P. trichocarpa x P. deltoides) x P. deltoides, which were submitted to two treatments of PGPM inoculation (Inoculated, Inoc_1 vs. non-inoculated, Inoc_0) and two irrigation treatments (full water vs. water restric-tion). The water restriction decreased shoot growth, photosynthetic rate (Asat), and stomatal conductance (gs); increased intrinsic water use efficiency (WUEint) of the hybrid clones but it did not change the root-to-shoot ratio. Un-like our expectations, by the end of the study, treatment Inoc_1 slightly de-creased basal diameter (D) and height (H) relative to Inoc_0 (5.8% and 5.2 %, respectively). Moreover, seven clones significantly decreased the root biomass by 37% to 62% in the Inoc_1 relative to Inoc_0 treatment, while the other clones showed no response to the inoculation. Oppositely, while most of the clones showed no response to the Inoc_1 treatment on leaf-physiological traits compared to Inoc_0, some of them exhibited an increase of Asat of 15% to 39%. Overall, the consortium applied did not improve the responses to the water restriction, and responses to the inoculation were more associated with a deleterious than a growth-promoting effect, which is discussed in the context of nutrient immobilization, application method, and timing.